ZyXEL Communications IES-708-22A - Manual (Page 120)

Chapter 14 xDSL Profiles Setup
IES-708-22A User’s Guide
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Traffic shaping controls outgoing (downstream) traffic, not incoming
(upstream).
14.5.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0
Specification.
14.5.1.1 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.
14.5.1.2 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.
14.5.1.3 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.
14.5.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.
14.5.2.1 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 maximum speed of 832Kbps gives a maximum PCR of 1962 cells/sec. This rate is not
guaranteed because it is dependent on the line speed.