ZyXEL Communications Prestige 650M-6x User Manual page 57

Prestige 650M-6x User's Guide
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.
The following figure illustrates the relationship between PCR, SCR and MBS.
Figure 5-4 Example of Traffic Shaping
Cell Transfer Delay (CTD) is the average time for a cell to be transferred from its source to its destination
over a virtual connection.
Cell Delay Variation (CDV) is difference between the maximum and minimum CTDs experienced during the
connection.
Cell Delay Variation Tolerance (CDVT) is the acceptable range of the CDV.
7.5.2 Service Categories
Service categories ensure that high priority transmissions get the bandwidth they need.
CBR (Constant Bit Rate) provides a fixed amount of bandwidth that is always available even if no data is
being sent. A PCR is specified and if traffic exceeds this rate, cells may be dropped. An example application
is a T1 circuit.
rt-VBR (real-time Variable Bit Rate) also provides a fixed amount of bandwidth (a PCR is specified) but is
only available when data is being sent. An example application is real-time videoconferencing.
nrt-VBR (non-real-time Variable Bit Rate) is commonly used for "bursty" traffic typical on LANs. PCR and
MBS define the burst levels, SCR defines the minimum level. An example application is multimedia e-mail.
WAN Setup
7-3