ZyXEL Communications P-861H-G1, User Manual, Page: 74 / 341

Prestige 861H Series User’s Guide
73 Chapter 5 WAN Setup
5.3 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.
5.3.1 ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0
Specification.
5.3.1.1 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.
5.3.1.2 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.
5.3.1.3 Unspecified Bit Rate (UBR)
The Unspecified Bit Rate (UBR) ATM traffic class is used for bursty data transfers. UBR
doesn’t guarantee any bandwidth and only delivers traffic when the network has spare
bandwidth. End devices using UBR get feedback from the network and can use flow-control to
dynamically adjust transmission rates.
UBR uses RM (Resource Management) cells to send feedback information from the
connection’s destination and/or intervening network switches to the connection’s source. A
source generates forward RM cells, which the destination returns to the source as backward
RM cells. Along the way, network switches can adjust the fields in the RM cells depending on
network conditions. Number of Resource Management (NRM) is the maximum number of
cells a source may send for each forward Resource Management cell.