When planning to implement VoIP on a network, a network designer needs to pay particular attention to ensuring that sufficient bandwidth is available to support voice traffic on WAN links. In previous sections you already learned that the codec chosen will impact the bandwidth requirements associated with a voice call. However, other elements that need […]

Packet loss can occur on any network for a variety of reasons including congested links (packets dropped when buffers are full), routing problems, incorrect equipment configuration issues, and more. Because voice traffic uses UDP as its transport protocol, dropped packets are lost, and obviously not resent. On a voice conversation, this is recognized by what […]

The bullet points below each type of variable delay encountered with VoIP traffic, and how these issues can be compensated for where possible.

Queuing Delay. When a WAN interface is congested, traffic must be queued using any of the various methods looked at in this chapter. Although a method like LLQ can prioritize voice traffic, another […]

The bullet points below each type of constant or “predictable” delay encountered with VoIP traffic, and how these issues can be compensated for where possible.

Processing/Packetization Delay. Processing and packetization delays are a function of the time that it takes to actually create, code, compress, decompress, and decode packets. This is influenced by the codec(s) used […]

While a traditional circuit-switched voice call has a dedicated circuit and bandwidth allocated to it, packet-switched voice calls are subject to two main issues that impact the perceived quality of a call, namely delay and jitter. Delay comes in various forms, impacted by everything from the speed at which a voice packet is created using […]

In order for a queuing mechanism like LLQ or IP RTP Priority to queue voice packets into a priority queue correctly, they must be able to identify the traffic as VoIP. With IP RTP Priority, packets are matched and priority queued according to the UDP port numbers used by RTP voice traffic, which fall into […]

Implementing QoS mechanisms is another key consideration in order to ensure that VoIP traffic is forwarded across a network in a timely manner. A variety of different queuing mechanisms can be used on WAN interfaces to help prioritize voice traffic in order to ensure that it is serviced in this manner, and not delayed by […]

Network congestion is an issue that can lead to a variety of problems on any data network; when the data network is also supporting voice traffic, these issues are even more serious. For example, WAN interfaces on a router may already be at or very near to capacity, leading to queuing issues that may result […]

In order to support time-sensitive applications like VoIP, a network should currently be running at a high performance level, and have the capacity to shoulder the load that will be associated with adding an additional (and time-sensitive) service. As such, a network that already suffers from over-utilization, delay, instability, and latency issues would not be […]

In the H.323 article you learned that VoIP communications uses a combination of both TCP and UDP at the transport layer. TCP is the transport protocol used for primary call control functions and signaling including call establishment, flow control, codec negotiation, and so forth. Call control functions rely on reliable communications facilities in order to […]