Traffic Shaping - ZyXEL Communications VMG3312-T20A User Manual

Chapter 6 Broadband
Operationally, PPPoE saves significant effort for both you and the ISP or carrier, as it requires no specific
configuration of the broadband modem at the customer site.
By implementing PPPoE directly on the VMG (rather than individual computers), the computers on the
LAN do not need PPPoE software installed, since the VMG does that part of the task. Furthermore, with
NAT, all of the LANs' computers will have access.
RFC 1483
RFC 1483 describes two methods for Multiprotocol Encapsulation over ATM Adaptation Layer 5 (AAL5).
The first method allows multiplexing of multiple protocols over a single ATM virtual circuit (LLC-based
multiplexing) and the second method assumes that each protocol is carried over a separate ATM virtual
circuit (VC-based multiplexing). Please refer to RFC 1483 for more detailed information.
Multiplexing
There are two conventions to identify what protocols the virtual circuit (VC) is carrying. Be sure to use the
multiplexing method required by your ISP.
VC-based Multiplexing
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.
VMG3312-T20A User's Guide
92