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Exam 3 of Computer Networks (ICE 1230)
2008.5.6
Answer in English and Total 16 points
1. Suppose two nodes start to transmit at the same time a packet of
length L over a broadcast channel of rate R. Denote the
propagation delay between the two nodes as dprop. Will there be
a collision dprop < L/R ? Why or why not?
There will be a collision in the sense that while a node is transmitting it will start
to receive a packet from the other node.
2. Why is an ARP query sent with a broadcast frame? Why is an
ARP response sent within a frame with a specific destination
MAC address? (1 point)
An ARP query is sent in a broadcast frame because the querying host does not
which adapter address corresponds to the IP address in question. For the
response, the sending node knows the adapter address to which the response
should be sent, so there is no need to send a broadcast frame (which would have
to be processed by all the other nodes on the LAN).
3. Define switches and routers and explain pros (at least two
items) of switches comparing with routers.
A router is a layer-3 switch and a switch is a layer-2 packet
switch. The pros are switch’s plug-and-play, high filtering and
forwarding.
4. Why are acknowledgements used in 802.11 but not in wired
Ethernet? (1 point)
The adapter would not be able to detect all collisions due to hidden
terminal problem and fading.
5. What are the purposes of the HLR and VLR in GSM networks?
What elements of mobile IP are similar to the HLR and VLR?
The home network in GSM maintains a database called the home location
register (HLR), which contains the permanent cell phone number and
subscriber profile information about each of its subscribers. The HLR also
contains information about the current locations of these subscribers. The
visited network maintains a database known as the visitor location register
(VLR) that contains an entry for each mobile user that is currently in the
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portion of the network served by the VLR. VLR entries thus come and go as
mobile users enter and leave the network.
The edge router in home network in mobile IP is similar to the HLR in
GSM and the edge router in foreign network is similar to the VLR in GSM.
6. What is the purpose of IEEE 802.11 RTS and CTS frames? (1
point)
The purpose is to reserve an access to the channel.
7. Define terminologies such as streaming, packet jitter, policing
and real-time.
Streaming: playing-out while downloading a file
Packet jitter: the variation of packet delay
Policing: the regulation of the rate at which a class or flow is
allowed to inject packets into the network
Real-time:
8. Give an example from queues you experience in your everyday
life of FIFO, Round Robin, and WFQ.
FIFO: line at Starbucks. RR: merging traffic (taking 1 vehicle from first
lane then 1from the second, then 1 from the first, and so on). WFQ: ticket
counter at airport providing service to 2 people from first class and 1 from
economy, and again 2 from first class, and so on.
9. What are some of the difficulties associated with the Intserv
model and per-flow reservation of resources? (1 point)
Scalability: Per-flow resource reservation implies the need for a router to
process resource reservations and maintain per-flow state for each flow
passing through the router. Flexibly service: The Intserv framework
provides for a small number of prespecified service classes.
10. List and explain at least three items which make the ICU
campus more global with respect to protocols between people.
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