ZyXEL Communications MSC1000G Series, User Manual

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Chapter 11 Profile Screens
Management Switch Card User’s Guide
360
11.12 Traffic Shaping
Traffic shaping is an agreement between the carrier and the subscriber to regulate the average rate
and fluctuations of data transmission over an ATM network. This agreement helps eliminate
congestion, which is important for transmission of real time data such as audio and video
connections.
Note: Traffic shaping controls outgoing (downstream) traffic, not incoming (upstream)
traffic.
11.12.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0
Specification.
11.12.1.1 Constant Bit Rate (CBR)
Constant Bit Rate (CBR) is an ATM traffic class that provides fixed bandwidth. CBR traffic is
generally time-sensitive (doesn't tolerate delay). CBR is used for connections that continuously
require a specific amount of bandwidth. Examples of connections that need CBR would be high-
resolution video and voice.
11.12.1.2 Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections. Connections that use
the Variable Bit Rate (VBR) traffic class can be grouped into real time (rt-VBR) or non-real time
(nrt-VBR) connections.
The rt-VBR (real-time Variable Bit Rate) type is used with bursty connections that require closely
controlled delay and delay variation. An example of an rt-VBR connection would be video
conferencing. Video conferencing requires real-time data transfers and the bandwidth requirement
varies in proportion to the video image's changing dynamics.
The nrt-VBR (non real-time Variable Bit Rate) type is used with bursty connections that do not
require closely controlled delay and delay variation. An example of an nrt-VBR connection would be
non-time sensitive data file transfers.
11.12.1.3 Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is similar to the ABR traffic class for bursty data
transfers. However, while ABR gives subscribers a set amount of bandwidth, UBR doesn't guarantee
any bandwidth and only delivers traffic when the network has spare bandwidth.
11.12.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.
11.12.2.1 Peak Cell Rate (PCR)
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This parameter may
be lower (but not higher) than the maximum line speed. 1 ATM cell is 53 bytes (424 bits), so a