Chapter 21

Review Questions

1. Which parameters can be used to describe a virtual circuit?

- end points which this virtual circuit connects;

- the sequence of intermediate nodes along the virtual circuit path;

- virtual circuit type - unidirectional or bidirectional;


- reserved bandwidth and QoS parameneters for traffic goes through the virtual circuit.

2. What should be done in case of failure of the physical link through which the virtual circuit passes?

The virtual circuit should be re-established if the network topology allows to have a new path between the circuit end points.

3. List all main stages of establishing a virtual circuit.

4. Can an X.25 network operate without PADs?

Yes, it can if all terminal nodes support the besic networking functionality (as PAD is used for primitive terminals without any intelligence)

5. What can happen to the best effort traffic if priority traffic coming into the input Frame Relay interface is not limited by average intensity?

Best effiort traffic could experience long delays and drops because of lack of bandwidth during the periods of priority traffic bursts

6. How can the user connect to the built-in PAD using the telephone network, provided that he works at a terminal that does not support automatic calls through the
telephone network?

The user can dial the destination manually

7. If your company needs to connect multiple remote office networks to the central network and to one another but you have only leased analog links with installed synchronous 19.2 Kbps modems, which technology would you choose: X.25, Frame Relay, or ATM? Justify your solution and explain the factors that influenced it.

It is rather an imaginary situation as analog modems 19.2 Kbps were widespread in the past, not now; however it is a good question because it illustrates that there are no bad technologies in general but each technology does its best under the conditions it was designed for. The correct answer is: X.25 as it was designed to work on noisy channels such as analog 19.2 modem channels. More modern technologies Frame Relay and ATM would work worse on such channels as they were designed for good quality optical links.

8. Which function of the token bucket algorithm is not supported by the leaky bucket algorithm?

The leacky bucket algorithm doesn't allow traffic to accumulate the burst which hadn't been used during the previous period of time and use it at the current one.

9. What category of services is it expedient to choose for transmitting voice using the ??? network?

CBR or rtVBR

10. How many virtual circuits is it necessary to establish in each direction between each pair of ??? switches, provided that you need to transmit three traffic classes with different QoS levels?

Six unidirectional circuits or three bidirectional circuits with different QoS parameters for each circuit.


11. For which service category does the ATM network explicitly control the data flow? Why doesn't it use flow control for other types of services?

ATM network explicitly control the data flow for the ABR category of service. It is necessary because ABR gives much of freedom to the data flow in terms of rate limits which can cause congestion. Other service categories don't have such a freedom; that's why they can work without such a mechanism.

12. Assume that you need to manually establish a PVC in two enterprise-wide ATM networks connected by a public ATM network. You do not need your VCI numbers to depend on the VCI numbers used by the administrator of the public ATM network. What kind of switching would you request from the public ATM service provider?

Virtual Path service

13. Suppose that you have connected two LANs by a remote bridge operating using a PVC in the Frame Relay network. NetBEUI sessions between computers belonging to different networks often are broken. When computers belong to the same LAN, there are no such problems. What reasons can cause such a situation?

It is very likely that NetBEUI packets are transmitted by Frame Relay network with big delays - one of the reason of such a situation might be using sattelite channels whoith a network. When a delay value exceeds the NetBEUI time-out limit the session is cancelled.


1. Compare the number of frames generated by exchanging two TCP messages (sending the data and receiving an acknowledgment) between two end hosts connected by one switch when this switch is an X.25 switch and when it is a Frame Relay switch.

When it is an X.25 network under IP layer, the data exchange in question generates 8 frames as a tramsmission of a TCP message (a data message or an acknowledgment) between an end node and switch generates 2 LAP-B frames: one which transfers TCP messge itself and another which is a LAP-B acknowledgement. So, TCP data message generate 4 messages when travelling from one end node to the swutch and from the switch to another end node. Similarly, a TCP acknowledgement also generates 4 messages when travelling along the reverse route.

When it is a Frame Relay network, the same data exchange generates only 2 frames, one of which transfers a TCP data message and another which transfers an TCP acknowledgement.

2. In which case will the percentage of frames delivered to the destination node using the Frame Relay network be higher: when the service is requested based on CIR, Bc, and Be parameters or when the service is requested only on the basis of CIR and Bc (it is assumed that CIR and Bc values for both cases are the same)? Assume that the Frame Relay network is underloaded, and the sender node sends the data at a rate that often significantly exceeds the CIR value.

When the service uses only CIR and Bc parameters as in this case frames exceded the limit Bc will be only marked with DE=1 and will be discasrded only if some network switche along the frame route experiences congestion which is very unlikely event for the underloaded network. In case of the service on the basis of CIR, Bc, and Be parameters frames which exceeded the limit Bc + Be are being discarded immidiately.

3. Suppose that the Frame Relay switch and the IP switch (router) are based on the same architecture and processors with the same clock rate. Will the Frame Relay switch provide better performance than the IP router? Justify your opinion.

Switch will have better performance as it fulfill less operations for a frame forewarding then a router for a packet forwarding. On top of that switch operations have lower computational complexity.

4. Solve the traffic engineering task for the ATM network shown in Fig. 21.12. You must ensure the most uniform load for all network resources for the offered load provided in Fig. 21.13.

Fig. 21.12. ATM network

Fig. 21.13. Offered load