ZyXEL Communications P-660H-T1 v2, Руководство пользователя

Страница: 71 / 312

Chapter 5 WAN Setup
P-660H-Tx v2 User’s Guide
71
Sustained Cell Rate (SCR) is the mean cell rate of each bursty traffic source. It specifies the
maximum average rate at which cells can be sent over the virtual connection. SCR may not be
greater than the PCR.
Maximum Burst Size (MBS) is the maximum number of cells that can be sent at the PCR.
After MBS is reached, cell rates fall below SCR until cell rate averages to the SCR again. At
this time, more cells (up to the MBS) can be sent at the PCR again.
If the PCR, SCR or MBS is set to the default of "0", the system will assign a maximum value
that correlates to your upstream line rate.
The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 33 Example of Traffic Shaping
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) provides fixed bandwidth that is always available even if no data is
being sent. CBR traffic is generally time-sensitive (doesn't tolerate delay). CBR is used for
connections that continuously require a specific amount of bandwidth. A PCR is specified and
if traffic exceeds this rate, cells may be dropped. 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 (VBR-RT) or
non-real time (VBR-nRT) connections.
The VBR-RT (real-time Variable Bit Rate) type is used with bursty connections that require
closely controlled delay and delay variation. It also provides a fixed amount of bandwidth (a
PCR is specified) but is only available when data is being sent. An example of an VBR-RT
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.