hat to ask for-that is, if they are to install a turnkey solution. There are two choices: Alternate Mark Inversion (AMI) and Extended Super Frame (ESF)/Binary Eight Zero Substitution (B8ZS). The digital telephone network is synchronized, in order to manage the stream of zeros and ones that run across it. Synchronization relies on clocking, which uses cues from the bit streams to maintain order. Generally, a voltage indicates a one bit and no voltage indicates a zero bit. The network uses the voltages of the 1 bits as clocking pulses. In signaling, no more than eight bit times should pass without a signal. However, it is not usual for a customer's data to have eight or more successive 0s and thus provide no heartbeat for clocking. Without synchronization, the network shuts down. One solution has been to dedicate one bit out of every eight as a control bit. If all the other bits in an eight-bit string are zeros, then the control bit will be set to one and synchronization will be maintained. The need for this control bit is the reason for switched 56 service (rarely used anymore), which runs over 64 Kbps DSO circuits, but provides only 56 Kbps service. Synchronization borrows the eighth (least significant) bit to maintain clocking. Thus, only seven out of eight bits are available to users. An alternate solution to the synchronization problem was developed in the mid- 1980s. It involves forcing a deliberate violation (called a bipolar violation or BPV). The BPV is used to maintain network clocking, but networking knows not to interpret BPV pulses as data. This technique, binary eight zero substitution, is called B8ZS. DSO circuits that support B8ZS from end to end can carry 64Kbps of user data-in other words, offer clear-channel service. The point of this digression is this: if channels of a T-1 carrier will be used for some of the newer clear-channel digital services offered today (e.g., ISDN), it is best to order ESF/B8ZS. ESF/B8ZS also supports 56 Kbps service. If the T-1 carrier will be used for 56 Kbps service e