Wireless ADSL Modem User’s Manual 31
•
Sustainable Cell Rate:
This is the sustained rate at which a PVC enabled with
VBR-nrt can transmit ATM cells. Sustainable Cell Rate (SCR) can be considered as
the true reserved bandwidth for a PVC.
Range for Sustainable Cell Rate field is 0-32767, default is 0.
•
Max Burst Size:
This is the number of cells a PVC enabled with VBR-nrt can
transmit continuously at peak cell rate (PCR).
Range for Max Burst Size field is 0-32767, default is 0.
Example: CBR and UBR
This example is provided to further explain the dynamics of UBR and CBR and how
different PVCs with different service category specifications coexist. In this example, the
ADSL upstream is 900 kbps.
Example: VBR-nrt
This example is provided to further explain the dynamics of VBR-nrt A PVC has a
service category of VBR-nrt with the following parameters:
1.
PCR = 400 kbps
2.
SCR = 100 kbps
3.
MBS = 22 cells (Note that 22 cells * 48 bytes/cell = 1056 bytes)
If the PVC has been idle for a while (meaning it has accumulated a MBS of 22 cells), and
it just has two packets of the same size (1000 bytes) to send. It can transmit the first
packet of size (1000 bytes) in 20ms: (1000 bytes * 8bit/byte / 4000kbps). Immediately
after the first second packet is transmit, it will take about 80ms to transmit the second
packet because the PVC can only transmit the second packet at SCR (100kbps).