home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
QRZ! Ham Radio 8
/
QRZ Ham Radio Callsign Database - Volume 8.iso
/
pc
/
files
/
p_misc
/
netconf.arc
/
TEXNET.001
< prev
next >
Wrap
Text File
|
1988-12-17
|
7KB
|
132 lines
[ Note: This series of articles was found on Compuserve and downloaded
from HAMNET there on 21 July 1985 by Dwight Ernest KA2CNN 70210,523. ]
An Introduction to networks
by T.C. McDermott, N5EG
networks SIG, TPRS
[ part 1 ]
This article is an introduction to the subject of a packet network.
It describes what a network is, why a network is necessary to support
amateur packet radio activity, and considerations that govern how a
network may be constructed. There are many ways that a network can be
designed, and it is beyond the scope of this article to elucidate them
all. Rather, this article will focus on the simplifing assumptions that
may be made in describing a network, and more specifically will
concentrate on some suggestions for the Texas Packet Radio Society
network which is called "TEXNET".
Most of us are familiar with packet radio activity through our
operations with the TAPR TNC board. This board implements what is
called a "Local Area Network", or LAN for short. When we wish to
communicate, we ask our TNC to CONNECT to another station. If that
station is not within range of our transmitter, then we may connect to
that station through a digi-peater, or through several digipeaters.
This is a convienent extension of the X.25 protocol, and forms a large
part of the difference between X.25 and the amateur version called
AX.25.
It would be possible to construct a network of stations that are
all within range of the next station, and then to connect to any station
in the network using this digipeat method, up to 8 stations distant.
This would not require the extension of any of the TAPR software, nor
would it require the development of any new hardware. Why then is this
not an acceptable method to construct a network? Basically this method,
although simple to implement has a serious flaw, it lacks "robustness".
That is, the method fails to support adequate communications in the
presence of a radio path that is not perfect. Secondly, it assures
communications integrity through a method known as "end- to-end ACK".
To understand this, it is necessary to understand how a TAPR
digipeater works. The TAPR digipeater is a "dumb" digipeater. That is
- the digipeater does not understand anything about the state of the two
stations that are trying to communicate to each other through it. When
one station wishes to connect to another station through a digipeater,
it simply adds the digipeater's address in series with the address field
of each and every packet. When the digipeater recognizes it's callsign,
it repeats the packet. The digipeater does not know what kind of packet
is being digipeated, and does not really care. The packet could be a
call-request packet, or user-data, or an acknowledge packet, it really
doesn't matter, it digipeats them all, blindly. Why is this important?
Because it affects how the transmission and acknowledgment of data is
handled between the two end stations trying to communicate with each
other.
When the sender, S, tries to send data to the receiver, R, through
one or more intervening digipeaters, D1, D2, ... , Dn, it does this as
follows:
S : sends a packet
D1: digipeats packet
D2: digipeats packet
R : receives packet,
R : sends acknowledge back
D2: digipeats acknowledge
D1: digipeats acknowledge
S : receives acknowledge,
S : sends next packet.
Although there can be several packets sent per acknowledge, it
requires that a packet, and the acknowledge (ACK) make the round-trip
from the sender to the receiver. Thus, the more digipeaters, the longer
the round-trip time, and the lower the packet throughput.
The above example assumed that there were no errors in the
transmission. What happens if one of the packets and one of the
acknowldeges is corrupted during transmission?
S : sends packet
D1: digipeats packet
D2: doesn't hear packet from D1, so doesn't do anything
S : still waiting for ACK from the receiver
S : still waiting for ACK from the receiver
S : still waiting for ACK from the receiver
S : still waiting for ACK from the receiver
S : still waiting for ACK from the receiver
S : times out waiting for ACK, and re-transmits packet
D1: digipeats the packet
D2: digipeats the packet
R : receives the packet,
R : sends ACK back to sender
D2: digipeats the ACK
D1: doesn't hear packet from D2, so doesn't do anything
S : still waiting for ACK from the receiver
S : times out waiting for ACK, and re-transmits packet
D1: digipeats the packet
D2: digipeats the packet
R : receives the packet, but it's a duplicate - throw away
R : sends ACK back to sender
D2: digipeats the ACK
D1: digipeats the ACK
S : receives the ACK,
S : sends the next packet
How long did this take? About 25 packet times. The situation gets
worse when 8 digipeaters are chained together. In fact with 8
digipeaters the round-trip time reduces the channel throughput by a
factor of approximately 16 (8 hops to R, and 8 hops for the acknowledge
to come back) if there are no channel errors. If the probability that
any single transmission is corrupted is about 70 percent, then with 8
hops the average round trip will take about 1000 packet times. In other
words, nothing will get through.
Why is the TAPR TNC built this way you might ask? For a very good
reason - simplicity. To build a digipeater that behaves in a more
coordinated fashion turns out to be a very complicated problem. The
TAPR digipeater extension is far superior to the other alternative - no
digipeater at all. The TAPR digipeater is elegantly simple, and a
reliable way to improve the communications between two stations that are
reasonably close, but not able to communicate directly. We have seen
that one or two digipeaters may not degrade the throughput terribly,
provided that the RF paths are highly reliable. Thus the digipeater
solution may be called an LAN solution. That is, it is an acceptable
network for small numbers of digipeaters, and high-quality circuits. A
single digipeater in a superior location allows many stations within the
coverage area of the digipeater to communicate. But the digipeater is
not an acceptable solution when the need is to communicate over long
distances, and with less than high-quality communications circuits.
Thus is born the requirement for a NETWORK. This will be discussed
in the next article of this series.