Information Technology Reference
In-Depth Information
Foreign Exchange Office (FXO)
allows a switch such as a PBX to us a standard ana-
log connection (FXS) from the PSTN or from another switch. In this case the PBX is
emulating an endpoint device. Because this is a standard endpoint connection it uses
two-wire connections just like a standard phone and often uses an RJ-11 connector
interface.
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Ear and Mouth (E&M)
connects private switches. It is an analog trunk used to con-
nect to a voice switch; it supports tie-line facilities or signaling between phone
switches. E&M can be connected with two-wire and four-wire. E&M is also called
Earth and Magnet.
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Channelized T1 (or E1)
circuits can be used to connect PBXs and the PSTN or 2
PBXs. These circuits can be provisioned in two different formats:
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Channel Associated Signaling (CAS) circuits provide 24 (for T1) or 32 (for E1)
channels (1 per DS0). CAS circuits get their name from the fact that switch to
switch signaling (dialed digits, caller ID, and so on) occurs in band along on each
individual channel with the voice traffic. Each DS0 channel is fixed at 64 Kbps,
so this type of signaling takes a small number of bits away from the voice trans-
mission to carry signaling data. Because of this CAS is also known as robbed-bit
signaling.
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Common Channel Signaling (CCS) circuits also use T1/E1 circuits. However,
unlike CAS circuits, CCS circuits set aside one channel specifically for carrying
signaling information for all of the other channels. This architecture allows for
CCS circuits to provide a more robust feature set between switches. ISDN PRI
uses CCS signaling and is the preferred connection type for PSTN-to-PBX or
PBX-to-PBX connections. CCS support is not always available, and therefore CAS
circuits are still widely used.
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Major Analog and Digital Signaling Types
Signaling is needed to provide the state of telephones, digit dialing, and other information.
For a c all to be placed, managed, and closed, all of the follow ing sig naling c ategories have
to occur:
Supervisory
provides call control and phone state (on-hook and off-hook).
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Addressing
provides dialed digits.
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Informational
provides information such as dial and busy tones and progress indicators.
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These different signaling categories are provided by analog and digital circuit types.
The signaling type depends on the type of connection. The major areas are
Loop start (CO to phone) is an analog signaling type commonly found in residential
applications. The circuit is “started” when the loop is closed (connected) completing
the circuit.
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start. Ground start allows for signaling between switches that allows one switch to
