Traffic Rates Shaping in the 7600 Card
13-5
Q
Traffic shaping is a mechanism that alters the traffic characteristics of a
stream of cells on a VCC or a VPC to achieve a desired modification of
those traffic characteristics. The VBR quality of service class makes use
of traffic shaping.
Q
EFCI is a congestion notification mechanism that the ATM layer service
user may make use of to improve the utility that can be derived from
the ATM layer. The ABR quality of service class makes use of Explicit
Forward Congestion Indication.
Q
Explicit Rate uses RM-cells to control the traffic rate through the
network. The ABR quality of service class makes use of explicit rate.
Traffic Rates
Shaping in the 7600
Card
The 7600 Card implements Traffic Management that adheres to the
emerging ATM Forum standards. The Traffic Management makes use of
RM-cells for flow control to allow large networks to be created around
ATM technology without having cell loss due to congestion.
The 7600 Card Traffic Management hardware module contains an ASIC
and local memory for traffic management tables. It performs processing
on ATM cells emerging from or received by the
ZipChip
. This provides
traffic rate shaping, additional statistics and flexibility in ATM network
configuration.
The 7600 Card checks connected devices for Traffic Management
modules and only activates its own TM module if a corresponding one is
found in the connected device.
RM Cell Usage
RM-cells are used by the network to control the rate flow. The RM cell
format is as defined in the TM specification. These cells are sent at regular
intervals from the 7600 Card and make their way through the network.
In the event that there is congestion, the congestion bit of the RM cell is
set by the destination end station. The RM cell is then returned to the
source 7600 Card. In case the RM-cell is not returned to the source 7600
Card or the CI bit is set, the traffic rate is automatically decreased. A
received RM-cell with the CI bit cleared enables the sending 7600 Card to
increase its sending rate.
No RM-cells are sent on point-to-multipoint connection. Multicast rates
are set in advance to avoid flooding the source with backward RM-cells.
abtthgde.book Page 5 Tuesday, June 23, 1998 10:29 AM
