' 5.6 Digital Local Loops
FIGURE 5.9 Spectrum of a
typical ADSL system
Summary
'
The main points to remember from this chapter are:( The public switched telephone network is of great importance for wireless communication because it links wireless users with other users—both wireless and wired.
( Many of the specifications for telephone equipment have their basis in nineteenth century technology and have remained almost unchanged for reasons of compatibility.
( Common-channel signaling allows the telephone network to set up calls economically, without tying up voice lines, and allows the system to pro-vide a considerable amount of data about calls.
( Wireless systems that connect with the telephone system can use its com-mon-channel signaling system to carry information about subscribers roaming out of their local service areas.
( Modern telephone systems are digital except for the subscriber loop, and progress is being made towards digitizing the local loop as well.
( Digital telephone signals are time-division multiplexed with a data rate of 64 kb/s per voice channel.
( Signaling can be done by robbing bits from the voice signal but com-mon-channel signaling using Signaling System Seven is more efficient and allows more information to be shared.
( ISDN allows the telephone system to be completely digital from end to end.
( ADSL allows for a conventional analog voice local loop and a high-speed data link to be combined on one twisted pair.
( Key Terms
Asymmetrical Digital Subscriber Line (ADSL) method of providing high-speed data transmission on twisted-pair telephone loops by using high-frequency carriers
B (bearer) channels in ISDN, channels that carry subscriber communication
bit robbing use of bits that normally carry payload information for other purposes, such as controlling the communication system bit stuffing addition of bits to a bitstream to compensate for timing
variations
call blocking failure to connect a telephone call because of lack of system capacity
central office switch in a telephone system that connects to local subscriber lines
circuit-switched network communication system in which a dedicated channel is set up between parties for the duration of the
communication
common-channel signaling use of a separate signaling channel in a telephone system, so that voice channels do not have to carry signaling information
crosstalk interference between two signals multiplexed into the same channel
D (data) channel in ISDN, a communication channel used for setting up calls and not for user communication
dual-tone multi-frequency (DTMF) dialing signaling using combinations of two audio tones transmitted on the voice channel
end office see central office
fiber-in-the-loop (FITL) use of optical fiber for telephone connections to individual customers
hybrid coil a specialized transformer (or its electronic equivalent) that allows telephone voice signals to travel in both directions
simultaneously on a single twisted-pair loop
in-band signals control signals sent in a voice channel at voice frequencies in-channel signals control signals using the same channel as a voice signal integrated services digital network (ISDN) telephone system using
digital local loops for both voice and data, with the codec in the telephone equipment
justification addition of bits to a digital signal to compensate for differences in clock rates; informally known as bit stuffing
local access and transport area (LATA) in a telephone system, the area controlled by one central office switch
local area network a small data network, usually confined to a building or cluster of buildings
local loop in a telephone system, the wiring from the central office to an individual customer
out-of-band in telephone signaling, a control signal that is outside the voice frequency range
packet-switched network a communication system that works using data divided into relatively short transmissions called packets; these are
routed through the system without requiring a long-term connection between sender and receiver
point of presence (POP) place where one telephone network connects to another
private branch exchange (PBX) small telephone switch located on customer premises
public switched telephone system (PSTN) the ordinary public wireline phone system
ringback signal in telephony, a signal generated at the central office and sent to the originating telephone to indicate that the destination telephone is ringing
sidetone in telephony, the presence in the receiver of sounds picked up by the transmitter of the same telephone
signaling system seven (SS7) system used in telephony which transmits all call setup information on a packet-data network that is separate from the voice channels used for telephone conversations
tandem office telephone switch that connects only to other switches, and not to individual customers
trunk lines transmission line carrying many signals, either on multiple pairs or multiplexed together on a single twisted-pair, coaxial cable, or optical fiber
( Questions
1. Explain briefly how the telephone network differs from a broadcasting network.
2. Explain the difference between circuit-switched and packet-switched networks. Is the PSTN mainly circuit-switched or packet-switched?
3. What is a central office?
4. What is the difference between a tandem office and an end office?
5. What is a trunk line?
6. What is meant by a LATA?
7. How do wireless telephone providers connect to the wired telephone network?
8. How has the breakup of the Bell monopoly changed the North Ameri-can telephone network?
9. What is meant by call blocking, and why does it happen?
10. How many wires are needed (for a single line) from the individual tele-phone set to the central office?
11. How many wires are normally contained in the cable from an individual residence subscriber to the network? Why is this number different from the answer to Question 10 above?
12. Explain the meaning of the terms tip and ring. Which has negative polarity?
13. Explain how pulse dialing works.
14. What is meant by DTMF dialing, and why is it better than pulse dialing?
15. What is the function of the hybrid coil in a telephone instrument?
16. What is sidetone and why is it used in a telephone instrument?
17. Approximately how much bandwidth, at baseband, is needed for one channel of telephone-quality audio?
18. List the steps required in originating a local call. Include the appropri-ate voltages and frequencies that appear at the telephone instrument.
19. Describe the difference between in-band and out-of-band signaling, and give an example of each.
20. Describe the difference between in-channel and common-channel sig-naling. Which is the more modern system?
21. How does common-channel signaling reduce the vulnerability of the telephone system to fraudulent use?
22. Name some types of information that are carried by Signaling System Seven.
23. What type of data channel is used by SS7?
24. What type of modulation is used in FDM telephony?
25. What type of modulation is used in TDM telephony?
26. What is meant by bit robbing? What is its function and why is it undesir-able for data connections?
27. What is meant by bit stuffing? When and why is it necessary?
28. Compare basic-rate ISDN and ADSL as technologies for voice telephony.
Compare number of lines and type of equipment needed.
29. Compare basic-rate ISDN and ADSL as technologies for data communi-cation. Compare data rates and connection type.
( Problems
1. Suppose the voltage across a telephone line, at the subscriber, drops from 48 V to 10 V when the phone goes off hook. If the telephone in-strument has a resistance of 200 ohms when off hook and represents an open circuit when on hook, calculate:
(a) the current that flows when the phone is off hook
(b) the combined resistance of the local loop and the power source at the central office
2. The local loop has a resistance of 650 ohms and the telephone instru-ment has a ringer voltage of 80 volts when the voltage at the central of-fice is 100 V. Calculate the impedance of the ringer in the telephone.
3. Find the DTMF frequencies for the number 8.
4. What number is represented by tones of 770 and 1209 Hz, in the DTMF system?
5. Calculate the overhead of a DS-4 signal:
(a) in bits per second
(b) as a percentage of the total bit rate
6. By what percentage does the use of bit robbing reduce the data capacity of a DS-1 signal?
7. What is the proportion of overhead in a basic rate ISDN signal? (Assume the D channel is part of the overhead.) Compare with the overhead in a DS-1 signal.
8. The fastest modem for use on an analog telephone line operates at 56 kb/s in both directions. If an ADSL system has an upstream rate of 640 kb/s and a downstream rate of 1.5 Mb/s, by what factor does it exceed the modem data rate in each direction?
9. Compare the data rate available with basic-rate ISDN and that available with the ADSL system described in the previous problem. By what factor does ADSL exceed basic-rate ISDN in each direction:
(a) if a voice call is being made simultaneously with data?
(b) if no voice call is being made so that the whole available ISDN rate can be used for data?
10. Telephone signals can be carried by radio using either analog or digital modulation schemes. Compare the bandwidth required to carry each of the following signals:
(a) an analog voice signal with a baseband bandwidth of 4 kHz using SSBSC AM (often used for terrestrial microwave links where many voice signals are transmitted together)
(b) the same analog voice signal using FM with a frequency deviation of 12 kHz (used for cellular phones)
(c) a standard digital voice signal using QPSK and assuming a channel with a signal-to-noise ratio of 20 dB. (Your answer to this question may suggest why data compression and vocoders are often used when digital signals are to be transmitted by radio.)