In a message dated 1/16/01, Praveen Rao writes: > We have a PABX which supports 'Loop-start' signalling. > How widely is this type of signalling accepted in the USA and Canada ? > Is 'Ground-start' much widely used and preferred in these countries ? What's > the scenario ? >
Hi Praveen: Loop start signalling is the standard type of single line service, where a terminal device requests dial tone from the central office by connecting tip to ring to close the loop. For PBXs, ground start signalling was developed to reduce the probability of "glare," which is the collision that arises when in outbound call is initiated at the same time that an incoming call comes in. Both loop start and ground start lines are widely available from central offices in the USA and Canada. In the loop start mode incoming calls are signalled by power ringing, with long silent intervals between the ring bursts. If the timing conditions are unfavorable, the application of incoming ringing can start during a silent interval, so that the terminal device does not know that an incoming call is pending for up to four seconds. During this initial silent interval, someone on the terminal end might pick up the phone expecting to place an outbound call, only to find themselves connected to the incoming call instead. This is "glare," and I'm sure it has happened to most of us every now and then on our home telephones. In a PBX, typically several extension lines are concentrated onto a smaller number of central office trunks that run between the PBX and the central office, so there is much more traffic on the central office trunks. This greatly increases the statistical probability of glare. To address this problem, the ground start protocol was developed. I won't go into the details of the protocol, but as the name implies, the terminal device (PBX) requests dial tone by momentarily grounding the ring lead. The protocol is designed such that the statistical probability of glare is extremely low. In general, a large business PBX that uses analog trunk lines to the central office will use ground start lines to avoid glare. In my observation, it seems that many of the smaller PBX and key systems (2 to 4 analog trunks to the central office) being made today are only supporting loop start, even though ground start would provide better performance. I'm not sure why this is, but it may have something to do with the added cost of supporting ground start or with the additional complexity that connecting to ground start line can create for user installed equipment. These days it seems that most large PBXs are using T1/E1 trunks to the central office, so ground start is a moot issue for them. This leaves only smaller systems where the ground start vs. loop start issue may be a problem. As I said above, many small systems being shipped today do not support ground start. So, I guess you will have to judge for yourself whether the lack of ground start support is a problem for your system. Ground start is a nice feature to have on a PBX, but many of the smaller systems in the market do not support it. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
