System information

Troubleshooting ATM Switching Environments 21-449

ATM Switching: Partial Data Delivered over Virtual Circuit

Symptom: Partial data is delivered over a Frame Relay, frame forwarding, UNI, or CBR virtual

circuit.

Table 21-7 outlines the problems that might cause this symptom and describes solutions to those

problems.

Table 21-7 ATM Switching: Partial Data Delivered over Virtual Circuit

Possible Problem Solution

Network congestion Check whether the network is congested. Check your trafﬁc management

conﬁguration and make adjustments as appropriate. Use the show chassis

congestion command to display the maximum and minimum intervals

between permit limit updates and the minimum interval between CA updates.

For detailed information, refer to the LightStream 2020 System Overview.

Target depth and maximum depth

parameters misconﬁgured (CBR

only)

1 CBR = constant bit rate

Use the set port c.p cbrpvc PVC# {targetdepth | maxdepth} bytes

command to control the reassembly buffer at the point where input cells are

converted back into a CBR stream. An adaptive control loop maintains data in

the buffer close to the level speciﬁed by targetdepth bytes. Data in excess of

maxdepth bytes is discarded.

The default values of the targetdepth and maxdepth attributes are usually

best left unchanged. If the target depth is set too high or if the maximum depth

is set too far above the target, end-to-end delay for the entire circuit increases.

With voice trafﬁc, such delay can cause annoying echo. If the target depth is

set too low or if the maximum depth is set too close to the target depth,

random CDV

may cause the circuit to overﬂow or underﬂow sporadically,

causing data errors and reframe events for equipment downstream. For certain

applications, such as video and phone, where some discarding of overﬂow

data is an acceptable cost of maintaining a constant bit rate, it may be

preferable to set these two parameters closer together.

2 CDV = cell delay variation