he receiver. The receiver performs the same mathematical algorithm on the block, and it comes up with its own 16- or 32-bit code. The receiver then compares its code with the one the terminal sent, and if they match, the receiver sends an ACK, a positive acknowledgment that everything's okay, and it moves on to sending the next block. If the two codes don't match, the receiver sends a NACK, a negative acknowledgment, which says there was an error in transmission and the previous block needs to be resent before anything else can happen. If that error is not cor- rected within some number of attempts that the user specifies, the receiver will dis- engage the session. This ensures that errors are not introduced. Yet another benefit of synchronous transmission is that it operates at higher speeds than asynchronous transmission, and today you commonly see it performing at 2Mbps. These two types of transmission make sense in different applications. For machine-to-machine communications where you want to take advantage of high speeds and guarantee accuracy in the data flow-such as electronic funds trans- fer-synchronous communication is best. On the other hand, in a situation in which a human is accessing a database or reading today's horoscope, speed may not be of the essence, and error control may not be critical, SO the lower-cost asynchro- nous method would be appropriate. Keep in mind that things are never simple in telecom, and you rarely deal with simple alternatives; rather, you deal with layers and combinations of issues. For example, you can think of an escalator as being a synchronous network. The steps are presented at the same rate consistently, and they all travel up the ramp at the same speed. Passengers alight on steps, and all passengers are carried through that network at the same speed; therefore, the network is synchronous. However, each passenger alights on the escalator at a different rate, which makes the access to the network asynchronous. For example, an eight-year-old child might run up to the 