Mid-Sem 2017

Question 1

In what circumstances does error correction offer benefits over using only error detection

Answer 1

• When the link only supports uni-directional (simplex) transmissions
• When the it is too expensive, too slow (long latency), too insecure to retransmit original data

Question 2

Under what circumstances would the use of DL layer frame pipelining be unnecessary?

Answer 2

When the latency between 2 directly connected nodes is very short, an ACK frame will return to the seder before the second frame can be assembled and transmitted

Question 3

Must sliding window protocols use windows whose sizes are powers of 2?

Answer 3

Not necessary - it just simplifies the implementation in software/hardware

Question 4

Explain the need for both a min and max frame length in IEEE 802.3 Eth std

Answer 4

• Min frame ensures that the nodes at the extremities of the etc segment will sense ALL frames before frame’s sender finishes sending the frame
• Max frame prevents single sending node from monopolising the use of the Eth segment

Question 5

Assuming that nodes use CSMA/CD and bin exp backoff arg have each attempted their first transmission, and all 3 collide.

• Prob that all 3 re-trans collide again in first time slot?
• Prob that exactly 2 nodes will collide again in the first time slot after orig collision?
• Prob that all 3 nodes can successfully transmit their frames in a max of 3 time slots after the orig?

Answer 5

• 1/8 (if retransmission immediately)
• 3/8
• 0 (there is always a collision in the second slot)

Question 6

3 Distinct DoS attacks that can be performed on IEEE802.11 wireless networks on different layers of the protocol stack

Answer 6

• Physical, device may generate random RF noise, making the medium appear constantly busy (saturated)
• DL (MAC), device may generate random packs, which violates the cooperative RTS/CTS sequence
• Appl, an application may NAIVELY perform a large file transfer, dominating the medium and slowing down connections

Question 7

List 2 distinct security-delated design requirements of the WEP protocol

Answer 7

• optional for running of wireless network
• employ reasonably strong encryption
• self-synchronising, as nodes move in and out of range
• computationally efficient
• conform to government encryption regulations

Question 8

To avoid eavesdroppers employing passive scanning to identify an IEEE802.11 network, the owner of an AP disables transmission of beacon point frames. Explain why this is ineffective against a determined eavesdropper seeking the network’s identity

Answer 8

• Only disables the SSID which is transmitted in beacon frames
• Determined eavesdropper just needs to wait until a valid mobile client device, wishing to associate with the network, transmits a probe request frame carrying the required SSID.

Question 9

As a brute force form of congestion control, the NL may choose to deliberate discard some packets when its maximum permissible memory allocation is reached.

• What packets are best candidates for removal?
• What effects does their removal have on the total network’s congestion?

Answer 9

• Low priority packets, not ACK packets, packets that haven’t travelled far (many router hops), packets for applications that tolerate unreliability
• Hopefully sender wait’s a while before retransmission, so that temporary congestion can clear.