Sample Exams Questions and Answers on Computer Networks

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CCM 4300 Lecture
Computer Networks, Wireless and Mobile Communication Systems

Dr P. Shah

Show that, according to the Shannon-Hartley law, if the signal power (S) is equal to the noise power (N), the channel capacity (C) in b/s is equal to the bandwidth B Hz. (6 marks) C= B log2 (1+S/N) If S=N C=B log2 (1+1) C=B log2 (1+1) C= B log2 (2) C=B 2

What are the purposes of the following GSM networks? VLR HLR MSC (home and visited) (6 marks) VLR: it is a database maintained by the visited network that contains an entry for each mobile user that is currently in the portion of the network served by VLR. HLR: a database that is maintained by the home network which contains permanent cell phone number and subscriber profile information for each of its subscribers. It also contains information about the current location of those mobile nodes under its care. Home MSc: point of contact to obtain routable address of mobile users. Visited MSC: is responsible for setting up calls to/from mobile nodes in cells associated with MSC.

For Bluetooth based networking, state the three power saving mechanisms and discuss their negative effects on the transmission QoS. (8 marks) Active Mode, Sniff mode, Hold mode, and Park mode. All systems save power by periodic sleep functions. In particular Bluetooth systems, which offer several low power modes, as they are typically, battery operated. Negative effects of power saving are the increased latency for spontaneous transmission – the devices have to wakeup first. Thus, the shorter access delay should be less power a device can save. Furthermore, high data rates require high power. If the periodic sleep function is not synchronised with i.e., periodic data transfer, heavy jitter will result

What are the different types of cells in a GSM cell structure and where are they used? (7marks) Different types of cells in a GSM cell structure and their specific uses are as below: 1.  Hot spots – Cell within a cell. Mainly used in extremely densely populated areas. 2.  Macro cells – Large cells. Used in sparsely populated areas, or rural areas. 3.  Micro cells – By splitting the existing areas into smaller cells, the number of channels available are increased as well as capacity of the cells. Power level of the transmitters used is reduced, in order to prevent the possibility of interference between neighbouring cells. Used in densely populated areas. 4.  Selective cells – They are created using special antennas. They do not provide 360-degree coverage. They are used at the entrance of tunnels where a selective cell could provide coverage of 120-degrees. Umbrella cells – They cover several micro-cells. Used for ‘high-speed’ (highly mobile) mobile stations, e.g. trains, cars, etc.

i.  Define Source Routing ii.  How Dynamic Source Routing would work for node A (Sender) to node G (Destination) work for the network shown below. Show diagram in order to explain the route search. [10 marks] B G D A E C F H

i.  In Source Routing, the sender determines the complete sequence of nodes through which the packets will be forwarded before they reach the destination. ii.  Node ‘A’ (source) would broadcast RREQ (Route Request) packet in search for a path to the destination. For any neighbouring nodes (‘B’ and ‘C’) receiving the RREQ packet, a.  If found in the node’s list of recent request, then discard the RREQ packet b.  Return a RREP (Route Reply) to the source if node matches the destination address in RREQ. c.  Append the node’s address to route record of RREQ. In the diagram, node ‘A’ sends a RREQ broadcast to ‘B’ and ‘C’. ‘B’ forwards the request to the neighbouring node ‘D’. ‘D’ forwards the request to neighbouring node ‘G’ (Destination). Each time, the node is added, the node address is appended to route the record of RREQ. Hence, in the RREP packet, the whole address or route is described as ‘A-B-D-G’. Likewise, node ‘A’ also sends a request to neighbouring node ‘C’....
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