Repeaters

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Repeaters

Copper wire can carry voltage only so far before the integrity of that voltage begins to deteriorate. This deterioration of a voltage signal is referred to as attenuation. It more or less means that the difference between a clean 1 (voltage on) and a clean 0 (voltage off) becomes muddled. Figure 5.2 shows the distinction between fresh, clean signals and what the signals look like as the distance increases from where the voltage was applied. The big problem is that this seems to occur after only a couple of hundred feet, certainly not the distance that is necessary for very large networks to successfully communicate. Something had to be done to extend the length of a network segment.

Extending the length of a network segment is difficult because distance is not the only factor that affects voltage. Other sources of interference can alter voltage on a wire. For instance, a copper wire can pick up voltage just from being in the same proximity as a magnetic or electrical source. This means that if a machine wants to communicate a “0” on the wire, but somewhere along the path of the wire it crosses another source of voltage, it picks up that voltage, resonating to that same frequency. Depending on the strength of that secondary source, this could make the “0” look like a “1” to the receiving machine(s). It may be easiest to think of the copper wire as a rambunctious partier who really likes to dance. If the wire hears music anywhere in the neighborhood, it picks up the beat and starts to dance. Imagine how chaotic it would seem to see a dancer try to do the waltz, the cha-cha, ballet, and disco all at the same time.

This troubled the engineers who were trying to design ethernet specifications; they had to figure out how to make sure only one dance was interpreted, while still being able to extend network segments. The network cards with which they were experimenting could transmit and interpret voltage very quickly (approx. 10 MB/sec, an enormous amount of data) and sat around idle most of the time, so speed didn’t seem to be the problem. Interpreting whether the voltage was real, on the other hand, was much more difficult.

They experimented with twisting the copper wiring and shielding it from outside interference. They also wrote software to try to make the network devices “pseudo-smart” Along this line, special algorithms were written in the network card logic that basically stated, “If the voltage is close to a 1, make it a 1; if it’s close to a 0, make it a 0. We’ll have to perform some error checking afterward.” After these algorithms were written and the wiring seemed fairly safe, engineers could finally turn to the task of extending the network.

This was fairly simple: design a piece of hardware between two wire segments; if the hardware hears voltage on one side, clean it up and retransmit it on the other side. String as many of these together as you want and you can extend a network for miles and miles, right? Well, no. Machines can’t wait forever for a reply to figure out whether a machine receives the voltage; remember, the sending machine has no idea repeaters are on the network. So, after a certain time-out period, the network card just says, “Hey, forget it,” or worse, “Hey let’s retransmit!”

So, how many repeaters can be strung together, repeating voltage from one segment to another, before the time elapses for a response? The general networking rule is the 5-4-3 rule. This rule states that you can connect up to five network segments with four repeaters where only three of the segments are populated with machines. Standard rules for segment lengths of 100 feet or more, depending on the type of cable, such as coax or UTP, also had to be followed.

All this developed out of the necessity to weed garbage from the data. Repeaters were simply designed and implemented to freshen up the voltage on a network segment and retransmit it, all nice and clean again. This type of conditioning of the line occurs at the first layer of the networking model. Although no true error-correction and retransmission utilities are running here, algorithms determine how degraded a signal is, how best to boost the signal that will be rebroadcast, or whether to simply ignore the signal that’s been received. Figure 5.3 illustrates at what layer of the networking model a repeater operates.

Given how low the repeater works in the networking model, it should be fairly clear that the TCP/IP protocol suite is not terribly concerned about whether there are repeaters in your networking environment—assuming of course, they’re working. IP and ARP, the lowest working protocols in the suite, don’t even care whether you’re on an ethernet, token ring, or other type of network, so long as the underlying network infrastructure is functioning. The one important disadvantage of repeaters occurs when two machines are on the same network segment. When they need to communicate with each other, there is no need for every other segment to receive the same voltage. Figure 5.4 illustrates where repeaters fall a little short.

The great thing about repeaters is that they retransmit any kind of voltage, including broadcasts, throughout the network to any machine that is listening. Unfortunately, this is also one of their flaws. Repeaters retransmit—throughout the entire network—even when it is unnecessary. For instance, when two machines on the same network segment want to send directed packets between themselves, the repeater will still retransmit those signals throughout the network. This creates unnecessary traffic on the other network segments. Repeaters simply do not know any better. To correct this problem, a smarter device was needed.

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