Queuing Delay

A packet-based network experiences delay for other reasons. Two of these are the time necessary to move the actual packet to the output queue (packet switching) and queuing delay.

When packets are held in a queue because of congestion on an outbound interface, the result is queuing delay. Queuing delay occurs when more packets are sent out than the interface can handle at a given interval.

Cisco IOS software is good at moving and determining the destination of a packet. Other packet-based solutions, including PC-based solutions, are not as good at determining packet destination and moving the actual packet to the output queue.

The actual queuing delay of the output queue is another cause of delay. You should keep this factor to less than 10 ms whenever you can by using whatever queuing methods are optimal for your network. This subject is covered in greater detail in Chapter 9, "Quality of Service."

The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.114 recommendation specifies that for good voice quality, no more than 150 ms of one-way, end-to-end delay should occur, as shown in Figure 8-1. With Cisco's VoIP implementation, two routers with minimal network delay (back to back) use only about 60 ms of end-to-end delay. This leaves up to 90 ms of network delay to move the IP packet from source to destination.

Figure 8-1. End-to-End Delay

OB Zone

Satellite Quality

High Quality

Fax Relay, Broadcast

M M \ \ \ { M I I M M M I I I M M I I M I I M M \ \ M M 0 100 ¿200 300 400 500 6 00 700 300

Time (msec)

Delay Target

As shown in Figure 8-1, some forms of delay are longer, although accepted, because no other alternatives exist. In satellite transmission, for example, it takes approximately 250 ms for a transmission to reach the satellite, and another 250 ms for it to come back down to Earth. This results in a total delay of 500 ms. Although the ITU-T recommendation notes that this is outside the acceptable range of voice quality, many conversations occur every day over satellite links. As such, voice quality is often defined as what users will accept and use.

In an unmanaged, congested network, queuing delay can add up to two seconds of delay (or result in the packet being dropped). This lengthy period of delay is unacceptable in almost any voice network. Queuing delay is only one component of end-to-end delay. Another way end-to-end delay is affected is through jitter.

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