ZyXEL Communications P-861H-G1, User Manual, Page: 76 / 341

Prestige 861H Series User’s Guide
75 Chapter 5 WAN Setup
Figure 24 PCR, SCR, MCR and MBS in Traffic Shaping
5.3.3 Cell Delay Variation Tolerance (CDVT)
Cell Delay Variation Tolerance (CDVT) is the accepted tolerance of the difference between a
cell’s transfer delay and the expected transfer delay. CDVT controls the time scale over which
the PCR is enforced. CDVT is used to determine if a cell arrived too early in relation to PCR.
5.3.4 Burst Tolerance (BT)
Burst Tolerance (BT) is the maximum number of cells that the port is guaranteed to handle
without any discards. BT controls the time scale over which the SCR is enforced. BT is used to
determine if a cell arrived too early in relation to SCR. Use this formula to calculate BT: (MBS
– 1) x (1 / SCR – 1 / PCR) = BT.
5.4 Permanent Virtual Circuit
Permanent Virtual Circuit (PVC) is a logical point-to-point circuit between customer sites.
PVC's are low-delay circuits because routing decisions do not need to be made along the way.
Permanent means that the circuit is preprogrammed by the carrier as a path through the
network. It does not need to be set up or torn down for each session.
5.4.1 PVC Example
In the following example, a fast Internet connection uses Ethernet ports three and four to
communicate through PVC 0. The other PVCs use Ethernet ports one and two to communicate
with a set top box or home gateway. This is the default remote node setup.