An FTP server running at host A must transmit a file of length F bytes to host B
ID: 3882031 • Letter: A
Question
An FTP server running at host A must transmit a file of length F bytes to host B over a network path consisting of N links (N–1 routers in the path). Assume the links all have the same transmission speed of R bits/second and that processing, queuing, and propagation delays are all negligible relative to the transmission delays.
(a) Suppose the network operates as a message-switched datagram network and that a header of h bytes (combined headers for all protocol layers) is added to the entire message when it is being transmitted. How long does it take to send the file from A to B?
(b) Suppose the network operates as a packet-switched datagram network, the F bytes are segmented into M packets of P bytes each, and a header of h bytes is added to each packet when it is being transmitted. How long does it take to send the file from A to B?
(c) Suppose the network operates as a packet-switched virtual-circuit network, the F bytes are segmented into M packets of P bytes each, and a header of h/2 bytes is added to each packet when it is being transmitted. The virtual circuit set-up time is Ts. How long does it take to send the file from A to B?
(d) Suppose the network operates as a pure circuit-switched network with an end-to-end transmission speed of R bits/second once the circuit has been established. A header of h/2bytes is added to the entire file when it is transmitted and the circuit set-up time is Ts. How long does it take to send the file from A to B?
(e) Under what conditions (what values of Ts) does the packet-switched datagram network provide a faster transfer of file F from A to B than the circuit-switched network?
Explanation / Answer
(a) Suppose the network operates as a message-switched datagram network and that a header of h bytes (combined headers for all protocol layers) is added to the entire message when it is being transmitted. How long does it take to send the file from A to B?
Total size of packet = (F+h) bytes = (F+h)*8 bits
transmission speed of R bits/second
In a message switching Total time for Transmitting a message =
N*propagation-delay + N*(T)
Where in,
N = Number of links
propagation-delay = propagation delay per link
T = time for transmitting complete message
Here propagation-delay is negligible so
total time = N*0 + N*(((F+h)*8)/R) = N*(((F+h)*8)/R) secs.
(b) Suppose the network operates as a packet-switched datagram network, the F bytes are segmented into M packets of P bytes each, and a header of h bytes is added to each packet when it is being transmitted. How long does it take to send the file from A to B?
Each packet size = (P+h)*8 bits
transmission speed of R bits/second
time taken for each packet = ((P+h)*8)/R secs
In packet switching, Total time for Transmitting a message =
N*propagation-delay + N*t + (n -1)*t
Where in,
N = Number of links
propagation-delay = propagation delay per link
n = Number of packets which represent each message
t = time for transmitting each packet
Here propagation-delay is negligible so
total time = N*0 + N*(((P+h)*8)/R) + (M-1)*(((P+h)*8)/R)
total time = (((P+h)*8)/R)*(N+(M-1)) secs
(c) Suppose the network operates as a packet-switched virtual-circuit network, the F bytes are segmented into M packets of P bytes each, and a header of h/2 bytes is added to each packet when it is being transmitted. The virtual circuit set-up time is Ts. How long does it take to send the file from A to B?
Each packet size = (P+(h/2))*8 bits
transmission speed of R bits/second
time taken for each packet = ((P+(h/2))*8)/R secs
The time to deliver the first of M packets to the destination is N * *((P+(h/2))*8)/R) secs
Every ((P+(h/2))*8)/R seconds a new packet from the M - 1 remaining packets arrives at the destination.
total time = ts + (((P+(h/2))*8)/R)*(N+(M-1)) secs
(d) Suppose the network operates as a pure circuit-switched network with an end-to-end transmission speed of R bits/second once the circuit has been established. A header of h/2bytes is added to the entire file when it is transmitted and the circuit set-up time is Ts. How long does it take to send the file from A to B?
Total size of packet = (F+(h/2)) bytes = (F+(h/2))*8 bits
total time = Ts + ((F+(h/2))*8)/R
(e) Under what conditions (what values of Ts) does the packet-switched datagram network provide a faster transfer of file F from A to B than the circuit-switched network?
For Ts > 12 he packet-switched datagram network provide a faster transfer of file F from A to B than the circuit-switched network