Computer Communication Network
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1- Discuss the concept of switching as it relates to the problem involved in the connection of devices. What are the three switching methods? 1- Long distance transmission is typically done over a network of switched nodes 2- Nodes not concerned with content of data 3- End devices are stations (computer – terminal – phone) 4- A collection of nodes and connections is a communication network. 5- Data routed by being switched from node to node. The three methods of switching are 1- Circuit Switching 2- Packet Switching 3- Message Switching 2- What are the two types of switching used in circuit switching? 1- Space-Division Switching 2- Time-Division Switching 3- Draw a block diagram for the public circuit-switched network?
4- What are the circuit switching elements? 1- Subscriber 2- Subscriber line
3- Exchange 4- Trunks
5- What is the function of the control unit in circuit switching? 3- Disconnect 1- Establish connection 2- Maintain connection 6- What is the limiting factor in a crossbar switch? How does a multistage switch alleviate the problem? - Number of crossbar grows as square of number of stations - For large number of users which use the crossbar switch. The crossbar becomes impractical coz its hug size and its expensive price (use fewer than 25% of crosspoints) - Crossbar switch limited to small offices only. ☺ Multistage switch solve the limitations by combine crossbar switches in several stages and hence reduce numbers of crosspoints. 7- Define what is meant by:
Blocking and non-blocking switch?
Blocking switch: which may be unable to connect stations because all paths are in use Used on voice systems (Short duration calls) -Multistage switchNon-Blocking switch: which permits all stations to connect (in pairs) at once. Used for some data connections -Crossbar switch-
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Computer Communication Network
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0008- Compare between crossbar switches and multistage switches? Crossbar switches - Connect n inputs to m outputs in a grid, using (transistors) at each cross-point - n input and m output need n x m cross-points (the major limitation of this design is the number of cross-points required) - For large m & n crossbar becomes impractical - Useful only for relatively small end offices. - Inefficient because only fewer than 25% of the cross-points are in use at a given time.
Multistage switches - Connect n inputs to m outputs by combining crossbar switches in several stages.
- The solution to limitations of the crossbar because it need fewer cross-points than single stage crossbar. - Its recommended for large m & n design. - Useful for large end office. - Efficient. ( have high utilization)
9- Draw a three-stage switch, that uses three switches in the first and last stages and two switches in the middle stage, to do the job of a single 15-by-15 crossbar switch. Each of the first-stage switches is 5-by-3 switches. Each of the last-stage switches is 3-by-5 switches. Each of the intermediate switches is 3-by-3 switches. a. Show a pathway between input line 4 and output line 9.
b. Show a pathway between input line 13 and output line 2.
c. Compare between the number of cross-points in 15-by-15 single stage crossbar
switch, with 15-by-15 multistage switch that describe above. Cross-points (Crossbar) = 15*15 = 225 Cross-points (Crossbar) = (5*2)*6 + (3*3)*2 = 78
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Computer Communication Network
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10- Compare between the mechanism of a space-division switch and time-division switch? Space-division mechanism is to separate physical paths. Time-division mechanism is to use time-division multiplexing (TDM). 11- What is the function of the control unit in time-division with TSI switch? Reorder the input frames and output frames in the time slots on the desired connections. 12- Compare between space-division switching and time-division switching? Space Division -Instantaneous -Large number of cross-points -Multistage (large n&m) accept blocking
Time Division (TSI) -Each connection need delay -Each Time slot must stored in RAM -No need to cross-points -Delay rather than blocking
13- Why is circuit switching inefficient for the transmission of non-voice data? Coz channel capacity dedicated for duration of connection, so if no data, capacity wasted. 14- Design a TST switch with 12 inputs and 12 outputs. The input multiplexers should be 4x1; the output multiplexers should be 1x4.
15- Design a TSSST switch with 48 inputs and 48 outputs. The input multiplexers should be 4x1; the output multiplexers should be 1x4.
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Computer Communication Network
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16- Design a STS switch with 10 inputs and 10 outputs. The first-stage switches should be 5x2; and the last-stage switches should be 2x5.
17- Compare between a circuit-switched network and a packet-switched network, from the point of view. Dedicated path Store and forward Connection Establishment Routing table Delay
Circuit-switched Make dedicated path Use store and forward Need No Small
Packet-switched Don't Make dedicated path Don't use store and forward Need in VC only Yes Relatively big delay
18- Compare between a datagram approach and a virtual circuit approach of packetswitched network, from the point of view. Datagram Same route Connection Establishment Table lookup Arrive out of order
No No No Yes
Virtual circuit Yes Yes Yes No
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