PM7340 S/UNI-IMA-8
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
10.2.4.2
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 14, 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
effected by the presence of stuff cells and ICP cells, but these effects are
transitory).
Figure 14
- 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
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
62
PMC [ PMC-SIERRA, INC ]
