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Ethernet
Linux supports two of the major network
types: Ethernet
and token-ring. Ethernet could be
labeled as the great grand-father of all the other network
types. It was developed in the 1970s by
Xerox for linking computers to printers. Although not very wide spread at first, Ethernet
has since
expanded to be (perhaps) the most widely spread type of network.
The principle behind
Ethernet is called Carrier Sensing, Multiple Access with Collision Detection (CSMA/CD). What this
means is that every machine on the net sits quietly listening for messages. When one of the machines
needs to talk, it waits for a pause and jumps in to send its message. What if two machines
simultaneously see the pause and start to send? Well, a collision occurs. This is detected by both
machine which wait a random amount of time before they will try again. Although the random amount of
time could be the same for both machines, it doesn't happen too often and each machine eventually
gets to send its message. The one that didn't get it's turn will see that the other one is talking
and waits.
Because there is no guarantee that a specific machine will ever get a
turn on the net, this type of mechanism is referred to as a probabilistic access system, since each
machine will probably get access to the system someday. Keep in mind that the busier a network
is,
the greater the chance for collisions and the greater the likelihood that there will be more
waiting. This does not mean that more machines mean more collisions. If I am sitting at my machine
doing all of my work locally, then the traffic on the network
cause by my machine is minimal.
However, once I make a connection, the traffic increases.
Ethernet appears in several
different forms, depending on it's physical characteristics. Primarily, these fall into the IEEE
specification 802.3, with an average speed of 10MHz. One thing I need to point out is that the
original specification developed at Xerox is not what most people think about when they think about
Ethernet. Rather it is the IEEE 802.3 standard.
The most popular ways Ethernet
appears is
10Base5 (Thicknet), 10Base2 (Thinnet) and 10Base-T (Twisted-Pair) and the 100-Mbit equivalents. The
general format of these labels is StypeL, where S is the speed of the cable in
megahertz, type is the transmission system, in this case baseband versus broadband and the
L is the maximum length of the cable in 100 meters. I have also heard that the last number
indicates the thickness of the cable in tenths of an inch. Thicknet, as one would guess, is thicker
than thin net, but both are coax cable. Twisted pair is similar is format to normal phone cable, but
may often have eight separate wires.
Often times, the topology (layout) of your network
is dependent on what kind
of cable you are using. Because it requires a central hub, twisted-pair is usually laid out in a
star, with the hub at the center. This is a star topology.
Thin- and thickwire are usually be spread
out in a line, or linear topology. This is also called a bus
topology.
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