S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
Link Addition and Differential Delay
Once a group is started, the delay profile for the group is determined. In order to add links, the
delay on the new links must be compatible with the existing links in the group and be able to be
synchronized with the existing links within the DCB constraints.
There are two mechanisms regarding delay that can be used.
The first method uses the guardband capability. At group start-up, a guardband is added to the
link with the longest transport delay. This guardband results in additional delay to be queued in
the DCBs for each link in the group. The guardband allows for links with a longer transport
delay to be added in the future without introducing any additional CDV. Note that new links
with the same or slightly shorter transport delay than the existing link with the longest transport
delay (by up to 3 cells) may be rejected from link addition, if the guardband is programmed to
0. This is due to differential delay measurement jitter associated with ICP and stuff cell
processing. For this reason, if the first method is used, the guardband should be programmed to
a value of 3 or greater.
The second method allows the delay accumulated per link to be increased dynamically. This
method will introduce additional delay to all of the links within the group when a link with a
larger transport delay is added to the group. The process of adding additional delay to the links
within a group will cause additional CDV to be introduced when the playout of data is stopped
while the delay is accumulated.
The RIPP determines the delay of the links that are being added and performs the appropriate
action to either include the link in the group or to reject the link if the link cannot be
synchronized within the DCB constraints. If a link is rejected due to delay, a LODS defect will
be reported on the link.
When links are deleted from the group, the delay of the remaining links is not adjusted.
See “Delay Compensation Buffers” in section 10.2.4 for more details on the management of the
DCB buffers.
Removing accumulated delay
In some situations, removal of accumulated delay may be desired. This usually occurs after a
group has been operational for a period of time and the link characteristics in terms of transport
delay have changed. The adjust_delay command is provided to remove delay from the group.
The execution of this command will affect the CDV of the group while the delay is reduced.
Any delay adjustment to the group will also affect the CDV of a connection carried on that
group. This is additive, if 20 ms of delay is removed from the group, a particular connection
within the group will experience an additional 20 ms of CDV. This will generally only be a
concern to CBR or VBR-rt traffic flows. This increase in CDV may cause traffic to be policed
out or real time applications to experience slips.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
74
PMC [ PMC-SIERRA, INC ]
