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Problem: Access Link Congestion

Access links are typically the bottleneck between a high bandwidth LAN and a high bandwidth IP network. An increase in traffic can cause the queue in the edge/access router to fill, which increases jitter and causes a short term increase in delay. High levels of congestion can also introduce packet loss due to buffer overflow or Random Early Detection (RED).

As an example, it takes 8 milliseconds to send a typical data packet over a T1 connection. If two data packets arrive at the access router ahead of a voice packet, then the voice packet would be delayed by 16 milliseconds. If the access link speed is slower than T1 then the delay would be greater - for a 512 kilobit/sec link the delay would be 24 milliseconds per packet.

Access link congestion can be a particular problem for ADSL and Cable Modem connections.


High levels of jitter resulting from access link congestion cause excessive numbers of packets to be discarded by the receiving Voice over IP end system's jitter buffer, which leads to degraded voice quality. As the level of congestion varies with traffic then the jitter level will vary, hence users may report that the call becomes garbled intermittently.


Access link problems can be reduced by

  1. Using priority queuing for delay sensitive voice and video traffic
  2. Reducing the maximum MTU size on low speed links (512 kbits/s or less)
  3. Increasing the capacity of the access link
  4. If multiple links are used, then applying load sharing to maximize use of capacity
  5. Applying call admission control to limit the number of calls
  6. Using fragmentation and interleaving.

Tools: Traceroute, VQmon

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