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Chapter 6 Broadband
VMG5313-B10A/-B30A Series User’s Guide
102
In this case, by prior mutual agreement, each protocol is assigned to a specific virtual circuit; for
example, VC1 carries IP, etc. VC-based multiplexing may be dominant in environments where
dynamic creation of large numbers of ATM VCs is fast and economical.
LLC-based Multiplexing
In this case one VC carries multiple protocols with protocol identifying information being contained
in each packet header. Despite the extra bandwidth and processing overhead, this method may be
advantageous if it is not practical to have a separate VC for each carried protocol, for example, if
charging heavily depends on the number of simultaneous VCs.
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 27 Example of Traffic Shaping