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Ground-start signaling is an adaptation of loop-start. Instead of the circuit being closed only at the phone end, both ends
of the circuit have the capability to detect current, and both ends can request and confirm the use of the circuit. This is
achieved by both ends being able to ground one of the wires in the circuit. These wires (or leads) are referred to as Tip
and Ring. These terms date back to the use of 1/4-inch jacks with a positive contact at the tip and a negative conductor in
the ring. The advantage is that it makes glare much less likely, and consequently ground-start is appropriate for trunk
connections that are heavily used. However, it is very rare to see a ground-start trunk in a VoIP network or indeed in any
new trunk deployment.
FIGURE 9
Ground-Start
Signaling
The ground-start process as it occurs on a trunk between a PBX and the CO switch is described next; refer to the diagram
for each step:
1. The PBX has a call that it must send to the PSTN. It signals to the CO switch that there is an inbound call by
grounding the ring lead.
2. The CO senses the ring lead as grounded and grounds the tip lead to signal the PBX that it is ready to receive the
call.
3. The PBX senses the tip ground and closes the loop between tip and ring in response; the PBX also removes the ring
ground.
4. The voice circuit is complete, and communication can begin.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
E&M Signaling
Variously called "Ear and Mouth," "RecEive and TransMit," and "Earth and Magneto," E&M analog trunks were typically
used to interconnect PBXs (tie-trunks). E&M connections have separate leads for signaling and voice; the signaling
leads are known as the E and M leads.
In an E&M connection, one side is called the trunk side; this is usually the PBX side. The other side is called the
signaling-unit side; this is the CO, channel-bank, or Cisco gateway E&M interface. The E lead is used to indicate to the
trunk side that the signaling-unit side has gone off-hook; conversely, the M lead is used to indicate to the signaling-unit
side that the trunk side has gone off-hook.
Five types of E&M signaling exist, numbered Type I through Type V. In a Cisco Gateway application, Types II and V can
be connected back-to-back and Type I cannot be. Cisco does not support Type IV.
Three main techniques are employed in E&M circuit signaling:
• Wink Start: The terminating side (for example, a Cisco Gateway) uses a brief off-hook-on-hook "wink" to
acknowledge that the originating side (for example, a PBX) has gone off-hook. Upon receipt of the wink, the originating
side begins sending digits. When the far-end device answers the call, the terminating side goes off-hook and
the voice circuit is then set up.
• Immediate Start: The originating side goes off-hook, waits a set time (perhaps 200ms), and then begins sending
digits whether or not the terminating side is ready.
• Delay Dial: Assume that a PBX is placing a call outbound to the PSTN: First, the PBX goes off-hook. The CO then
goes off-hook until it is ready to receive digits; it then goes on-hook. (This time period is the delay dial signal.) The
PBX sends digits. When the far-end device answers the call, the CO goes off-hook (called Answer Supervision), and
the voice circuit is then set up. The advantage of Delay Dial is that some equipment is not ready to receive digits
instantly, even though it has sent the wink; the delay compensates for this.