PM7340 S/UNI-IMA-8
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
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 shouldspread throughout the frame. To aviod 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.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
53
PMC [ PMC-SIERRA, INC ]
