Occasionally one of our customers will experience quality problems with VoIP calls. I thought the following observations were worth sharing.
A. Identify the Symptoms and Circumstances
The following important criteria are necessary for thorough trouble isolation and resolution, this is the data that your support staff, vendor and carrier will need:
1. Description of Voice Quality problems:
a. Clipping
b. Static
c. Echo
d. Call drops
e. Unable to place or receive calls
2. Call examples
a. TOD
b. Calling #
c. Called #
3. Network observations
a. Any other data traffic problems
B. Possible Causes of Voice Quality Problems
1. Circuit path degradation
Transmission errors on any part of the circuit path will cause TCP retransmits, this will result in out of sequence packets and latency beyond limits for acceptable voice quality. Attenuation deficiencies can cause echo and static problems, and could be indicative of a problem at the client premise or on the carrier network.
2. End to end QOS problems
If VoIP packets are competing for the same bandwidth as other data traffic, the voice packets must be marked and given priority over the other traffic. This is known as Quality of Service (QOS).These markings are used to place traffic into one or more classes of services (COS). Each COS is then allocated some portion of the available bandwidth. Each COS is then processed according to industry standard queuing algorithms. The call compression algorithms that are used will have a big impact on how effective your QOS and COS configuration will be. Two frequently used algorithms are :
G.711 uses about 82Kb per call
G.729 – uses about 34Kb per call.
3. Lack of call admission control
The phone systems should work in conjunction with the QOS configuration by limiting the number of VoIP calls permitted on the IP trunk groups to a quantity that is less than the number of calls that can be treated successfully by the bandwidth allocation set in the QOS configuration.
4. Network congestion
Although QOS can ensure that VoIP traffic will be given preferential treatment over other data, and provide desired quality, it does have limitations. In addition to Queuing algorithms, routers also have to deal with congestion control. Congestion control involves the monitoring and allocation of queues based upon real existing bandwidth utilization and anticipated needs. Sustained congestion of links will lessen the effectiveness of a router to control congestion and administer QOS queue allocations.
Filed under: VoIP | Tagged: call admission, cos, qos, voip | Comments Off
