13-4
C
HAPTER
13: T
RAFFIC
M
ANAGEMENT
IN
THE
ATM N
ETWORK
the sending rate is reduced. When the source end station receives a
backward RM cell with the CI bit cleared it may increase the sending
rate on that particular VC.
Explicit Rate Control
Explicit rate control enhances the implicit rate control by adding the
explicit rate
(ER) field to the RM cell. In this field the source end station
indicates the rate at which it would like to transmit. If an explicit rate
switch exists in the virtual connection (VC) route it may reduce the ER
field in the backward RM-cells in case of congestion. In this case it
indicates explicit rate for the source end station. The source end station,
upon receiving the backward RM-cells, adjusts its sending rate according
to the ER field.
The traffic management standard as defined by the ATM forum supports
VC routes with both explicit and implicit switches. The standard specifies
how to handle lost RM-cells as well as fairness between competing ABR
connections and enhancements for better link utilization.
Traffic and
Congestion Control
Functions
The following describes the available traffic and congestion control
functions:
Q
Network resource management may be carried out by using virtual
paths. Virtual path connections may be used to separate traffic,
thereby preventing statistical multiplexing with other types of traffic.
The CBR and VBR quality of service classes make use of network
resource management.
Q
Connection admission control is defined as the set of actions taken by
the network in order to establish whether a virtual channel or virtual
path connection can be accepted. The CBR and VBR quality of service
classes make use of connection admission control.
Q
Usage parameter control is defined as the set of actions taken by the
network to monitor and control traffic in terms of traffic ordered and
validity of the ATM connection at the user access. The CBR and VBR
quality of service classes make use of usage parameter control.
Q
Selective cell discarding by network elements of the CLP=1 flow while
still meeting network performance objectives on both the CLP=0 and
CLP=1 flows. The VBR quality of service class makes use of selective
cell discarding.
abtthgde.book Page 4 Tuesday, June 23, 1998 10:29 AM
