Network(6) -- Delay, Loss, and Throughput in Packet-Switched Networks

Delay, Loss, and Throughput in Packet-Switched Networks

Queuing Delay and Packet Loss

The queuing delay can vary from packet to packet. Typically use statistical measures, such as average queuing delay, variance of queuing delay, and the probability that the queuing delay exceeds some specified value.

The queuing delay depends on the rate at which traffic arrives at the queue, the transmission rate of the link, and the nature of the arriving traffic, that is, whether the traffic arrives periodically or arrives in bursts.

traffic intensity: La/R, 每秒进来多少bits，比上每秒发射出去多少bits，就是流量密度。

If La/R > 1, then the average rate at which bits arrive at the queue exceeds the rate at which the bits can be transmitted from the queue.

Design your system so that the traffic intensity is no greater than 1.

If la/R <= 1, the nature of the arriving traffic imacts the queuing delay, that is, periodically or bursts.

Typically, the arrival process to a queue is random.

Packet Loss

With no place to store such a packet, a router will drop that packet.

End-to-End Delay

Suppose there are N-1 routers between the source host and the destination host. Also suppose for the monent that the network is uncongested(so that queuing delays are negligible), the processing delay at each router and at the source host is d_proc, the transmission rate out of each router and out of the source host is R bits/sec, and the propagation on each link is d_prop. The nodal delays accumulate and give an end-to-end delay,

dend-end = N(d_proc + d_trans + d_prop)

dend-to-end=NL/R

Traceroute

IMPORTANT: In Windows or Linux, use tracert hostname to see network path.

Throughput in Computer Networks

The instantaneous throughput at any instant of time is the rate (in bits/sec) at which Host B is receiving the file.

If the file consists of F bits and the transfer takes T seconds for Host B to receive all F bits, then the average throughput of the file transfer is F/T bits/sec.

min{R , R } is the transmission rate of the bottleneck link