Traffic Shaping - ZyXEL Communications Prestige 650M User Manual

Scenario 1. One VC, Multiple Protocols
PPPoA (RFC-2364) encapsulation with VC-based multiplexing is the best combination because no extra
protocol identifying headers are needed. The PPPoA protocol already contains this information.
Scenario 2. One VC, One Protocol (IP)
Selecting RFC 1483 encapsulation with VC-based multiplexing requires the least amount of overhead (0
octets). However, if there is a potential need for multiple protocol support in the future, it may be safer to
select PPPoA encapsulation instead of RFC 1483, so you do not need to reconfigure either computer later.
Scenario 3. Multiple VCs
If you have an equal number (or more) of VCs than the number of protocols, then select RFC 1483
encapsulation and VC-based multiplexing.

4.4.4 Traffic Shaping

Traffic Shaping is an agreement between the carrier and the subscriber to regulate the average rate and
"burstiness" or fluctuation 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
832 Kbps 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 a bursty, on-off traffic source that can be sent at the peak
rate, and a parameter for burst-type traffic. SCR may not be greater than the PCR; the system default is 0
cells/sec.
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.
Internet Access
Prestige 650M ADSL Bridge
4-3