COMP 327 Computer Networks Term Test II – November 16, 2010
(Total 20 marks)
1. (2 marks) Given the dataword 100101 and the divisor 1101, show the generation of the codeword at
the sender site, and the codeword.
1101
|
100101000
1101
1000
1101
1011
1101
1100
1101
100
The codeword (dataword + redundancy) is 100101100
2. (2 marks) In the Stop-and-Wait ARQ algorithm, what is the proper action of the receiver when the
receiver receives a DATA frame of a wrong sequence number.
The receiver discards the DATA frame and sends an ACK frome with the correct sequence number.
3. (2 marks) In Selective-Repeat ARQ, the sender sliding window has frames 2, 3, 4, 5 and 6 over 0, 1, 2,
3, 4, 5, 6, 7, 0, 1, ... The sender sent frames 2, 3 and 4, and received ACK 4. List all the frames that the
sender can send without waiting for the next ACK. We assume that the sender would not send any
unnecessary frames again.
4, 5, 6, 7, 0
Need to remember that ACK 4 means the receiver waits for DATA frame 4, and
that the sender’s sliding window slides after the ACK arrived.
4. (2 marks) A sender needs to send the two data items 0x3456 and 0xCDEF. Find the checksum at the
sender site.
3456
CDEF
Add
0245
1
0246
Add the carry
Take the complement
5. (2 marks) Explain how CSMA/CD protocol is different from CSMA.
CD (Collision Detection): While the sender is sending a frame, it keeps checking the link to see if
there is any collision. If so, the sender sends a jamming signal, and retransmit the frams later.
6. (4 marks) An organization is granted the block 22.0.8.0/22. The administrator wants to create 3
subnets. The size of the first subnet is 1/2 of the total address block, the size of the second is 1/4 of
the total address block, and the size of the third subnet is 1/4 of the total address block. Find the
addresses of the 3 subnets.
1/2
22.0.8.0/23
1/4
22.0.10.0/24
1/4
22.0.11.0/24
7. (2 marks) What do we mean when we say that a bridge can filter traffic? Why is filtering important?
A bridge receives a frame and checks the desination address. If the destination address does not
belong to other network segment, then the frame will not be forwarded to any other network
segments. This filtering helps to reduce the traffic, and eventualy packet collision.
8. (2 marks) Find the netid and the hostid of the network address 208.34.5.12/12.
208.32.0.0;
0.2.5.12
9. (2 marks) An organization has 4 networks – 192.122.40.0/24, 192.122.41.0/24, 192.122.42.0/24 and
192.122.43.0/24. The administrator wants to decide the address of a supernet of the 4 networks, so
that the router connected to the 4 networks can propagate the routing information to other routers.
What is the address of the supernet?
192.122.40.0/22