Repeaters

Repeaters

As you learned in Chapter 3, “Transmission Media,” all media attenuate the signals they carry. Each media type, therefore, has a maximum range that it can reliably carry data. The purpose of a repeater is to extend the maximum range for the network cabling.

A repeater is a network device that repeats a signal from one port onto the other ports to which it is connected (see Figure 6.4). Repeaters operate at the OSI Physical layer. (Refer to “The OSI Reference Model” section in Chapter 2.) A repeater does not filter or interpret—it merely repeats (regenerates) a signal, passing all network traffic in all directions.

A repeater doesn’t require any addressing information from the data frame because a repeater merely repeats bits of data. This means that if data is corrupt, a repeater will repeat it anyway. A repeater will even repeat a broadcast storm caused by a malfunctioning adapter (see Chapter 13, “Troubleshooting”).

The advantages of repeaters are that they are inexpensive and simple. Also, although they cannot connect networks with dissimilar data frames (such as a Token Ring network and an Ethernet network), some repeaters can connect segments with similar frame types but dissimilar cabling.

Figure 6.5 shows the use of a repeater to connect two Ethernet cable segments. The result of adding the repeater is that the potential length of the overall network is doubled.

Some repeaters simply amplify signals. Although this increases the strength of the data signal, it also amplifies any noise on the network. In addition, if the original signal has been distorted in any way, an amplifying repeater cannot clean up the distortion.

Certainly, it would be nice if repeaters could be used to extend networks indefinitely, but all network designs limit the size of the network. The most important reason for this limitation is signal propagation. Networks must work with reasonable expectations about the maximum time a signal might be in transit. This is known as propagation delay—the time it takes for a signal to reach the farthest point on the network. If this maximum propagation delay interval expires and no signals are encountered, a network error condition is assumed. Given the maximum propagation delay allowed, it is possible to calculate the maximum permissible cable length for the network. Even though repeaters enable signals to travel farther, the maximum propagation delay still sets a limit to the maximum size of the network.