21.4 The t carrier framing and coding formats

1. The superframe format

The T carrier was designed to carry 24 independent digitized voice channels, with each channel encoded as a 64kbit/s data stream. One of the earliest framing formats was the D4 framing pattern. This Tl frame format evolved principally to carry voice streams, and data to be transmitted over a T carrier system must conform to this format. The frame consists of 193 bits, with the last bit always being a framing bit. The first 192 bits correspond to 24 conversations, or channels, sampled with PCM type methods and generating 8-bit words. The combined signal is word interleaved, providing one frame.

A superframe (CCITT calls it a multiframe) is a repeating se­quence of 12 such frames and thus contains 12 framing/signalling bits. In order to keep track of the frame structure, at least 1 bit in 15 bits of the combined stream (information plus signalling) must be a 1, and at least 3 bits in 24 bits of the stream must be Is. The bandwidth used on voice frequency (VF) signalling is minimised by putting signalling information only in the LSB in the sixth and twelfth frames.

As illustrated in Figure 21.5, each superframe consists of 12 repeating frames. One frame corresponds to 125 microseconds; one superframe is thus 1.5 milliseconds in duration. Each frame contains one synchronisation bit to allow the receiving equipment to decode, demultiplex, and allocate the incoming bits to the appropriate chan­nels.

From the above follows that each super frame contains a 12-bit word, composed of individual bits coming from each of the 12 frames. The framing bits are called BFf, and the signalling bits BFs. BFfs are the odd numbered framing bits, whilst BFss are the even numbered bits. The 12-bit word is used for synchronisation and for identifying frames number 6 and 12, which contain channel signall­ing bits. Each frame thus contains a BFfor BFs framing/signalling bit on the 193rd position. The resulting pattern of the 12-bit word is 10ОО11О1ПОО. This entire process repeats every 1.5 milliseconds. The BFf bit alternates (1, 0, 1, 0, 1, 0, ...) every other frame. The receiving multiplexer can identify this unique sequence in the in­coming digital data stream to maintain or re-establish frame boun­daries. Frame 6 is identified by the fact that it occurs when the BFs is a 1 preceded by three BFs which were 0s. Frame 12 is identified by the fact that it occurs when the BFs is 0, preceded by three BFs which were Is. The receiver can easily identify this sequence, in the absence of severe line errors or impairments, and thus identify the desired frames which contain signalling information.

The 193rd bit signalling described above involves the manage­ment of the DS1 facility itself. As indicated in Section 21.3.2, each individual voice channel requires its own signalling information (call set-up, call completion, etc.). The signalling information must be transmitted along with the PCM samples. To achieve this the multiplexer will rob, or share the least significant bit, i.e. bit 8 from the user data stream. Consequently, this bit alternatively carries information or signalling data.

For five consecutive frames bit 8 will contain voice bits, and on the sixth, it will contain a signalling bit. This should not be confused with the mechanism described above for the superframe and the 193rd bit, though both mechanism and principle are similar. This bit robbing occurs totally within one voice sample, namely within one

octet of bits. The sixth bit is also referred to as the A bit, while the twelfth bit is also called the В bit. These combinations of bits allow the end-user station equipment to carry out its signalling protocol, which involves indicating such states as idle, busy, ringing, no ringing, loop open, etc.

For data applications the A/B signalling has no relevance. How­ever, when transferring data, the impact of losing the LSB every six frames is not acceptable. There are two ways to get around this inherent difficulty. Instead of transmitting data in 8-bit quantities, they are transferred in 7-bit quantities. Therefore, the fact that the LSB is robbed does not affect the overall performance because it is not used. To achieve this, data must be transmitted 7 bits at a time every 125 usec. In practical terms this involves either using a device that is capable of outputting at 7-bit increments or providing a special timing pulse to enable the data output.

Many communications controller devices can be operated in the latter mode but only a few in the former. As only 7 bits are transmitted every 125 µsec, the data rate is reduced from the 64kbit/s available to 56kbit/s. With this method, 12% of the poten­tial bandwidth is lost (see also Section 21.4.3).