Problem: ADSL and Cable Modem Congestion
Although ADSL and Cable Modems are regarded as 'Broadband', they are not fast enough to ensure adequate Voice over IP performance. Very high levels of jitter can be seen on many DSL and cable connections due to interference from data traffic.
The chart below shows the serialization delay (time taken to send) of an packet (of size MTU) on links of various speeds. Jitter would be directly proportional to this delay (e.g. may be 2-3 times the serialization delay).
Impact
High levels of jitter resulting from access link congestion cause excessive numbers of packets to be discarded, which leads to degraded voice quality.
Resolution
ADSL and Cable Model congestion can be mitigated by limiting MTU size to 512 bytes or less, using priority queuing for real-time traffic and limiting the number of Voice over IP streams that are simultaneously carried.
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