Delay Impairment

Overall one-way delay, particularly media connected over IP network, consists of:

• Sender packetization delay
• Jitter buffer in the receiver (fixed, absolute or adaptive)
• Transmission delay in the network equipment
• Propagation delay

Although not to be neglected, Transmission/propagation delay normally takes a small portion of delay budget, but should not be neglected when low speed connections are involved.

Sender packetization delay and receiver delay due to jitter buffer are usually main causes of delay impairment for VoIP communication and described in more details below.

Sender packetization delay

The following formulas can be used for calculating the minimum and maximum codec-related processing delay:

Minimum packetization delay for high-speed connections = N x frame size + Toh (ms)

Maximum packetization delay for low-speed connections = 2N x frame size + Toh (ms)

Where: N = number of speech frames per packet; speech frame size is in ms;

Toh= look-ahead, PLC, and additional firmware/hardware delay in ms

For instance, when a packet with 3 frames of G.729A vocoder is used in a connection, the minimum sender delay becomes 35 ms (= 3 x 10 + 5) and maximum delay becomes 65 (= 2 x 3 x 10 + 5 ms) assuming overhead is 5 ms.

Receiver’s delay

Receiver delay is a sum of decode processing, jitter buffer delay and other overhead.

Normally decoding process and overhead should not exceed 5 ms and, therefore, the receiver delay is largely dependent on jitter buffer size.

Jitter buffer can be sized frame-based or absolute jitter buffer size.

When jitter buffer size is frame-based, the size of the buffer is a multiple of speech-frame size.

It can significantly increase receiver delay if the packet size is large. The reason is that the jitter buffer should normally be two times the packet size for frame-based buffers.

Absolute jitter buffers are sized (fixed) to the maximum delay of the transport caused by PDV - maximum packet delay variation in time. It can reduce receiver delay since the buffer size can be set optimally when the network condition is known, instead of choosing multiple value of frame size.

For instance, 22 ms max PDV can be set to 22 + safety margin instead of 30 ms. Adaptive jitter buffer can be considered as a more enhanced method of absolute jitter buffer.

In summary, the following formulas can be used to estimate receiver delay:

• Frame-based jitter buffer delay = 2N x frame size (ms), recommended
• Absolute jitter buffer delay = actual end-to-end delay variation + margin (ms)

Where: N = number of speech frames per packet; speech frame size is in ms.

Absolute jitter buffer delay is always smaller than (or equal to) frame-based jitter buffer.

Refer to the Media Gateway Unit, MGU description for more technical details.

Improvement of delay

From delay impairment point of view, it will result minimum delay to choose minimum packet size containing a single frame and minimum jitter buffer size on receiver. But badly dimensioned buffer size will cause frequent packet loss if PDV, packet delay variation, exceeds buffer size causing that overall speech quality is affected negatively by the combined impairment factor.

It is an issue of balancing minimum delay and the price to pay for other factors working against minimum delay, that is, more bandwidth requirement and more DSP processing capacity.