The 8448 kbit/s interface represents the third level in the G.700 series digital hierarchy, designed primarily for multiplexing four 2048 kbit/s signals into a single, higher-speed stream.

The basic frame characteristics are a frame length of 1056 bits, numbered sequentially 1 to 1056, with a frame repetition rate of 8000 Hz.

The frame alignment signal for this interface is 11100110 100000. This signal is distributed across two parts of the frame, occupying bit positions 1 to 8 and 529 to 534.

A CRC procedure is specified to provide end-to-end quality monitoring of the 8 Mbit/s link. A 6-bit CRC is used, with the six bits (C₁ to C₆) inserted into bit-positions 1 to 6 of channel time slot 99. The CRC-6 block size is 132 octets (1056 bits), and the computation is made 8000 times per second. The generator polynomial is x⁶ + x + 1.

For channel-associated signalling, 64 kbit/s channel time slots 67 to 70 are used. The capacity of these time slots is sub-multiplexed across a 16-frame multiframe to provide signalling for up to 120 telephone channels.

Table 11: Bit Allocation of 64 kbit/s Channel Time Slots 67 to 70

This document will now introduce the highest bit rate covered in this specification, 44 736 kbit/s.