ne instruments used for phones, but it can cause problems on trunks between plain old telephone service (POTS) use a simple PBXs and COs with high activity. In that situation, the two-wire interface to the network. They rely on an in- remote end and the CO switch can both try to seize the ternal two-wire/four-wire hybrid circuit to both transmit line at the same time. This situation, called glare, can and receive signals. This economical approach has been freeze the trunk until one side releases it. The solution is effective but requires special engineering regarding echo. to short tip or ring to ground as a signal for line seizure, rather than looping it. This is called ground start. After the line is seized, it is necessary to dial the Basic telephony number. Normal human fingers cannot outrun the dial receivers in a modern switch, but digits dialed by a PBX Three types of signaling are required for traditional te- can. In that case, many analog trunks use a delay start or lephony: supervision, alerting, and addressing. Supervi- wink start method to notify the calling device when the sion monitors the state of the instrument-for example, switch is ready to accept digits. allowing the central office or PBX to know when the Another analog interface often used for trunking is receiver has been picked up to make a call or when a call E&M. This is a four- or six-wire interface that includes is terminated. Alerting concerns the notification of a user separate wires for supervision in addition to the voice that a call is present (ringing) or simple call progress pair. E&M stands for "ear and mouth" or "Earth and tones during a call (such as busy, ringback, and SO on). magneto" and is derived from the early telephony days. Finally, addressing enables the user to dial a specific The E&M leads are used to signal on-hook and off-hook extension. states. In addition to signaling, telephony services also An analog interface works well for basic trunk con- provide secure media transport for the voice itself, nections between 