1 Dar es Salaam institute of Technology (DIT) ETT 06208 Communication Systems

2 DIT Introduction to Telecommunications

3 DIT What is Telecommunication? Technology that enables Multi-media Communication between subscribers located anywhere, at anytime. Deals with Communication of Voice / Data / Video Message signals from Source to Destination at acceptable quality levels, using various Wireline//Wireless Transmission Media. Viewed as Infrastructure like Power, Roads, Ports, Transport etc.

4 DIT Sections of Telecommunications

5 DIT Significance of Telecommunications Telecommunications networks make up the most complicated equipment in the world. Telecommunications services have an essential impact on the development of a community. The operations of a modern community are highly dependent on telecommunications. Telecommunications plays an essential role on many areas of everyday living.

6 DIT Examples of Telecommunication Services Banking, automatic teller machines, telebanking Aviation, booking of tickets Sales, wholesale and order handling Credit card payments Booking of hotel rooms by travel agencies Material purchasing by industry Government operations, such as taxation.

7 DIT Major Milestones in Telecom Evolution Invention of Telegraph & Telephone Instruments Birth of Telephone Exchanges & Wireline Communication Advent of Wireless Communication Establishment of Long Distance Communication Networks (Submarine Cables / Fiber Optics / Microwave / Satcom) Use of Computers in Electronic Exchanges Advances in DSP / Data Communication & Convergence Evolution of Internet & World Wide Web Mobile Communication Services (GSM & CDMA) expand with focus on Introduction of Broadband Services, improving Teledensity / QOS and Cost reduction. New Technologies such as ATM / GbE, IP / MPLS, Optical Networking, 3G / 4G Mobile, Wi-Fi / Wi-MAX, VoIP, IPTV etc emerging.

8 DIT Electromagnetic Spectrum (Frequency Bands) Wavelength = Velocity of Light (c) / Frequency (f )  RF ( UHF) -Wireless Medium for Mobile communication.  Infrared (IR) – For Fiber Optic Core Transmission Networks.

9 DIT Basic Telecommunications Network The basic purpose of a telecommunications network is to transmit user information in any form to another user of the network. These users of public networks, for example, a telephone network, are called subscribers. User information may take many forms, such as voice or data, and subscribers may use different access network technologies to access the network, for example, fixed or cellular telephones. The telecommunications network consists of many different networks providing different services, such as data, fixed, or cellular telephony service.

10 DIT Elements of Communication Switching System The purpose of a telecommunication switching system is to provide the means to pass information from any terminal device to any other terminal device selected by the originator. Telecommunication system can be divided into four main parts.  End system or Instruments  Transmission system  Switching system  Signaling.

11 DIT End Systems or Instruments The end system or instruments are a transmitter or receiver that are responsible for sending information or decoding or inverting received information or message into an intelligible message. End systems in the telephone network have evolved from analog telephones to digital handsets and cellular phones. However, endless arrays of other devices are being attached to telephone lines, including computer terminals used for data transmission.

12 DIT Transmission System Transmission is the process of transporting information between end points of a system or a network. Transmission systems use four basic media for information transfer from one point to another:  Copper cables, such as those used in LANs and telephone subscriber lines.  Optical fiber cables, such as high-data-rate transmission in telecommunications networks.  Radio waves, such as cellular telephones and satellite transmission.  Free-space optics, such as infrared remote controllers. In a telecommunications network, the transmission systems interconnect exchanges and, taken together, these transmission systems are called the transmission or transport network. Note that the number of speech channels (which is one measure of transmission capacity) needed between exchanges is much smaller than the number of subscribers because only a small fraction of them have calls connected at the same time.

13 DIT Copper Cables Copper cable is the oldest and most common transmission media. Its main disadvantages are high attenuation and sensibility to electrical interference. Attenuation in copper cable increases with frequency approximately according to the following formula: where A dB is attenuation in decibels, f is the frequency, and k is a constant specific for each cable. This formula gives us approximate attenuation at other frequencies if the attenuation at one frequency is known. The speed of signal propagation in a copper cable is approximately 200,000 km/sec. The three main types of copper cables are: twisted pair, open-wire lines, and coaxial cable.

14 DIT Twisted Pair A twisted pair consists of two insulated copper wires that are typically 0.4 to 0.6 mm thick or about 1 mm thick if insulation is included. These two wires are twisted together to reduce external electrical interference and interference from one pair to another in the same cable. The twisted pair is symmetrical and the difference in voltage (or to be more accurate, electromagnetic wave) between these two wires contains the transmitted signal. Twisted pair is easy to install, requires little space, and does not cost a lot. Twisted pairs are used in the telecommunications networks in subscriber lines, in 2-Mbps digital transmissions with distances up to 2 km between repeaters, in DSLs up to several megabits per second, and in short-haul data transmissions up to 100 Mbps in LANs. Unshielded twisted pair (UTP) cables used in LANs are categorized as UTP Cat 3, 4, and 5. Cat 3 is a voice-grade cable designed for voice frequency applications, such as local loops. The characteristics of Cat 5 cable are specified up to a 100-MHz frequency and they are suitable for high-speed LANs operating at 100 Mbps or 1 Gbps.

15 DIT Open-Wire Lines The oldest and simplest form of a two-wire line uses bare conductors suspended at pole tops. The wires must not touch each other, otherwise short circuit occurs in the line and communication will be interrupted. New open-wire lines are rarely installed today but they are still in use in rural areas as subscriber lines or analog carrier systems with a small number of speech channels.

16 DIT Coaxial Cable In a coaxial cable, stiff copper wire makes up the core, which is surrounded by insulating material. The insulator is encased by a cylindrical conductor. The outer conductor is covered in protective plastic sheath. The construction of the coaxial cable gives a good combination of high bandwidth and excellent noise immunity. Coaxial cables are used in LANs (original 10-Mbps Ethernet), in antenna systems for broadcast radio and TV, and in high capacity analog and digital transmission systems in telecommunications networks and even in older generation submarine systems.

17 DIT Applications of Copper Cables

18 DIT Optical Fiber Cables Purpose:  Propagation of Light waves carrying electrical signals. Function:  Transmission of Electrical signals through light waves in a glass medium. Application:  For Broadband & Long distance communication. Telephone Trunk circuits / Backbone Networks, WAN, CATV Networks, FTTH etc. Submarine Cables provide International Broadband Communication. Technology:  Uses Total Internal Reflection inside an Optical Fiber, consisting essentially of inner Core & Cladding outside; Refractive Index, μ, decreases from Core to Cladding, gradually or abruptly. Fiber Types:  Single Mode Smaller Core dia) for long haul applications.  Multimode (Larger Core dia) for shorter distance coverage; Step Index & Graded Index versions available.

19 DIT Optical Fibers Cables (2) Advantages  High transmission capacity  Low cost  Tolerance against external interference  Small size and low weight  Unlimited material resource  Low attenuation Disadvantages  Optical fibers is more difficult to install than copper cables  Radiation of light from a broken fiber may cause damage to the human eye

20 DIT Radio Transmission The most important advantage of radio transmission over cable transmission is that it does not require any physical medium. One important factor that restricts the use of radio transmissions is the shortage of frequency bands. The most suitable frequencies are already occupied and there are many systems with a growing demand for wider frequency bands. Examples of other systems using radio waves are public cellular systems, professional mobile radio systems, cordless telephones, broadcast radio and TV, satellite communications, and WLANs. The use of radio frequencies is regulated by the ITU-R at the global level and, for example, by ETSI at the European level, FCC in the United States, and TCRA in Tanzania. To implement a radio system, permission from a national telecommunications authority is required.

21 DIT Wireless Transmission Electromagnetic Waves traveling in free space with velocity of light, are used as wireless media. Message signals superimposed on carrier signals (Transverse EM Waves) are radiated by antennas for transmission and reception. Frequency Bands (Radio Frequency (RF), Microwave, Infra Red (IR) etc.) are allocated for various applications ( Fixed & Mobile).

22 DIT Satellite Transmission In satellite communications a microwave repeater is located in a space. An Earth station transmits to the satellite at one frequency band and the satellite regenerates and transmits the signal back at another frequency band. The frequencies allocated by ITU for satellite communications are in the frequency range of 1 to 30 GHz. Figure below illustrates point-to-point transmission with the help of a geostationary or geosynchronous satellite using the 6/4-GHz satellite band.

23 DIT Switching System In principle, all telephones could still be connected to each other by cables as they were in the very beginning of the history of telephony. However, as the number of telephones grew, operators soon noticed that it was necessary to switch signals from one wire to another. Then only a few cable connections were needed between exchanges because the number of simultaneously ongoing calls is much smaller than the number of telephones. The first switches were not automatic so switching was done manually using a switchboard. Modern exchanges usually have quite a large capacity—tens of thousands subscribers—and thousands of them may have calls ongoing at the same time.

24 DIT A Basic Telecommunications Network

25 DIT Signaling Signaling is the mechanism that allows network entities (customer premises or network switches) to establish, maintain, and terminate sessions in a network. Signaling is carried out with the help of specific signals or messages that indicate to the other end what is requested of it by this connection. Some examples of signaling examples on subscriber lines are as follows:  Off-hook condition: The exchange notices that the subscriber has raised the telephone hook (dc loop is connected) and gives a dial tone to the subscriber.  Dial: The subscriber dials digits and they are received by the exchange.  On-hook condition: The exchange notices that the subscriber has finished the call (subscriber loop is disconnected), clears the connection, and stops billing.

26 DIT Telephone Calls-The “Traditional” Way When you place a call, the circuit-switched network dedicated 64Kbps circuit for the duration of your call. This means that if you are calling from Tanga to Mwanza, a dedicated circuit is set up from one end of the line to the other. When you talk, 64Kbps of bandwidth is utilized, and when you are silent, you still consume 64Kbps. No matter what you do, you are trying up 64Kbps as long as you remain connected. If a switch goes down or someone cuts a fiber, your calls ends. This is not very efficient or cost effective. A driving force behind VoIP network is the desire to reduce or eliminate such bandwidth waste and expense for calls.

27 DIT Basic Telephone Telephone systems convert sound waves, vibrations that move in the air, into electrical signals. Each telephone handset contains a transmitter covered by a diaphragm and a receiver composed of a coil attached to a speaker cone that vibrates, producing sound waves. When the telephone handset is lifted to make a call, the switchhook contacts are closed, energizing a relay, which initiates a search for an open line. A connection to the telephone central office (CO) is established and a dial tone is established. The line finder then prepares the telephone company switching equipment to receive a telephone number. Standard telephone cabling is a single pair of twisted-pair copper wiring. The telephone network termination point is an RJ-11 jack. The two connection has two pins, designed tip and ring.

28 DIT The Process of Making a Call 1. Both telephone sets are on hook with circuits open. 2. Lifting the handset (off hook) closes the switch and causes current to flow, which sends a signal to the CO, generating a dial tone. 3. Customer dials a destination telephone number, which are received in band by the CO switch. 4. Destination CO sends a ringing signal to the destination receiver and to the sender to indicate that the call request has been completed and the call is proceeding. 5. When the call is answered, another signal is sent to the CO to stop ringing and start the accounting process. The time it takes for the CO to complete the connection between the person calling and person called is referred to as call setup time. 6. Hanging up the phone (on hook) opens the switch, which stops current and dial tone.

29 DIT Public Switched Telecommunications Network

30 DIT Explanation The figure presents a simplified diagram of a regional or national Public Switched Telephone Network (PSTN) that has connections to the global Internet and PSTN. The network contains the Public Land Mobile Network (PLMN), which provides wireless access for cellular subscribers and is connected to the PSTN/ISDN network at the trunk exchange level. Internet users are connected to the global Internet via the hosts of their ISPs. Networks of national ISPs are connected and this interconnection is extended to the networks of ISPs of neighboring countries, and these networks together make up the global Internet. A telephone or ISDN network is used for dial-up connections and ADSL provides permanent higher rate Internet service. Digital PBX is connected to a local exchange with a 1,544/2,048-Kbps digital line that has the capacity of 23/30 simultaneous calls. This connection is called the primary rate interface in the case of ISDN. PBX is a dedicated small exchange that provides telephone service to the personnel of a company. Analog PBX uses analog telephone lines, one for each simultaneous external call. Each analog line (twisted pair) carries one telephone call with signaling.

31 DIT Explanation The corporate-wide PBX/PABX service can also be implemented without any equipment investments in the company, that is, without physical PABX equipment. Network operators provide a service called Centrex and for that the public exchange is programmed to behave as a PBX/PABX. One of the subscriber lines is set to operate as a switchboard line and the others make up a user group with abbreviated dialing and other PBX/PABX services. For data communication via an analog network or digital network with analog subscriber interfaces, a modem is required. The term modem comes from modulator/demodulator and it transmits data through a speech channel in voice frequency tones. If a subscriber has ISDN service, which is fully digital, no modem is needed and an end-to-end bidirectional 64- or 128-Kbps digital circuit is available with the help of a network terminal that takes care of the digital bidirectional transmission over the subscriber loop. For active Internet users who require continuous connection or higher data rates, circuit-switched services are expensive because the cost is based on the duration of the call and they do not provide high enough performance. An attractive access method for these types of users is ADSL, which provides data rates up to a few megabits per second with a fixed monthly fee. One office site of a company has high-data-rate access to its ISP. All employees have access to the Internet via the company’s private LAN. Leased lines, which interconnect two offices, are often the most economical solution for high-data-rate circuits that are needed, for example, for LAN interconnections.

32 DIT PSTN Architecture ( Exchange Hierarchy) PSTN: A wide area, circuit switched, partially connected mesh network of star connected sub networks.

33 DIT Basic Signal Concept Voice Signal  Analog (continuous)  Low bandwidth (15 KHz Band limited to 4KHz for conventional telephony)  Transmission can be Analog or Digital. Data Signal  Digital (discrete), Bursty  Low & High Speed; PC / LAN / FAX  Variable bandwidth (Few bps to 10 Gbps & beyond)  Transmission can be Analog or Digital Video Signal  Analog (continuous)  High bandwidth (5 MHz Typical)  Transmission can be Analog or Digital

34 DIT Thanks! Technology changes but communication lasts.