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COM347J1 Networks and Data Communications Lecture 3: The Physical layer Ian McCrum Room 5D03B Tel: 90 366364 voice mail on 6th ring Email: [email protected] Web site: http://www.eej.ulst.ac.uk 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/1/61 Today physical media: • • • • • Serial and Parallel connections Connectors Cables Coaxial, twisted pair Optical fibers Radio waves 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/2/61 Modes of serial data transfer • Simplex communications – Unidirectional data path from transmitter to receiver in the manner of radio broadcasts • Half Duplex – Unidirectional at any one time in the manner of a conversation over radio link with change of direction signaled by ‘over’. • Full Duplex – two computers using two comms channels one for transmission and one for reception both working simultaneously. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/3/61 Parallel data transfer • Most data in the form of bytes or wider. – Transfer all of the bits at the same time however one conductor for each bit, more copper etc. suitable for short distances and very high data rates, used inside computer where groups of conductors are called busses . – synchronisation between each bit on different conductors becomes difficult specially as distance increases due to tiny differences between conductors and their environment. Start End Transmission --> 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/4/61 Serial slower but cheaper Serial-to-parallel conversion Parallel data in Transmit Buffer Register Control Signals Serial data out Transmit Data Register Control Control Parallel data link Unit Signals Serial data in Receive Data Register Control Signals Parallel data link Receive Buffer Register Parallel data out 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/5/61 Connectors and cables • Standards… often specify details • D-type 25way used for RS232 serial links in old days (and in the “official standard”) Modern usage dictated by PC design … 9 pin Dtype connector – consider computer- modem cable with straight through cable connecting DTE and DCE. Necessary because uni-directional line drivers all that were available in the old days… • RJ45 – telephone type connectors. • Ribbon Cables and IDC connectors • Network connectors and cables 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/6/61 Cables for data transmission Twisted pair PVC/Teflon Braid Foil Braid Insulation Tin-plated solid copper core Centre conductor Dielectric Jacket 29/09/04 Electrostatic shielding www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/7/61 Typical Coaxial connection T-piece Segment Connector Network interface Transceiver circuit Terminator 10base2 Cable Terminator Maximum cable length 200m 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/8/61 Benefits of coaxial and Twisted pair • Shielding against induced noise. • Common mode rejection. • Speeds of each (cat 5e 100m bits/sec) 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/9/61 Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/10/61 Coaxial Cable A coaxial cable. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/11/61 Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/12/61 Fiber Optic Networks A fiber optic ring with active repeaters. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/13/61 Fibre optic cable is available in three basic forms: 1. Stepped-index fibre. In this type of fibre, the core has a uniform refractive index throughout. This generally has a core diameter of to . This is a multi-mode fibre. Stepped-index fibre 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/14/61 Graded-index fibre. In this type of fibre, the core has a refractive index that gradually decreases as the distance from the centre of the fibre increases. This generally has a core diameter of . This is a multi-mode fibre. Graded-index fibre 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/15/61 Mono-mode fibre. As the name suggests, the distinguishing characteristic of this fibre is that allows only a single ray path. The radius of the core of this type of fibre is much less than that of the other two, however it does have a uniform refractive index. From, 1 to 3, we find that the cost of production increases, the complexity of transmitter and receiver increases, while the dispersion decreases. This latter property change means that the mono-fibre also has the potential to provide greater bandwidth. As it becomes cheaper to produce mono-mode fibre technology, we will see an increased use of this type of optical fibre 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/16/61 Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/17/61 Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/18/61 Fiber Cables A comparison of semiconductor diodes and LEDs as light sources. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/19/61 Optical fibre is a waveguide. The fibre (in its simplest form) consists of a core of glass of one refractive index, and a cladding of a slightly lower refractive index (Figure ). The fibre is then surrounded by a refractive sheath. Typical fibre dimensions are to diameter. The basic structure of a fibre optic waveguide 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/20/61 In simple terms, the action of a waveguide can be partially understood by considering the rays down the fibre. A light-wave entering the fibre is either refracted into the cladding, and attenuated, or is totally internally reflected at the core/cladding boundary. In this manner it travels along the length of the fibre. The maximum angle at which it may enter the guide and travel by total internal reflection is termed the acceptance angle It is also possible for the wave to follow a helical path down the guide. These rays are called skew-rays. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/21/61 However, this view is too simple to explain all features of waveguide behaviour. In fact, it is not possible for the wave to take any ray down the guide. Only certain rays can be taken. These rays are called modes. For any particular frequency, there is a different ray. The modal action of a waveguide is a consequence of the wave nature of the radiation. A mono-mode fibre is a fibre that only has one acceptable ray-path per frequency. A multi-mode fibre has a number of possible rays that light of a particular frequency may take. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/22/61 Snell’s Law n1 y 1 1 n1 n2 n2 2 1 Sin 1 Sin 29/09/04 2 = 2 n n2 n1 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/23/61 Total Internal Reflection n1 y 1 1 2 n2 as then 29/09/04 1 n1 2 Sin1 n 2 Sin 2 n1 n2 1 2 n Cos1 n 2 Cos 2 n1 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/24/61 From the diagram n1 is greater than n2 so 2 decreases as until as 1 2 1 , for a finite value of 1 decreases 2 0 1 . is now the critical angle c beyond which Total Internal Reflection occurs and n2 c Cos n1 1 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/25/61 Light Acceptance cone y n2 n2 n1 c n1 m 1 as then n2 n1 c Cos 1 2 n Sin c 1 m Sin 1 when Snell is applied therefore the light acceptance cone is 2 m 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/26/61 n Propagation of light by total internal refection n2 n1 c a c l See attenuation profile Fig 2.6 A.T. and then Fig 2.7 for fibre construction 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/27/61 Copper v F.O. • repeaters 5km • reactive • E.M. R.F. problems • bulky • tappable 29/09/04 • repeaters 30km • relatively inert • no E.M. R.F. problems • >bandwidth in duct • no tapping www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/28/61 Wireless Tx c f • Wavelength* frequency = speed of light • therefore Atlantic 252 where the 252 refers to the frequency in kilohertz .. leads to the wavelength being 1190m long where the speed of light is taken to be 300,000,000 m/s 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/29/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/30/61 Variety see Fig 2.11 for spectrum • Radio VLF,LW,MW 9kHz bandwidth, long dist, earth hugging Fig 2.12 • Radio HF,VHF various bandwidths, straight lines and ionosphere bounce up to 60MHz • Microwave line of sight, large bandwidths (418MHz) • Infra Red line of sight, good for LAN in rooms • Light - building to building good bandwidth Fig 2.13 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/31/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/32/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/33/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/34/61 Communication Satellites • • • • 29/09/04 Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/35/61 Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/36/61 Communication Satellites (2) The principal satellite bands. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/37/61 Communication Satellites (3) VSATs using a hub. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/38/61 Globalstar (a) Relaying in space. (b) Relaying on the ground. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/39/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/40/61 Telephone system for data comms: • Why telephone system for data communications • Structure of PSTN • How it can carry digital data 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/41/61 Public Switched Telephone Network • It exists everywhere and is relatively cheap to establish contact • It is slow and error prone. • It is improving rapidly and costs are falling • allows access for many home users to Internet and enables home working. • Vast investment • Relies on Circuit switching 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/42/61 PSTN Structure • pairs of handsets therefore a conductor per pair, n houses implied n conductors! Fig2.14a • first manual centralised switching office with jumpers being placed by operators Fig2.14b • the interconnection of switching offices(cities) led to the same problem one conductor per office pair same problems as fig 2.14a • hierarchy developed as in fig 2.14c 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/43/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/44/61 PSTN Structure Fig 2.15 • Subscriber linked to local exchange by local loop by a pair of copper wires, distance can be small or up to many kilometres. • thus a local call is switched with the local exchange. • Local exchanges are connected by trunk lines in an ascending hierarchy. • medium and long distance calls are carried on multiplexed high bandwidth links and managed through switching higher up the hierarchy. • International connections demand interfaces and standardisation 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/45/61 Transmission • Local loops consist of twisted pairs and signalling is analogue. • trunks are higher bandwidth and employ coaxial(ageing), microwave and fibre optics. This uses multiplexing for analogue(ageing) and digital signals. • Amplification of an analogue signal can also amplify the noise arising as it propagates thus noise can predominate over a long connection. • Amplification of a digital signal is merely the regeneration of the original digital signal, thus only noise is that which was originally present. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/46/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/47/61 Digital v Analogue • Digital -Predictable attenuation therefore regenerators can be reliably sited to restore the signal to either 0 or 1, therefore no loss of signal even over long distances c.f. international telephone calls. • Analogue amplification is imperfect and cumulative over long distances. • Many sources can produce digital signals using the same connections • Data rates are increasing • digital is cheaper • digital more readily maintained. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/48/61 Transmission and reception • Attenuation, loss in signal strength, increases as a proportion to the length of conductor. dB/km. varies with wavelength distorts wave shape. • delay distortion also varies with wavelength, overlaps different bits, can limit bandwidth. • noise, random and burst. • crosstalk 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/49/61 Modem • MOdulator DEModulator • Change a wave in such a manner that the changes represent another signal • recognise the changes in the received wave and deduce what the modulating signal was. • falling prices. • high speeds. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/50/61 Modulation techniques • • • • • • • • • amplitude modulation frequency modulation phase modulation frequency shift keying combination Quadrature amplitude modulation QAM constellation patterns upto 64 points for 6 bits per baud compression (more later) echos supression and cancellation full and half duplex in-band signalling. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/51/61 Signal Energy Distribution for Human Speech Bypass Filter O Hz 29/09/04 300 Hz ~3,400 Hz www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt 20 kHz L3/52/61 Modems (a) A binary signal (c) Frequency modulation (b) Amplitude modulation (d) Phase modulation 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/53/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/54/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/55/61 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/56/61 Modems (2) (a) QPSK. (b) QAM-16. (c) QAM-64. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/57/61 Modems (3) (a) (b) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/58/61 Trunk Line SONET/SDH* OC3/STM1 OC12/STM4 OC48/STM16 OC192/STM64 OC768/STM256 Speed 156 Mbps 622 Mbps 2.5 Gbps 10 Gbps 40 Gbps speeds are multiples of 51.84 Mbps 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/59/61 1. Normally, One Ring is Used in Each Ring Telephone Switch SONET/SDH Ring Telephone Switch 2. Rings Can Be Wrapped if a Trunk line Is Broken. Still a Complete Loop. 29/09/04 Break Telephone Switch SONET/SDH Ring www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/60/61 Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL. 29/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/COM347J1_L3.ppt L3/61/61