Wireless ADSL Modem User’s Manual
30
ATM
•
VPI:
Virtual Path Identifier is a virtual path used for cell routing that is identified
by aneight bit field in the ATM cell header. The VPI field specifies this eight bit
identifier for routing.
Range for VPI field is 0-255, default is 0.
•
VCI:
A Virtual Channel Identifier is a virtual channel that is identified by a
unique numerical tag that is defined by a 16-bit field in the ATM cell header. The
purpose of the virtual channel is to identify where the cell should travel. The VCI
field specifies this 16 bit numerical tag that determines the destination.
Range for VCI field is 0-65535, default is 38.
•
Service Category:
This field allows you to select from the following service
categories, with UBR as the default.
o
UBR (Unspecified Bit Rate): When configured as UBR, traffic is
delivered with best efforts but with no guarantee. This allows for
fluctuation in times of temporary increase of available bandwidth. For
example, if a PVC with CBR is temporarily inactive, the PVC(s) with
UBR will utilize that bandwidth while it is available. UBR is intended for
applications that do not require any maximum bound on the transfer delay.
o
CBR (Constant Bit Rate): When a PVC is specified as a CBR, that PVC is
guaranteed a certain bandwidth, characterized by the Peak Cell Rate
(PCR). The CBR does not have to transmit with a peak cell rate, and when
it does, it is only when the bandwidth specified by the PCR is guaranteed.
o
VBR-nrt (Variable Bit Rate - non real time): An PVC enabled with VBR-
nrt can transmit a cell only if the PVC has a token available. The PVC
accumulates tokens at the rate of the
Sustainable Cell Rate
, and the PVC
can only accumulate a maximum of the value specified by
Maximum
Burst Size
tokens. When a PVC has a token available, it can transmit cells
at the rate of PCR. After a cell is transmitted, the PVC loses the token it
has accumulated.
Note: In the case of multiple PVCs, CBR specified PVCs will have higher
priority than PVCs with UBR. For example, the CBR PVCs will take their
bandwidth and the remaining bandwidth will be split among the UBR
PVCs. In the case of total PVC CBR bandwidth exceeding ADSL upstream,
the total upstream bandwidth will be shared proportionally to the
bandwidth allocated for each CBR PVC.
•
Peak Cell Rate:
This value specifies the maximum, and in some cases guaranteed,
cell rate for CBR and VBR-nrt. Peak Cell Rates are typically measured in
Cells/Second, however, the user entered value is in kbps and is then converted by the
firmware.
Range for Peak Cell Rate field is 0-32767, default is 0.
