MT8979

Data Sheet

11

Zarlink Semiconductor Inc.

Framing Algorithms

There are three distinct framers within the MT8979. These include a frame alignment signal framer, a multiframe

framer and a CRC framer. Figure 13 shows the state diagram of the framing algorithms. The dotted lines shows

optional features, which are enabled in the maintenance mode.

The frame synchronization circuit searches for the first frame alignment signal within the bit stream. Once

detected, the frame counters are set to find the non-frame alignment signal. If bit 2 of the non-frame alignment

signal is not one, a new search is initiated, else the framer will monitor for the frame alignment in the next frame. If

the frame alignment signal is found, the device immediately declares frame synchronization.

The multiframe synchronization algorithm is dependent upon the state of frame alignment framer. The multiframe

framer will not initiate a search for multiframe synchronization until frame sync is achieved.

Multiframe

synchronization will be declared on the first occurrence of four consecutive zeros in the higher order quartet of

channel 16. Once multiframe synchronization is achieved, the framer will only go out of synchronization after

detection of two errors in the multiframe signal or loss of frame alignment synchronization.

The CRC synchronization algorithm is also dependent on the state of the frame alignment framer, but is

independent of the multiframe synchronization. The CRC framer will not initate a search for CRC framing signal

until frame alignment synchronization is achieved. Once frame alignment synchronization is acquired, the CRC

framer must find two framing signals in bit 1 of the non-frame alignment signal. Upon detection of the second CRC

framing signal the MT8979 will immediately go into CRC synchronization. When maintenance feature is enabled

(maint bit = 1) the CRC framer will force a complete reframe of the device if CRC frame synchronization is not found

within 8 ms or more than 914 CRC errors occur per second.