Which of the following signaling methods can be used by FXS and FXO interfaces? (Choose two.)
- delay dial
- ground start
- immediate start
- loop start
- wink start
Loop start and ground start signaling can be used by foreign exchange station (FXS) and foreign exchange office (FXO) interfaces. An FXO interface is typically used to connect an analog device to the public switched telephone network (PSTN). In addition, if a private branch exchange (PBX) is configured with an FXS port, the FXO interface on an analog device can terminate an analog trunk from a PBX. FXO interfaces are commonly found on standard telephones, fax machines, and analog modems. Thus devices that have FXO interfaces typically connect to the PSTN by using plain old telephone service (POTS) lines.
Cisco Analog Voice Gateways, such as the VG202 and the VG204, come with FXS ports that enable you to connect FXO devices, such as corded analog telephones or cordless analog telephone bases, to Cisco Unified Communications Manager. The VG202 supports up to a maximum of two analog devices. The VG204 supports up to a maximum of four analog devices.
When a loop start phone handset is picked up, two wires are connected, which completes the electrical circuit loop. The PSTN central office (CO) detects the closed circuit and sends a dial tone to the phone. By default, FXS and FXO interfaces are configured to use loop start signaling. Loop start signaling is susceptible to a problem called glare, which occurs when an incoming call seizes the same line as an outgoing call. Although glare is not typically a problem on residential lines, it can be a problem on business lines that receive high call volumes.
Ground start signaling was developed to address the problems caused by glare on FXS and FXO interfaces. Trunk lines and PBXs typically use ground start signaling to separate incoming calls from outgoing calls. When a ground start PBX detects a phone going off-hook, the PBX grounds two wires to alert the PSTN CO that an outgoing call is about to occur.
Immediate start, wink start, and delay dial signaling are used by ear and mouth (E&M) interfaces, not by FXS and FXO interfaces. E&M interfaces have two signaling paths: an E-lead and an M-lead. When an outgoing call is made with immediate start signaling, the phone goes off-hook on the E-lead, pauses for a short time, and sends dual-tone multi-frequency (DTMF) tones or pulses to specify the call destination. Like loop start signaling, immediate start signaling is susceptible to glare.
Wink start signaling was developed to address the problems caused by glare on E&M interfaces. When an outgoing call is made with wink start signaling, the phone goes off-hook on the E-lead, waits for a pulse, or wink, to be sent over the M-lead to indicate that the line is clear, and then sends DTMF tones or pulses to specify the call destination. By default, E&M interfaces are configured to use wink start signaling.
When an outgoing call is made with delay dial signaling, the source detects whether the destination is on-hook or off-hook. If the destination is on-hook, the source sends DTMF tones or pulses to specify the call destination. If the destination is off-hook, the source will wait until the destination is on-hook before sending DTMF tones or pulses.