Network Edge

Question 1

Store and forward packet switching

Answer 1

Means that the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link

Question 2

Transmission delay

Answer 2

Time it takes for bits to be pushed in the wire

If L is the size of the packet, and R is the transmission rate across the wire, then the transmission delay is equal to L/R

Question 3

Propagation delay

Answer 3

The time it takes for a bit to travel across a wire. If s is the speed that the bit is travelling, and D is the length of the wire, then the propagation delay is equal to D/s

Question 4

What is throughput?

Answer 4

Throughput is the rate that bits get transferred between and a sender and receiver.

Question 5

Queueing delay

Answer 5

The time waiting at the output link for transmission

Depends on:

• A = packet arrival rate (A packets/sec)
• L = packet length (bits)
• R = link bandwidth (bits/sec)

Some key conditions:

• If LA/R = 0, then the average queueing delay is small
• If LA/R -> 1, then the queueing delay becomes large
• If LA/R>1, then the average queueing delay is infinite

Question 6

What is the formula for calculating end to end delay when packet switching?

Answer 6

E2E = t_setup + N(d_proc + d_trans + d_prop) + (k-1)(d_trans)

• t_setup is the set up time of the network
• N is the number of hops
• k is the number of packets sent

Question 7

What is the formula for calculating end to end delay when circuit switching?

Answer 7

E2E = t_setup + N(d_prop + d_proc) + d_trans

• t_setup is the set up time of the network
• N is the number of hops