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1 Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr. Modified by Sunantha Sodsee © 2008 The McGraw-Hill Companies 2 Telephones The telephone system The largest and most complex electronic communication system in the world. The primary purpose Provide voice communication. Widely used for Facsimile transmission Computer data transmission. © 2008 The McGraw-Hill Companies 3 Telephones The telephone system Full-duplex analog communication of voice signals. Telephone can connect with any other telephone in the world. Identification code Telephone number Country code + Subscriber numbers : +66 XXXX XXXX Trunk prefix + Subscriber numbers: 02 XXX XXXX Subscriber numbers: area code, local number © 2008 The McGraw-Hill Companies 4 Telephones The Local Loop Single central office 10,000 telephone lines can be connected The two-wire, twisted-pair connection Telephone and central office local loop or subscriber loop. © 2008 The McGraw-Hill Companies 5 Telephones Telephone Set Analog baseband transceiver Handset: a microphone and a speaker, transmitter and receiver. Ringer and a dialing mechanism ringer: bell or an electronic oscillator connected to a speaker. A switch hook a double-pole mechanical switch Dialing circuits : dual-tone multifrequency (DTMF) system. Hybrid circuit special transformer used to convert signals from the four wires from the transmitter and receiver into a signal suitable for a single two-line pair to the local loop. http://electronics.howstuffworks.com/telephone1.htm © 2008 The McGraw-Hill Companies 6 Telephones Basic telephone set. © 2008 The McGraw-Hill Companies 7 Telephones DTMF © 2008 The McGraw-Hill Companies 8 Telephones Standard Telephone and Local Loop Telephone wires: color coded: tip wire is green and usually connected to ground, and the ring wire is red. © 2008 The McGraw-Hill Companies 9 Telephones Subscriber interface. © 2008 The McGraw-Hill Companies 10 Telephone System Telephone Hierarchy a telephone call, your voice is connected through your local exchange to the telephone system. Several other facilities may provide switching, multiplexing, and other services required to transmit your voice. The telephone system is referred to as the public switched telephone network (PSTN). © 2008 The McGraw-Hill Companies 11 Telephone System © 2008 The McGraw-Hill Companies 12 Telephone System Trunk: A communications path between two switching systems Organization of the telephone system in the United States. © 2008 The McGraw-Hill Companies 13 Telephone System Private Telephone System Telephone service among the telephones in a company or organization The two basic types : Key systems Private branch exchanges © 2008 The McGraw-Hill Companies 14 Telephone System Private Telephone System: Key Systems serve 2–50 user telephones within an organization. individual telephone units called stations, all of which are connected to a central answering station. The central answering station is connected to one or more local loop lines, or trunks, back to the local exchange. The telephone sets in a key system typically have a group of pushbuttons that allow each phone to select two or more outgoing trunking lines. © 2008 The McGraw-Hill Companies 15 Telephone System Private Telephone System: Private Branch Exchange For larger organizations: thousands of individual telephones within an organization. private automatic branch exchanges (PABXs) computer branch exchanges (CPXs). Advantages of efficiency and cost reduction when many telephones are required. An alternative to PBX is Centrex. This service performs the function of a PBX but uses special equipment and special trunk lines. © 2008 The McGraw-Hill Companies 16 Telephone System A PBX. © 2008 The McGraw-Hill Companies 17 Circuit Switch © 2008 The McGraw-Hill Companies 18 Circuit-Switching PSTN is a circuit-switched network Circuit establishment Transfer of information point-to-point from endpoints to node internal switching/multiplexing among nodes Circuit disconnect Circuit switching is well suited for analog voice communications as in the telephone network. in-efficient for data networks due to its resource allocation nature data traffic is BAD © 2008 The McGraw-Hill Companies 19 Setting up a Path Before any data can be sent, the path between the caller and callee must be established. It can easily take 10 seconds to set up the path (more if its an international call). During this time interval, the switching equipment is searching for a ‘copper’ path through the network. © 2008 The McGraw-Hill Companies Advantages of Circuit Switching 20 The advantages are: For the duration of the call, the communicating computers have exclusive use of a connection. The full bandwidth of the connection can be used. Data can be sent at a constant rate (there are not unexpected delays and data arrives in the order it was sent). Circuit switching is also easier to administer, charge for and maintain. © 2008 The McGraw-Hill Companies Disadvantages of Circuit Switching 21 The disadvantages are: There is along delay while the circuit is set up and acknowledgement sent. The connection can be tapped (thus a potential security problem). No error checking or flow control is done by network, the computers must to it themselves. Traffic often consists of short bursts of data followed by long periods of inactivity (thus line utilization is low). © 2008 The McGraw-Hill Companies 22 Examples of Circuit Switching Public Switched Telephone Network – PSTN Telephone service carried by the PSTN is often called plain old telephone service (POTS). Private Automatic Branch Exchange – PABX Integrated Services Digital Network - ISDN © 2008 The McGraw-Hill Companies 23 POTS POTS standard telephone service that most homes use. The main distinctions between POTS and nonPOTS services speed and bandwidth. POTS : about 33.6 kbps (33,600 bits per second) (modem manufacturers : rates of 56.6 kbps). © 2008 The McGraw-Hill Companies Public Switched Telephone Network (PSTN) Elements Subscribers Local loop Connects subscriber to local telco exchange Exchanges Telco switching centers Also known as end office >19,000 in US 24 Trunks Connections between exchanges Carry multiple voice circuits using FDM or synchronous TDM Managed by IXCs (inter-exchange carriers) © 2008 The McGraw-Hill Companies 25 Telephone Network Structure 25 © 2008 The McGraw-Hill Companies 26 Circuit Switching Connection © 2008 The McGraw-Hill Companies 27 Signaling 1 Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook 2 Dial Tone 3 Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Audible Ring Answer Ringing Off-hook Disconnect © 2008 The McGraw-Hill Companies 28 Signaling 1 Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) 6 Dialed Digits Audible Ring Answer Ringing Off-hook Disconnect © 2008 The McGraw-Hill Companies 29 Signaling 1 Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) 6 7 Dialed Digits Audible Ring Answer Ringing 8 Off-hook Disconnect © 2008 The McGraw-Hill Companies 30 Signaling 1 Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 On-hook (wink) 6 7 Dialed Digits Audible Ring Answer 10 Disconnect Ringing 8 Off-hook 9 10 © 2008 The McGraw-Hill Companies 31 PABX PBX = Private Branch Exchange connect customer telephones (and related equipment) to LEC central office lines (trunks), and to switch internal calls within the customer's telephone system. Modern PBX numerous software-controlled features such as call forwarding and call pickup. A PBX uses technology similar to that used by a central office switch (on a smaller scale). (The acronym PBX originally stood for "Plug Board Exchange".) © 2008 The McGraw-Hill Companies 32 ISDN Integrated services digital network sending voice, video, and data over digital telephone lines. requires special metal wires and supports data transfer rates of 64 Kbps (64,000 bits per second). Most ISDN lines offered by telephone companies give you two lines at once, called B channels. one line for voice and the other for data, or both lines for data: data rates of 128 Kbps © 2008 The McGraw-Hill Companies 33 B-ISDN B-ISDN, broadband transmission support transmission rates of 1.5 million bits per second and higher. requires fiber optic cables It is not widely available. © 2008 The McGraw-Hill Companies Issues in Circuit Switched Networks Alternate Routing 34 Switch selects the best route for each call Routes listed in preference order Different sets of routes may be used at different times Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic) Need to use algorithms to determine paths dynamically, based on load/congestion vectors © 2008 The McGraw-Hill Companies 35 Alternate Routing © 2008 The McGraw-Hill Companies 36 Message Switching © 2008 The McGraw-Hill Companies 37 Message Switching message switching all the connections are permanently set up. Message header containing address of the source destination computer. routing information. Each message is sent to the local switching office that stores the message (checking it for errors) and then forwards it on to the next appropriate switching office (this technique is called store-and-forward). © 2008 The McGraw-Hill Companies Advantages of Message Switching 38 The advantages are: no waiting for setting up connections. Flow control and error checking Messages can be sent even when the receiving computer is not ready they can be stored until it is ready. © 2008 The McGraw-Hill Companies Disadvantages of Message Switching 39 The disadvantages are: no limit to the length of a message single message may block a link for a long time. If messages are too long, intermediate switching offices may not have sufficient memory to store them they cannot be passed on. © 2008 The McGraw-Hill Companies 40 Packet Switching © 2008 The McGraw-Hill Companies 41 Packet Switching Packet switching, like message switching, uses permanent connections. messages are broken up into smaller messages called packets (typically 512 bytes long). header containing Address routing information position in the original message. © 2008 The McGraw-Hill Companies 42 Packet Switching Packets are reassembled by the receiving computer to form the original message. Packet switching widespread in many computer networks and the internet. © 2008 The McGraw-Hill Companies Advantages of Packet Switching 43 The advantages are: take less time to transmit across links. less memory to store and forward. More secure because line taps will reveal only fragments of messages. © 2008 The McGraw-Hill Companies Disadvantages of Packet Switching 44 The disadvantages are: Packets may arrive at their destination out-of-order long delay while a small number of slow packets find their way through the network. It is not certain how long it will take a packet to pass through the network or how long to wait before deciding to request its retransmission). Packet switching is not ideal for supplying streams of data (as required for radio or T.V). © 2008 The McGraw-Hill Companies 45 Virtual Circuits Virtual circuit is a fixed path through a network establish when a call starts. Data is transmitted as packets. The packets follow the fixed path through the network. packets from other sources can share common links. The packets are guaranteed to arrive in the correct order. It is usually left to the receiving computer to ask for damaged or missing packets to be retransmitted this reduces the workload of the network and allows higher transfer rates in general. © 2008 The McGraw-Hill Companies 46 Virtual Circuits transmitting video and speech data occasional missing or damaged packets are ignored. file transfers. When a packet is lost, it’s absence is detected immediately because of the guaranteed order of packets. © 2008 The McGraw-Hill Companies 47 Crossbar Switches Several kinds of switches are (or were) common within the telephone system. The simplest kind is the crossbar switch (sometimes called a crosspoint switch). The switch has N inputs and N outputs for N full duplex lines. There are N2 intersections, called crosspoints. © 2008 The McGraw-Hill Companies 48 The connection is a direct electrical connection jumper Every line can be connected to every other line. © 2008 The McGraw-Hill Companies 49 Space Division Switches smaller connected crossbar switches. Theses are called space division switches. For example, if we had 16 lines, we could have four crossbar switches each taking 4 lines. The output of the crossbar switches can themselves be fed into crossbar switches. © 2008 The McGraw-Hill Companies 50 Space Division Switches Each stage of the space division switch is fully connected to the next stage. This means that an electrical connection can be made from any input to any output. © 2008 The McGraw-Hill Companies Pros and Cons of Space Division Switches 51 Because the space division switches use many smaller crossbar switches, if one fails it can be easily replaced without disrupting all the calls. it is possible for a Space Division Switch to be jammed i.e. a lot of calls had to go through one crossbar switch, all its input or output lines may be used up. Setting k=2n-1 will ensure this will not happen © 2008 The McGraw-Hill Companies 52 Time Division Switches the n inputs are scanned in sequence to build a frame with n slots. For T1 switches, the slots are 8 bits including 1 control bit. 8,000 frames are processed every second. © 2008 The McGraw-Hill Companies 53 Time Division Switches Each input is mapped (using an n word mapping table) to one of the n output lines. The slots are reordered so that they are sent to the correct output lines. © 2008 The McGraw-Hill Companies Finally….Advantages of Time Division Switches 54 Time Division Switches use digital technology. The number of switches involved (be they electronic gates) grows linearly with the number of inputs. The Time Division switch must, however, store and forward the n inputs within 1/8000 of a second(125 sec). © 2008 The McGraw-Hill Companies 55 Facsimile Facsimile, or fax, an electronic system for transmitting graphic information by wire or radio. Facsimile send printed material by scanning it converting it into electronic signals that modulate a carrier to be transmitted over the telephone lines. Since modulation is involved, fax transmission can also take place by radio. © 2008 The McGraw-Hill Companies 56 Facsimile Components of a facsimile system. © 2008 The McGraw-Hill Companies 57 Facsimile How Facsimile Works? High-tech electro-optical machine. Scanning is done electronically the scanned signal is converted into a binary signal. Digital transmission with standard modem techniques is used. image scanner that converts the document into hundreds of horizontal scan lines. all incorporate a photo- (light-) sensitive device to convert light variations along one scanned line into an electric voltage. The resulting signal is then processed in various ways to make the data smaller and faster to transmit. © 2008 The McGraw-Hill Companies 58 Facsimile How Facsimile Works? The signal is sent to a modem it modulates a carrier set to the middle of the telephone voice spectrum bandwidth. The signal is then transmitted over the public-switched telephone network. The receiving machine’s modem demodulates the signal processed to recover the original data. The data is decompressed and printed out. © 2008 The McGraw-Hill Companies 59 Facsimile Block diagram of modem fax machine. © 2008 The McGraw-Hill Companies 60 Facsimile Most fax machines use charge-coupled devices (CCDs) for scanning. A CCD is a light-sensitive semiconductor device that converts varying light amplitudes into an electrical signal. Data compression is a digital data processing technique that looks for redundancy in the transmitted signal. Every fax machine contains a built-in modem that is similar to a conventional data modem for computers. © 2008 The McGraw-Hill Companies