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Chapter 12: Telecommunications Systems

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1 Chapter 12: Telecommunications Systems

2 Objectives Identify the basic elements of a telephone system, and discuss the limitations of telephone signals Describe the composition of the telephone industry before and after the 1984 Modified Final Judgment, and explain the differences Describe the difference between a local exchange carrier and an interexchange carrier, and list the services each offers

3 Objectives (continued)
Differentiate between the roles of the local telephone company before and after the Telecommunications Act of 1996 List the types of leased lines that are available and their basic characteristics Outline the features of ISDN, and distinguish a basic rate interface from a primary rate interface List the basic characteristics of frame relay, such as permanent virtual circuits, committed information rate, and switched virtual circuits

4 Objectives (continued)
Identify the main characteristics of Asynchronous Transfer Mode, including the roles of the virtual path connection and the virtual channel connection, the importance of the classes of service available, and ATM’s advantages and disadvantages Identify the main characteristics of digital subscriber line, and recognize the difference between a symmetric system and an asymmetric system

5 Introduction Students used to go into either data communications or voice communications Today, the two fields are merging Voice systems transfer computer data and data networks support voice Anyone studying the field of data communications and networks must learn some basic telecommunications too

6 The Basic Telephone System
POTS is the plain old telephone system that connects most homes and small businesses POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz A telephone conversation requires two channels, each occupying 4000 Hz

7 The Basic Telephone System (continued)

8 The Basic Telephone System (continued)
A 4000 Hz analog signal can only carry about 33,600 bits per second of information while a 4000 Hz digital signal can carry about 56,000 bits per second If you want to send information faster, you need a signal with a higher frequency OR you need to incorporate more advanced modulation techniques POTS cannot deliver faster signals

9 The Basic Telephone System (continued)
Local loop is the telephone line that runs from the telephone company’s central office to your home or business Central office is the building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone If you place a long distance call, the central office passes your telephone call off to a long distance provider

10 The Basic Telephone System (continued)

11 The Basic Telephone System (continued)
The country is divided into a few hundred local access transport areas (LATAs) If your call goes from one LATA to another, it is a long distance call and is handled by a long distance telephone company If your call stays within a LATA, it is a local distance call and is handled by a local telephone company

12 The Basic Telephone System (continued)
A trunk is a special telephone line that runs between central offices and other telephone company switching centers A trunk is usually digital, high speed, and carries multiple telephone circuits A trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit

13 The Basic Telephone System (continued)
A trunk is not associated with a single telephone number like a line is A telephone number consists of an area code, an exchange, and a subscriber extension The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services

14 The Basic Telephone System (continued)

15 The Basic Telephone System (continued)
When the telephone company installs a line, it must not proceed any further than 12 inches into the building This point is the demarcation point, or demarc Modular connectors, such as the RJ-11, are commonly used to interconnect telephone lines and the telephone handset to the base When the handset is lifted off the base (off-hook), an off-hook signal is sent to the central office

16 The Basic Telephone System (continued)
When the off-hook signal arrives at the central office, a dial tone is generated and returned to the telephone When the user hears the dial tone, they dial (or press) the number The central office equipment collects the dialed digits, and proceeds to place the appropriate call

17 The Basic Telephone System (continued)
Foreign exchange service (FX) - customer calls a local number which is then connected to a leased line to a remote site Wide area telecommunications services (WATS) - discount volume calling to local and long distance sites Off premises extensions (OPX) - dial tone at location B comes from the PBX at location A

18 Before and After 1984 The Telephone Network
In 1984, the U.S. government broke up AT&T Before then, AT&T owned a large majority of all local telephone circuits and all the long distance service With the Modified Final Judgment of 1984, AT&T had to split off the local telephone companies from the long distance company The local telephone companies formed seven Regional Bell Operating Companies Today, there are only 4 left: Bell South, SBC, Qwest (US West), and Verizon (Bell Atlantic)

19 Before and After 1984 (continued)
The Telephone Network Before and After 1984 (continued)

20 Before and After 1984 (continued)
The Telephone Network Before and After 1984 (continued) Another result of the Modified Judgment was the creation of the LATA (local access and transport area) Local telephone companies became known as local exchange carriers (LECs) Long distance telephone companies became known as interexchange carriers (IEC, or IXC) Calls that remain within a LATA are intra-LATA, or local calls Calls that pass from one LATA to another are inter-LATA, or long distance

21 Before and After 1984 (continued)
The Telephone Network Before and After 1984 (continued) Before 1984, the telephone network in the U.S. resembled a large hierarchical tree, with Class 5 offices at the bottom and Class 1 offices at the top Users were connected to the Class 5 offices The longer the distance of a telephone call, the further up the tree the call progressed Today’s telephone structure is a collection of LECs, POPs, and IECs

22 Telephone Networks after 1996
Telecommunications Act of 1996: opened up the local telephone market to competitors Now cable TV companies (cable telephony), long distance telephone companies, or anyone that wanted to start a local telephone company could offer local telephone service Local phone companies that existed before the Act are known as incumbent local exchange carriers (ILEC) while the new companies are competitive local exchange carriers (CLEC)

23 Telephone Networks after 1996 (continued)
LECs are supposed to allow CLECs access to all local loops and switching centers / central offices If a local loop is damaged, the LEC is responsible for repair The LEC is also supposed to provide the CLEC with a discount to the dial tone (17-20%) LECs can also provide long distance service if they can show there is sufficient competition at the local service level

24 Other Players in the Market
Alternate operator services - pay phones, hotel phones Aggregators - pulls a bunch of small companies together and goes after phone discounts Reseller - rents or leases variety of lines from phone companies, then resells to customers Specialized mobile radio carriers - mobile communication services to businesses and individuals, including dispatch, paging, and data services ARDIS and RAM Mobile Data two good examples

25 Provides advanced intelligent features to users, such as:
PBX Private branch exchange (PBX) - common internal phone switching system for medium to large-sized businesses Provides advanced intelligent features to users, such as: 4-digit internal dialing Special prefixes for WATS, FX, etc (private dialing plans) PBX intelligently decides how to route a call for lowest cost

26 More PBX Features Voice mail Routes incoming calls to the best station set (automatic call distribution) Provides recorded messages and responds to touch-tone requests (automated attendant) Access to database storage and retrieval (interactive voice response) VoIP

27 PBX Components CPU, memory, telephone lines, trunks Switching network Supporting logic cards Main distribution frame Console or switchboard Battery back-up system

28 Automated Attendant Plays a recorded greeting and offers a set of options Lets the caller enter an extension directly (touch tone or voice) and bypass an “operator” Forwards the caller to a human operator if the caller does not have a touch tone phone Available as an option on a PBX

29 Automatic Call Distributor
Automatic Call Distributor: perhaps you’ve experienced this when you call a business, are told all operators / technicians / support staff etc. are busy, and that your call will be answered in the order it was received Used in systems where incoming calling volume is large, such as customer service, help desk, order entry, credit authorization, reservations, and catalog sales Early systems used hunt groups Original systems routed call to first operator in line (kept person very busy!)

30 Automatic Call Distributor (continued)
Modern systems perform more advanced functions, such as: Prioritize the calls Route calls to appropriate agent based on the skill set of the agent If all agents busy, deliver call to waiting queue and play appropriate message (like how long they may have to wait) Forward calls to another call center, or perform automatic return call

31 Interactive Voice Response
IVR is similar to AA EXCEPT: IVR incorporates a connection to a database (on a mainframe or server) IVR allows caller to access and/or modify database information IVR can also perform fax on demand

32 Interactive Voice Response (continued)
Common examples of IVR include: Calling your bank to inquire about an account balance University online registration system Brokerage firm taking routine orders from investors Investment fund taking routine requests for new account applications A company providing employees with info about their benefit plans

33 Key Telephone System Used within a small office or a branch office, a key telephone system (KTS) is an on-premise resource sharing device similar to a PBX For example, a key system might distribute 48 internal telephone sets over 16 external phone lines The business would pay for the 16 individual lines but have 48 telephone sets operating User selects outside line by pressing corresponding line button on key set (phone)

34 What if you need a faster service, or need one that is always on?
Leased Line Services Most home computer users use POTS lines and conventional modems to connect to other computer systems What if you need a faster service, or need one that is always on? You can get a leased line service A basic leased line, or tie line, gives you a 56 kbps data transfer rate T-1 (or T1) service provides Mbps rate Used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world

35 A T-1 service is always on and always transmitting
A T-1 service is a digital, synchronous TDM stream used by businesses and telephone companies A T-1 service is always on and always transmitting One T-1 service can support up to 24 simultaneous channels These channels can be either voice or data (PBX support) A T-1 service can also be provisioned as a single channel delivering Mbps of data (LAN to ISP connection)

36 T-1 Service (continued)
A T-1 service requires 4 wires, as opposed to a 2-wire telephone line A T-1 can be either intra-LATA (local) which costs roughly $350-$400 per month, or inter-LATA (long distance) which can cost thousands of dollars per month (usually based on distance) A customer may also be able to order a ¼ T-1 or a ½ T-1

37 T-1 Service (continued)
T-1constantly transmits frames (8000 frames per second) Each frame consists of one byte from each of the 24 channels, plus 1 sync bit (8 * = 193 bits) 8000 frames per second * 193 bits per frame = Mbps If a channel is used for voice, each byte is one byte of PCM-encoded voice If a channel is used for data, each byte contains 7 bits of data and 1 bit of control information (7 * 8000 = 56 kbps)

38 Integrated Services Digital Network (ISDN)
ISDN is another leased service that provides a digital telephone or data connection into a home or business With ISDN you can have a digital telephone line and a 64 kbps data line, or one 128 kbps data line The basic rate interface (BRI) is the service for homes and small businesses, while the primary rate interface (PRI) is the service for larger businesses

39 ISDN (continued) BRI ISDN consists of two B channels and one D channel
A B channel can carry 64 kbps of data or PCM-encoded voice The D channel is 16 kbps and carries signaling information The B channels are dialable, and the D channel can be always on Many users combine both B channels for a 128 kbps data channel

40 ISDN (continued) PRI ISDN is used by larger businesses and contains 23 B channels and one 64 kbps D channel PRI ISDN is essentially equivalent to a T-1, but with ISDN, the 23 channels are dialable! The appropriate ISDN modems / multiplexors are necessary to support this service What could you use an always-on D channel for?

41 A business only has to connect itself to the local frame relay port
Frame relay is the leased service that can provide a high-speed connection for data transfer between two points either locally or over long distances A business only has to connect itself to the local frame relay port Hopefully this connection is a local telephone call Once the data reaches the local frame relay port, the frame relay network, or cloud, transmits the data to the other side

42 Frame Relay (continued)

43 Frame Relay (continued)
A connection between two endpoints is called a permanent virtual circuit (PVC) PVCs are created by the provider of the frame relay service User uses a high-speed telephone line to connect its company to a port, which is the entryway to the frame relay network The high-speed line, the port, and the PVC should all be chosen to support a desired transmission speed

44 Frame Relay (continued)

45 Frame Relay (continued)
Consider a company that has four office locations and currently has six leased lines interconnecting the four locations To install frame relay, the company would ask for six PVCs in place of the six leased lines The company would also need four high-speed telephone lines and four ports connecting the four locations to the frame relay cloud

46 Frame Relay Setup

47 Frame Relay Setup (continued)

48 Committed Information Rate (CIR)
The user and frame relay service would agree upon a committed information rate (CIR) CIR states that if the customer stays within a specified data rate (standard rate plus a burst rate), the frame relay provider will guarantee delivery of 99.99% of the frames The burst rate cannot be exceeded for longer than 2 seconds

49 CIR (continued) Example: If a company agrees to a CIR of 512 kbps with a burst rate of 256 kbps, the company must stay at or below 512 kbps, with an occasional burst up to 768 kbps, as long as the burst does not last longer than 2 seconds If the company maintains their end of the agreement, the carrier will provide something like 99.99% throughput and a network delay of no longer than 20 ms If the customer exceeds its CIR, and the network becomes congested, the customer’s frames may be discarded

50 Frame Relay vs. the Internet
Frame relay has many advantages over the Internet, including guaranteed throughput and minimum delay as well as better security Internet has the advantage of being practically everywhere Cheaper and simpler to create connections (no PVCs necessary) Internet tunnels (VPNs) are also attractive

51 Voice over Frame Relay (VoFR)
Frame relay is also capable of supporting voice communications High transfer speeds adequately support the needs of interactive voice If a company requires multiple voice circuits, frame relay is an interesting solution

52 Frame Relay Switched Virtual Circuits
Frame relay can also provide switched virtual circuits (SVC) An SVC can be created dynamically by the customer Good for short-term connections, but more expensive

53 Asynchronous Transfer Mode (ATM)
Asynchronous transfer mode (ATM) is a very high speed packet delivery service, similar in a number of ways to frame relay Both send packets of data over high speed lines Both require a user to create a circuit with a provider One noticeable difference between ATM and frame relay is speed ATM is capable of speeds up to 622 Mbps while frame relay’s maximum is typically 45 Mbps

54 ATM (continued) Similar to frame relay, data travels over a connection called a virtual channel connection (VCC) To better manage VCCs, a VCC must travel over a virtual path connection (VPC) One of ATM’s strengths (besides its high speeds) is its ability to offer various classes of service If a company requires a high-speed, continuous connection, they might consider a constant bit rate service

55 Variable Bit Rate (VBR): less demanding service
ATM (continued) Variable Bit Rate (VBR): less demanding service Can also support real time applications (rt-VBR), as well as non-real time applications (nrt-VBR), but does not demand a constant bit stream Available bit rate (ABR): used for bursty traffic that does not need to be transmitted immediately ABR traffic may be held up until a transmission opening is available Unspecified bit rate (UBR): for lower rate traffic that may get held up, and may even be discarded part way through transmission if congestion occurs

56 Advantages and Disadvantages of ATM
Advantages of ATM include very high speeds and different classes of service Disadvantages include potentially higher costs (both equipment and support) and a higher level of complexity

57 Digital Subscriber Line
Digital subscriber line (DSL) is a relative newcomer to the field of leased line services DSL can provide very high data transfer rates over standard telephone lines Unfortunately, less than half the telephone lines in the U.S. are incapable of supporting DSL There has to be a DSL provider in your region

58 DSL (continued) DSL, depending on the type of service, is capable of transmission speeds from 100s of kilobits into single-digit megabits Because DSL is highly dependent upon noise levels, a subscriber cannot be any more than 5.5 kilometers (2-3 miles) from the DSL central office Service can be symmetric, in which downstream and upstream speeds are identical, or asymmetric in which downstream speed is faster than upstream speed

59 DSL (continued) A DSL service often connects a user to the Internet
A DSL service can also provide a regular telephone service (POTS) The DSL provider uses a DSL access multiplexer (DSLAM) to split off the individual DSL lines into homes and businesses A user then needs a splitter to separate the POTS line from the DSL line, and then a DSL modem to convert the DSL signals into a form recognized by the computer

60 DSL (continued)

61 A DSL service comes in many different forms:
DSL (continued) A DSL service comes in many different forms: ADSL - Asymmetric DSL CDSL - Consumer DSL (trademarked version by Rockwell) DSL.Lite - Slower form than ADSL HDSL - High-bit rate DSL RADSL - Rate adaptive DSL (speed varies depending on noise level)

62 DSL (continued)

63 Computer Telephony Integration
Computer telephony integration (CTI): emerging field that combines more traditional voice networks with modern computer networks Consider a system in which a customer calls a customer support number: Customer’s telephone number appears on customer support rep’s terminal and immediately pulls up customer’s data Rep answers phone by clicking on an icon on the screen and helps the customer Rep transfers the call by clicking on another icon on the computer screen

64 (continued) Computer Telephony Integration
CTI can also integrate voice cabling with data cabling PBX talks directly to the LAN server PBX can direct the LAN server to provide a telephone operation to the user through the user’s computer Telephones may still be connected to the PBX or they may be connected to the LAN via the LAN wiring

65 Computer-Telephony Integration (CTI)
CTI applications could include the following: *Unified messaging *Third party call control *Interactive voice response *PBX Graphic User Interface *Integrated voice recognition *Call filtering and response *Fax processing and fax-back *Customized menuing systems *Text-to-speech and speech-to-text conversions

66 Company Makes a Service Choice
Telecommunication Systems in Action: A Company Makes a Service Choice Better Box Corporation has offices in Seattle, San Francisco, and Dallas, with headquarters in Chicago Better Box wants to connect Chicago to each of the other three offices Better Box needs to download 400 kbyte files in 20 seconds Requires a transmission speed of 160,000 bps Better Box could use three separate T-1 lines, use a frame relay service, or use asynchronous transfer mode

67 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice

68 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice

69 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice Better Box Corporation might also consider dial-up access lines, ISDN BRI service, and leased 56k lines Dial-up, ISDN BRI, and leased 56k lines will not meet the company’s requirements for a 160 kbps download Typical various prices for these services are shown on the next table

70 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice To provide T-1 service to all four offices: Seattle to Chicago: $6325 ($ $2.50 per mile) San Francisco to Chicago: $6625 Dallas to Chicago: $3500 Total interLATA T-1 costs = $16,450 / month

71 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice To provide frame relay service: Three ports at 256K = 3 x $495 One port at 768K = $1240 Three 256K PVCs = 3 x $230 Four intraLATA T-1s = 4 x $350 Total charge = $4815 / month

72 A Company Makes a Service Choice
Telecommunications Systems in Action: A Company Makes a Service Choice To provide asynchronous transfer mode service: Four ports at Mbps ABR = 4 x $1750 Three channels = 3 x $250 Three paths = $2 per mile x 5140 miles = $10,280 Four intraLATA T-1s = 4 x $350 Total ATM charges = $19,430 / month

73 Summary Telephone system 1984 Modified Final Judgment
Local exchange carrier vs. interexchange carrier Telecommunications Act of 1996 Leased lines ISDN and basic rate interface vs. primary rate interface Frame relay Asynchronous Transfer Mode Digital subscriber line and symmetric vs. asymmetric system

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