Question

For packet-based transmissions, the queueing portion of an end-to-end transmission will typically be very small. In fact, due to the finite capacity of a router's buffer, there is an absolute upper limit on this delay for a successful communication. In spite of this, excessive queueing could potentially lead to tremendous delays in receiving data. ...how?

Answer 1

Having now briefly considered the major "pieces" of the Internet architecture – the applications, end systems, end-to-end transport protocols, routers, and links – let us now consider what can happen to a packet as it travels from its source to its destination. Recall that a packet starts in a host (the source), passes through a series of routers, and ends its journey in another host (the destination). As a packet travels from one node (host or router) to the subsequent node (host or router) along this path, the packet suffers from several different types of delays at each node along the path. The most important of these delays are the nodal processing delay, queuing delay, transmission delay and propagation delay; together, these delays accumulate to give a total nodal delay.

End-to-End Delay

Our discussion up to this point has been focused on the nodal delay, i.e., the delay at a single router. Let us conclude our discussion by briefly considering the delay from source to destination. To get a handle on this concept, suppose there are Q−1 routers between the source host and the destination host. Let us also suppose that the network is uncongested (so that queuing delays are negligible), the processing delay at each router and at the source host is dproc, the transmission rate out of each router and out of the source host is R bits/sec, and the propagation delay between each pair or routers and between the source host and the first router is dprop. The nodal delays accumulate and give an end-to-end delay,

dendend = Q (dproc + dtrans + dprop),

where once again dtrans = L/R, where L is the packet size. We leave it to the reader to generalize this formula to the case of heterogeneous delays at the nodes and to the presence of an average queuing delay at each node.