iers. By the time of the breakup, the other long distance carriers already had networks running from indepen- dent clocks. With AT&T's network running on its own clock, this left each of the seven local exchange carriers no choice except to design and build their own clock systems into each LATA's network. Original T-carrier or T-1 clocking was a simple matter of two channel banks inter-operating with each other over a four-wire connection. As long as clock stability and recovery were within range, there was no problem. When the clock signal ranged out of bounds of clock recovery, something called a slip, meaning an out-of-lock condition occurred, followed by re-synchronization of the receiving clock. From a practical perspective, this causes a barely audible click in speech. The impact is hardly noticeable in telephone conversation and will likely cause a re-send in data applications, but it can be a killer in content transport applications, especially where the application is something like an MPEG transport stream. Over time as digital components made their way into switching systems and higher-order multiplex equipment, clocking signals were distributed through the network itself. When that happened, all of a sudden, network clocking was dependent on the network. If the network failed, then clocking failed. To solve this issue, the concept of clock hold-over was created. Hold-over simply set some design and operational rules in place that said, "When clocking signal is lost, switch to a local source and hold the stability within specified range until the master clock signal is restored, then switch back to the master clock signal." Most pieces of equipment in use have at least one clock. Typically, the design includes external reference capability and configuration parameters that allow the user to control the operation SO that it either stays on internal reference, or alternatively locks to an exter- nal source. If it is programmed to lock to an external source and it fails, the equipment automatically rever