Delay
Overall or absolute delay can affect VoIP. You might have experienced delay in a telephone
conversation with someone on a different continent. The delays can cause entire
words in the conversation to be cut off and can therefore be very frustrating. Figure 2-2
illustrates various areas in the network that can introduce delay.
Figure 2-2 Sources of Delay
When you design a network that transports voice over packet, frame, or cell infrastructures,
it is important to understand and account for the predictable delay components in
the network. You must also correctly account for all potential delays to ensure overall
network performance is acceptable. Overall voice quality is a function of many factors,
including the compression algorithm, errors and frame loss, echo cancellation, and delay.
Following are the two distinct types of delay:
■ Fixed delay: Fixed-delay components are predictable and add directly to overall
delay on the connection. Fixed-delay components include the following:
■ Coding: The time it takes to translate the audio signal into a digital signal
■ Packetization: The time it takes to put digital voice information into packets and
remove the information from packets
■ Serialization: The insertion of bits onto a link
■ Propagation: The time it takes a packet to traverse a link
■ Variable delay: Variable delays arise from queuing delays in the egress trunk buffers
that are located on the serial port connected to the WAN. These buffers create variable
delays, called jitter, across the network.
E1 E1
Fixed:
Switch
Delay
Fixed:
Switch
Delay
Fixed:
Switch
Delay
Fixed:
Serialization
Delay
Fixed:
Dejitter
Buffer
Fixed:
Coder
Delay
Fixed:
Packetization
Delay Variable:
Output
Queuing Delay
64
kbps