PM7341 datasheet(113/413 Pages) PMC

113 / 413 page

PRELIMINARY

INVERSE MULTIPLEXING OVER ATM

PM7341 S/UNI-IMA-84

DATASHEET

PMC-2000223

ISSUE 4

INVERSE MULTIPLEXING OVER ATM

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

115

10.3 Link FIFOs

In the transmit direction, per link FIFOs exist to provide an elastic store to ensure

proper operation. The number of cells in the FIFO at any particular time varies

as a function of the CDV introduced by the insertion of stuff cells and ICP cells,

the effects of the physical link interface, and the smoothing of data from the ATM

layer for group level CDV. During operation, these FIFOs are loaded a cell at a

time and emptied in a TDM fashion. The Link FIFOs in the transmit direction are

to be 8 cells deep.

In the receive direction, the link FIFOs serve as an interface between the TDM

domain and the ATM cell domain. Cells are gathered in the FIFO for each link

and then burst out to the external memory by the RDAT. The Link FIFOs in the

receive direction are 2 cells deep.

A diagnostic loopback capability is provided to loopback data from the receive

link FIFOs to the transmit link FIFO; this will loopback all links when enabled.

10.4 TC Layer

10.4.1 TX TC Layer (TTTC)

The TX TC layer (TTTC) performs the TC layer functions. These functions consist

of optional HEC generation, optional payload scrambling, and cell-rate

decoupling through physical layer (idle) cell insertion.

This function removes data byte by byte from the per-link FIFOs as required to

provide data to the physical layer function. When the physical layer function

needs a byte of data, it will request data from the (TTTC). The TTTC will then

read a byte of data from the per-link FIFO. If that byte is the first byte of a cell

and the logical channel FIFO is empty, the TTTC will format the next 53 bytes as

a physical layer (idle) cell. If the byte is the fifth byte of a cell, the byte is

optionally overwritten by the CRC-8 calculation over the previous four bytes for

that logical channel. The sixth through 53

rd bytes may be scrambled by a x43+1

self-synchronous scrambler.

Note: Since the Link FIFOs are cell based, an underrun cannot happen in the

middle of a cell.

PM7341 Datasheet(PDF) 113 Page - PMC-Sierra, Inc