Dialogic 6.2 manual General Information, November 265

General Information

These protocols apply either to digital (T1 or E1) or analog lines. The different types of lines simply provide a different mechanism for conveying the signal-electric signals (loop current and ring voltage) on analog lines and bits on digital lines.

On digital lines, these protocols are sometimes referred to as Channel Associated Signaling (CAS) or Robbed-Bit Signaling (RBS) due to the nature of the signaling. Each channel's state is represented as a set of 2 signaling bits, and these bits are transmitted on the line at constant intervals. The LEC protocols are normally associated with T1 lines, where they were originally used, but nowadays it is possible to find these protocols used in PBX's using E1 lines (which provide 4 signaling bits per channel).

A different technique is to reserve a full channel on a T1 or E1 line to carry information about all channels, and to use the available bits as a continuous stream carrying information “packets” instead of repeating a number of signaling bits over time. This technique is called Common-Channel Signaling (CCS), and is used on ISDN lines among others.

In T1 lines, audio and line state information is grouped in frames, each frame consists of 8 bits of data for each of the 24 channels, plus a framing bit, adding to 193 bits/frame (8 bits/byte * 1 byte/channel * 24 channels/frame + 1bit). The sampling rate is 8000 Hz, so the bit rate is 8000x193=1,544,000, or 1.544 Mb/s.

The technique used for carrying the signaling bits on T1 is to use (“rob,” hence the term Robbed-Bit Signaling) some of the bits normally intended to represent data (voice/fax/data) on a channel for the purposes of call control. Research has shown that robbing the least significant bit of each channel's sample every 6th frame causes a virtually imperceptible (for humans) level of distortion for voice. However, when raw data is being sent, this loss becomes unacceptable, so for simplicity only 7 of the 8 bits are used for data applications. This technique allows for all 24 channels to be used for calls, as opposed to 23 channels used for calls and one for call control (23B+D), as with ISDN.

In E1 lines (32 timeslots per line), one of the timeslots (0) in each frame is reserved for framing and synchronization data, and another (timeslot 16) is used for the signaling bits — in the first frame it carries information about audio timeslots 1 and 17, in the second frame for timeslots 2 and 18 and so on until all audio channels are covered, and then the process starts over.

November 2009 265