NOTES, RESOURCES, ADDITIONAL INFORMATION | 53
Actually, there are two types of off-hook signaling. On a loop-start line, when a phone goes
off-hook, the circuit is closed, and the central office detects the change in current. This is the
common residential format. Ground-start signaling is a small modification to the scheme to
permit disconnect supervision and remove the possibility of glare, where a PBX mistakenly
takes a ringing-in line for an outgoing call. . In an idle circuit, the central office provides -48v on
the Ring wire and an open on the Tip wire. From the PBX side, Ring is grounded first, then the
central office circuit must respond by grounding Tip. The PBX senses this, releases its ground,
and maintains the connection by drawing loop current.
E&M Trunks
E&M trunks use two extra wires for signaling (the so-called Ear and Mouth connections). These
solve the problems with glare and disconnect supervision. This scheme is nearly obsolete,
but occasionally E&M interface cards are used to connect music-on-hold to VoIP PBXs. They
are convenient for this purpose because the audio path is transformer isolated and there is no
need to supply talk battery from the MOH source.
T1/E1
These are basic digital interfaces to the switched voice network, and are widely used. This is
especially so in the USA, where T1 is nearly standard for large PBXs. In Europe, ISDN PRI is
more widely employed. T1 transports up to 24 voice channels, while E1 supports as many as
32. T1s are common in the USA and Japan, while E1s are provided by Telcos in most of the
rest of the world. In addition to the audio, these digital circuits also carry basic signaling in
CAS (Channel Associated Signaling) bits. This signaling emulates loop-start, ground-start, or
E&M, depending upon configuration. Over a T1, 23 speech channels are offered, while an E1
provides 30. They actually have 24 and 32 channels, respectively, but one or two of the chan-
nels are reserved for signaling communications.
T1s can also be used for IP connections. In this case, all or part of a T1’s 1.544kbps capacity is
used as a transparent pipe from the local IP router to the ISPs equipment. The phrase channelized
T1 is sometimes used to distinguish a T1 that is intended for circuit-switched voice application.
A fractional T1 is a service that uses a portion of the line’s full capacity. It is sometimes pos-
sible to order a T1 that is divided into a channelized portion and a data-transparent part for IP
connectivity.
ISDN PRI
ISDN PRI (Integrated Services Digital Network Primary Rate Interface) uses the same underly-
ing circuits as T1s and E1s. However, it employs sophisticated out-of-band signaling, which
allows transfer of information such as calling number, codec type, clearing causes, and such.
(It’s rather strange that non-ISDN T1 sends caller ID modem-encoded in the audio channel,
no?) The speech paths are called B (bearer) channels, while the signaling is carried in D (data)
channels. Almost all large VoIP gateways and PBXs support ISDN PRI lines. The signaling in
the USA is a slightly different protocol than that used in Europe and other parts of the world.
Your gateway will need to be set to match the protocol on your line. Normally in the USA, this
would be NI-1 (National ISDN - 1), while Europe would use the Euro ISDN standard.
You might hear the term QSIG in the context of ISDN PRI gateways. This is a signaling protocol
that is yet more sophisticated than ISDN’s usual Q.931 protocol, and is layered on top of it. With
the ascendancy of SIP, QSIG looks to be yet another valiant attempt falling by the wayside.
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