S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
Delay Compensation Buffers
Since IMA must re-create the original cell stream in the proper order, delay compensation
buffers (DCBs) are used to remove the differential delay between the links in a group. As cells
arrive from each link, they are placed in that link’s DCB. Links with the least transport delay
will have the largest amount of data in the DCB, while links with the largest amount of transport
delay will have the least amount of data in the DCB.
At group start-up, all the links are compared to determine the link with the largest transport
delay and the link with the least transport delay. The difference between these is the differential
delay. Data is queued for all links until the corresponding data arrives for the link with the
largest transport delay. Figure 16, shows a group with 3 links with a differential delay of 5 cells.
Link 0 has the shortest transport delay and link 2 has the longest transport delay. Once the data
has arrived for all of the links, it is played out to the ATM layer at the IDCC rate, thus keeping
the depths of each DCB at a nominally constant level. (Depths are instantaneously affected by
the presence of stuff cells and ICP cells, but these effects are transitory).
Figure 16 Snapshot of DCB Buffers
DCB
DCB
DCB
Link 0
Link 1
Link 2
Write
Pointer 0
19
16
13
10
7
Write
Pointer 1
11
8
Write
Pointer 2
4
5
6
3
Group
Read
Pointer
1
2
When a group is already started, IMA supports the addition of links to the group. As illustrated
by Figure 16, adding a link with a transport delay that is within the range of the existing links
does not present any problems. The DCB for the new link must be aligned with the existing
links and added to the round robin for playout.
Adding a link with a smaller transport delay increases the differential delay of the group. This
requires that the depth of the DCB buffer be larger than any of the existing links. As long as the
differential delay is within acceptable bounds, the new link can be accepted. The DCB for the
new link is aligned with the existing links and added to the round robin for playout.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
62
PMC [ PMC-SIERRA, INC ]
