ve repeaters eliminate the noise that may accumulate on the signal during transmission. In addition, there is greater use of fiber optics as the transmission media, and fiber is impervious to many of the impairments that affect electrical systems. As a result, all the error control proce- dures are removed from Frame Relay networks in order to make them faster. Further- more, in Frame Relay we're not very concerned about having to retransmit information. There's less likelihood that errors or noise in the network will cause the need for retrans- mission. Thus, Frame Relay uses a larger packet size than X.25, and the result is improved bandwidth efficiency. A Frame Relay packet contains less control information for a larger group of bytes of information, and this means the packet makes better use of the available bandwidth. If we jump ahead one more generation, to ATM switching, we find that packets are called cells. They're small-only 53 bytes-which gives them the capability to cross inter- sections quickly (i.e., it reduces the latency). And, more importantly, the cells are all the same size, which is particularly important to multimedia and interactive applications. They can transition through network nodes very quickly, but because they are so small and each cell still requires basic control information, the result is a greater percentage of overhead, which is sometimes referred to as cell tax. However, with ATM, there's an underlying assumption that bandwidth isn't an issue due to the presence of high-speed fiber infrastructures, so you're not trying to conserve on bandwidth; instead, the prevail- ing condition that you're trying to satisfy is low latencies and guaranteed QoS. You can see that the optimum size of packets depends on a number of factors, such as the performance of the network, the cost of the bandwidth, and the demand of the applications involved. Packet-Switched Networks Frame Relay The second generation of packet switching, Frame Relay, was introduced in 1991. Frame Relay assumes that there's