Chapter 12 Telecommunications Systems

1. Chapter 12 Telecommunications Systems Data Communications & Computer Networks, Second Edition

2. Introduction Students used to study either data communications or voice communications. Today, the two fields are merging. Most voice systems are computer controlled and data networks support voice. Anyone studying the field of data communications and networks must learn some basic telecommunications too. Data Communications & Computer Networks, Second Edition

3. Basic Telephone Systems 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. Data Communications & Computer Networks, Second Edition

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5. Basic Telephone Systems A 4000 Hz analog signal can only carry about 33,600 bits of information per second 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. POTS cannot deliver faster signals. Data Communications & Computer Networks, Second Edition

6. Basic Telephone Systems The local loop is the telephone line that runs from the telephone company’s central office to your home or business. The 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. Data Communications & Computer Networks, Second Edition

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8. Basic Telephone Systems 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 call and is handled by a local telephone company. Data Communications & Computer Networks, Second Edition

9. Basic Telephone Systems 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. Data Communications & Computer Networks, Second Edition

10. Basic Telephone Systems 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. Data Communications & Computer Networks, Second Edition

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12. Basic Telephone Systems 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. Data Communications & Computer Networks, Second Edition

13. Basic Telephone Systems 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. Data Communications & Computer Networks, Second Edition

14. Basic Telephone Systems Services 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. Data Communications & Computer Networks, Second Edition

15. The Telephone Before and After 1984 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). Data Communications & Computer Networks, Second Edition

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17. The Telephone Before and After 1984 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), and 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. Data Communications & Computer Networks, Second Edition

18. The Telephone Before and After 1984 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. Data Communications & Computer Networks, Second Edition

19. The Telephone After 1996 Another landmark ruling affecting the telephone industry was the Telecommunications Act of 1996. This act 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). Data Communications & Computer Networks, Second Edition

20. The Telephone After 1996 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. Data Communications & Computer Networks, Second Edition

21. PBX Private branch exchange (PBX) - a common internal phone switching system for medium to large-sized businesses. Provides advanced intelligent features to users, such as: 4-digit, special prefixes for WATS, FX, etc. (private dialing plans) PBX collects dialed digits and intelligently decides how to route this call for lowest cost Data Communications & Computer Networks, Second Edition

22. PBX Additional advanced 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) Data Communications & Computer Networks, Second Edition

23. 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. Data Communications & Computer Networks, Second Edition

24. Interactive Voice Response Common examples of IVR include: Call your bank to inquire about an account balance DePaul’s online registration system Brokerage firm taking routine orders from investors Investment fund taking routine requests for new account applications A company providing employees with information about their benefit plans Data Communications & Computer Networks, Second Edition

25. 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. A T-1 (or T1) service gives you a 1.544 Mbps rate and is used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world. Data Communications & Computer Networks, Second Edition

26. T-1 Service 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 1.544 Mbps of data (LAN to ISP connection). Data Communications & Computer Networks, Second Edition

27. T-1 Service A T-1service requires 4 wires, as opposed to a 2-wire telephone line. A T-1 can be either intra-LATA (local) which costs roughly $300-$700 per month, or inter-LATA (long distance) which can cost thousands of dollars per month. A customer may also be able to order a ¼ T-1 or a ½ T-1. Data Communications & Computer Networks, Second Edition

28. T-1 Service A 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 * 24 + 1 = 193 bits). 8000 frames per second * 193 bits per frame = 1.544 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). Data Communications & Computer Networks, Second Edition

29. 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. Data Communications & Computer Networks, Second Edition

30. ISDN 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. Data Communications & Computer Networks, Second Edition

31. ISDN 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? Data Communications & Computer Networks, Second Edition

32. Frame Relay 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. Data Communications & Computer Networks, Second Edition

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34. Frame Relay A connection between two endpoints is called a permanent virtual circuit (PVC). PVCs are created by the provider of the frame relay service. The 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. Data Communications & Computer Networks, Second Edition

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36. Frame Relay 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. Data Communications & Computer Networks, Second Edition

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39. Frame Relay The user and frame relay service would agree upon a committed information rate (CIR). The 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. Data Communications & Computer Networks, Second Edition

40. Frame Relay For 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 milleseconds. If the customer exceeds its CIR, and the network becomes congested, the customer’s frames may be discarded. Data Communications & Computer Networks, Second Edition

41. Frame Relay vs. the Internet Frame relay has many advantages over the Internet, including guaranteed throughput, minimum delay, and better security. The Internet has the advantage of being practically everywhere, cheaper, and simpler to create connections (no PVCs necessary). Data Communications & Computer Networks, Second Edition

42. Voice over Frame Relay (VoFR) Frame relay is now capable of supporting voice communications. The high transfer speeds of frame relay adequately support the needs of interactive voice. If a company requires multiple voice circuits, frame relay is an interesting solution. Data Communications & Computer Networks, Second Edition

43. Frame Relay Switched Virtual Circuits Frame relay can now also provide switched virtual circuits (SVC). An SVC can be created dynamically by the customer. Good for short-term connections. Data Communications & Computer Networks, Second Edition

44. Asynchronous Transfer Mode 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. Data Communications & Computer Networks, Second Edition

45. Asynchronous Transfer Mode 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. Data Communications & Computer Networks, Second Edition

46. Asynchronous Transfer Mode A less demanding service is variable bit rate (VBR). VBR can also support real time applications, as well as non-real time applications, but do not demand a constant bit stream. Available bit rate (ABR) is 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) is for lower rate traffic that may get held up, and may even be discarded part way through transmission if congestion occurs. Data Communications & Computer Networks, Second Edition

47. Asynchronous Transfer Mode Advantages of ATM include very high speeds and the different classes of service. Disadvantages include potentially high costs (both equipment and support) and a high level of complexity. Data Communications & Computer Networks, Second Edition

48. 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. And there has to be a DSL provider in your region. Data Communications & Computer Networks, Second Edition

49. Digital Subscriber Line 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. A DSL service can be symmetric, in which the downstream and upstream speeds are identical, or asymmetric in which the downstream speed is faster than the upstream speed. Data Communications & Computer Networks, Second Edition

50. Digital Subscriber Line 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. Data Communications & Computer Networks, Second Edition