PXF4333 Datasheet, PDF (81/361 Pages) Infineon Technologies AG – ABM 3G ATM Buf fer Manager

ABM-3G
PXF 4333 V1.1
Functional Description
3.4.2.4 Traffic Shaping
Traffic shaping is a mechanism that alters the characteristics of a cell stream in order to
make better use of network resources or to enforce conformance to the negotiated traffic
contract at an interface. Conformance is defined operationally in terms of a Generic Cell
Rate Algorithm GCRA(T,tau) which specifies the upper limits, in terms of a given
tolerance tau, for cells arriving in excess with respect to a given reference cell rate (1/T).
The ITU-T Recommendation I.371 [ 1 ] or the ATM Forum TM Specification 4.1 [ 2 ]
provide the details.
A situation that is particularly prone to produce non-conforming traffic is congestion in a
network. Figure 3-26 shows the need for shapers at the output of a congested network
for nrt-VBR traffic. An nrt-VBR cell stream originally shaped to conformance by the
terminal (1) traverses Network A, which exhibits burst level congestion. At the output of
Network A the cell stream is accumulated into a single large burst, which by far exceeds
even the Peak Cell Rate (PCR) of the original connection (2). It is no longer conforming
to the traffic contract and therefore would not pass through the subsequent policer.
Hence, at the output of Network A, an SCR shaper is enabled, which regenerates a
conforming cell stream to match a given burst tolerance BT (3). This cell stream is
accepted by the policer and traverses Network B which exhibits cell level congestion
only. As a result PCR and SCR vary slightly due to the cell clumping effect (4). This Cell
Delay Variation (CDV) is reduced to match a given tolerance (CDVT) by the PCR shaper
at the output of Network B (5).
Terminal
Shaper
Network A
Policer
Shaper
burst level congestion
rate
BT
PCR
SCR
1
2
3
Network B
Policer
Shaper
cell level congestion
CDVT
Policer
4
5
Figure 3-26 Shaping and Policing at Network Boundaries
Note that the outcome of Network B has a very different shape when compared to the
input to Network A and to the outcome of Network A. Nevertheless, due to the shapers
implied, the traffic is conforming on both the User-Network interface (UNI) and the
subsequent Network-Network Interfaces (NNI).
Data Sheet
81
2001-12-17