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Presentation on theme: "OVERVIEW OF TELEPHONE NETWORK"— Presentation transcript:

What is a telephone and how does it work? How does the phone network work? How are calls routed in the phone network? Newer technologies: IVR, DOV.

2 Central office (CO) Hub of the city’s phone network.
Large city might have several central offices (Boulder: 492, 494, 499, 449, etc..) Central office: provides power for your phone routes your calls bills you Pair of wires from your home to CO is called the local loop.

3 Telephone instrument Rotary or pulse dialing:
Send as many pulses as the number being dialed, e.g.., 5 pulses for a five, 9 pulses for a nine, etc. Touch-tone or Dual tone multifrequency: Each row and column has a different tone associated with it. Two tones are produced simultaneously, associated with the corresponding row and column.

4 Touch Tone Telephone

5 Signals on telephone network
Off-hook signal Dial Tone Pulses or touch-tones Phone Company Central Office Ringback tone /busy signal Conversation Call waiting tone Flash signal On-hook signal

6 AT&T Telephone Network Hierarchy

7 The Phone Network

8 Hierarchical Routing Structure
Regional Center Sectional Center Primary Center Toll Center End Office Class 1 Class 2 Class 3 Class 4 Class 5 Most traffic is carried by the trunks at the regional and sectional levels. The trunks shown by dashed line represent redundant paths.

9 on the telephone network
Call routing methods on the telephone network Hierarchical routing: Only one fixed path for each call in the 5-level hierarchy, e.g.., Redundant links between different levels of the hierarchy provide alternate routes, but the network is still static.

10 on the telephone network
Call routing methods on the telephone network Dynamic routing: Several alternate paths. Selects route based on the current state of the network. Considers time of day, and day of week. Strategies Centralized versus distributed Time-dependent versus Adaptive.

11 J I Route d Route b Y Route a X Route c K Dynamic Routing Chicago
Seattle J I Route d Route b Y Route a New York X Route c Denver K Pittsburgh Dynamic Routing

12 Possible Routes from X to Y

13 Echo Suppression

14 Interactive voice response (IVR)
NEWER TECHNOLOGIES Interactive voice response (IVR) Provide specific information based on caller’s input (as opposed to simple routing): Credit card companies (Visa) Banks Parts ordering (dealerships, Avon, etc.) Brokerage companies (stock quotations) Automated fax back services IVR is another form of disintermediation

15 NEWER TECHNOLOGIES Data-over-voice
Data-over-voice: superimpose data and voice on the same copper wire. Separate and direct them properly. Digital Data over voice: similar to DOV, but in this case both data and voice are sent digitally. This is more reliable, and less error-prone.

16 Digital Data over Voice (DDOV) from Pacific Bell
Pacific Bell Digital Data Over Voice (DDOV) service allows the simultaneous transmission of digital data at speeds of 2.4, 4.8, 9.6 or 19.2 Kbps and voice frequency operation over the same two-wire exchange voice grade non-loaded loop facility. DVM: Data Voice Multiplexer CDVM: Central Office Data Voice Multiplexer. (Source: Pacific Bell)

17 New Services Most phone companies provide the following services:
Call forwarding, 3-way calling, voice mail, distinctive ring, call waiting, etc. Advanced services Unified messaging: Integration of fax, , voice mail) Number portability for cell phones: Allows subscribers to change service provider, location and type of service, and keep the same telephone number (coming!?). Five minutes: What are some other new services that you care about?

What is a Switch? Switching Principles? What is a PBX? What is a call center? What are special phone services?

19 Point to point lines If we could have point to point lines between all senders and all receivers, then we would not need switching.

20 Switching Imagine point to point lines between all pairs of people:
# of people # of lines million In general, for n people, # of lines = n(n-1) 162 mill. Hosts > 13 Quadrillion 2

21 The Good Old Days

22 The Even Better Present Time
A technician replaces a line card in the telephone switch. A card handles 24 telephones. Source:

23 Public Telephone Network
NY City Denver

24 Switch Bob Bob Alice Alice Jane Dick Dick John Beth Dave Dave
Lines or wires from everybody's house go into the switch When a call is placed, the switch creates a temporary link between these lines.

25 Types of switching CIRCUIT SWITCHING: creates a continuous,
dedicated path between sender and receiver. Must set up an end to end path before any data can be sent. PACKET SWITCHING: No dedicated path between sender and receiver. Examples:

26 Circuit versus Packet Switching
Circuit Switching Packet Switching Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

27 A simple switch

28 Circuit switching Space division switches: Signal paths are physically separate from one another (i.e. divided in space). Single-stage Multi-stage Time-division switches: Same path is shared by multiple streams (division in time) Modern switches are combination switches

29 Single Stage space Division Switch
Output Lines Single Stage space Division Switch 1 2 3 4 Input Lines 5 6 7 8 9 10 Crossbar Switch (also called Crosspoint switch)

30 Blocking Blocking means that even if two parties are free they cannot call one another because of congestion in the switch (“All circuits are busy, try later!!”). If one or both parties are busy, then it is not a case of blocking. Single stage, space division switches are non-blocking Multi-stage, space division switches are blocking Time-division switches are usually non-blocking.

31 Three Stage Space Division Switch
1 1 2 2 3 3 4 4 4 5 5 6 6 7 7 8 8 8 9 9 10 10 Three Stage Space Division Switch

32 Examples of Crossbar Switches
Number of crosspoints = Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

33 Time Division Switch 1 2 3 4 5 6 Control Logic
Memory contains the calls in progress. Control logic opens and closes gates. Each call gets a slot in a cycle. 1 cycle = 125 micro sec. 6 Control Memory 1-5 2-3 3-2 4-6 5-1 6-4 Control Logic

34 Time Division Switch

35 Both Time and Space Switching Needed
TSM N x k 1 N STS Both time and space switching is required in large (greater than 10,000) digital switching offices because with the current technology, a time slot interchange (TSI) cannot be built fast enough to handle more than lines. (There are also TSSST switches for larger number of channels.) Space Switch Inlet Memory N 1 2 TST STS – simpler control requirements TST – better for large switches with heavy load traffic (ie. No.4 ESS can service over 100,000 lines) (TSM: Time Switch Module) Source: Bellamy, John. Digital Telephony. 2nd Edition.

36 TSSST Switching Structure
TSM n x k (N/n) x (N/n) k x n TSM TSM TSM n x k (N/n) x (N/n) k x n TSM TSM TSM TSM Inlet time stage Outlet time stage Space stage Space stage Space stage Source: Bellamy, John. Digital Telephony. 2nd Edition.

37 PBX (Private Branch Exchange)
Switchboard for a company or university. First generation : late 1800s, manual operator Second generation: 1930, no operator required Second and half generation: 1970, Computerized Business Exchange. computer controlled switches programmable, more intelligent least cost routing of phone calls 3rd/4th generation: , AT&T Definity 75/85. “All digital” switching Both computer data and voice Simultaneous voice and data Non-blocking operation.

38 CENTREX A Centrex (Central Office Exchange Service) is similar to PBX, but owned and maintained by phone company. Cost is 20 to 50% higher than the cost of plain telephone lines.

39 Call Center More than $50 Billion business in North America.
Growing at 20% per year. A central place where customer calls are routed by an organization, usually with some amount of computer automation. Sophisticated Routing: Ability to handle a considerable volume of calls at the same time, to screen calls and forward them to someone qualified to handle them, and to log calls. Call centers are used by: mail-order catalog organizations, telemarketing companies, computer product help desks, and any large organization that uses the telephone to sell or service products and services. Achieve economies of scale associated with mass call handling and yet give callers a tailored feel.

40 Bus_136: IVR=Interactive Voice Response Advanced call routing Improve speed and quality of call based on information in the database, and input provided by caller (e.g., IVR). Examples: Skills based routing instead of “next-available-agent.” Key customers can move higher in the queue or be routed to agents assigned to their account. Callers who placed service calls within the last 24 hours can be routed to the agent they originally spoke to. Calls can be routed to a properly skilled agent, depending on the products or services previously purchased. A telemarketing group can route calls to agents with the highest closing ratios in order to increase sales revenues. Challenges: supporting multiple types of transactions; multiple skills.

41 Summary Switching is a very important telecommunications technology.
Switching technology has become very fast (nano-second speeds). Switches are essentially computers with millions of lines of software. Computer telephony integration (CTI) has made it possible to develop very sophisticated PBXs and call centers. Vendors are coming out with newer products and features every day in telephony equipment. The telephone system is moving closer to becoming all digital except for the local loop which is still analog.


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