Computer Networks

Tutorial #14

1. What is the conceptual difference between Telstra Big Pond and Telstra On Ramp?

2. In the lecture, it was noted that the Optus Spinnaker service was a "wholesale" Internet Service Provider, as was (for example) Big Pond Direct and connect.com.au. What does this mean, and what is the alternative?

3. Two aspects of ISDN service which are emphasised by the marketing people are dial on demand routing, and bandwidth on demand data transfer. What do you think these terms refer to? A local ISDN data call has a flagfall cost (day rate) of 20 cents (which includes the first 180 seconds of the call) plus 0.05 cents per second after 180 seconds has elapsed. Under what conditions do you think it would be reasonable to configure a local-call ISDN-based system to use "dial on demand" and/or "bandwidth on demand"?

4. A Bendigo business is investigating an ISDN SPC connection to its head office in Melbourne. The SPC charge for this distance is $9720 per annum^[1] for one B channel. How does this compare to using dial-up ISDN over a year? (NB: NDD3 call costs are 12 cents flagfall plus 0.43483 cents/sec (8am-6pm Mon-Fri), 0.31339 cents/sec (6pm-10pm Mon-Thurs) and 0.18853 cents/sec (other times).

5. A Bendigo business wants to build a computer network connecting its Bendigo branch office with its Melbourne HQ. Explain briefly the economics of directly connecting the two sites with a "leased-line" service compared to using the Internet. What is the "downside" of using the Internet? Use information from Question 3 and Question 4 to support your argument. NB: this leads us into a more advanced topic, that of extranets, or Virtual Private Networks (VPNs). We should get to talk about these later in the semester.

6. Explain how a frame relay service at a port speed of 64 kbps differs from an ISDN semi permanent link at the same speed. In particular, why would you expect the frame relay service to be cheaper?

7. How is it possible for a frame relay service to drop frames? How can a user recover if this happens?

8. Why would a frame relay customer choose a CIR of zero bps?

^[1] 1998 prices. These figures are no longer available on Telstra's Web Pages.