In Best Effort networks propagation delay is fixed processing and queuing delays are unpredictable

The figure illustrates the impact that a network has on the end-to-end delay of packets going from one end of the network to the other. Each hop in the network adds to the overall delay because of these factors:

■ Propagation delay is caused by the speed of light traveling in the media; for example, the speed of light traveling in fiber optics or copper media.

■ Serialization delay is the time it takes to clock all the bits in a packet onto the wire. This is a fixed value that is a function of the link bandwidth.

■ There are processing and queuing delays within a router, which can be caused by a wide variety of conditions.

Propagation delay is generally ignored but it can be significant; for example, about 40 ms coast-to-coast, over optical. Internet Control Message Protocol (ICMP) echo (ping) is one way to measure the round-trip time of IP packets in a network.

Example: Effects of Delay

A customer has a router in New York and a router in San Francisco, each connected by a 128-kbps WAN link. The customer sends a 66-byte voice frame across the link. To transmit the frame (528 bits), it will take 4.125 ms to clock out (serialization delay). However, the last bit will not arrive until 40 ms after it clocks out (propagation delay). The total delay equals 44.125 ms.

Now, change the circuit to a T1. To transmit the frame (528 bits), it will take 0.344 ms to clock out (serialization delay). However, the last bit will not arrive until 40 ms after transmission

(propagation delay) for a total delay of 40.344 ms. In this case, the significant factor is propagation delay. In the same situation—but for a link between Seattle and San Francisco— serialization delay remains the same and propagation delay drops to about 6 ms, making 528 bits take 10.125 ms (on a 128-kbps link), and a total delay of 6.344 (on a T1 link).

Both serialization and propagation delays must be taken into account in determining total delay.

Types of Delay

Processing Delay: The time it takes for a router to take the packet from an input interface, examine it, and put it into the output queue of the output interface Queuing Delay: The time a packet resides in the output queue of a router Serialization Delay: The time it takes to place the "bits on the wire" Propagation Delay: The time it takes to transmit a packet

Processing Delay: The time it takes for a router to take the packet from an input interface, examine it, and put it into the output queue of the output interface Queuing Delay: The time a packet resides in the output queue of a router Serialization Delay: The time it takes to place the "bits on the wire" Propagation Delay: The time it takes to transmit a packet

In summary, there are four types of delay, as follows:

■ Processing delay: The time it takes for a router to take the packet from an input interface and put the packet into the output queue of the output interface. The processing delay depends on these factors:

— CPU utilization

— IP switching mode

— Router architecture

— Configured features on both input and output interfaces

■ Queuing delay: The time a packet resides in the output queue of a router. Queuing delay depends on the number of and sizes of packets already in the queue, the bandwidth of the interface, and the queuing mechanism.

■ Serialization delay: The time it takes to place a frame on the physical medium for transport.

■ Propagation delay: The time it takes to transmit a packet, which usually depends on the type of media interface.

Ways to Reduce Delay

Upgrade the link; the best solution but also the most expensive. Forward the important packets first. Compress the payload of Layer 2 frames (it takes time). Compress IP packet headers.

Assuming that a router is powerful enough to make a forwarding decision rapidly, most processing, queuing, and serialization delay is influenced by these factors:

■ Average length of the queue

■ Average length of packets in the queue

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