VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC8021/VSC8022

2.5Gb/s SONET-Compatible

8-Bit MUX/DEMUX Chipset

Page 8

G52028-0, Rev 4.1

05/25/01

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VSC8022 SONET Frame Recovery and Detection

The SONET framing sequence is a string of A1 bytes followed by a string of A2 bytes. (A1 = 11110110 and A2

= 00101000). The first serial bit starts at the left of the byte. Table 6 shows the number of A1 and A2 bytes in each

SONET frame for different line rates. The VSC8022 contains a frame recovery circuit and a frame detection circuit.

Table 6: A1/A2 Byte Count

Frame Recovery Circuit

The frame recovery circuit is designed to scan the serial data stream, looking for the A1 byte. When it finds

the A1 pattern, it adjusts internal timing so that the serial data is properly demultiplexed onto the eight parallel

outputs. Subsequently, the MSB of the A1 byte will appear in the D1 position and LSB of the A1 byte will

appear in the D8 position. This word boundary alignment causes the BYCKO, BYCKON output to be resyn-

chronized. While the frame aligner is hunting for the frame, BYCKO and parallel data are invalid. Frame recov-

ery circuits are disabled by frame detection (resulting in FP) or by a falling edge on the OOFN input while FDIS

is high.

Frame Detection Circuit

The frame detection circuit monitors the demultiplexed data, and senses the boundary between A1 and A2

bytes. This pulse on the FP output will reset the frame recovery circuit, so that no further resynchronization will

occur until permission is given through OOFN.

Circuit Operation

The frame recovery circuits are initialized and enabled on the falling edge of the OOFN ECL input with

FDIS held low. The OOFN must be at least one byte clock period wide. It must occur at least four byte clock

periods before the A1/A2 boundary. The circuit requires at least three A1 bytes followed by 3 A2 bytes for suc-

cessful alignment. The first A1 byte is used by the frame recovery circuit to obtain initial word boundary align-

ment, while the following two A1 and three A2 bytes are used to reset the frame recovery circuit and maintain

alignment for the subsequent bit stream. Frame recognition will occur for each word boundary aligned

A1A1A2A2A2 sequence in the data stream. Frame recognition is signaled by a one byte clock period high pulse

on the FP ECL output pin. This FP pulse will appear one byte period after the first A2 byte appears on the paral-

lel data output pins.

STS Level

Line Rate (Mb/s)

# of A1 Bytes

# of A2 Bytes

STS-3

155.520

3

3

STS-12

622.080

12

12

STS-48

2488.32

48

48