Prestige 861 VDSL Modem
VDSL Commands
5-3
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.
5.4.2 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.
Unspecified Bit Rate (UBR) does not guarantee bandwidth or throughput. Cells are dropped if there is not
enough bandwidth. Only the PCR is set. An example application is background file transfer.
