Principles of Electronic Communication Systems

1. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr. Modified by Sunantha Sodsee

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.

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

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.

5. Telephones Telephone Set • Analog baseband transceiver • Handset: a microphone and a speaker, transmitterand 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

6. Telephones Basic telephone set.

7. Telephones • DTMF

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.

9. Telephones Subscriber interface.

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).

11. Telephone System

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

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

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.

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.

16. Telephone System A PBX.

17. Circuit Switch

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

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.

20. 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.

21. 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).

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

23. POTS • POTS • 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).

24. 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) Public Switched Telephone Network (PSTN) Elements

25. Telephone Network Structure

26. Circuit Switching Connection

27. 1 2 3 Signaling Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

28. 1 2 3 4 5 6 6 Signaling Terminating Switching Office Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

29. 1 2 3 4 5 6 6 7 8 Signaling Terminating Switching Office Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

30. 1 2 3 4 5 6 6 7 8 9 10 10 Signaling Terminating Switching Office Originating Switching Office Originating CPE Terminating CPE Idle Off-hook Dial Tone Dialed Digits Off-hook Off-hook (wink) On-hook (wink) Dialed Digits Ringing Audible Ring Answer Off-hook Disconnect

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".)

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

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.

34. 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

35. Alternate Routing

36. Message Switching

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).

38. 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.

39. 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.

40. Packet Switching

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.

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.

43. 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.

44. 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).

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.

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.

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.

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

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.

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.