Principles of Electronic Communication Systems Third Edition Louis E. Frenzel, Jr. Modified by Sunantha Sodsee

Telephones The telephone system The primary purpose Widely used for 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.

Telephones The telephone system Identification code 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

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.

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

Telephones Basic telephone set.

Telephones DTMF

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.

Telephones Subscriber interface.

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).

Telephone System

Telephone System Trunk: A communications path between two switching systems Organization of the telephone system in the United States.

Telephone System Private Telephone System Telephone service among the telephones in a company or organization The two basic types : Key systems Private branch exchanges

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.

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.

Telephone System A PBX.

Circuit Switch

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

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.

Advantages of Circuit Switching 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.

Disadvantages of Circuit Switching 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).

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

POTS POTS The main distinctions between POTS and non-POTS services standard telephone service that most homes use. The main distinctions between POTS and non-POTS services speed and bandwidth. POTS : about 33.6 kbps (33,600 bits per second) (modem manufacturers : rates of 56.6 kbps).

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 Trunks Connections between exchanges Carry multiple voice circuits using FDM or synchronous TDM Managed by IXCs (inter-exchange carriers)

Telephone Network Structure 25

Circuit Switching Connection

Originating Switching Office Terminating Switching Office Student Notes Global Wireless Education Consortium  July, 2001 Signaling Originating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits Off-hook Off-hook (wink) Signaling Stages A typical telephone call requires several stages of signaling before a conversation can begin. The idle state of the CPE is a supervisory state called on-hook. In the on-hook state a customer line will be connected to the common voltage supplied by the switching office. The most common line voltage is –48v. The –48v is the historical nominal voltage supplied by the switching office. However, depending on the operating state of the power supply in the switching office the power supplied to a customer’s line may vary from –42.5v to –56v. In the on-hook state the customer’s line appears as a high resistance, essentially an open circuit. To originate a call the customer must change the supervisory state to off-hook. Off-hook occurs when the CPE closes the customer loop back to the switching office causing current flow in the loop. The off-hook state is also called line seizure. In response to the off-hook the switching office returns an information signal called dial tone giving the customer the indication to start dialing. On-hook (wink) Dialed Digits Audible Ring Ringing Answer Off-hook Disconnect S-PSTN Page 27

Signaling 1 2 3 4 5 6 Student Notes Global Wireless Education Consortium  July, 2001 Signaling Originating Switching Office Terminating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 Signaling Stages (Continued) The next stage of signaling is the addressing stage when the customer sends the dialed number to the switching office. The most common method is with a series of tones referred to as Dual Tone Multi Frequency (DTMF). The DTMF tones are generated by the dialing keypad on the CPE, each dialed digit sending a combination of two tones to the switching office. Still in use today is another older method called dial pulse. With dial pulse signaling a series of opens and shorts are returned to the switching office by the CPE. Each digit dialed would send a corresponding number of opens and shorts, or pulses to the switching system. At the switching office the addressing continues as the dialed digits are collected and translated to switch the call to the serving switching office of the dialed number. The originating switching office selects an available trunk to the terminating switching office and sends a supervisory off-hook condition on the selected trunk. The terminating switching office will return a “wink” back to the originating switching office which consists of a short off-hook returning to on-hook. The wink serves as an indication to the originating switching office to send the dialed digits. On-hook (wink) Dialed Digits Audible Ring Ringing Answer Off-hook Disconnect S-PSTN Page 28

Signaling 1 2 3 4 5 6 7 8 Student Notes Global Wireless Education Consortium  July, 2001 Signaling Originating Switching Office Terminating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 Signaling Stages (Continued) In response to the wink the originating switching office sends the dialed digits to the terminating switching office. There are two types of signaling used for inter-office signaling, inband and out-of-band signaling. Inband signaling sends the signals on the same transmission path that will be used for the call. Multi Frequency (MF) signaling is the most common method of sending inband signals by using a combination of two frequencies for every digit sent over the transmission path. Out-of-band signaling uses a dedicated data link transmission path to send signaling information separate from the transmission path used for the call. Inband signaling will be used for the remainder of this call example. Upon receiving the dialed digits the terminating switching office will translate them to customer’s number receiving the call and check the busy/idle status of the called customer’s line. If the called customer’s line is busy the terminating switching office will return busy tone to the originating switching office. If the called line is idle the terminating switching office will return audible ringing tone back to the originating switching office and also send ringing signal over the customer loop to the called customer’s CPE. The ringing signal is a-84vrms to –104vrms 20Hz signal superimposed on the nominal –48v on the customer loop. On-hook (wink) Dialed Digits 7 Audible Ring Ringing 8 Answer Off-hook Disconnect S-PSTN Page 29

Originating Switching Office Terminating Switching Office Student Notes Global Wireless Education Consortium  July, 2001 Signaling Originating Switching Office Terminating Switching Office Originating CPE Terminating CPE Idle 1 Off-hook 2 Dial Tone 3 Dialed Digits 4 5 Off-hook Off-hook (wink) 6 Signaling Stages (Continued) When the called customer answers the call it will send an off-hook signal back to the terminating switching office. The terminating switching office will disconnect both the ringing signal to the called customer and the audible ring to the the originating switching office. The terminating switching office will then send an answer signal back to the originating switching office and connect the call through to the called customer’s CPE. When the connection is released by either customer on-hook signals will be sent from the serving switching office and the connection will be disconnected. On-hook (wink) Dialed Digits 7 Audible Ring Ringing 8 Answer Off-hook 9 Disconnect 10 S-PSTN Page 30

PABX PBX = Private Branch Exchange Modern PBX 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".)

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

B-ISDN B-ISDN, It is not widely available. broadband transmission support transmission rates of 1.5 million bits per second and higher. requires fiber optic cables It is not widely available.

Issues in Circuit Switched Networks Alternate Routing 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

Alternate Routing

Message Switching

Message Switching message switching Message 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).

Advantages of Message Switching 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.

Disadvantages of Message Switching 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.

Packet Switching

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.

Packet Switching Packets are reassembled by the receiving computer to form the original message. Packet switching widespread in many computer networks and the internet.

Advantages of Packet Switching 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.

Disadvantages of Packet Switching 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).

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.

Virtual Circuits transmitting video and speech data file transfers. 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.

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.

The connection is a direct electrical connection jumper Every line can be connected to every other line.

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.

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.

Pros and Cons of Space Division Switches 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

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.

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.

Finally….Advantages of Time Division Switches 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).

Facsimile Facsimile, or fax, Facsimile 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.

Facsimile Components of a facsimile system.

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.

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.

Facsimile Block diagram of modem fax machine.

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.