Chapter 7 Managing Module Hardware
96 Avaya M770 ATM Switch User’s Guide
Note:
•OC-3 Modules (M15-155F/SF/MS):
In order to provide DEFINITY support, all the OC-3 modules in the chassis
must be C/S 2.0 or higher. The C/S of the module can be located on the unit’s
box or on the module itself. If the module is already inserted in the M770
switch, type the command summary info from the ATM module’s
Command Line Interface to see the C/S version.
• Only ports 1 and 2 can be used by the M-SPS to synchronize its clock. All ATM
ports can be used to connect the DEFINITY to the M770 ATM Switch.
•OC-12 Modules (M3-622F/SF):
Currently the DEFINITY does not support OC-12 connections, however the M3-
622 module may be used as backbone links between ATM switches. Only M3-
622 modules with C/S 2.0 or higher can provide clock synchronization to the
network or deliver clock to the switch. Newer and older C/S’s of the modules
can be mixed in the same chassis, but the older modules will not be
synchronized to the rest of the ATM network.
• All ports (of C/S 2.0 modules) can be used by the M-SPS to synchronize its
clock.
Managing Packet Discard Thresholds for a Module
There are two packet discard thresholds for each module, called Early Packet
Discard (EPD), and Partial Packet Discard (PPD). By default, both thresholds have a
set limit (see below) and are enabled.
The M770 ATM Switch uses EPD to discard entire AAL5 frames for UBR traffic
(rather than random cells fr om different frames), when it determines that it is about
to become congested. The M770 ATM Switch still passes through the last cell of each
AAL5 frame so that end-stations are aware of the discard that has taken place. You
can configure a threshold at which the EPD will be invoked. The threshold is a
percentage of the overall shared buffer space. The default for EPD is set at 80% of
the buffer fill.
PPD works similarly to EPD, but it is generally invoked at higher levels of
congestion. The difference is that, while with EPD the M770 ATM Switch can wait
for the start of a suitable frame, PPD is involved when congestion is too serious to
wait that long. Therefore, packet discarding will start in the middle of the frame.
The value for PPD threshold cannot be set by the user. It is higher than the default
for EPD. Under normal circumstances, EPD would deal with the congestion before
the buffer capacity is reached, and PPD should never be invoked.