Review Questions
1. What types of multiplexing and switching are used in telephone networks?
Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM); circuit switching.
2. Which properties of circuit-switched networks can be considered drawbacks?
· Possibility of setup request blocking;
· Impossibility to redistribute a fixed bandwidth of a circuit dedicated to a particular user between other users of a network during periods when this user doesn’t use all amount of the circuit bandwidth.
3. Which properties of packet-switched networks negatively affect the transmission of multimedia information?
A source of multimedia such as voice or video generates a regular (even) stream of packets. While transferring through a packet-switched network these packets can experience uneven delays due to congestion and hence queues of different sizes. As a result stream of arriving at destination node packets is uneven, i.e. with different inter-packet intervals. This condition results in distortions in playback of video or voice at a destination node.
4. Is buffering used in circuit-switched networks?
No
5. Which element of the circuit-switched network can deny the node requesting to establish circuit?
A. None, the network is always ready to receive data from a user
B. Any transit node - correct
C. The destination node – correct
6. Which concepts are characteristic for the Ethernet technology?
7. Do datagram networks take into account the existing flows?
No, packets are processed independently in a packet-switched network. At the same time all packets that have the same destination address can be considered as a flow and an existence of such kind of flows is taken in account when routes of packets are being determined.
8. Give a definition of a logical connection.
9. Is it possible to provide a reliable data transfer without a logical connection between end nodes?
No
10. Which logical connections could be named as a virtual circuit?
Those which determine a route of packets belonging to a given virtual circuit
11. Which networks use the virtual circuit technology?
X.25, ATM, Frame Relay
12. Specify which of the devices listed here are functionally similar:
A. Hub
B. Switch
C. Concentrator
D. Repeater
E. Router
F. Bridge
Hub, concentrator and repeater.
Switch and bridge.
13. List the differences of a bridge and a switch.
Bridge does not support forwarding frames between pairs of its ports in parallel whilst switch does.
14. Is the following statement true? Ethernet built according to the star topology with the hub in the center is more reliable than the same network built on coaxial cable with the common bus topology.
The statement is true as generally reliability of hubs is better than reliability of cables.
15. How could you increase the bandwidth available to each end-user computer in a network built on hubs?
Two ways of such an upgrade are possible:
- to use faster technology, e.g. Fast Ethernet instead of Ethernet;
- replace hubs by switches.
Problems
1. P1. Determine how the data transmission time will increase in the packet-switched network as compared to the circuit-switched network given the following data:
r Total size of transmitted data — 200 KB
r Total length of the connection link — 5,000 km
r Assumed signal propagation speed — 0.66 light speed
r Link bandwidth — 2 Mbps
r Packet size (without header size) — 4 KB
r Header size — 40 bytes
r Interpacket interval — 1 msec
r Number of transit switches — 10
r Switching time in each switch — 2 msec
Assume that the network operates in the underloaded mode. Therefore, there are no queues in the switches.
The data transmission time will increase on 420 ms.
2. P2. If all communications devices in the network fragment shown in Fig. 3.22 are concentrators, on which ports will the frame sent from computer A to computer B appear?
1. 5 and 6
2. 4, 5, and 6
3. 4, 5, 6, and 7
4. 4, 5, 6, 7, and 12
5. On all ports
Fig. 3.22. Fragment of the network
On all ports
3. Provided that all communications devices in the network fragment in Fig. 3.22 represent switches, on which port will the frame sent from computer A to computer B appear?
On ports 5 and 6.
4. If all communications devices in the network fragment in Fig. 3.22 are switches, except one concentrator to which computers A and B are connected, on which ports will the frame sent from computer A to computer D appear?
On ports 4, 5, 6, 7, 12, 15, 17, 16, 11, 8, 3, 1.
5. In a datagram network, between nodes A and B there are three flows and three alternate routes. Is it possible to forward each flow via a different route?
No, as in a datagram network routing takes into account only destination address while in our case packets of all flows have the same destination address.
6. In a virtual circuit network, between nodes A and B there are three flows and three alternate routes. Is it possible to forward each flow via a different route?
Yes, as in a virtual circuit network a routing takes into account a number of flow attributes. For example, node A can send three different flows into a network each of which belongs to a certain application. An application tag is included in a header of a packet so that a network can recognize a type of application and route a flow accordingly.
7. A network is based on a shared medium with the bandwidth of 10 Mbps and comprises 100 nodes. What will be the maximum exchange rate between two computers in such a network?
Maximum exchange speed will be 10 Mbps in case when only two computers use a network – a sender and receiver.
8. A network can transmit data in two modes: datagram and virtual circuit. What would you take into account when choosing a specific mode for transmitting your data, if the main criteria are the speed and reliability of data delivery?
Longevity of data flows.
1. Do you think circuit-switched networks will soon be replaced by packet-switched networks? Or the contrary, that packet-switched networks will be replaced by circuit-switched networks? Or that the technologies will coexist? Provide arguments that support your opinion. Consider various areas of application for these technologies.