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.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.
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.”
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.
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.
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.
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.
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.
The nice thing about T1 lines is that they have well established standards to support both voice and data services. If you need telephone trunk lines for an in-house PBX or IP PBX systems, chances are that some configuration of T1 line will do the job. You may be surprised by how competitive T1 prices are now. Check T1 and ISDN PRI prices online to compare with your other telephone service options.
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