PM7341 datasheet(82/413 Pages) PMC

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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

10.2.3.1

IMA Frame

The Transmit IMA processor creates the IMA frame by inserting an ICP cell after

every (M-1) cells per link. Values of M supported are 32, 64, 128, and 256. The

ICP cell is offset within the IMA frame. This offset is programmable on a per-link

basis, and the offsets should spread throughout the frame. To avoid interaction

between groups, the offsets within a group may not be aligned at the same offset.

If offsets are aligned at the same offset within a group, the CDV experienced by

other groups will be increased. Each frame is identified with an IMA frame

sequence number (IFSN); this number is the same for every link in the group that

is within the same frame and increments with each frame. The TIMA is

responsible for aligning the transmission of the IMA frame on all links within a

group.

10.2.3.2

Stuffing Procedure

The TIMA can support both Independent Transmit Clock (ITC) and Common

Transmit Clock (CTC) modes. The difference between these modes is the

stuffing protocol. The method of stuffing is set independently from the clocking

mode present in the ICP cell.

In CTC mode, a stuff cell is added after 2048 cells on each link. The stuff cell is

identical to the ICP cell and is inserted immediately following the ICP cell. The

stuff cell events will occur on the same frame on all links; however, the

programmed ICP offsets determine at which cell in the frame the stuff event will

occur.

In ITC mode, a stuff cell is added to the reference link after 2048 cells on the

reference link. On all other links in the group, stuff cells are added as necessary

to compensate for data rate differences between the link and the reference link of

the group. The added stuff cells (or lack of stuff cells) keep the data rate between

links equalized.

The stuff cell is generated immediately after the ICP cell and both the ICP cell

and the stuff cell are identified as stuff cells via the Link Stuff Indication (LSI) field

of the ICP cell.

In CTC mode, the stuff event is always advertised in the ICP cell of the preceding

frame. The stuff event may also be advertised in the 4 preceding frames. It is

programmable per group whether the ICP cell is advertised starting 1 frame or 4

frames prior to the occurrence of the stuff event.

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