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Chapter 6

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  1. Chapter 6 Modern Telecommunications Systems

  2. Introduction • Telematique – the integration of computers and telecommunications systems • Computers are changing roles from computing machines into communications machines

  3. Telecommunications • The science and technology of communication by electronic transmission of impulses through telegraphy, cable, telephony, radio, or television either with or without physical media • Tele is Greek for distance • Communicate has its roots in the Latin word “to impart”

  4. Voice Networks • Interactive - Bidirectional networks that provide on-demand communication • The first telephone networks were deployed widely following World War II • By the late 1950s in the United States, telephones were a permanent fixture in most homes

  5. Circuit Switched Networks • Telephone networks use circuit switching that creates a complete, dedicated, end to end connection before voice data begins to flow • Circuit creation results in exclusive allocation of specific data transmission resources for the duration of the call

  6. Circuit Switching • Guarantees that each successful connection owns all the resources necessary to deliver a high quality link • When the call ends, the circuit is torn down, and the resources are freed; these resources can then be utilized for a new connection

  7. Switched Network • It is the capacity of the network to interconnect any two endpoints

  8. Legacy • The telephone network is one of the largest legacy systems ever created and maintained • Phone handsets over 50 years old can still interoperate seamlessly with current equipment • Some basic design specifications date back to the early 1900s

  9. Telephone Signals • Original telephone specifications were based on analog signal technology • Analog signals vary in amplitude (signal strength) and in frequency (pitch) • The telephone handset converts sound into continuously varying electrical signals with the microphone • The speaker at the other end converts electrical signals back to sound

  10. Analog Signal

  11. Digital Signals • These signals are discrete and discontinuous • They exist in predetermined states • Binary signals are digital signals limited to only two states, 0 and 1

  12. Digital Signal

  13. Multiplexing • Multiplexing is subdividing the physical media into two or more channels • Telephone lines use frequency multiplexing to carry both voice and DSL signals simultaneously • The frequencies between 0 and 4000 Hz carry voice, and those between 25 kHz and 1.5 MHz carry DSL

  14. Digitizing Voice Signals • By converting analog voice signals into a digital format, voice can then be processed like other digital data by computers • The economies of Moore’s law and semiconductor economics can be brought to bear on voice applications

  15. Pulse Amplitude and Pulse Code Modification

  16. Analog to Digital Conversion • Generally a two step process • First, the analog signal is sampled at regular intervals; measurements taken at these periods are converted to a discrete value • Second, the discrete values are converted to a binary format; this is called pulse code modulation

  17. Fidelity • Translating a signal from analog to digital format results in loss of data. By increasing the number of discrete values produced per second (sampling more often) and increasing the range of discrete values produced by sampling, the digitized waveform more closely represents the analog original. This is fidelity.

  18. Nyquist’s Theorem • A mathematical formula that will quantify the fidelity of the signal given the rate and resolution of sampling • For a 4000 Hz signal, fidelity will be acceptable if the signal is sampled 8000 times per second with a resolution of 8 bits per sample • A 4000 Hz signal is equivalent to a 64000 bit per second data stream

  19. The Digital Telephone • When a voice signal enters the local switch, it is digitized • The local switch is located physically close to the end users of the telephone line (usually within 10000 ft) • The switch is capable of handling 500 to 1000 copper lines • It is connected via high speed digital links back to the central office

  20. Central Office • Handles the telephone traffic for a number of small communities or a small city • Commonly central offices are responsible for 100000 lines

  21. Central Office Network Configuration

  22. Customer Premise Equipment • CPE is the device found at the customer termination of a telephone connection (fax, telephone, modem, etc.)

  23. Local Loop • Also known as the access line • Identified by the last four digits of the telephone number • It is the physical connection between the CPE and the local switch • The first three digits of a seven digit telephone number identify the local switch to the central office

  24. Local Switch • A local switch is a smart router. It can independently connect calls from any two lines terminating directly into it. • This helps to keep local calls confined to the local switch • It identifies and routes outbound calls quickly to the central office

  25. Topology • Topology is the configuration of elements in a network • The local exchange (local switch and all attached CPE and trunks) form a switched star network • This is an effective arrangement when most of the lines are idle at any one time • At peak hours 15% of a given set of lines are in use

  26. Regional Connections • A Central Office is connected to other Central Offices by high speed links; it also has connections to other higher level centers and long distance networks • These links in the US form a network of 150 million lines

  27. Regional Telephone Switching Networks

  28. Call Setup • When the handset is raised, the local switch issues a dial tone • When the user inputs the destination phone number, the local exchange uses it to set up the circuit • A leading 1 signals the local switch that the call is long distance and routes the call immediately to the Central Office

  29. T-Services • T-services are high speed digital links using time-division multiplexing (TDM) to move multiple signals • TDM successively allocates time segments on a transmission medium to different users • It combines multiple low speed streams into one high speed stream

  30. T-1 • The T-1 line is capable of carrying 1.544 Mbps • The T-1 frame is composed of 24 time slices. Each time slice is a channel. Each channel is capable of carrying one phone circuit.

  31. Time-Division Multiplexing and the T-1 Frame

  32. T-1 Frame • Multiplexing equipment aggregates the incoming individual channels and constructs a frame • Each channel can transmit 8 bits per frame • Each frame contains 24 channels and one “framing” or start bit • 8000 frames are transmitted per second yielding 1.544 Mbps

  33. The T-Service Hierarchy • The T-1 connection is composed of 24 channels called B channels • They are able to carry the digitized audio data for one voice circuit • A T-1 connection can carry 24 Bs • A T-3 connection can carry 672 Bs (45 Mbps)

  34. T-Services

  35. E-Services • Europeans use a slightly different standard called the E series • 8000 frames per second with each frame composed of 32 channels • Only 30 of the channels can be used for data, the other two are reserved for signaling information and signaling the framing start sequence • Carries 2.048 Mbps

  36. Corporate Use of T-Services • T-services are available to customers • T-lines can be configured to create a high speed private point-to-point network • Internally, data and voice can be mixed, so that a T-1 line can be provisioned to carry 12 voice circuits and 12 data circuits • T-1s allow rapid connection of fixed locations with high speed private links

  37. Data Communication Networks • Voice networks have hard requirements for network latency (the amount of time needed for data to move from one end to the other) • Data that arrives late or out of order is worthless • Pure data networks have looser time constraints opening the door to different topologies and technologies

  38. Packet Switching • In traditional voice networks, circuits are established that provide for a continuous stream of data; packet switching takes outgoing data and aggregates it into segments called packets • Packets carry up to 1500 bytes at a time • Packets have a header prepended onto the front of the packet that contains the destination address and sequence number

  39. Packet Routing • In circuit switched networks, the entire data pathway is created before data transmission commences; in packet networks, the packet travels from router to router across the network • At each router, the next hop is chosen, slowly advancing the packet toward its destination

  40. Packet Routing • Given moment to moment changes in network loading and connections, packets may or may not take the same route • In taking different routes, packets may arrive in a different order than the order they were transmitted • The destination uses the sequence number in the header to reassemble the incoming data in the correct order

  41. Local Area Networking • Until the 1990s, local area networking used vendor specific protocols that made interoperability difficult • With widespread deployment of personal computers, networking to the desktop became more imperative for companies, so that they could fully leverage their IT infrastructure investments

  42. Metcalfe’s Law • Robert Metcalfe is the patent holder for Ethernet networking • He asserted that the value of a network increases as a square function to the number of attached nodes

  43. OSI Model • OSI was the Open System Interconnection model that attempted to modularize and compartmentalize networking interfaces • The result was a seven layer model • As data passes down from layer 7 to layer 1 it is broken into smaller pieces and encapsulated with wrappers of additional information used at the corresponding layer by the recipient to reconstruct the original data and destination

  44. Open System Interconnection Model

  45. OSI is a Model • OSI was intended to be the final structure and framework for global networking • Widespread implementation of the entire OSI model has never taken place • It took years to develop • It was the product of a committee • It was extremely rigid

  46. ARPANET • In the early 1970s, the Department of Defense saw the need to make heterogeneous networks of information systems communicate seamlessly • They needed networks that were self healing and had a distributed intelligence • ARPA (Advanced Research Projects Agency) took the OSI layering concept and built an operational system with layers 3, 4, and 5 only

  47. The Internet • From this nucleus of networked machines grew the Internet • ARPA called the OSI layer 4 protocol TCP (Transmission Control Protocol) and layer 3 IP (Internet Protocol), hence the Internet networking standard TCP/IP • This has become the de facto global standard, and OSI has been relegated to a reference model

  48. Internetworking Technology • The Internet Protocol Suite is a group of helper applications that standardizes interactions between systems and assists users in navigating the Internet • These helper applications work at many different levels of the OSI model from seven all the way down to two

  49. Internet Protocol Suite • Layer seven applications include • FTP – File Transfer Protocol • HTTP – HyperText Transfer Protocol • SMTP – Simple Mail Transfer Protocol • Layer two protocols include • ARP – Address Resolution Protocol

  50. Internet Protocol • The Layer three protocol is responsible for the standard dotted decimal notation used for computer addressing • Each machine has a unique address specified by a set of four numbers ranging from 0 to 255 • These numbers are separated by decimal points in the format 216.39.202.114