Computer Networks

Tutorial #13

1. What is the conceptual difference between a supplier of telecommunications services and an Internet Service Provider (ISP)? NB: In Australia, these differences are exemplified by Telstra's BigPond service (which is an ISP) and Telstra On Ramp, which is a telecommunications service.

2. Some notable "wholesale" Australian Internet Service Providers include C&W Optus Spinnaker, Telstra BigPond Direct and connect.com.au. What does mean to say these services are "wholesale", 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 Telstra local ISDN data call in Australia 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 Microlink 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 install a point-to-point link connecting its Bendigo branch office with its Melbourne HQ. Explain briefly the economics of directly connecting the two sites with a "leased-line" service such as an ISDN SPC 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 may 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?

9. Want a challenge? Try and figure out how much you'd pay for a Telstra OnRamp Xpress data service in a typical small business.