Review Questions

1. What factors determine the complexity of organizing remote access?

2. How can a provider that doesn't own cabled local loops ensure network access services to its users?

To lease local loops from an owner or to make a contract with the local loops owner to direct users' traffic to a provider network.

3. Which characteristics of remote access clients must be taken into account when organizing remote access service for those clients?

The additional services' set that clients would like to have

4. What kind of a local loop can be considered universal?

The one which provides transmission of all kinds of traffic: data, voice (telephony) and video (television)

5. In what network does the PC of the remote access user participate?

The Internet and/or her/his corporate network .

6. What is the difference between remote node mode and remote control mode?

In the remote control mode a user's computer is used only as a terminal of a remote computer when the latter does all data processing and stores results. In the remote node mode a user's computer is used in full as if it were connected to LAN instead of WAN.

7. What kind of access is used when configuring remote routers?

Remote control; usually it involves telnet or SSH protocols.

8. Why are the rates of dial-up modems considerably lower than the access rates of ADSL and cable modems?

Because dial-up modems work through a telephone channel which has a fixed narrow bandwidth of 3.4 KHz determined by telephone network switches and multiplexors. The bandwidth restricts the speed of dial-up modems. ADSL modems work through a local loop (no telephone switches and multiplexors are involved) which has a bandwidth in 1MHz range (it depends on a local loop length). Cable modems use a dedicated bandwidth channel of a local TV cable loop which is usually has 10 -30 MHz width for data downloading. The wider available bandwidth provides higher acces rates for ADSL and cable modems.

9. What are the differences between modems and DSU/CSU devices?

A modem provide righer functionality compared to DSU/CSU; foe example, a modem does data framing and retransmitts corrupted or lost frames doing in fact the job of the data link layer while DSU/CSU carries out only physical layer functions.

10. Suppose that you have made sure that the operation of your modem is stable on a dedicated two-wire line in both synchronous and asynchronous mode. Which mode would you prefer and why?

Synchronous mode is preferrable in this situation as it provides higher user data transmission speed when bit rate is fixed.

11. To what layer of devices (according to the OSI model terms) is it possible to classify a dial-up modem?

It depends on a modem functionality. If a modem doesn't recognise frames and works only with bits it belongs to physical layer devices; otherwise it is the data link layer device.

12. What are DSLAM functions?

13. What are the differences between the requirements for the LAN of a dial-up acees service provider and those for the LAN of an ADSL access service provider?

The latter provider shpuld have much higher speed LAN as ADSL clients exchange data with much higher data speed.

14. What method of access to the shared medium is used by cable modems?

Arbitrator controlled access.

15. Is it possible to use the same coaxial cable for ensuring access to the residents of one apartment building (more than 400 apartments)?

Yes, it is, but when most of users work at the same time they will rfeceive small part of shared bandwidth and hence will have a slow access to the Internet.

16. Why is the 802.11 technology rarely used for organizing fixed wireless access?

Because of uncertainty in the part of shared bandwidth which a client has at any particularly moment.

17. What is the difference between MMDS and LMDS technology?

MMDS uses the lower frequency range compared to LMDS, hence MMDS covers larger areas than LMDS but provides lower access speed.

18. Why won't most home users be satisfied with mobile wireless access?

Because mobile wireless access is much more expencive and provides much less access speed than fixed access. As mobility is not an issue for a home user a fixed access is more suitable for her/him.

Problems

• Suppose that you have purchased a V.90 modem and are attempting to establish a connection, using a telephone network, to your colleague that also uses V.90 modem. You are sure that all telephone switches between you and your colleague operate in a digital mode. At what rate will you connect?

33,6 Kbit/s

• Which of the ISDN services is it expedient to use if two PCs are connected to this network using TAs, if they need to constantly exchange data at a rate of 2,400 bps with bursts reaching 9,600 bps, and if packet delays are not critical?

D-channel use for data transfer

• Which of the ISDN services is it expedient to use if two LANs are connected to this network through routers and internetwork traffic has rates from 100 Kbps to 512 Kbps during long periods?

Permanent connection via PRI

• In which cases is it more expedient to organize remote access through ISDN with a B+D interface? In which cases is it better to use a leased 64 Kbps digital line? When is it more profitable to use a persistent frame relay virtual circuit with a CIR equal to
  64 Kbps?
• What needs to be changed if an ADSL modem operates on a given local loops with an exceedingly high percentage of errors?

To decrease the data rate