A repeater is a device that extends the inherent distance limitations of various transmission media, including wireless links, by boosting signal power so that it stays at the same level regardless of the distance it must travel. As such, the repeater operates at the lowest level of the Open Systems Interconnection (OSI) reference model—the Physical Layer.
Repeaters are necessary because signal strength weakens with distance: The longer the path a signal must travel, the weaker it gets. This condition is known as “signal attenuation.” On a telephone call, a weak signal will cause low volume, interfering with the parties’ ability to hear each other. In cellular networks, when a mobile user moves beyond the range of a cell site, the signal fades to the point of disconnecting the call.
In the LAN environment, a weak signal can result in corrupt data, which can substantially reduce throughput by forcing retransmissions when errors are detected. When the signal level drops low enough, the chances of interference from external noise increase, rendering the signal unusable. Repeaters also can be used to link different types of network media—fiber to coaxial cable, for example.
Often LANs are interconnected in a campus environment by means of repeaters that form the LANs into connected network segments. The segments may employ different transmission media—thick or thin coaxial cable, twisted-pair wiring, or optical fiber. The cost of media converters is significantly less than full repeaters and can be used whenever media distance limitations will not be exceeded in the network.
Hubs or switches usually are equipped with appropriate modules that perform the repeater and media conversion functions on sprawling LANs. But the use of hubs or switches also can eliminate the need for repeaters, since most cable segments in office buildings will not run more than 100 feet (about 30 meters), which is well within the distance limitation of most LAN standards, including 1000BaseT Gigabit Ethernet running over Category 5 cable.
Often the terms repeater and regenerator are used interchangeably, but there is a subtle difference between the two. In an analog system, a repeater boosts the desired signal strength but also boosts the noise level as well. Consequently, the signal-to-noise ratio on the output side of the repeater remains the same as on the input side. This means that once noise is introduced into the desired signal, it is impossible to get the signal back into its original form again on the output side of the repeater.
In a digital system, regenerators are used instead of repeaters. The regenerator determines whether the information- carrying bits are 1s or 0s on the basis of the received signal on the input side. Once the decision of 1 or 0 is made, a fresh signal representing that bit is transmitted on the output side of the regenerator.
Because the quality of the output signal is a perfect replication of the input signal, it is possible to maintain a very high level of performance over a range of transmission impairments. Noise, for instance, is filtered out because it is not represented as a 1 or 0.
Stand-alone repeaters have transceiver interface modules that provide connections to various media. There are fiberoptic transceivers, coaxial transceivers, and twistedpair transceivers. Some repeaters contain the intelligence to detect packet collisions and will not repeat collision fragments to other cable segments. Some repeaters also can “deinsert” themselves from a hub or switch when there are excessive errors on the cable segment, and they can submit performance information to a central management station.