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Chapter 6 Broadband
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.
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 84 Example of Traffic Shaping
ATM Traffic Classes
These are the basic ATM traffic classes defined by the ATM Forum Traffic Management 4.0 Specification.
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.
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
VMG/XMG Series User's Guide
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