Network Traffic

VoIP traffic has a number of issues that must be carefully considered, such as traffic parameters and network design. Without such due diligence, you could be faced with service that does not function reliably or is severely degraded.

These important considerations are as follows.

• Latency
  
  
• Bandwidth
  
  
• Jitter
  
  
• Packet loss
  
  
• Reliability
  
  
• Security
  
  

Latency (Delay)

Latency (or delay) is the time that it takes a packet to make its way through a network end to end. In telephony terms, latency is the measure of time it takes the talker's voice to reach the listener's ear. Large latency values do not necessarily degrade the sound quality of a phone call, but the result can be a lack of synchronization between the speakers such that there are hesitations in the speaker' interactions. Generally, it is accepted that the end-to-end latency should be less than 150 ms for toll quality phone calls. To ensure that the latency budget remains below 150 ms, you need to take into account the following primary causes of latency. When designing a multiservice network, the total delay that a signal or packet exhibits is a summation of all the latency contributors.

• One source of latency is the time it takes for the endpoints to create the packets used in voice services. These packetization delays are caused by the amount of time it takes to fill a packet with data. Generally, the larger the packet size, the greater the amount of time it takes to fill it. Packetization delay is governed by the CODEC standard being used. This problem also exists on the receiving side because the media gateway must remove and further process the packet data. If the packets are kept small, this amount of delay in both directions is usually quite small depending on the hardware / software implementation of the media gateways. All considerations being equal, nominal operation of any media gateway unit should not exceed 30 ms.
  
  
• Another source of latency is the delay it takes to serialize the digital data onto the physical links of the interconnecting equipment. This delay is inversely proportional to the link speed. In other words, the faster the media, the lower the latency. This value is somewhat dependent on the link technology used and its access method. For example, it takes 125 microseconds to place one byte on a 64-Kb circuit. The same byte placed on an OC-3/STM-1 circuit takes 0.05 microseconds. Although this delay is unavoidable (regardless of the bandwidth used), keeping the number of intervening links small and using high bandwidth interfaces reduces the overall latency.
  
  
• Propagation delay is the time it takes an electrical (or photonic) signal to traverse the length of a conductor. The speed of these signals is always slower than that of the speed of light. There is always propagation delay; however, it only becomes an issue when the signal (or packet) travels a great distance. The accepted formula for calculating propagation delay is as follows.
  
  Propagation delay = Circuit km / (299,300 km x .6)
  
  Example: Calculation of one-way propagation delay of a 6,000 km fiber run
  
  (Discounting any signal repeaters in between)
  
  0.0334 sec = 6000 km / (299,300 km x .6)
  
  By this calculation, the latency contributed by just propagation delay would be 33.4 ms.
  
  
• A queuing delay, which is a large source of latency, is the amount of time that a packet remains buffered in a network element while it awaits transmission. Network traffic loads result in variable queuing delays. The amount of buffer that a queue uses is usually a configurable parameter, with a smaller number being better for latency values. However, his delay is also based on the amount of traffic the element is trying to pass through a given link, and therefore it increases with network load. Hence, you need to set aside adequate bandwidth and resources for voice traffic. If the queue used for voice traffic is not serviced fast enough and that queue is allowed to grow too large, the result is greater latency.
  
  
• Packet switching delay, yet another source of end-to-end latency, is the time it takes a router or switch to buffer a packet and make the decision on which interface the packet is to be directed. Although this delay is usually small, the architecture of the router or switch is the deciding factor. If a packet must be further buffered as a part of its processing, greater latency is incurred