Chapter 8 Broadband
VMG/EMG/AM/DM/GM Series User’s Guide
208
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.
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 101 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.
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