1.4 Delay, Loss, and Throughput in Packet-Switched Networks Flashcards
Define processing delay.
Time required to examine a packet’s header and determine where to direct the packet. Can also include other factors, such as time to check bit-level errors.
~microseconds or less.
Queuing delay.
The packet experiences this delay as it waits to be transmitted onto the link.
It depends on the number of earlier-arriving packets that are queued and waiting for transmission onto the link.
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.
~microseconds to milliseconds.
Transmission delay.
Denote the length of the packet by L bits, and denote the transmission rate from router A to router B by R bits/sec, the trasnmission delay is L/R.
~microseconds to milliseconds.
Propagation delay.
The time required for each bit to propagate to router B once it’s pushed into the link.
~speed of light (or a little less).
When is the queuing delay large and when is it insignificant?
a: Avg. rate at which packets arrive at the queue [packets/sec]
R: transmission rate [bits/sec]
Suppose for simplicity that all packets consist of length L bits. Then, avg. rate at which bits arrive at the queue: L*a bits/sec.
Assume a very big queue (it can hold inf. number of bits). Then:
traffic intensity = L*a / R
If L*a / R > 1, then: Problem! (divergence).
Packet loss
Because the queue capacity is finite, a packet can arrive to find a full queue. With no place to store the packet, a router will drop the packet.
Performance at nodes is often measured not only in terms of delay but also in terms of the probability of packet loss.
End to end delay.
Suppose N-1 routers.
Suppose uncongested network (negligible queuing delays).
- Processing delay at each router and at the source host is d_proc.
- transmission rate our of each router and our of the source host is R bits/Sec
- propagation on each link is d_prop
d_end-to-end = N(d_proc + d_trans + d_prop)
where d_trans = L/R
Define Throughput.
Instantaneous throughput at any instant of time is the rate at which host B is receiving the file.
