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Prestige 650M-6x User's Guide
7.3
Multiplexing
There are two conventions to identify what protocols the virtual circuit (VC) is carrying. Be sure to use the
multiplexing method required by your ISP.
7.3.1 VC-based Multiplexing
In this case, by prior mutual agreement, each protocol is assigned to a specific virtual circuit; for example,
VC1 carries IP, etc. VC-based multiplexing may be dominant in environments where dynamic creation of
large numbers of ATM VCs is fast and economical.
7.3.2 LLC-based Multiplexing
In this case one VC carries multiple protocols with protocol identifying information being contained in each
packet header. Despite the extra bandwidth and processing overhead, this method may be advantageous if it
is not practical to have a separate VC for each carried protocol, for example, if charging heavily depends on
the number of simultaneous VCs.
7.4
VPI and VCI
Be sure to use the correct Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) numbers
assigned to you. The valid range for the VPI is 0 to 255 and for the VCI is 32 to 65535 (0 to 31 is reserved
for local management of ATM traffic). Please see the appendix for more information.
7.5
Introduction to ATM
ATM (Asynchronous Transfer Mode) is a connection-oriented switching technology that uses fixed-size cells
to transmit data across a dedicated path (permanent virtual circuit (PVC)).
An ATM service contract is an agreement between the carrier and the subscriber to regulate the average rate
and fluctuations of data transmission over an ATM network. This agreement helps eliminate congestion,
which is important for transmission of real time data such as audio and video connections.
7.5.1 Quality of Service (QoS) Parameters
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This parameter may be lower
(but not higher) than the maximum line speed. 1 ATM cell is 53 bytes (424 bits), so a maximum speed of 832
Kbps gives a maximum PCR of 1962 cells/sec. This rate is not guaranteed because it is dependent on the line
speed.
Sustained Cell Rate (SCR) is the mean cell rate of each bursty (data flows followed by idle periods) traffic
source. It specifies the maximum average rate at which cells can be sent over the virtual connection. SCR
may not be greater than the PCR.
7-2
WAN Setup
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