Repeaters

Like any type of network, 10BASE5 and 10BASE2 had limitations on the total length of a cable. With 10BASE5, the limit was 500 m; with 10BASE2, it was 185 m. Interestingly, these two types of Ethernet get their name from the maximum segment lengths—if you think of 185 m as being close to 200 m, then the last digit of the names defines the multiple of 100 m that is the maximum length of a segment. That's really where the 5 and the 2 came from in the names.

In some cases, the length was not enough. So, a device called a repeater was developed. One of the problems with using longer segment lengths was that the signal sent by one device could attenuate too much if the cable was longer that 500 m or 185 m, respectively. Attenuation means that when electrical signals pass over a wire, the strength of the signal gets smaller the farther along the cable it travels. It's the same concept behind why you can hear someone talking right next to you, but if that person speaks at the same volume and you are across the room, you might not hear her because the sound waves have attenuated.

Repeaters allow multiple segments to be connected by taking an incoming signal, interpreting the bits as 1s and 0s, and generating a brand new, clean signal. A repeater does not simply amplify the signal because amplifying the signal might also amplify any noise picked up along the way.

NOTE Because the repeater does not interpret what the bits mean, but does examine and generate electrical signals, a repeater is considered to operate at Layer 1.

So, why all this focus on standards for Ethernets that you will never work with? Well, these older standards provide a point of comparison to how things work today, with several of the features of these two early standards being maintained today. Now, on to an Ethernet standard that is still found occasionally in production networks today—10BASE-T.

0 0

Post a comment