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May 24, 2018
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T1 CAS vs ISDN PRI Telephone Trunks
Channel Associated Signaling and Clear Channel Signaling for T1 Telephone lines

By: John Shepler

T1 telephone trunk lines are popular with PBX and IP PBX phone systems. Most current equipment offers an interface connection so that you don’t need separate a CSU/DSU box to connect to the line. What you may not know is that there are two type of T1 telephone lines available. One is CAS and the other is PRI, also known as ISDN PRI.

How T1 Lines Are Organized
T1 CAS is the original design for T1 lines used as telephone trunks. The line is channelized or divided into 24 separate segments of 8 bits each sampled at 8 Kbps. Multiply 8 bits by 8 Kbps and you get 64 Kbps, which is the capacity of one T1 channel or DS0. The designation DS0 stands for Digital Signal level 0, the basic building block of a T1 line.

The DS0 Voice Channel
What goes into a DS0? It’s one digitized telephone call. The process of converting from analog to digital is called PCM or Pulse Code Modulation. The 8 bits are enough to represent 256 different voice levels and provide excellent voice quality.

The T1 Frame
A collection of 24 channels of 8 bits each strung together end to end plus 1 extra bit defines a T1 frame. The extra bit is used to synchronize the equipment so that it knows where the frame starts. Hence, the designation “framing bit.”

Channel Associated Signaling
Now, here’s where CAS comes in. Notice that each of the telephone channels is filled up with digitized voice. The framing bit is occupied in synchronizing the line. So, how do you communicate dialing and switching information? The dialing is handled by transmitting the DTMF dial-tones through the voice channels just like you would over an analog line. Knowing whether the channel is on-hook, off-hook, ringing, etc, needs to be handled by some extra bits that aren’t designed into the system. The way this is done is by “robbing” one bit from each channel periodically to create a signaling channel. This degrades the voice channels by such a small amount that it isn’t noticed.

Robbed Bits
The robbed bit signaling technique combined with DTMF dialing means that each channel is responsible for its own operation. This is what is called Channel Associated Signaling or CAS. Another name for this is in-band signaling. Each channel on a T1 line is equivalent to one analog telephone line. There are separate transmit and receive paths on the T1 so that you can talk and listen at the same time. This is full duplex operation.

Using a Channel Bank
Here’s something interesting. You don’t need a PBX system to use a T1 line to bring in multiple business phone lines. Instead, you buy a device called a Channel Bank that converts 24 phone lines to one T1 line. The channel bank handles all the interfacing and conversion back and forth between analog and digital protocols.

ISDN PRI Alternative
The other type of T1 telephone line that has become even more popular than T1 CAS is T1 PRI or ISDN PRI. ISDN is a set of digital standards that include a low speed and a high speed option (BRI and PRI). PRI stands for Primary Rate Interface. You might think that it is something completely different from T1, but actually PRI is just a reorganization of the T1 channels.

Clear Channel Signaling
With ISDN PRI, 23 of the 24 available voice channels are kept exactly the same as T1 voice channels, but are called B channels or Bearer channels. The remaining channel is called a D channel or Delta channel, sometimes called a Data channel. With ISDN, all the control and signaling is assigned to the D channel. There are no robbed bits from the voice channel. What that does is speed up the dialing and switching, an important feature for high volume call centers. The other advantage gained by having a D channel is that it can be used to supply digital data, such as Caller ID. ISDN signaling is known as CCS or Clear Channel Signaling to distinguish it from CAS. This is also referred to as out of band signaling as opposed to in-band signaling.

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