Consider two hosts are linked by a 4Mbps (4 times 10^6) channel and RTT is 0.04
ID: 3809587 • Letter: C
Question
Consider two hosts are linked by a 4Mbps (4 times 10^6) channel and RTT is 0.04 sec. Assume the size of each packet is 2000 bits. Answer the following questions for ARQ schemes: a) Assume that the link is error-free, what is the possible maximum rate of transmission for Stop-and-wait, GBN, and SR, respectively? Why? b) For GBN and SR, in order to allow sender to continuously send packets without any waiting, what is the minimum window size in terms of the number of packets? c) For b), what should be the minimum number of bits for the sequence number for GBN and SR, respectively? d) Suppose that we are continuously transmitting packets end-to-end start from the 2nd packet, and the 6^th packet is lost. Assume there is no other packet lost or ACK lost. For stop-and-wait, GBN, and SR, which packets need to be retransmitted?Explanation / Answer
a)
For sliding window protocols - stop and wait, GBN, SR ARQ,
when there are no transmission errors, maximum rate of transmission is maximum throughput.For the highest possible throughput, it is important that the transmitter is not forced to stop sending by the sliding window protocol earlier than one round-trip delay time (RTT).
Maximum throughput = packet size in bits / Round trip time
= 2000 / 0.04 bits per sec
= 2000 / 4 * 10^-2 bits per sec
= 200000 / 4 bits per sec
= 50000 bits per sec
= 50 kbps
b)
For GBN and SR ARQ,
RTT = 0.04 sec = 40 ms
Bandwidth = 4 Mbps
Packet size = 2000 bits
in 2*10^3 ms 4*10^6 bits
so, in 40 ms X bits;
X = 50*(4*10^6)/2*10^3 = 100*10^3 bits
now, 1 packet size = 2000 bits
so total number of packets in window will be
= 100*10^3 / 2000
= 50. this will be a window size.
c)
To represent 50 packets uniquely we need 6 bits.
d)
In the given scenario,
For stop and wait, we need to re-transmit 6th packet. Then the subsequent packet transmission resumes.
For GBN, we need to re-transmit 6th packet and rest of the packets transmitted.
For SR, we need to re-transmit only 6th packet.