THE LAN TUTORIAL SERIES


PART 2: PROTOCOLS

Definition
The LAN Magazine "Glossary of LAN Terms" defines a protocol this
way: A set of rules for communicating between computers.
Protocols govern format, timing, sequencing and error control.
Without these rules, the computer will not make sense of the
stream of incoming bits.

But there is more. Communicating data from computer to computer
takes many steps. For example, suppose you are sending a file
from one computer to another. The file has to be broken into
pieces. The pieces have to be grouped in certain fashion.
Information must be added to tell the receiver where each group
belongs in relation to others. Timing information must be added.
Error correcting information must be added, and so on.

Because of this complexity, computer communication is usually
broken down into steps. Each step has its own rules of operation,
its own protocol. These steps must be executed in a certain
order, usually from the top down on transmission and from the
bottom up on reception. Because of this hierarchical arrangement,
the term protocol stack is used to describe the different steps
of computer communication. A protocol stack is simply a set of
rules for communication, only it can be broken down into sets of
rules for each step in the sequence.

Protocols, Really
What is a protocol, really? It is software that resides either in
a computer's memory or in the memory of a transmission device
like a network interface card. When data is ready for
transmission, this software is executed. It prepares data for
transmission and sets it in motion. At the receiving end, it
takes the data off the wire and prepares it for the computer,
taking off all the information added by the transmitting end. So,
protocols are just software that performs data transmission.

But there is more. Confusion is caused by the fact that there are
many protocols, many different ways of getting data from one
place to another. Novell does it one way. 3Com does it another.
DEC does it a third way. And since the transmitter and the
receiver have to "speak" the same protocol, these three can't
talk directly to each other. That's where the term protocol
standard and the OSI Model fit in.

A protocol standard is a set of rules for computer communication
that has been widely agreed upon and implemented by many vendors,
users and standards bodies. Ideally, a protocol standard should,
when implemented, allow people to talk to each other, even if
they are using equipment from different vendors.

Of course, you don't have to have a "standard" protocol to
communicate. You can make up your own. The only problem is that
you are limited to talking to yourself. 

Let's look at some of the protocol standards that exist and see
if we can't get a feel for how protocols work. As you will see,
there are many standards -- none of which can be called
universal.

The OSI Model
The OSI Model is the best place to start because it is a full
protocol stack. It is a set of protocols that attempt to define
and standardize the entire process of data communications (some
protocol standards only define part of the process). The OSI
Model -- which stands for the Open Systems Interconnection Model
of the International Standards Organization (ISO) -- has the
support of most major computer and network vendors, along with
many large customers and the U.S. government.

The OSI Model is really nothing more than a concept, describing
how data communications should take place. It divides the process
into seven layers. Into these layers fit protocol standards
developed by the ISO and by other standards bodies. At each
layer, there are numerous protocols. That is, the OSI is not a
single definition of how data communications actually takes place
in the real world. It just says, "This is the way things should
be divided and these are the protocols that you can use at each
layer." As long as a network vendor chooses one of the protocols
at each layer, the network should work with other vendors'
offerings.

Nobody really believes the hype that the OSI Model will lead to
complete, transparent intercommunication between all computers.
We are just hoping it is a step in the right direction.

Each successive layer of the OSI Model works with the one below
it. Remember, protocol stacks are not democratic; they are
rigidly hierarchical. Each layer of the OSI Model is modular.
That is, you may (theoretically) substitute one protocol for
another at the same layer without affecting the operation of
layers above or below. For example, you should be able to use a
Token Ring board or an Ethernet board and still use all the other
pieces of your network, including network operating system,
transport protocols, internetwork protocols, applications
interfaces, etc. Of course, vendors must create these products to
the OSI Model specifications for this to work.

The OSI Model's modularity should become clear as we describe the
major protocols that conform to it. First a look at what each
layer is supposed to do.

1. Physical Layer. The first, or Physical layer, of the OSI Model
conveys the bits that move along the cable. It is responsible for
making sure that the raw bits get from one place to another, no
matter what shape they are in. It deals with the mechanical and
electrical characteristics of the cable.

2. Data Link Layer. The second, or Data Link, layer of the OSI
Model is responsible for getting data packaged and onto the
network cable. It manages the physical transfer, providing the
blocks of data, their synchronization, error control and flow
control. The Data Link layer is often divided into two parts --
Logical Link Control (LLC) and Medium Access Control (MAC) --
depending on the implementation.

3. Network Layer. The third, or Network, layer of the OSI Model
establishes, maintains and terminates connections. It is
responsible for translating logical addresses, or names, into
physical addresses.

4. Transport Layer. The fourth, or Tranport, layer of the OSI
Model ensures data is sent successfully between the two
computers. If data is sent incorrectly, this layer has the
responsibility to ask for retransmission.

5. Session Layer. The fifth, or Session, layer of the OSI Model
decides when to turn communication on and off between two
computers. It coordinates the interaction between them. Unlike
the network layer, it is dealing with the programs running in
each machine to establish conversations between them.

6. Presentation Layer. The sixth, or Presentation, layer of the
OSI Model does code conversion and data reformatting. It is the
translator of the network, making sure the computer is talking in
the right language for the network.

7. Application Layer. The seventh and final, or Application,
layer of the OSI Model is the interface between the software
running in the computer and the network. It supplies functions to
the software in the computer, like electronic mail or file
transfer.

Unfortunately, protocols in the real world do not conform
precisely to these neat definitions. Some network products
combine layers. Others leave out layers. Still others break apart
layers. But no matter what, all working network products achieve
the same result, getting data from here to there. The question
is, do they do it in a way compatible with the rest of the
world's networks? More important, do they care?

Popular Physical Protocols
Hopefully, all of this will become clearer if we look at some
real protocols and compare them to the OSI Model.
The best known physical layer standards of the OSI Model (there
are a few), are those from the IEEE, the Institute of Electrical
and Electronic Engineers. That is, the ISO adopted some of the
IEEE's physical network standards as part of its OSI Model. These
are IEEE 802.3, or Ethernet, IEEE 802.4, or token- passing bus
and IEEE 802.5, or Token Ring.

These three standards define the physical characteristics of the
network and how to get raw data from one place to another. Each
is a Layer 1 standard. They also define how people can use the
network at the same time without bumping into each other.
Technically, this last part is a job for the Data Link layer,
Layer 2. We will deal with this below. For now, let's see just
what these standards mean.

IEEE 802.3 defines a physical network that has a bus (straight
line) layout. Data is broadcast throughout the network in no
particular physical direction. All machines receive every
broadcast, but only those meant to receive the data respond with
an acknowledgement. 

Network access is determined by a protocol called Carrier Sense
Multiple Access With Collision Detection, or CSMA/CD. It lets
everyone send whenever they want. If they bump into each other,
they back off, wait, and send again until they get through. Thus,
the more users, the more crowded and slower the network -- like
the freeway. (More on network access next month).

IEEE 802.4 defines a physical network that has a bus layout. It
is also a broadcast network. All machines receive all data but do
not respond unless data is addressed to them.

Network access is determined by a token that moves around the
network in a logical fashion. It is broadcast to every machine
but only the machine that is next for the token gets it. Once a
machine has the token, and not before or after, it may transmit
data. The MAP/TOP (Manufacturing Automation Protocol/Technical
Office Protocol) standard uses this protocol.

IEEE 802.5 defines a physical network that has a ring layout.
Data moves around the ring from station to station. Each station
regenerates the signal from the previous station. In this way it
is not a broadcast network.

The network access protocol is token-passing. The difference is
that the token moves about in a ring, rather than over a bus.
IBM, Texas Instruments and Ungermann-Bass are the only vendors of
the chips needed to make Token Ring network interface cards.
Nevertheless, it is fast becoming one of the most popular network
hardware options.

There are other Physical and Data Link layer standards, some that
conform to the OSI Model and others that don't. The most famous
that does not is Arcnet. It uses a token-passing bus access
method, but not the same one as IEEE 802.4. A new physical
standard called Fiber Distributed Data Interface (FDDI) is a
100M-bits-per-second physical protocol using token ring over
fiber optic cable. It will probably be OSI-compatible.

Data Link Protocols
As we said, the IEEE protocol standards are not confined to the
Physical layer but also work at the Data Link layer. We also said
that the Data Link layer is often divided into two parts. The
upper part is called Logical Link Control (LLC) and the lower
part is called Medium Access Control (MAC). As it turns out, the
IEEE standards define the lower, or MAC, half of the Data Link
layer -- the part that determines how network users keep from
bumping into each other.

Medium Access Control is just what it sounds like. It is the
protocol that determines which computer gets to use the network
cable when many computers are trying. We saw that IEEE 802.3 lets
everyone simply bump into each other and keep trying until they
get through. IEEE 802.4 and 802.5 are more ordered, limiting
conversation to the computer with the token. Remember, all of
this is done in fractions of a second. So even when the network
is crowded, no one really waits very long for access on any of
the three types of networks.

The other half of the Data Link layer, LLC, provides reliable
data transfer over the physical link. In essence, it manages the
physical link.

There are two reasons why the IEEE split the Data Link layer in
half (and why the ISO accepted it). First of all, the Data Link
layer has two jobs to do. The first is to coordinate the physical
transfer of data. The second is to manage access to the physical
medium. Splitting the job allows for more modularity, and
therefore flexibility.

The second reason also has to do with modularity, but in a
different way. The type of Medium Access Control has more to do
with the physical requirements of the network than actually
managing the transfer of data. In other words, the MAC layer is
"closer" to the physical layer than the LLC layer. By splitting
the two, it is possible to create a number of MAC layers
(corresponding to physical layers) and just one LLC layer that
can handle them all. This increases the flexibility of the
standard. It also gives LLC an important role in providing an
interface between the various MAC layers and the higher-layer
protocols.

By the way, Logical Link Control is the more common name of the
IEEE's 802.2 specification. The numbers give it away. 802.2 works
with 802.3, 802.4 and 802.5. It should also work with emerging
standards, like FDDI.

There are other protocols that perform the LLC functions.
High-level Data Link Control (HDLC) is the protocol from the ISO.
Like LLC, it conforms to the OSI model. IBM's SDLC (Synchronous
Data Link Control) is a Data Link layer standard that does not
conform to the OSI Model but does perform similar functions. IBM
has many products that do not follow the OSI Model or its
hierarchical setup. IBM has pledged support of OSI, however.

Transport Protocols
The ISO is in the process of establishing protocol standards for
the middle layers of the OSI Model. As of yet, none of these have
been implemented on a widespread basis, nor has the complete OSI
protocol stack been established. To make matters more confusing,
most of the middle-layer protocols on the market today do not
conform neatly to the OSI Model's network, transport and session
layers. They were created before the ISO started work on the
model.

The good news is many existing protocols are being incorporated
into the OSI Model. Where existing protocols are not
incorporated, interfaces between them and the OSI Model are being
implemented. This is the case for TCP/IP, NetBIOS and APPC, the
major middle-layer protocols available today.

In the PC LAN environment, NetBIOS is the most important
protocol. It stands for Network Basic Input/Output System. IBM
developed it as a BIOS for networks. It is essentially a Session
layer (Layer 5) protocol that acts as an applications interface
to the network. It provides the tools for a program to establish
a session with another program over the network. Hundreds of
programs have been written to this interface, making it the most
widespread protocol in the PC network arena.

NetBIOS does not obey the rules of the OSI Model in that it does
not talk only to the layers above and below it. As we said,
programs can talk directly to NetBIOS, skipping the application
and presentation layers. This doesn't keep NetBIOS from doing its
job. It just makes it incompatible with the OSI Model, which is
not the end of the world. Someone will write an interface between
the two, soon.

NetBIOS is limited to working on one network. Therefore, some
network vendors have established an interface between NetBIOS and
TCP/IP, a protocol from the Department of Defense for use over
large combinations of networks (internetworks).

TCP/IP stands for Transmission Control Protocol/Internet
Protocol. TCP is a Transport protocol (Layer 4), corresponding to
the definition we gave above. Its job is to get data from one
place to another without errors. It forms an interface between
the protocols above and below -- shielding the upper layers from
concern about the connection and the lower layers from concern
about transmission content.

The IP protocol is for getting data from one network to another.
Its main concern is bridging the differences between networks so
they don't have to be modified to talk to each other. It does
this by providing rules for the breakdown of data to conform with
a given network. Gateways, which are the physical translators
between networks, use IP's rules to take data from one network,
modify it and route it correctly over another network.

TCP/IP enjoys enormous support in government, scientific and
academic internetworks. These computers use UNIX and other
large-computer operating systems. In the past few years, business
internetworks have begun to approach the size of those in
government and universities. This has driven these businesses to
look for internetwork protocol standards. They have found TCP/IP
useful and it has become a de facto standard. Many see it as an
interim solution until the OSI transport and internetwork
protocols are finished. TCP/IP products for DOS-based networked
PCs are also available.

Often when TCP/IP is discussed, acronyms like SMTP, FTP and
TELNET are tossed around. These are applications that have been
written for TCP/IP and are widely used. They work at the
Applications layer (Layer 7). SMTP stands for Simple Mail
Transfer Protocol. FTP stands for File Transfer Protocol. TELNET
is the name for a terminal emulation protocol. These protocols,
written for TCP/IP, do exactly what they say they do.

Advanced Program-to-Program Communications, or APPC, is another
protocol for large networks. It comes from IBM and is part of Big
Blue's Systems Network Architecture (SNA). It is similar to
NetBIOS in that it provides an interface to the network for
programs so they may communicate, but it is not limited to one
network as is NetBIOS. APPC is geared toward mainframe computers,
though IBM is offering it as part of its OS/2 Extended Edition.
Using APPC, all computers communicate as peers, even PCs.
Previously in the IBM world, PCs were forced to emulate terminals
when communicating with mainframes. A number of other vendors,
mini and micro, also offer APPC.

APPC has received much publicity. Unfortunately, there are not
many applications for APPC in the PC network arena. There are
more in the minicomputer and mainframe network market.
Nevertheless, IBM and others are promoting APPC as a protocol
standard for the future. Its robustness, flexibility and
reliability make it worth the extra development effort.

There are other middle-layer protocols. XNS, IPX and NetBUEI are
all transport protocols. XNS is short for Xerox Network System.
It was one of the first local area network protocols used on a
wide basis, mainly for Ethernet (802.3) networks. 3Com and many
others use it. IPX is Novell's implementation of XNS. It is not
completely compatible with the original, but very widely used.
NetBUEI is IBM's transport protocol for its PC networking
products. All of these protocols perform similar tasks.

Many More
If it seems like the number of protocols is idiotic, it is and it
isn't. Different protocols have different advantages in different
environments. No single protocol stack will work better than
every other in every setting. NetBIOS seems to work fantastically
in small PC networks but is practically useless for communicating
with mainframes. APPC works well in mainframe environments.
TCP/IP excels in large internetworks.

On the other hand, much more is made about the differences in
protocols than is actually warranted. Proprietary protocols are
perfect solutions in many cases. Besides, if the proprietary
protocols are widespread enough, they become standards, and
gateways between them and other standards are built. This is
happening with some of the major protocols we have not covered.
These protocols include many de facto standards in minicomputer
and scientific workstation communications. They include DEC's
entire protocol suite, Sun Microsystems' NFS, AT&T's protocols
and many others. We have also left out Apple's AppleTalk and AFP.
While these enjoy widespread use, that use is based on the
computers these companies are selling and not the proliferation
of the protocols throughout the networking industry.

Unfortunately, whether proprietary or standard, users are still
faced with the dilemma of choice. This choice is made slightly
easier by the shakeout and standardization that has occurred over
the past few years at the lower Physical and Data Link layers.
There are three choices, Token Ring, Ethernet or Arcnet. Right
now, the same is happening at the higher layers. Can you guess
which way things will go? 

-- Aaron Brenner