United Public Domain Gold 2/United Public Domain Gold 2.iso/utilities/pu555.dms/pu555.adf/--------------

Text File | 1994-08-24 | 91KB | 2,036 lines

     Amigatech does it again, the complete COMMS for beginners file.
     Everything you need to know to get started in comms!!!!!!!!!!!!

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                                         Bristol UK


   ==================================================================








                         COMMS FOR BEGINNERS
                         -------------------

                           BY DAVID FOSTER
                           ---------------

                      PART 1: IN THE BEGINNING!
                      =========================

Comms can be a  confusing  subject  for  newcomers,  with all sorts of
jargon, buzzwords and abbreviations to be  found, not only with regard
to setting up  and  using  comms  software,  but  also  logging  on to
different services.

In this series, I  intend  to  look  at  the  the  various aspects and
explain the mumbo-jumbo  in  simple  terms.  You  won't  find detailed
technical descriptions about how modems  or error correction work   my
object is to make all facets of comms sufficiently understandable that
you will know enough about  the  jargon  to  find  your way around the
various on-line services without wasting time and money.

Let's start right at the beginning. Communications provides a means of
passing data (anything  from  text  files  to  programs) backwards and
forwards  over  long   distances,   usually   over   telephone  lines.
Unfortunately, phone lines are  not  really  the  ideal means of doing
this, for a number of reasons.

One problem with the telephone  network  is  the  poor quality of some
lines, which can cause data corruption.  This is being improved as new
lines and digital exchanges are installed throughout the country.

The old, Strowger, type of telephone exchange is purely mechanical and
the noise made by the relays clicking away as people dial out is often
enough to cause noise on nearby lines.

Another problem is that the  way  the  telephone system works makes it
unsuitable for passing data in a  form that a computer can understand,
so some means is required to convert the  data. This is the job of the
modem.

The modem (MOdulator DEModulator)  converts  the  data at the computer
end into a form suitable  for  transmission  over the phone (modulates
it) and reconverts received data back into computer-digestible form by
demodulating it.

Data is transferred between the  computer  and  the modem via a serial
link, where each bit of data is  passed  down the same wire, one after
another, unlike the parallel connection,  used by most printers, where
several bits are passed down a  number  of  parallel lines at the same
time.

The serial port is often referred  to  as  the  RS232 port, or in some
cases, a RS423 port. The latter is  really a sub-set of the RS232 with
enough, though not all,  of  its  facilities.  It's important that the
modems at each end of a line know  the  exact form in which data is to
be transmitted and received and this is done by the carrier tone.

When you dial another modem, one of the modems tries to determine what
form the transfer is  to  take  by  listening  to  the carrier. Having
recognised the signal, it switches  into  the  same mode, so that both
modems can 'talk' to each other and know what each other is doing.

I shall look at the serial port  in  more detail next time, as well as
the settings required when using it.

                     PART 2: THE RS232 INTERFACE
                     ===========================

The RS232 Serial port standard was  one of those wonderful ideas which
never quite came off.

It was originally intended  to  provide  a  standard for serial ports,
with all items of equipment using  the  same type of plugs and sockets
wired in the same way,  so  that  all  equipment which used the serial
port could  just  be  plugged  in,  using  the  appropriate  cable and
everything would work.

So much for theory! As many  of  you  will have long since discovered,
this ideal hasn't really been met, for a number of reasons.

The socket on the back of  a  computer,  for example, should be a male
25-pin D-type, but some portable manufacturers decided that having all
those male pins hanging out the  back  was  not very safe, so chose to
use the female variety of socket instead.

IBM, in its wisdom, decided that  AT  computers  should use a 9-pin D-
type connector and Acorn  settled  on  the  5-pin  DIN connector and a
sub-set of the RS232 standard, called the RS423.

I won't go into  any  great  detail  about  all the possible different
set-ups, other than to say  that  matters  seem  to be improving, with
many computers now using not only the same connectors, but even wiring
them the same way (more or less).

The  way  the  cable  is  wired  is  not  only  important  as  far  as
communicating with another modem is concerned,  but also because a lot
of software reads the various signals to determine whether it is on or
off line, among other things.

The main thing that the modems at each  end  of a line need to know is
the speed at which data is  to  be  transmitted down the line. This is
often referred to as the Baud rate,  but there are also standard terms
used to describe them.

The most common rates are 300/300, also known as V21, 1200/1200 (V22),
2400/2400 (V22bis) and 1200/75 and 75/1200 (V23 and reverse V23) bps.

Bps means bits per second and is  the accurate description of the rate
of data transfer.

V23 differs from the others in that the transmit and receive rates are
different. When such split  baud  rates  are  in  use, one computer is
always set to one rate and the other  to its reverse mode, so that the
1200 rates match and so do the 75bps rates. The effect of this is that
data flows quickly in one direction, but slowly in the other.

This is often not such a problem as  it might appear, as usually a lot
of data will transfer in one direction,  with only a limited number of
responses the other way.

Prestel is a good example,  where  you  will receive a complete screen
full of characters in return for  a  single  press of a numeric key or
the hash key.

Split baud rates are rarely used outside  the UK. At one time, Prestel
and also many  bulletin  boards,  used  split  baud rates exclusively,
although many now support a range of speeds.

Having now got a suitable cable and a suitable data transfer speed, it
is important that the serial port is  set to transmit and receive data
in the same format that the other end  is using. The format of data is
determined by three main items: word  size,  parity and number of stop
bits.

Most services and software  use  either  7-bit  or  8-bit words   each
character sent down  the  wire  consists  of  7  or  8  bits. An ASCII
character consists of only 7 bits, as  this is all that is required to
produce any character between 0 and 127 in the ASCII character set.

If you want to produce characters in the extended character set, which
includes such symbols as the pound sign,  then 8 bits are required for
each character. It is therefore important that each computer knows how
many bits make up each character it receives.

If you are connecting to a  service  such as Prestel, Telecom Gold, or
even a bulletin board, you will not generally have any say in the word
size, as this is determined by the system.

For some obscure reason, most of  the commercial services like Prestel
and TG use a 7-bit format, while most BBs use 8-bit.

Parity is, to a great extent,  obsolete. It was originally intended as
a crude form of error-checking, but  there are now far better methods.
The usual  options  are  odd,  even  and  no  parity.  Again,  this is
determined by the service you are using.

The principle behind this error-checking  is  that  the number of bits
which are set in a character are totalled up and, depending on whether
the number  is  an  odd  or  an  even  number,  another  bit  is  sent
afterwards, to indicate whether the number was odd or even.

When the character is received, the number of set bits is added up and
compared with the parity bit to  see  whether  the number is still the
same. As a means of error-checking,  it  is largely redundant, but you
still have to use the correct settings.

The final setting is the number of  stop bits. Nowadays the setting is
nearly always  1,  but  in  the  early  days  of  communications, when
response was often sluggish, it was not uncommon to use 2 or more stop
bits, to ensure that  at  least  one  was  picked  up by the receiving
computer.

Again, you must use the correct setting,  as the number of bits passed
down the wires for a single  character  is determined by a combination
of all three settings, so both ends must know what to expect.

Next week I shall look at  different  methods of ensuring that data is
received and transmitted without corruption.

                        PART 3: LINE NOI!%SE
                        =====================

One of the problems with using  phone lines for comms, particularly at
high speeds,  is  that  characters  are  sometimes  lost  or  text  is
corrupted. This is usually due to line noise.

Line noise may be caused by  a  number  of things   noisy equipment in
old fashioned exchanges, a fault on the line, or just too much traffic
using the main trunk route lines.

Whatever  the  cause,  line  noise   can   be  anything  from  a  mild
inconvenience to a major problem. If it only happens occasionally while
you are reading text, it may not  be  a problem, but if you are trying
to upload or  download  an  important  text  file,  or,  even worse, a
program file, the  effects  of  even  one  bit  of  corruption  can be
disastrous.

The reason why it appears to be  worse  at faster line speeds is that,
for a given duration of  a  bout  of  line noise, more characters will
have been transferred and corrupted.

Until recently, there wasn't  really  any  solution  to the line noise
problem. A number of file transfer  protocols have now been developed,
to solve the problem  of  transferring  data  program files. There are
lots of file transfer protocols, but  the most common is Xmodem, which
is available on the vast majority of services.

Xmodem is far from the most efficient  in terms of transfer speed, but
has proved to be very  reliable  and  is  so well established that you
cannot afford to ignore it.

Many of the other protocols  which  have  appeared are developments of
the original Xmodem, aiming to  improve  transfer rates or enable more
than one file at a time to be transferred.

One of Xmodem's disadvantages is that  it  will  only work with 8 data
bit connections. This  means  that  it  cannot  be  used with services
requiring 7 data bits,  including  many  of  the major online systems,
such as Telecom Gold.

Another file transfer  protocol  you'll  meet  quite  often is Kermit.
(Yes, it was named after the  frog.)  Kermit will work with both 7-bit
and 8-bit systems.

Transfer protocols follow  the  general  principle  of  reading a pre-
determined amount of data and transmitting it as a block (often called
a packet) of a fixed length,  complete with start-of-block and end-of-
block markers and, usually,  a  checksum.  When the receiving computer
received a  packet  it  strips  out  the  markers,  re-calculates  the
checksum and compares it with the checksum sent with the packet.

If the checksums  don't  match,  the  receiving  computer  transmits a
request to re-send the packet.

If the checksums  match,  the  block  was  alright  and  the receiving
computer sends an  acknowledgement,  at  which  point the transmitting
computer sends the next block and  so  on  until the complete file has
been transferred.

Many protocols are much  more  sophisticated  than  this. I shall look
more fully into the pros and cons of the more frequently-met protocols
in a future article.

File transfer protocols are ideal  for  their intended purpose, but do
nothing to eliminate line noise encountered when just browsing around.
The recent development of  error  correction  protocols addresses this
problem.

The currently most popular error  correction  protocol in this country
is MNP. It stands for  Miracom   Networking Protocol and was developed
by the US modem manufacturer, Miracom.

I'll examine MNP more fully next week.

                       PART 4: ERROR CORRECTION
                       ========================

As briefly mentioned last  week,  MNP  is  currently the most commonly
encountered error  correction  protocol  and  is  to  be  found either
implemented as a part of the modem  the most efficient method or, in a
few cases, as part of the comms software the cheapest way.

There are many levels  of  MNP  error  correction, each more efficient
than its predecessor. Microcom  put  the  original, lower, levels into
the Public Domain, presumably to  encourage the protocol's development
and popularity.

Initially, only level 2 was available,  but  I understand that level 4
is now available and that level  5  may  be before long. Levels higher
than this are still the property of Microcom.

The original versions were not  very  efficient in terms of throughput
of  data,  with  the  rate  of  transfer  reducing  to  about  80%  of
uncorrected  when  using  level   2.   Later  versions  have  improved
efficiency, with level  4  producing  approximately  the same transfer
rate as uncorrected.

The big jump comes with level  5, which introduces data compression to
error correction. Theoretically,  with  the  right  type  of data, the
transfer rate can rise to about twice that of uncorrected transfer.

Many newer modems are available  with  MNP  up  to level 5 (or higher)
but, so far, the  highest  level  available  in  software  is level 5.
Depending on the power of the computer, level 5 in software may not be
as efficient as the same level  incorporated  into a modem, due to the
extra processing that has to be done by the computer.

Everything isn't  quite  as  rosy  as  it  may  seem  though,  as  the
efficiency of the data compression is very much determined by the type
of data being transferred. Plain  ASCII  text usually compresses well,
program files less so.

Compressed files,  such  as  ARCed  or  ZIPped  files,  are  even less
efficient, even to  the  extent  of  being  slower,  because  MNP goes
through the process of  trying  to  compact,  but  fails, only wasting
time.

Another problem is that some of  the  file transfer protocols, such as
Xmodem and Kermit, don't  work  very  efficiently  in conjunction with
MNP. The reason for  this  is  that  the  file transfer protocol wraps
everything up into packets, then passes  it  on to MNP, which proceeds
to do the same before sending it.

At the other end, it has to be unpacked twice, then an acknowledgement
has to be packed up and sent back to the transmitting computer.

There are  a  number  of  file  transfer  protocols  (called streaming
protocols) which are designed for use  with error correction and these
merely pass the  data  through,  leaving  MNP  to  do  the correction.
Ymodem-g is one such and allows greater throughput than most.

Another file transfer protocol  which  works  quite  well  with MNP is
Zmodem, which does  so  by  virtue  of  the  fact  that  it just keeps
transmitting data until  it  receives  a  message  from  the other end
requesting a re-send  because   a   packet   doesn't   check out. This
ought never to happen, as  MNP  should  ensure  that it always arrives
intact.

The same rules apply (even  with  the streaming protocols) with regard
to efficiency  reducing  when  transferring  already  compacted files.
Generally speaking, it is better to use level 4 rather then level 5 if
you want  to  transfer  already  compacted  files,  which  should then
transfer at about the same rate they would do if uncorrected.

MNP is easy to use, only requiring you to tell the modem, or software,
whether you want to use it  and,  if  so,  what level you want to use.
When you call an online  service,  as  soon  as the carriers have been
detected, the modems automatically try  to  agree not only whether MNP
will be used, but the level to be used.

If the modems fail to agree,  either  because  one doesn't have it, or
the line is too bad, it is possible to continue without correction, or
the modem may be configured to drop the line in such a situation.

MNP is a fallback protocol which  means  that if both modems recognise
that the other wants to  use  MNP,  they  attempt to agree the highest
common level of correction, falling  back  to  lower levels until both
can agree.

There are other protocols. Some are  standards which have been agreed,
but  not  used  to  any  great   extent  EPAD,  proprietary  to  modem
manufacturers, requiring both modems to be the same make.

At the moment, MNP is the  leader  in  this country, though things may
well change in the future.  MNP  already  has  a good following and it
will require something special to  persuade  so many people to abandon
it for something else.

                        PART 5 BULLETIN BOARDS
                        ======================

After our brief looks at connecting  a  modem and error correction, it
is time for a complete change, with a look at scrolling services.

There are two main types of scrolling services (those which do not use
the Prestel "paged" format) electronic  mail services, such as Telecom
Gold, which  are  generally  commercial  and  Bulletin  Board Services
(BBS), which are mostly privately run and free.

There is a degree of overlap.  Some  BBs are commercially run and make
charges;  some  electronic  mail  services  also  provide  many  other
facilities.

Bulletin boards can be fascinating  and  very worthwhile, but they can
also be extremely expensive and,  in  some  cases, a complete waste of
time and money. Long-distance calls to  a  good BB can be very costly,
so beware. There are over 300 BBs  in  the UK, but the good ones never
seem to be a local call away!

Most BBs are run as a  hobby  by  comms enthusiasts and this should be
remembered and respected. The person in  charge of a bulletin board is
known as the sysop SYStem OPerator.  In  most  cases the sysop is also
the owner.

The majority of BBs offer a  mixture  of messaging and file/upload and
download facilities and, occasionally, online  games.  At least one BB
Merkinstead, in Manchester enables  users  to  view  Ceefax and Oracle
online, even though it is  a  scrolling  system rather than a videotex
service. Very ingenious.

Depending how sophisticated it is, a  BB  may have many message areas,
each  covering  a  different  subject  comms,  programming  and  micro
specific discussions, for example.

Some BBs also allow messages to be  passed to other bulletin boards in
a network, but it can take  time  for  messages  to get around and, in
terms  of  speed,  is  no  substitute  for  Prestel  or  Telecom  Gold
messaging.

Bulletin boards  may  also  have  a  number  of  files  available  for
download, using suitable file  transfer  protocols.  These are usually
classified by types  of  software  (comms,  utilities)  or  by make of
computer.

All software on BBs should be either  public domain or shareware it is
an offence to distribute  copyrighted  programs without the permission
of the authors.

Public domain describes  software  which  the  author  has made freely
available to everyone, usually on  the  condition  that no charges are
made for it. Often, they still retain the copyright of the program.

Shareware is different (though many people don't seem to think so). It
is commercial software which the  author  makes available for users to
try out.

If, after a reasonable period of time,  you decide you want to use the
program regularly, you  are  morally  obliged  to  pay  the author the
amount of money he or she suggests as a registration fee.

Many BBs also have news areas,  containing information on all sorts of
subjects and doors which provide access to  some of the many games and
programs written specially for the purpose.

Doors are  separate  programs  which  can  be  linked  in  to  most BB
software. The vast majority of these  programs are written for IBM PC-
compatibles.

Setting up a BB is  something  which  should  not  be done lightly   a
considerable  amount  of  time,  money   and  effort  is  needed.  The
requirements for setting up a BB  are  a  computer, a modem with auto-
answer capability, a phone line and suitable BB software.

Most BBs have their own  phone  line,  so  that  they are available 24
hours a day, but some, often  newly-opened BBs, are only available for
certain hours of the day or    more commonly   night because the phone
is needed for normal purposes the rest of the time.

The only satisfactory solution is a completely separate line. Once you
have paid the installation charge,  costs are relatively small   since
people will be calling the BB, the  line  will qualify for a low usage
rebate.

Bulletin board software  is  available  for  most  computers, from the
Spectrum to the Amiga and even  the  BBC  Micro,  with its puny 32K of
memory. Far and away the most popular computer for running a BB is the
PC.

There are several reasons for this.  Not  only  is  a wide range of BB
software available, but hard discs are  relatively cheap and many add-
ons, like compact discs, are available for PCs.

While a hard disc is not  essential,  it  certainly makes for a better
BB, as the amount of data (messages  and  files) you can offer is very
restricted if  you  only  have  floppy  drives.  Hard  discs  are also
considerably quicker.

BB software varies widely, from basic  systems with few facilities, to
multi-user versions, handling several callers  at a time and providing
very sophisticated and customisable options.

The more sophisticated the software, the more time it will take to set
up and maintain. Maintenance  is  something  which  must be considered
it can take the sysop considerable  time to answer messages, check out
software uploaded to the BB by users  and locate it in the appropriate
areas.

Next week, I'll describe how to  set  up  your software to access a BB
and take you through a typical online session.

                    PART 6: HOW TO LOG ON TO A BB
                    =============================

Logging on to a  bulletin  board  for  the  first  time  can be rather
intimidating.

Logging on to Prestel  for  the  first  time  is  a  bit worrying, but
somehow it seems more impersonal than  a  BB, where you are aware that
the sysop may be watching what you are doing. There's no need to worry
though most sysops know that everyone  has  to start somewhere and are
only too willing to help.

Before you can log  on  to  a  bulletin  board,  you  must set up your
software to suit the system. Scrolling  BBs usually use 8-N-1 settings
8 bit words, No parity and 1 stop bit, so if you use the same software
as you use  for  Prestel,  you'll  need  to  re-configure  it from the
Prestel 7-E-1 setting.

If you know what Baud rates the BB supports, then you should set it to
the highest that both the BB and  your modem will handle. If you don't
know, then the only answer is to find out by trial and error.

If your modem handles 1200/1200,  try  that  first as most BBs support
it. If not, then  select  300/300  for  the  first attempt. If neither
works, then try 1200/75 Baud if you've got it.

Once you have successfully logged  on,  most BBs' opening screens tell
you which rates  they  support  and  whether  MNP  error correction is
available. Make a note for future occasions.

If your modem connects but nothing happens, press the Return key a few
times some BBs require "waking up" in this way.

If nothing still happens, one of  two things is probably wrong. Either
the BB hasn't reset the software  properly from the previous caller or
you've connected at 1200/1200 to a service which only supports 1200/75
this occasionally happens. If lots  of  garbage appears on-screen, try
again using a lower speed.

Sometimes a message will appear, informing  you that the BB is running
under "BinkleyTerm" or something similar and  asking you to wait a few
seconds. This  is  part  of  the  system  mentioned  last  week, where
messages are passed round a network of BBs.

After a few seconds, the BB  software  starts up and you are presented
with the Welcome screen. You can  often  speed this up by pressing the
Esc key.

The Welcome screen will usually ask for your name, either as first and
surname separately, or both at  the  same  time. Sometimes it will ask
you to enter NEW or NEW USER if it is the first time you have used the
board.

Most sysops actively  encourage  users  to  give  their  proper names,
rather than pseudonyms. Some software automatically rejects names like
Maggie Thatcher or Donald Duck.

The first time you use a  particular  BB,  it will repeat the name you
entered and ask you to confirm  it,  as  a safe-guard against the name
being corrupted by line noise. If  it  isn't correct, re-enter it. You
are then asked  for  a  password.  The  size  permitted  for passwords
varies, but most systems will  allow  you  to use punctuation marks as
well as letters and numbers.

You may sometimes by asked to  use  a  different name. This is because
there is already a user  with  the  same  name, so change it slightly.
Names must be exclusive as they are used when passing messages between
callers, among other things.

Make sure you remember the precise  name and password you used because
the next time you log on, the  software will check them against a user
list. If they match, you  will  be  allowed  to  continue into the BB,
otherwise you will be thrown out!

Do not log on under different names, unless you really have to it only
makes more work for the sysop and a mockery of the system.

Most BB sysops grant extra facilities  to  known  users, in the way of
either more time allowed you on the  BB   many BBs have time limits on
how long you may stay logged on to them, in any one day,  or access to
more areas.

Sometimes you  will  be  asked  to  complete  a  short  questionnaire.
Generally, you are not obliged to fill this in, but it can provide the
sysop with useful information  about  his  callers. Very occasionally,
the information is used by the sysop  to  contact you in writing or by
phone, to confirm you are who  you  said you are, before granting full
access to the BB.

Once into a BB  for  the  first  time,  particularly  if it is running
different BB software from  any  you  have  come  across,  it is worth
finding the help files and capturing them to study later, off-line. On
most BBs they will be found in a bulletin, news or general files area.

Next week, I'll look at some of  the  things you can expect to find on
BBs.

                       PART 7: WHAT'S ON BOARD?
                       ========================

If you are a newcomer to the  world  of bulletin boards, it is all too
easy to become confused  there  are  so  many  different  brands of BB
software, each with its own 'feel'.

Things are made  even  worse  because  some  programs  allow sysops to
change the appearance of the software completely!

This individuality is also one of  the  fun things about BBs, allowing
sysops to put their personal stamps on their bulletin boards.

Despite these variations  which  usually  extend to visually different
selection menus and help  screens   the  basic  operation  of most BBs
falls into one of two categories.

In nearly all cases, pressing ?  or  H  or  just Return when the BB is
waiting for input will provide  you  with  a help screen detailing the
commands the board will recognise.

The simplest BBs are those  which  just  have  Message and File areas,
where going to the area  results  in  your  being  offered a number of
different sections within each category.

Selecting the Message area from the  main  menu will normally take you
to the first message  area,  which  may,  for  example, be for General
Messages to and from the Sysop.

You can then change to a  different  area  by selecting from a list of
available message areas.  These  are  often  categorised  into special
subjects, such as  Comms  and  Programming,  or  sometimes  into areas
dedicated to different types of computer.

File areas tend to be  divided  in  a  similar fashion, into different
types of software, or software for specific makes of micro.

The second commonly found type of BB  is one has SIGs Special Interest
Groups. The main difference with these is that once you have logged on
to the BB, you are offered a number  of SIGs to select from. These may
be broken down into different Groups for  each make of computer, or by
subjects.

Once you select the SIG  you  want,  you  enter  what is, in effect, a
special BB devoted only  to  that  subject,  often  with just a single
message area, but often a number of different file areas.

The advantage of this is that  you  only  get  to see items related to
your chosen subject, whereas with  the  earlier system you must always
select the relevant areas when you switch between Files and Messages.

Another advantage of the SIG approach is that the sysop can (and often
does) allow other people to run the different SIGs as sub-sysops.

This relieves the pressure  on  him/her  to keep everything up-to-date
and also means that each  area  can  be  run  by  a specialist in that
subject.

Message areas provide a place  where  you  can read notices from other
users, reply to them and post new ones of your own for others to read.
This can result in some very interesting "conversations".

Most BB software allows users to follow the thread of linked messages,
instead of just reading them  in  the  order  they  were posted on the
board.

Most BBs also allow you to  send  private  messages to other users, in
addition to the public messages which can be read by anyone.

Files areas are extremely popular  and  often  contain vast amounts of
Public Domain and Shareware  software.  This  usually  consists of the
latest versions, as people are always uploading new versions they have
found, so they quickly spread round the bulletin boards.

It will be possible to  List  the  files  in  the area (usually with a
brief description of each program's purpose) and often to Search for a
file or files.

You could, for example, search  for  XMODEM  and  obtain a list of all
files  which  contain  the  word  XMODEM   in  either  their  name  or
description. You can  then  choose  to  download  one  or more program
files.

Provision is often made for you  to  view the contents of any archived
files before downloading, to make  sure  the  program  is what you are
looking for.

The range of  file  transfer  protocols  provided  by  BBs varies, but
usually consists of Xmodem, in its  various  guises, at the very least
and sometimes a choice of as many as fifteen.

A warning won't go amiss at this point. Most sysops are only too happy
for you to download software, but some  BBs suffer from what are known
as troughers people who download everything in sight, but never upload
anything, or even leave the sysop a message saying 'Thank you'.

Troughers are rarely  popular  and  the  least  you  can  do is upload
something that you notice isn't already  available on the BB and leave
a message when you log off. The whole BB system works on the principle
of give and take, so don't just take!

Some sysops operate a time  limit,  which  restricts  you to a maximum
time on line to the  BB  each  day,  while  others limit the number of
files you can download at any time.

Generally, if you prove yourself to be the responsible sort, your time
or file limit can be increased by the sysop.

As well as Message and  File  areas,  many  BBs have other interesting
areas such as Doors. These provide a means to run another program from
within the BB and a wide range is available.

Door programs can be almost  anything,  although  many are games which
you can play on-line and often  see  how you do compared with previous
callers.

Bulletin Boards can be fun  and  also  extremely  useful as a means of
obtaining information. They can  also  be  expensive  if you spend too
long on line, so be careful!

Finally, when you log off  a  bulletin  board,  try to do so properly,
using the menu option to do so. Don't just drop the line.

Most BB software copes quite  happily  happily  with dropped lines and
recovers for the next caller.

However, many  of  them  update  user  information  when  you  log off
properly. They can't do this if you just reset your modem or computer.

                     PART 8: A QUICK LOOK AT CIX
                     ===========================

I have looked at the  bulletin  board  scene  over the last few weeks.
These are mostly free,  but  there  are  also  a  number of commercial
services which make charges for accessing  them,  in much the same way
as you pay to use Prestel.

There are two main  types  of  commercial  service. The first includes
bulletin boards which make a charge for their facilities and the other
covers services like Telecom Gold and MicroLink, which also charge.

The big difference between the two types  is that services like TG and
MicroLink have nationwide (even international) access numbers, whereas
"commercial BBs" usually require  a  call  directly  to where they are
based.

CIX (CompuLink Information eXchange) is  an  example of the commercial
BB. There are many telephone lines into CIX, but they are all based in
the same exchange, meaning the cost  of  a long distance call for many
people in addition to the online charges <sigh>.

CIX has developed a reputation for being a 'techy' service, frequented
by computer  gurus.  To  a  certain  extent  this  is  true,  as  many
professional programmers use  CIX  and  it  can  provide  a  wealth of
technical information, but there are  also many "normal" users. (Sorry
all you programmers and techie types  no insult intended!)

The software which CIX uses  is  unusual  in  many  respects and has a
reputation for being "unfriendly". I'd  call it different, rather than
hostile and Help is there if you need  it. CIX is also a very powerful
system  with  lots  of  features  and  comms  novices  may  find  this
intimidating.

CIX is a conferencing system, which means that it is divided into many
sections (conferences), each  of  which  can  have  a  number  of sub-
sections. I believe there are  several  hundred conferences (I haven't
bothered to count  them!),  covering  every  imaginable  subject, from
Politics to Pascal.

One example is the  Arnor  conference,  moderated (maintained) by Mark
Tilley, who is  also  a  Netter,  which  has  topics  for  Protext and
Prodata.

Some conferences are private and  you  must  apply,  or be invited, to
join them, although the vast majority are  open to anyone who cares to
join in.

CIX doesn't have Message and File areas like most BBs. Instead, it has
files  stored  within  conferences.   Once   you   join  a  particular
conference, you can read the messages  in  any  of its topics. In many
cases you can also find  files  to  download  and  you can also upload
files to a conference topic.

CIX is command-driven,  eschewing  the  menu-driven  interface of most
BBs. Once you log on you  are  presented with a simple command prompt,
at which you have to issue the required commands.

This is  probably  what  has  gained  CIX  its  reputation  for  being
unfriendly but it is  also  one  of  the  features  which  makes it so
powerful.

Another feature is  the  scratchpad.  Instead  of  reading messages on
line, you can save them to  the  scratchpad using commands like File 1
to last, which  will  save  all  the  messages  in  the  topic  to the
scratchpad.

You can keep doing this  for  different  topics and conferences, then,
before logging off, you can compress  the  scratchpad into an ARC file
and download it, saving time and money.

CIX also provides a private messaging  system allowing you not only to
send message to and receive  messages  from  other  users, but also to
send and receive files.

The drawback is that unless you  are  lucky  enough to be within local
call reach of CIX, the cost can be extremely high  long-distance phone
call plus online time charges.

If you think Prestel is expensive, then forget about CIX!

I shall look  at  the  commercial  services  which  provide more local
access next week.

                      PART 9: LOGGING ON TO GOLD
                      ==========================

There  are  many  commercial  services.   Some  only  provide  private
facilities  for  companies   and   organisations   on   payment  of  a
subscription, but a number are  open  to  anybody  prepared to pay the
charges.

Two of the better known, particularly  to the home computing and small
business communities, are Telecom Gold and MicroLink.

MicroLink was part of  the  Telecom  Gold  network until October 1989,
when it moved onto the Istel  INet  network. I shall look at MicroLink
later.

Telecom Gold provides local call access to  users in many parts of the
country (guess who isn't one of  them!),  by  means of the PSS (Packet
Switched Stream) network.

Currently, the maximum baud rate  available  is 1200/1200 over the PSS
network, but I understand that work  is  in  hand to not only increase
this to 2400 baud, but also to make MNP error correction available.

2400 baud and MNP to level 5  are  available on TG, but currently only
by dialling a London number, which  also supports 300/300, 1200/75 and
1200/1200.

Like Prestel, TG is a 7-bit  system,  requiring the same settings of 7
bits, Even Parity and One Stop  bit (7-E-1), but unlike Prestel, which
is displayed as frames of text, TG is a scrolling terminal system.

The London direct multi-baud rate  number  also allows 8-N-1 access (8
bits, No Parity, One Stop bit) but this does require suitable software
capable of "stripping" the extra bit from text displayed on screen.

The advantage of being able  to  use  8-bit communications is apparent
when you consider that most file transfer protocols (except Kermit and
Modem7) require 8 bits to work.

Both Xmodem and  Ymodem  file  transfer  protocols  are available when
using the London number and the Ymodem-g "streaming" protocol, for use
with error-correcting  protocols  such  as  MNP,  is  also  available.
Further protocols will no doubt be added in time.

If you are dialling into  TG  via  PSS,  you  must press Return twice,
input A2 followed by a  couple  more  Returns  as soon as the carriers
connect. You will then be presented with the input prompt NUI?

This is asking you to enter your  Network User Identity, which you are
given when you join Telecom Gold. Note  that  when you key it in, only
the first part is echoed back to your screen.

The next prompt is  ADD?,  to  which  you  have  to reply with another
sequence of characters which you are also given when you join.

Things are a lot simpler if you  dial the London number direct you are
merely greeted with a PAD> prompt,  to  which you reply CALL nn, where
nn is the number of the TG  service  you  want  to use. In the case of
InterLink, Micronet's own Gold service, it  is 74. From this point on,
you are into the TG system and  everything  is the same, no matter how
you connected.

You are now asked to Please  sign  on  and should enter ID cccnnnnn at
the > prompt, where ccc  are  the  three  characters and nnnnn are the
numbers which make up your ID.  Finally,  you  are asked to enter your
password.

That may all sound a little complicated but most comms software either
provides the facility to send the  responses automatically, or you can
put them in macros, requiring only a keypress or two.

By the way, with  security  in  mind,  never  include passwords in any
automatic log-on sequences you have stored on disc.

There is another way for Micronetters to gain access to TG this is via
the InterLink Gateway on Prestel. InterLink is the service provided by
Micronet and allows the cheapest access  of  all to Telecom Gold, even
though you have to  pay  an  extra  charge  over  and  above your your
Micronet sub to access it.

The easiest way to access InterLink  from  within Prestel is by keying
*INTERLINK#, then following the routes from there.

Two gateways are available. The  first  of  these uses a semi-videotex
format,  where  everything  is  converted  into  40-column  width  and
displayed as frames.

The second is a scrolling gateway, which  allows you to use TG in much
the same way you would when accessing via PSS or dialling London.

This is the most  satisfactory  way,  but  does require comms software
which can switch  between  viewdata  and  scrolling  displays while on
line. Not all comms proggies can do this. Accessing TG via the Gateway
is an ideal way of receiving mail, as  you  can get it as fast as your
software and modem allow.

It is not ideal for  sending  messages,  though,  due to the fact that
Prestel does  not  use  handshaking  and  its  buffers  soon  fill and
overflow, as they do if you  try  uploading  text to a Prestel Mailbox
faster than 300 baud.

I find I use the gateway to read and download most of my mail and dial
either PSS or London to upload mail.

Next week, I shall look at what you can do once you are on line to TG.


                   PART 10: ON LINE TO TELECOM GOLD
                   ================================

Whereas Prestel is an  interactive  medium,  with  its  chat areas and
publicly readable letters  areas,  Telecom  Gold  is  mainly  used for
sending private messages.

Being a scrolling system,  TG  is  ideal  for  handling large lumps of
text. It permits full-screen-width messages  of unlimited length to be
sent and received. (To the  best  of  my  knowledge;  I have not tried
sending a never-ending message!) This article will find its way to the
Micronet office via TG.

Telecom Gold messaging  offers  many  options.  Sending  and receiving
messages is relatively simple once  you  have  sorted out what you are
supposed to be doing.

The same message can be sent  to  many  people at the same time. Blind
Copies, where the recipient is  not  aware  that  the message has been
sent to others and Carbon Copies, which list the other recipients, may
be sent.

Messages received can be forwarded to others, receipt of your messages
at the other  end  acknowledged  and  you  can  even  request that the
message recipient is placed in  "reply"  mode  as soon as she/he reads
your message.

Marking a  message  as  Express  will  send  it  to  the  top  of  the
recipient's queue of messages.

If you want more information on the command syntax used with messaging
(and other options),  there  is  a  considerable  amount  available on
Micronet. Type *INTERLINK# for the main InterLink menu, then Key 2 for
the InterLink User Guide.

One item barely mentioned  (alas)  in  the  User  Guide  is the PCMAIL
command previously called WPMAIL which  is  just about the most useful
command I know, enabling a lot  of  messages  to be uploaded at one go
extremely easily. Many people haven't even discovered it.

The user creates a single  text  file  (using  a word processor) which
contains the messages and  where  they  are  to  be sent. Once online,
she/he just types PCMAIL and sets their uploader going.

As long as the user has included  the .S command (which actually sends
the message) at the end of  each  message  and  .END at the end of the
file, TG will do the rest.

A useful tip is to add -ECHO to the PCMAIL command, so that everything
is displayed on your  screen  as  it  is  uploaded. Full details about
PCMAIL are available on TG by typing INFO PCMAIL at the > prompt.

INFO is a very useful  command,  providing online help and information
on a wide range of subjects. Some info  files are quite long, so it is
worth capturing them with your software and digesting the contents off
line.

INFO INFO tells you which  subjects  have  INFO files available. There
are so many  INFO  files  that  you  could  probably  spend a lifetime
reading them all!

Telexes may be received and sent worldwide via InterLink and full help
is available in the InterLink User  Guide.  Extra charges are made for
sending telexes, according to the number of characters sent.

Beware! Do NOT use the  TELEX  command  unless  you  intend to use the
service, as it creates a  file  called  TELEX2 which you cannot delete
yourself and which incurs a small storage charge.

Faxes may also be  sent  via  TG  and,  again,  the procedure is fully
described in the User  Guide.  Unfortunately,  it  is  not possible to
receive incoming faxes, nor to to  send faxes containing graphics, but
in many situations this is acceptable and far cheaper than using a fax
bureau.

Apart from messaging in its various  forms, TG has many other services
which can be accessed.  For  example,  there's Jordans, which contains
Companies House information on large companies.

Use INFO DATABASE  at  the  >  prompt  for  full  details of available
databases. You can even  get  information  on  the  BBC  TV Micro Live
series if you really feel you have to!

There is a lot of information available on  TG and some of it does not
appear to  be  particularly  well  publicised.  A  useful  command  is
GOLDNEWS, which periodically  provides  information  on  new items and
changes.

Using InterLink via the Prestel gateway is an ideal means of accessing
TG to receive mail and  to  indulge  in  a  bit  of browsing, as it is
cheaper than accessing it via PSS.

But until Prestel can handle the  uploading  of messages at a sensible
speed, it is better to use PSS to upload messages if you have prepared
them off line.

                          PART 11: MICROLINK
                          ==================

MicroLink has been going for a number  of years. Originally, it was on
Telecom  Gold  and  offered  a   range   of  facilities,  including  a
"traditional" BB for messages to be  posted  on and access to a number
of specialist services. Last year MicroLink  moved from TG to the Inet
Istel network.

There is a larger number of access points than for TG, but these are a
mixture of baud rates and you may  find,  as I do, that the only local
number  is  a  1200/75  number.  This   is  alright  for  reading  and
downloading, but is hopeless for uploading anything.

There are also several  1200/1200  and  2400/2400  access points and a
number of multi baud rate numbers. Some access points also provide MNP
error correction, but only to level 2, though some of the newer access
points offer level 4.

One strange thing I have found with  the system is that very often the
1200 and 2400 numbers are  considerably  slower  than the 1200/75 baud
numbers.

I suspect the reason for  this  is  that  the 1200/75 baud numbers are
actually a different network and may  well be less heavily loaded with
traffic.

A lot of new and  enhanced  features  were  promised when the move was
announced, but many  have  only  just  come  online,  or  are still in
abeyance. Access is available to a number of information databases and
also to multi-user games.

At first, a lot of people  found  the  new system very confusing for a
number of reasons.  Many  things  are  very  different  and  take some
getting used to. Caucus is one of these.

Caucus is the name of  the  Bulletin  Board  software and, though it's
very powerful, it is also  quite  confusing  and not very intuitive to
use. This hasn't been  helped  by  the  fact  that  only  a very basic
introduction to the system was provided  at the time of the changeover
and, as far as I know, full documentation  on how to use the system is
still not available.

Caucus offers a  "conferencing"  system  it  is  broken  into separate
sections devoted to different makes of computer and subjects.

Messaging on MicroLink vaguely resembles that  on TG, but without many
of the options and, again, is not very intuitive. Most systems use the
word Send to send a message MicroLink uses Compose!

This is one of  the  problems  with  MicroLink.  The various parts use
different software, requiring different syntax  to  be used to perform
the same tasks in different areas.

Sending a message  is  totally  different  from  posting  a message on
Caucus.  You  also  find  that  different  commands  are  required  at
different stages to move back up the tree structure.

Another problem is that  some  parts  of  the  system  actually run on
different computers. The effect of this is that if you are reading the
letters in the PC Link area, you cannot reply to them directly, as you
could before, but have to exit PC  Link, go back to the main messaging
area and send sorry, Compose a message from there.

If you thought TG was confusing,  then  MicroLink might make you think
again. This is all the  more  surprising  when  you consider the large
menus which appear at every possible opportunity and the volumes of on
line help available!

The  only  solution  is  to  use  MicroLink  often  enough  to  become
conversant with it. Repeatedly calling  up  the extremely verbose help
is an expensive pastime, as  you  are  charged  for every block of 500
characters received.

It is also possible  to  send  messages  to  a  number  of other email
networks. This used to  include  Telecom  Gold,  but I understand that
this facility has now been withdrawn.

File transfer is  currently  very  rudimentary,  consisting  only of a
non-windowing version of  Kermit,  which  appears  to  be very quirky.
Other protocols are promised for the future, but not yet implemented.

Overall, MicroLink promises a  lot,  but  still  seems to be suffering
from teething troubles.

While local call access is more  widely  available than with TG, there
appears to  be  a  fair  amount  of  trouble  with  keeping  them  all
functioning, with numbers periodically "going down".

Very frustrating if your next nearest number is a long distance call!

                   PART 12: FILE TRANSFER PROTOCOLS
                   ================================

Some weeks ago I  said  I  would  come  back  to  the  subject of file
transfer protocols, so I will take a break from looking at the various
on line services this week.

Depending on the sort of data you  want to transmit over the telephone
lines, some form of error  checking  is  important, even vital. If you
only send text files, it may  be  sufficient  to just transmit them as
ASCII files without any error  correction  and hope that noise doesn't
corrupt anything.

It is easy enough to see  where  corruption has occurred in text files
as bits will not make  sense  and  usually  you  can make sort out any
corrupted bits.

Many businesses transfer considerable amounts of data between branches
as part of their  business  and  in  such  cases  it is essential that
corruption does not occur. The same is also true of some text files.

Trying to detect corruption in a  text file containing lots of numbers
may well be difficult and if you are sending anything to a typesetting
company, they aren't going to  waste  their  time  reading the text to
check for corruption.

Bulletin Boards thrive on the passing backwards and forwards of PD and
Shareware programs and file transfer protocols  are part and parcel of
this process.

In simple terms, file  transfer  protocols  function by 'chopping' the
file into chunks, or packets,  and  transmitting  them with markers to
indicate the start and end of  the  packet, together with some form of
checksum figure.

Once the packet has been received  the  receiving end strips out start
and end markers, performs a  check  sum  analysis on the received data
and compares the result with the  checksum it received. If they match,
the data is alright and  an  acknowledgement  is  sent,  but if not, a
'request to re-send' is made.

The most commonly encountered file  transfer  protocol must be Xmodem.
This was originally developed  by  a  gentleman  by  the  name of Ward
Christiansen, who put the protocol into the Public Domain.

Xmodem is a relatively old protocol  now,  but remains popular for the
reason that just about every comms program includes it as at least one
of its options. There are one or  two that don't provide any protocols
at all and others,  mostly  specialist,  which  only provide their own
proprietary protocols, but, in general, if the program supports a file
transfer protocol, it will be Xmodem.

Even Xmodem is available  in  a  variety  of  flavours. There are both
Xmodem CRC Xmodem Checksum and Xmodem-1K types.

CRC and Checksum are essentially the same with the exception that they
use different methods of checking the integrity of the blocks.

CRC, incidentally is short  for  Cyclic  Redundancy  Check. I won't go
into any further details about the differences, other than to say that
CRC is  more  efficient  thanbChecksum,  so  if  it  supported  by the
software at both ends, use it. Most versions of Xmodem will attempt to
use CRC checking initially and fall  back  to Checksum if that is what
the transmitting program is using.

Xmodem-1K differs from Xmodem in that it transfers blocks in 1K chunks
rather than the standard 128  byte  blocks.  The  effect of this is to
considerably speed up the transfer of data for a number of reasons.

Firstly, as the  blocks  are  larger,  fewer  have  to  be  packed and
unpacked, which saves processing time  and  also reduces the number of
bytes transmitted, as the various packet markers and checksums all add
to the amount to be sent.

Secondly, Xmodem waits for an acknowledgement from the other end after
every block, before either re-sending  the  block if it was corrupted,
or sending the next block.

The disadvantage of using  Xmodem-1K  is  that  if  a packet fails the
check, the whole 1K must be  re-sent.  Unless the line is particularly
bad considerable  time  savings  can  result  from  using  the  larger
packets.

Xmodem is a single file transfer  protocol,  meaning that you can only
send one file at a time  and  you  have  to tell the receiving end the
name of the file before starting.

Over the years, many new protocols  have been developed, most of which
have their roots in Xmodem.  One  of  the most commonly encountered of
these is Ymodem which was developed by Chuck Forsberg.

Ymodem is basically the same as Xmodem-1K, but was originally intended
to be able to  transfer  multiple  files.  This  was  made possible by
Ymodem sending a header to  the  other  end, containing details of the
name of the file  being  sent,  its  size  and date/time stamp, before
commencing  the  true  file  transfer.   Ymodem  still  waits  for  an
acknowledgement from  the  other  end  before  transmitting  the  next
packet.

Unfortunately, some of the implement-ations  of Ymodem weren't as good
as they should have been and even though they sent the file name, many
of them would only receive  a  single file. Ymodem-Batch was developed
as a result and does allow  the  transfer  of  more than one file in a
session, with a new header block being transmitted between each file.

This was a big step forward,  allowing, as it did, unattended transfer
of many files.

I shall continue with the discussion  of  the main protocols and their
merits next week.

                           PART 13: ZMODEM
                           ===============

Perhaps one of the  most  common  variants  of  Xmodem is Zmodem, also
developed by Chuck  Forsberg.  This  addressed  several limitations of
Xmodem, Ymodem and also introduced a number of new features.

The first enhancement was  a  tightening  up  on the error-checking by
using a 32-bit CRC check,  which  reduced even further the possibility
of corruption.

With Xmodem there is a small chance  that two errors in a packet might
cancel each other out, giving  the  impression  that everything is OK.
Using a 32-bit check reduces that chance even further.

Zmodem also tackled  the  problem  of  speed  by  eliminating the wait
between  blocks  while  the   transmitting   program   waited  for  an
acknowledgement or a request to resend.

Basically, Zmodem keeps transmitting blocks one after another until it
receives a request to re-send  if  there  is  anything is wrong with a
block, or transmission and  reception  get  out  of step. This reduces
delays considerably.

Zmodem uses a sophisticated method of  coping  with bad lines and line
noise. Depending on the  speed  of  the  connection between computers,
Zmodem will typically transmit 1K blocks with 2000 bps connections (at
lower speeds it starts with a smaller block size).

If a request is received to resend  a block, depending on the cause of
the problem, Zmodem will reduce  the  block  size  by  half, so only a
512-byte block is sent. If  this  is  successful,  it will continue to
send 1K blocks for a while, but  if no more errors are encountered, it
will step up a gear again and start transmitting 1K blocks.

Conversely, if it meets failures with 1K  blocks, it will drop down to
128-byte blocks and then step  back  up  if everything is alright. The
smallest block size I have seen is 32 bytes.

Although sending  smaller  blocks  is  less  efficient,  because  more
processing is required, it is  still  relatively efficient as there is
no pause between blocks.  Another  advantage  is  that the smaller the
block, the less likelihood of a block being corrupted and the transfer
fail due to excessive re-tries of the same block.

I  have  seen  Zmodem  successfully  transfer  files  when  all  other
protocols have been beaten by line noise.

Not content with such  enhancements,  file  transfer recovery features
were included. If a  file  transfer  fails,  the the receiving program
closes down the file in an  orderly fashion, rather than abandoning it
as some protocols do.

If the original uploader rings back (it  can be at any later date) and
tries to re-upload the file, the receiving end checks whether the file
exists, then tells the transmitter how big it is.

If there is a difference in  file  size,  a  comparison is made of the
last block uploaded and transmission continues from where it left off.
A real boon if a transfer fails after 350K!

If the two files don't match  up  to  the point when failure occurred,
Zmodem re-transmits the complete  file.  This  can  also happen if you
tried to upload a different version of a file with the same name.

Zmodem also has many other bells  and whistles and may include options
to allow or restrict the  transmission  of  directory paths as well as
filenames, suppress the overwriting of existing files of the same name
at the destination and even automatically  to commence a download when
the other end transmits a special sequence of codes.

Even though the Zmodem protocol has  been  put into the Public Domain,
it's only fairly recently  that  versions  have appeared for computers
other than the PC. It is now available  for ST, Amiga and even in CP/M
versions as well as for UNIX-based machines.

Zmodem is also available in "add-on" form  so  that it may be run from
any comms program if it supports external protocols which many do.

I would say that, currently,  Zmodem  is  the most frequently found of
the newer protocols and is the one most people will choose.

Next week I shall look at some of the other popular protocols.

                   PART 14: KERMIT AND SOME OTHERS
                   ===============================

Kermit is a very popular protocol,  not  only  because there is a good
chance that it will be available for  most systems, but because it has
one other feature which almost makes it unique.

Whereas most file transfer  protocols  require 8-bit communications in
order to work, Kermit can work on  both  7-bit and 8-bit systems. As a
result, Kermit has been  popular  with  Telecom  Gold  and other 7-bit
systems for some time.

Kermit is American in origin (aren't  they  all?) and was developed by
Columbia University and, yes, it is named after the Muppet frog!

The protocol was placed into the  Public  Domain,  which is one of the
reasons why it has been implemented  in  so many operating systems and
comms programs.

In the UK, Kermit is  officially  represented by Lancaster University,
who can supply support and advice if necessary.

There are many different versions of Kermit available, but all of them
agree on the highest common features and use them during transmission.
This means that if you  have  a  modern  version  of Kermit and try to
communicate with an older  version  with  fewer features, the programs
will sort out the highest common features and use those.

Some of  the  developments  to  Kermit  include  data  compression and
sliding windows. This last  feature  makes  a big difference, allowing
full-duplex  communications,  with   the   program  transmitting  data
continuously, while at the same  time  scanning for responses from the
other end.

Many versions of Kermit include Server Mode, in addition to the normal
file transfer mode Telecom Gold uses it.

Selecting server mode  puts  you  in  control  of  the host computer's
protocol and use commands  such  as  GET,  SEND,  LOGOUT and FINISH to
control transfers, log off or return  to the normal mode on completion
of transfers.

The  different  versions  vary  wildly  in  efficiency;  the  standard
protocol isn't particularly quick,  but  sliding windows versions have
quite reasonable transfer rates.

There are also a number of  other  protocols which regularly appear on
bulletin boards, many of which are only available for use on PCs.

SEALink  is  a  proprietary  protocol,  based  on  a  sliding  windows
development of Xmodem. It  was  originally  developed  to overcome the
delays which occur when Xmodem is used with PSS networks and satellite
transmissions.

These have a disastrous effect on the transfer spped of normal Xmodem,
which always waits  for  an  acknowledgement  before  sending the next
block. SEALink appears to work alright,  but has few, if any, benefits
over the other streaming protocols.

CompuServe B  (CIS  B)  is  a  protocol  developed  for  users  of the
CompuServe Information Service in the USA.

MegaLink is yet another protocol  with  its  roots in Xmodem, the main
differences being that it uses 512-byte blocks for faster transfer and
a 32-bit checksum for more accurate error-checking.

Another interesting protocol is Jmodem.  What  makes it different from
most protocols is that it starts off  with small (512 byte) blocks and
if the line appears good, with no re-tries, it gradually increases the
size of the block in jumps of 512 bytes, up to a maximum of 8K blocks.

If a block fails to check,  it  is  re-transmitted, but reduced by 512
bytes, up to a maximum of 8K blocks.  If a block fails to check, it is
re-transmitted, but reduced by 512 bytes  in  size. If it still fails,
it is reduced again and so on.

The one big flaw with Jmodem is  that  if  you manage to build up to a
big block size and then have a failed block transfer, the whole 8K has
to be re-transmitted.

Other protocols which have appeared on the scene recently include Lynx
and Puma, both by the same  author  and trying to make improvements on
Zmodem, which is probably the current leader in the popularity stakes.

Both protocols provide similar  facilities,  but  appear  not to offer
anything really new except, possibly, a slight increase in speed.

BiModem is the most interesting of  the new protocols and is radically
different from all  the  others  I've  I've  described. BiModem allows
files to be transferred in both  directions simultaneously and with no
apparent loss in transfer speed.

On top of that, BiModem enables you  to  "chat" with the person at the
other end at the same time with  error correction! The program is very
complex  and  still  exhibits  one  or   two  minor  quirks,  but  is,
nonetheless, a remarkable achievement.

I have used it quite extensively  in  the  last month or two and never
cease to  be  impressed  with  the  speed  at  which  files  are being
transferred, with transfer rates  in  both  directions typically being
slightly faster than Zmodem.  I  think  we  shall  see  a  lot more of
BiModem in the future.

That concludes my look at  file  transfer protocols. I haven't covered
them all, but have tried to look at those which are readily found and,
to a great extent, those which  are  available  for  use on a range of
different computers.

                         PART 15: DUMB MODEMS
                         ====================

In general, the old saying that you  get  what you pay for stands true
for modems. Having said this, there  is  sometimes a wide variation in
the price of equally powerful units.

I do not intend to go into  great  detail about the actual workings of
modems, as Paul Blitz's excellent  'Advanced  Comms' series running at
the moment gives the low down on all the technical stuff.

There are two  types  of  modem:  intelligent  and  dumb. Intelligence
refers  to  whether  the  modem  actually  contains  any  software  to
interpret and control what is going on. Some of the latest intelligent
modems contain their  own  processors  which  are  as  powerful as the
average computer. Nevertheless, dumb  modems  are not entirely without
intelligence.

A typical dumb modem is the  Datachat  1223, which is free to Micronet
subscribers. There is absolutely nothing  wrong with dumb modems; they
work as well as  any  other  modem  within  their  restricted range of
speeds. In many cases,  the  only  speeds  available  are  300 bps and
1200/75 bps.

Generally speaking,  they  have  no  facility  to  auto-dial  numbers.
Instead, you have to dial  the  number  using  your telephone and then
press the online switch once you  hear  the  carrier at the other end.
You also have to set the  speed  manually  to the required rate before
going on line and, when a  call  finishes,  drop the line by switching
off.

There is one major  disadvantage  to  most  dumb  modems which becomes
apparent when you try to use  them  with modern computers. The 1200/75
split baud rate  is  not  used  in  the  USA  and,  since most popular
computers have their roots  in  America,  they  don't  split rates. In
addition, a fair percentage of comms software is American and does not
support 1200/75 bps either.

This means that the computer  can  only  be  connected to the modem at
300/300 bps, which  works  fine,  or  1200/1200,  which  will not work
properly. You may get a connection  and actually receive data, but you
won't be able to transmit anything.

Intelligent modems have a buffer  which  can  capture data and hold it
while transmitting it at 75  bps.  There's  no need to buffer received
data, as this is passed through at  1200  bps. Dumb modems do not have
buffers, mainly because they have no software to control them.

It is still possible to use  1200/75  bps  with dumb modems as long as
the software is  specially  written  to  do  the  necessary work. Many
British comms programs are written to  take this into account and when
used with a dumb modem, they perform some very clever switching of the
baud rates between 1200/1200  and  75/75  bps,  so  that they are able
receive at 1200 and transmit at 75 bps.

The more sophisticated dumb  modems  provide  a form of auto-dialling,
but this is not done  in  any  intelligent way. The software "rattles"
the lines in much the same way that you can rattle a telephone to dial
a number.

As long as the software  sends  the  appropriate  number of "highs and
lows" at suitable intervals,  the  telephone  line  can be fooled into
thinking that someone is  dialling  the  number manually. Examples are
the Magic Modem and Voyager.

Intelligent modems are a different matter  altogether and I shall look
at them next week.

                     PART 16: INTELLIGENT MODEMS
                     ===========================

Most people think of intelligent modems being Hayes-compatible modems.
This is not strictly true. Hayes  modems are intelligent, but they are
not the only intelligent ones.

An intelligent modem is one which contains its own software and is, to
a great extent, capable of  controlling  the  transfer  of data to the
other modem on its own.

Intelligent modems are told what to do by the software sending special
escape sequences down the serial cable.

The modem  initially  scans  the  data  received  from  the  computer,
watching for these escape sequences. Once  it finds one it recognises,
it interprets it and acts accordingly.

The Hayes command set is the  most  commonly encountered set of escape
codes, although there are others.

Many of these are proprietary  they  belong, technically, to the modem
manufacturer who invented them. Most of  these  have now fallen by the
wayside, leaving the Hayes set as the favourite.

There is one other command set  which  is found in reasonable numbers,
usually offered as an alternative to  the  Hayes set and this is known
as V25.

The reason for the inclusion  of  V25  probably  owes more to the fact
that it is a standard and readily available to anyone to use.

The Hayes set, however,  is  proprietary  and  it  may be possible for
Hayes Microcomputer Products Inc to  prevent other modem manufacturers
from using its command set.

Intelligent  modems  also  offer   speed-buffering:  this  allows  the
computer to be connected to the modem at one speed, while the modem is
connected to another modem at a different speed.

The modem's software handles the storing  of  data both from the other
modem and the computer and passing it on at the appropriate speeds.

Many intelligent modems  can  cope  with  split  baud  rates and speed
buffering is beneficial when the modem is connected to a service which
only supports 1200/75 baud  rates  while  the computer itself doesn't.
This includes IBM  PC  compatibles,  the  Atari  ST  and the Commodore
Amiga.

Speed-buffering also  becomes  an  important  factor  when  the  modem
incorporates MNP error correction.

The higher levels of MNP (5 and  above) allow data compression to such
an extent that it is possible to  double the rate of transfer with MNP
5 and even more with the higher levels.

The data may transferred between modems at  2400 bps, but if the modem
is also connected to the computer at 2400 bps, a bottleneck is formed.

Once a block of data has  been  transferred  using MNP, it is unpacked
and may double in size, which means that  twice as much data has to be
passed on to the computer.

If the modem-computer connection is at  2400  bps, the modem will have
to wait while this  is  transmitted  to  the  computer  and the buffer
cleared before it can request more data from the other modem.

Similarly, if the data is passed  from  the  computer to the modem for
transmission at 2400 bps, the  modem  will  have  to wait until it has
received twice as much data, before it  can compress it and pass it on
to the other modem.  The  effect  of  this  is  that  the gains of MNP
compression are lost.

The solution is to  set  the  computer-to-modem  connection  at a much
higher baud rate and let the modem buffer it as necessary, until it is
ready to transmit it, or pass it on to the computer.

Note that some comms programs  can  handle  MNP  in software, in which
case there is little  to  be  gained  by  increasing  the speed of the
modem-computer connection as the expansion  of  the data does not take
place until it has reached the computer.


                       PART 17: HAYES COMMANDS
                       =======================

In my look at  intelligent  modems  last  week,  I mentioned the Hayes
command set, which you'll come across quite often.

The true Hayes command  set  is  actually  quite  limited, with only a
small range of commands and small number of registers which may be set
to pre-configure certain features of the modem.

Most Hayes-compatible modems contain  a  number  of extra commands and
registers which, technically, are not part of the Hayes standard.

These have evolved as a result of a need for extra commands to control
new features in modern modems and also to handle the split baud rates,
which the Americans have no need for.

Unfortunately, though the basic Hayes  command set is largely standard
throughout all modem manufacturers, the same  can  not be said for the
extended commands and registers.

This can be a  cause  of  considerable  confusion  and  is  one of the
reasons why most comms  programs  offer  a  wide  range of alternative
modem setups for various models of  modem or the facility to configure
your own setup.

I understand that Hayes Microcomputer Products have been making noises
about the command set being its property and that modem makers wishing
to use it must pay Hayes for the privilege.

I think this claim is based  on  the  principle of the special on-line
escape sequence used by Hayes modems, rather than the actual commands.

When the modem is off-line no  carrier  has been detected from another
modem it scans all input from the computer for Hayes commands and acts
on them.

Once the modem is  on-line,  it  ceases  scanning  for these commands,
otherwise it would react to every occurrence of commands, even if they
were part of text being transmitted or received!

This is where the escape sequence comes into play and, while the modem
is on line, instead  of  scanning  for  Hayes  commands, it constantly
scans for the escape sequence.

By default, this escape sequence is three consecutive plus signs +++.

It isn't quite as simple as that though, as a "guard time" is required
before and after the sequence.

The default guard time demands a  delay  of  one second before and one
second after the sequence.  If  this  complete  sequence  of delay +++
delay is not received, or  any  other  characters are sent between the
plus signs, the modem starts its search again.

If the sequence is recognised, the  modem goes into local mode nothing
is transmitted to the other modem and it is virtually off-line, except
that the carriers are still  locked  between  the two modems. The user
can then send Hayes  commands  to  the  modem  to change settings, for
example.

With two exceptions, the  Hayes  commands  are  all  prefixed with the
characters  AT  (short  for  ATtention),   followed  by  one  or  more
characters to specify the  actual  command  and  any parameters it may
require.

The two  exceptions  are  AT  which,  used  on  its  own,  elicits  an
acknowledgment from the modem and A/, which causes the last AT command
sequence received by by the modem to be repeated.

A feature of Hayes commands is  that  a  number  of them may be strung
together into a sequence. When  Return  is  pressed, the whole command
line is checked and, if valid, each command is acted upon.

It is only necessary to  have  the  AT  sequence  at  the start of the
command, to initially get the modem's  attention, after which a number
of command letters may be used to perform various tasks.

I won't discuss every command in  detail, as these are usually covered
by the modem manuals, but next  week  I'll  list the main commands and
discuss some of  the  more  commonly  misunderstood  ones and problems
associated with them.

                 PART 18: HAYES COMMANDS (CONTINUED)
                 ===================================

I shall start this week with  a  list  of the Hayes commands which are
part of the true Hayes command  set  and, theoretically, common to all
Hayes-compatible modems, then  go  on  to  discuss  some  of  the more
commonly encountered problems.

AT
Get the modem's attention.

ATA
Answer. The modem goes on-line, emits  an answer carrier and waits for
an incoming carrier. If, after a  while, no carrier has been detected,
the modem will timeout and go off-line.

ATD
Dial. If the command is issued on  its own, the modem goes on-line and
attempts to connect with a modem at  the  other end. If the command is
followed by a telephone number,  the  modem  will  dial the number and
attempt to connect.

ATEn
Echo. Determines whether the command sent  to the modem is echoed back
to the computer.

ATE0
Not echoed.

ATE1
This is the default.

ATH
HangUp, or On/Off Hook. This  command  is  used  to force the modem to
drop the carrier. This is  normally  used  after going into local mode
with the escape sequence (+++) while on-line, to terminate the call.

ATIn
Identity. This command varies slightly from  modem to modem, but gives
details of the modem and its  software,  version etc, depending on the
number n following the command.

ATMn  Monitor.
This controls the  modem's  internal  speaker  and  normally has three
possible values, though occasionally more.

ATM0
No Speaker at all.

ATM1
Speaker on until carriers connect. Default.

ATM2
Speaker permanently on.

ATO
On-line. Normally used to return to on-line state from local mode.

ATP
Pulse dial  default.  Sets  the  default  mode  of  dialling  to pulse
dialling.

ATQn
Quiet. Determines whether result messages are returned to the computer
in response to Hayes commands received.

ATQ0
Result messages sent. Default setting.

ATQ1
No results sent.

ATSn
Set S registers. The S  registers  allow  certain modem settings to be
reconfigured and are discussed in more detail later.

ATT
Tone dial default. Sets the default mode of dialling to tone dialling.

ATVn
Verbose. Determines whether the result codes returned (if ATQ has been
set to return them) are verbose or terse.

ATV0
Terse. Numbers are sent according to the result of the command.

ATV1
Verbose. Plain English messages are sent back to the computer.

ATXn
Extended result codes. The number of valid values varies from modem to
modem but, typically, the higher the number, the greater the number of
different result messages. Zero  will  often  only  send CONNECT or NO
CARRIER (or their terse values of  0  and 1), while higher values will
report the speed of the connection, BUSY, NO DIALTONE etc.

ATZ
Software reset. Resets the modem to its default settings.

Some modems are provided  with  battery-backed  RAM  which  is used to
retain the default settings for a number of the commands.

If this is the case, two  special  commands  (not strictly part of the
standard command set) are available to set and reset the RAM.

AT&F
Resets the RAM to the factory defaults.

AT&W
Writes the current defaults to RAM.

Many modems  have  considerably  extended  command  sets,  some  to an
apparently absurd degree, where every feature  you would never want to
reconfigure can be configured, or all  manner  of on-line tests can be
made!

This is usually done with "extended  codes"  such as AT\, AT& and AT%,
followed by a character or characters.

The S registers have been briefly mentioned already and enable certain
new default values to be  set,  using  the  ATS= command. ATS0=10, for
example, would set S register zero to 10.

Most modems have  considerably  extended  S  registers,  but the basic
Hayes S registers,  common  to  all  Hayes  compatible  modems, are as
follows:

S0
The number of rings before the modem  auto answers an incoming call. 0
disables auto-answer.

S1
Indicates the number of  rings  before  a  call  was answered. This is
normally a read-only register.

S2
The Escape character. The default is 43 the + character.

S3
Carriage Return. Default 13.

S4
Line Feed. Default 10.

S5
Backspace. Default 8.

S6
Wait time for dial tone before dialling. Default 4 (secs).

S7
Wait  time  for  carrier  after  dialling  or  answering.  Default  40
(seconds)

S8
Length of pause when  a  comma  is  used  in  dialling string. Default
varies, according to modem.

S9
Carrier Detect Response  Time.  The  minimum  time  a  carrier must be
present for the modem to recognise  it  as  a valid carrier. Default 6
(1/10ths of a second).

S10
Lost Carrier Hangup Delay. The amount of  time the modem waits after a
carrier is lost before hanging up.  Default  varies, but is in 1/10ths
of a second.

S11
MF Tone duration and spacing. Only  relevant  to tone dialling and not
used by all modems.

S12
Escape Sequence Guard Time. The  length  required for the pause before
and after the sequence of  the  three  escape characters determined by
the S2 register. Default is 50, being 1 second.

S13-S16
These are reserved for special use.

As you will see, the standard  Hayes  command  set is quite small when
compared with those provided by most  modems,  but it supplies most of
the necessities, with the exception of  anything to do with split Baud
rates, which is one of the reasons for the extended sets.

                        PART 19: ONLY CONNECT!
                        ======================

Originally, Hayes  modems  determined  the  speed  of  modem  to modem
connection required by reading the speed of the connection between the
computer and the modem at the time when the most recent AT command was
received.

This meant that if you set your comms software to 1200/1200, the modem
would attempt to connect at 1200/1200.

As many computers are  incapable  of  handling  split  baud rates, the
modem to computer connection  is  usually  made  at  1200/1200 and the
modem will handle the buffering, but  some method of telling the modem
to attempt to connect at  1200/75 is required. Different manufacturers
have chosen different methods to do this.

In the case of the current Amstrad modems, the ATB command may be used
to set the baud rate and ATB23 forces the modem to connect at 1200/75.
In the case of the  Pro  24  modem,  the  ATF  command is used, with a
setting of ATF3.

Miracom, on the other hand, uses a  special S register on its WS range
of modems, requiring a setting of  S26=10,  while the Pace Linnet 1200
requires the S50=1, or S50=2 and S51=2.

Alternatively, in the case of the  Linnet  1200, '+75' may be added to
the end of the telephone  number  to  be  dialled  to achieve the same
result!

Confusing, isn't it?  The  matter  is  further  complicated when using
modems with MNP correction, as the connection between the computer and
the modem should be at a  higher rate than the required modem-to-modem
connection.

This means that in many cases  it  is  necessary  to send a special AT
command to the modem  to  tell  it  the  required speed of connection.
Again, the precise codes vary from modem to modem.

Many current modems now default  to auto-scanning when originating the
call (calling another service) and  attempt  to connect at the highest
possible baud rate. This may not be what you want on all occasions.

Sometimes it is possible to connect at 2400/2400 and have so much line
noise that you would have  been  better  at  1200/1200, but with auto-
scanning, it keeps connecting at 2400 bps!

Another problem with auto-scanning is that  if the modems at both ends
are scanning, it is possible for them  to  switch at the same time and
miss the connection.

By tradition, the calling (originating) modem sets a constant rate and
the receiving (host) modem does the scanning.

If you have trouble  connecting  at  the  baud  rate  you require, see
whether  your  modem  includes  a  command   to  force  the  speed  of
connection.

Another common problem occurs  when  software  waits  for result codes
from the modem  before  going  on-line.  There  are  three possible AT
commands which can affect this. ATE, ATQ and ATX.

Some software requires 'terse'  (numeric)  result codes, whilst others
expect 'verbose' messages, which means  that  the  ATQ setting must be
set to return the appropriate result.

It is also important to ensure that ATE  is set to echo the codes back
at all, otherwise the software will wait for them for ever!

The ATX settings may also  be  important,  particularly when running a
BB,  as  most  software  determines   the  speed  of  the  modem-modem
connection according to the result codes returned.

This means that the ATX setting must  be  set  to a high level so that
the result messages report the  speed  of  connection, such as CONNECT
1200 and not just CONNECT.

Sometimes, for example when connecting  to  a  Mercury line or ringing
out through an office's internal  switchboard  it  may be necessary to
dial part of the number  with  pulse  dialling  and the remainder with
tone dialling or to introduce a delay part way through.

The Hayes commands allow for this  by permitting certain characters to
be included in the telephone number in the dialling string.

The comma ',' may be used in the  middle of a number to make the modem
pause for a period of time determined by the S8 register setting. If a
longer delay is required, more than one  comma  may be used, or the S8
setting changed.

Dialling may be changed between pulse and tone by inserting P and T in
the telephone number where appropriate. If neither of these characters
are found the modem will dial using  the  default mode, or that set by
any ATT or ATP command.

Note that not all Hayes  compatible  modems  support tone dialling the
original Pace Linnet accepts the ATT command and the use of a T in the
dialling string, but ignores them!

Most software  provides  configuration  options,  or  examples  in the
documentation, for commonly encountered modems.  If yours isn't one of
them, there is little option but to experiment with settings.

If your modem has non-volatile RAM  to  store  modem settings, it is a
good idea to use AT&F, then  AT&W  to  reset  the RAM back to defaults
before experimenting. In  most  cases  the  default  settings  will be
fairly near those you require for your installation.

                    PART 20: WHICH COMMS SOFTWARE?
                    ==============================

In this final article I shall take a general look at comms software in
an attempt to make it easier for you to decide on the sort of software
you will find most useful.

Comms programs differ widely in the features they provide and some are
more suited to one purpose  than  others.  Many  are the result of one
person's "dream" of what a comms program  should be, which may, or may
not, coincide with yours.

The problem is that  people  use  comms  in  very different ways. Some
people are only interested  in  logging  on  to Prestel, while whereas
others may only want to access Telecom Gold.

Some people require a program which provides script files, so that the
complete on-line session  can  be  automated  and  carried  out with a
single keypress; others have no need for such things.

These are very basic differences between comms software, but there are
others, such as whether you wish to  save Prestel frames in ASCII text
form suitable for loading into a word processor, or whether you prefer
to save them as they appear, for later replay.

Another feature, particularly relevant to  Prestel, is whether you use
the messaging system a lot.

If you do, then look  for  a  program which can automatically download
all incoming mail, allow you tp read  and reply to them off-line, then
log on and upload the replies automatically.

With some makes of computer you do  not  have a lot of choice in comms
software, but others,  particularly  the  PC,  have  a  wide  range of
software available, covering just about every possible option.

I have yet to discover a single program which provides every feature I
would like to have, so  compromises  have  to  be made and the program
which is nearest to the ideal selected.

Some programs are very easy to  use,  often because there is little to
them; but it is possible for a program which contains many features to
be just as simple.

Take script files as an  example.  One  of  the  most popular PC comms
programs has quite a powerful, yet relatively simple, script language.

Unfortunately, the only way to log  on  to a service and automatically
send IDs and passwords is by  creating  a script file for each service
you want to use.

On the other hand, there are other programs which do not have a script
language as such, but which allow  you  to specify the ID and password
sequence to be sent at log-on as part of the dialling directory entry.

If all other features of the  programs  are  equal and you have no use
for script files, then it is  obviously sensible to choose the program
which does not require scripts.

If your main concern  is  with  logging  on  to  BBs and uploading and
downloading files, then it is sensible  to choose a program which will
allow you to use the file transfer protocol you prefer.

This does not  necessarily  mean  a  program  which  includes  lots of
protocols, but  a  program  which  allows  you  to  link  in  external
protocols, several of which are available for most computers.

Zmodem is currently "flavour  of  the  month";  lots of comms software
doesn't have it, but it is  available  as an external protocol and may
be hooked into any other program which can handle them.

It is worth taking time  and  even  spending  money to get the program
which suits your use best.  Not  only  is  it  more  pleasant to use a
program you feel comfortable  with,  but  it  is  also cheaper, as the
quicker you can carry out tasks, the less time you will spend on-line.

You might, for example, be prepared to  fore go the pleasures of full-
colour Prestel graphics in  return  for  a  faster  screen display and
simulated graphics in text mode.

The choice is yours and the  range  is  wide, so don't just stick with
the program you are using just  because  it  came with the computer or
modem. Find out what is available and, if possible, try them out.

Luckily for us, a  large  percentage  of  comms software is shareware,
allowing the opportunity to try  them  out  at little initial cost. Do
register the program if you use it regularly though.

The number of comms users  is  only  a  small  percentage of the total
number of computer users, so the authors do need as many registrations
as possible if they are to be  encouraged to support and develop their
software!

This is the last of  this  series  on  basic communications and I hope
that I have covered most  of  the  areas  which can cause confusion to
beginners and  even  the  more  experienced.  Paul  Blitz's  excellent
Advanced Comms series covers the technical side, which I have tried to
avoid!

I have tried to avoid any  unnecessary complications and jargon as far
as possible, but if there are any areas which you think I have missed,
or not covered as clearly as I  might,  I would welcome feedback to my
mailbox (219992706) and might be persuaded to to a follow-up article.

END....


Amigatech note: Although the modem specifications (baud ratings) may
be slightly out of date (faster modems are now generally available
since this text was written) the information is invaluable to anyone
new to comms and it may even enlighten seasoned users.

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