THE LAN TUTORIAL SERIES

PART 4: LAN Access Methods


Definition
Access method is the term given to the set of rules by which
networks arbitrate their use. It is the way the LAN keeps people
from crashing into each other as they use the network. Think of
the access method as traffic law. The network cable is the
street. Traffic law (access method) regulates the use of the
street (cable), saying who can drive (send data) where and at
what time.

Access method deals on the Physical layer of the network, the
lowest level of the OSI model. That's because it is worried about
the use of the cable that connects users. The access method
doesn't care what is being sent over the network, just like the
traffic law doesn't stipulate what you can carry. It just says
you have to drive on the right and obey the lights and signs.

Networks need access methods for the same reason streets need
traffic lights - - to keep people from hitting each other. On a
network, if two or more people try to send data at exactly the
same time, their signals will interfere with each other, ruining
the data being transmitted. The access method prevents this.

There are three major access methods in use today, though many
more exist. They are Ethernet, Arcnet and Token Ring. Actually,
these are wider-ranging standards that use particular access
methods. They also define other features of network transmission
besides the access method, like the electrical characteristics of
signals, the size of data packets sent, etc. Nevertheless, these
three standards are best known (and best distinguished) for the
access methods they employ.

Ethernet
Ethernet is the most common network access method. It was
developed by Xerox Corporation at its Palo Alto Research Center
facility in the mid-1970s. It is supported by Xerox, Digital
Equipment, Intel (the three of whom made it a standard) and many
other network vendors. At least half of the installed base of
network nodes (PCs, engineering workstations, minicomputers) use
Ethernet.

The Ethernet access method is Carrier Sense Multiple Access with
Collision Detection, or CSMA/CD. This is a broadcast access
method. That means every computer "hears" every transmission.
However, not every computer "listens" to every transmission.
Here's how it works.

When a computer wants to send a message it does, no questions
asked. The signal it sends moves up and down the cable in every
direction, passing every other computer on the network. Every
computer "hears" the message, but ignores it. Only the computer
to which the message is addressed recognizes the message and
sends an acknowledgement. The message is recognized because it
contains the address of the destination computer. The
acknowledgement can be correctly addressed because the original
message also contained the message of the sending computer.

What happens if two computers send at the same time? A collision.
This doesn't make any noise, but it does keep the messages from
going through. When it does happen, each of the colliding
computers backs off for a random amount of time and tries again.
This happens until they get through. Of course, the whole process
takes a small fraction of a second.

Computers can tell if a collision has occurred because they don't
"hear" their own message in a given amount of time, determined by
the "propagation delay" of the network (the time it takes for a
signal to go to the end of the network and back). Remember,
messages move up and down the network in all directions. Every
computer hears every message, even its own messages. That is the
Carrier Sense Multiple Access with Collision Detection access
method.

It would seem Ethernet is an inefficient access method, prone to
collisions. But while collisions do happen often, they don't mean
very much in most cases. Since the whole
transmission/collision/retransmission process takes place so
quickly, the delay a collision causes is minuscule. Of course, if
you have lots of traffic, from lots of computers, the number of
collisions can mount and the network can slow down. This happens
with some large-scale imaging applications or on Ethernet network
segments with more than 50 to 75 nodes. Few Ethernet networks,
however, have a traffic load of more than 10 to 20 per cent,
which means delay caused by collisions is unnoticeable.

Arcnet
Arcnet was developed by Datapoint Corporation (San Antonio, TX)
in the early 1970s. The main Arcnet hardware vendors in the PC
network arena today are Datapoint, Standard Microsystems
(Happauge, NY) and Pure Data (Markham, Ontario). After Ethernet,
Arcnet is the most installed network access method, supported by
most network software vendors.

Arcnet is a token passing access method that works on a star-bus
topology. That means the network cable is laid out as a series of
stars, with each computer attached to a "hub" as the center of
the star and the hubs connected in a bus, or line. Hubs can
connect four, eight, 16 or 32 computers.

When a computer wants to send on an Arcnet network, it must have
the "token." The token is simply a series of data bits created by
one of the computers on the network. (There is a whole process
for token creation that we need not go into). It moves around the
network in a given pattern, a logical ring. All computers on the
network are numbered with an address -- from 0 to 255, so the
maximum number of computers on an Arcnet segment is 256. The
token moves from computer to computer in numerical order, even if
adjacent numbers (e.g. 14 and 15) are at opposite ends of the
network. When the token reaches the highest number on the network
it moves to the lowest, thus creating a logical ring.

Once a computer has the token it can send one packet of data --
up to 512 bytes. It does so by attaching the destination address,
its own address, up to 508 bytes of data and some other
information to the token. This combination becomes the packet.
The entire packet then moves from node to node in sequential
order until it reaches the destination node. There the data is
removed and the token released to the next node in order.

Since one packet is often not enough for an entire message, the
token may need to make several rounds of the network to complete
a message.

The advantage of token passing is predictability. Because the
token moves through the network in a determined path, it is
possible to calculate how long it will take for it to move around
the network. Since the token will only carry up to 508 bytes at a
time, it is possible to calculate how long different sized
transmissions will take. This makes network performance very
predictable. It also means introduction of new network nodes will
have a predictable effect. This differs from Ethernet, where the
addition of new nodes may or may not seriously effect
performance.


The disadvantage of the token passing access method is the fact
that each node acts as a repeater, accepting and regenerating the
token as it passes around the network in a specific pattern. If
there is a malfunctioning node, the token may be destroyed or
simply lost, bringing down the whole network. There are, however,
provisions for token regeneration so that a lost or destroyed
token is not gone forever. The star topology also helps.

Token Ring
The Token Ring network was introduced by IBM in 1984. It is not
the first ring network, but it has had the most impact on the LAN
industry. It has evolved into IBM's ultimate connectivity
solution for all its computers -- personal, mini and mainframe.
IBM's specifications follow those of the IEEE's (Institute of
Electrical and Electronic Engineers) 802.5 standard. The other
major Token Ring hardware vendors are Proteon (Natick, MA), 3Com
(Santa Clara, CA) and Ungermann-Bass (Santa Clara, CA). The
network software vendors that support Token Ring hardware include
3Com, Novell (Orem, UT) and Univation (Milpitas, CA). The
installed base of Token Ring should surpass that of Ethernet and
Arcnet soon.

Like Arcnet, Token Ring networks use token passing. The
difference is computers are arranged in a physical ring. The
token moves around the ring, giving successive computers the
right to transmit. If a computer receives an empty token it may
fill it with a message of any length as long as the time to send
does not exceed the token-holding timer. This message moves
around the network with each computer regenerating it. Only the
receiving computer will copy the message into its memory, then
marking the message as received. It does not remove the message
from the ring. The sending computer does that when the message
comes back around.

Because each computer looks at the message and may act on it,
each computer can perform certain tests to make sure the message
is getting through correctly. Also, since the frame is copied and
marked rather than purged, the sending computer can see if the
destination computer exists and if the message was received when
the message comes back around.

Token Ring networks have a priority mechanism whereby certain
computers can get the token faster than others. They can also
hold it for longer.

Token Ring's advantages include reliability and ease of
maintenance. It uses a star-wired ring topology in which all
computers are directly wired to a multi-station access unit, or
hub. These are connected in a ring. The multi-station access unit
allows malfunctioning computers to be disconnected from the
network. This overcomes the disadvantage of token passing, namely
the way in which one malfunctioning computer can bring down the
network since all computers are active in regenerating the token
and passing signals around the ring. Malfunctioning computers are
simply disconnected by unplugging them from the multi-station
access unit.

-- Aaron Brenner





THE LAN TUTORIAL SERIES