Sources of Delay
Delay is the time it takes for a packet to travel from source to destination. Sources of delay include the following:
The physical path length
The time it takes to transmit data onto the wire (serialization delay)
Queueing when there is congestion between the source and destination
Jitter, which is the measure of difference in delay between packets
Physical Path Length
Physical path length is the actual distance that packets need to travel from the source to the destination. The physical path length is a fundamental factor in determining delay, as it contributes to the overall time it takes for a signal to traverse the distance. In general, longer physical paths result in higher delays.
Serialization Delay
Serialization delay refers to the time it takes to convert digital data into a stream of bits and transmit it onto the network. This process involves encoding the data and sending it out as a series of bits, as shown for the 8 bits in Figure 28-3.
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Figure 28-3 Converting Digital Bits to the Physical Medium
Queueing Delay
Queueing delay occurs when there is congestion or contention for resources within the network. When multiple packets are trying to traverse the same network link simultaneously, they may have to wait in a queue before they can be transmitted. This queueing delay is directly related to network traffic and the network’s capacity. Higher levels of congestion lead to longer queueing delays.
Jitter
Jitter is the measure of variation in delay between packets. In an ideal network, packets would all arrive at the destination with consistent and predictable delays, as shown for the top row of packets in Figure 28-4. However, in real-world networks, factors such as varying traffic loads, different routing paths, and queueing delays can introduce variation in the arrival times of packets, as shown in the bottom row of packets in Figure 28-4.
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Figure 28-4 An Example of Equally Spaced and Jittered Packets
Jitter can be a significant issue in real-time applications like Voice over Internet Protocol (VoIP) or video streaming applications, where consistent timing is essential. To mitigate jitter, network engineers often use techniques like quality of service (QoS) to prioritize certain types of traffic and reduce variability in delay.