S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
10.2.3 Transmit IMA Processor (TIMA)
The TIMA is responsible for the transmit IMA functions. This consists of distributing the cells
arriving from the ATM layer to links in a group and for inserting ICP cells, filler cells, and stuff
cells as required by the IMA protocol. Additionally, the TIMA can support cell transmission on
connections using only the Transmission Convergence (TC) sublayer without the use of the
IMA protocol sublayer.
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.
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 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.
In ITC mode, the stuff event may be advertised in the ICP cell of the preceding frame or in the
four preceding frames. If the stuff event needs to be advertised for four preceding frames, a
DS1/E1 clock tolerance of +/- 50 ppm, or better, is required. If a frequency tolerance of +/- 50
ppm cannot be met among the independent transmit clocks, the TIMA can provide the single
frame advertisement of stuff events.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
54
PMC [ PMC-SIERRA, INC ]
