 the switch is limited, re- quiring timing of the incoming and outgoing DS-1 signals to come from a reliable source. LECs and IXCs have strict synchronization plans to ensure that disruptions in the signal path caused by timing errors are minimized. 11.3.3 Managing calls The jobs we usually associate with a telephone switch are call processing and account- ing. These tasks are easier in a wired system than in a wireless system because for land telephones, the telephone numbers are related directly to physical presence on a spe- cific switch, and all accounting is associated with that switch. Wireless switches need do more work to process calls and handle accounting functions because of the mobil- ity of the wireless terminals. 11.3.3.1 Call processing. The local exchange office has the task of strobing each line to detect an off-hook condition, applying the internal resources needed to collect the dialed digits from the caller, and then interpreting the caller's intent. The local ex- change office also must locate and ring the called telephone and send tones that sound like ringing to the calling party's telephone. The local exchange office must detect if and when the called party accepts the call. Once the called party accepts, the originating lo- cal exchange office must be notified SO that the originating office can remove the locally generated ringing tone from the calling party's line and cut through the voice path. At this time, the local exchange office that connects the calling party initiates a billing record for the calling party. When either party hangs up, the on-hook condition must be sensed and communi- cated to the other local exchange office participating in the call so that a disconnect can be forced at that end and billing can be stopped. The switch also deals with call states other than normal call completion, such as busy signals, blocked trunks, and various telephone network failures. Failures of telephone equipment are rare, but the telephone company does not throw its arms in the air and gi