Types of Delay: Processing Delay - Queuing Delay

Types of Delay: Processing Delay - Queuing Delay

Let's look at these delays in the context of following figure. As part of its end-to-end route between source and destination, a packet is sent from the upstream node through router A to router B. Our object is to characterize the nodal delay at router A. Note that router A has an outbound link leading to router B. This link is preceded by a queue (also known as a buffer). When the packet arrives at router A from the upstream node, router A examines the packet's header to decide the appropriate outbound link for the packet and then directs the packet to this link. In this example, the outbound link for the packet is the one that leads to router B. A packet can be transmitted on a link only if there is no other packet currently being transmitted on the link and if there are no other packets preceding it in the queue: if the link is currently busy or if there are other packets already queued for the link, the newly arriving packet will then join the queue.

The nodal delay at router A

Processing Delay

The time required to examine the packet's header and decide where to direct the packet is part of the processing delay. The processing delay can also contain other factors, such as the time required to check for bit-level errors in the packet that took place in transmitting the packet's bits from the upstream node to router A. Processing delays in high-speed routers are normally on the order of microseconds or less. After this nodal processing, the router directs, the packet to the queue that precedes the link to router B. (In "The Network Layer" we'll study the details of how a router operates.)

Queuing Delay

At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The length of the queuing delay of a particular packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link. If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero. On the other hand, if the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long. We will see shortly that the number of packets that an arriving packet might expect to find is a function of the intensity and nature of the traffic arriving at the queue.  Queuing delays can be on the order of microseconds to milliseconds in practice.


nodal delay, processing delay, queuing delay

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