1. Network Technology CSE3020 - 2006 1 Network Technology CSE3020 Week 4

2. Network Technology CSE3020 - 2006 2 Transmission Media Guided Transmission: –Twisted Pair - Unshielded Twisted Pair (UTP) - Shielded Twisted Pair (STP) –Coaxial Cable –Optical Fiber Unguided (wireless) Transmission: –Terrestrial Microwave –Satellite Microwave –Broadcast Radio –Infrared

3. Network Technology CSE3020 - 2006 3 Transmission Media Guided (wire) and Unguided (wireless) use electromagnetic waves. Characteristics and quality determined by medium and the transmitted signal. –For guided, the medium is more important. –For unguided, the bandwidth of signal produced by the antenna is more important. Key concerns are data rate and distance.

4. Network Technology CSE3020 - 2006 4 Transmission Media Design Factors: –Bandwidth - Higher bandwidth gives higher data rate. –Transmission impairments - Attenuation limits the distance. –Interference (Noise) - Overlapping frequency bands can distort or wipe out a signal - Emanations from nearby cables –Number of receivers - More receivers (multi-point), each attachment introduces some attenuation in guided media.

5. Network Technology CSE3020 - 2006 5 Interference (Noise)  Additional signals inserted between transmitter and receiver.  Crosstalk: signal from one line is picked up by another.  Thermal: - Due to thermal agitation of electrons. - Uniformly distributed. - White noise.  Intermodulation: signals that are the sum and difference of original frequencies sharing a medium.  Impulse: - Irregular pulses or spikes - e.g. External electromagnetic interference - short duration & high amplitude

6. Network Technology CSE3020 - 2006 6 Electromagnetic Spectrum

7. Network Technology CSE3020 - 2006 7 Twisted Pair Two separately insulated copper wires twisted together form a pair. A wire pair acts as a single communication link Twisting reduces crosstalk between adjacent pairs Applications: –Common medium for both analog and digital. –Telephone network - Between house and local exchange (local loop) – 56kbps modem, ADSL modem, voice. –Within buildings - Private branch exchange (PBX). –For local area networks (LAN) - 10Mbps,100Mbps, 1Gbps

8. Network Technology CSE3020 - 2006 8 Twisted Pair Two categories –Unshielded Twisted Pair (UTP) –Shield Twisted Pair (STP)

9. Network Technology CSE3020 - 2006 9 UTP – TIA/EIA 568A Cat 1 Ordinary home telephone cable Cat 3 –Bandwidth 16 MHz. –Voice grade cable found in most offices. –Twist length of 7.5 cm to 10 cm. Cat 5 –Bandwidth 100 MHz. –Commonly pre-installed in new office buildings –100BaseTx – max 100m, then need repeater –Twist length 0.6 cm to 0.85 cm –Cat 6 – 250MHz –Cat 7 – 600MHz

10. Network Technology CSE3020 - 2006 10 STP Metal braid or sheathing that reduces interference. More expensive. Harder to handle and work with Near End Crosstalk Coupling of signal from one pair to another. Coupling takes place when transmit signal entering the link couples back to receiving pair. i.e. near transmitted signal is picked up by near receiving pair.

11. Network Technology CSE3020 - 2006 11 Coaxial Cable A coax cable consists of the following: - A center conductor – usually copper. - A metalic outer conductor serves as a ground. - An insulator covering the center conductor. - A plastic jacket. Two types: Thinnet and Thicknet.

12. Network Technology CSE3020 - 2006 12 Coaxial Cable Applications: –Television distribution - Cable TV –Long distance telephone transmission: - Can carry 10,000 voice calls simultaneously using FDM - Being replaced by fiber optic. –Short distance computer systems links. Transmission Characteristics: –Higher frequency characteristics than twisted pair. –Analog (Up to 500MHz): Amplifiers every few km, closer if higher frequency. –Digital: Repeater every 1km, closer for higher data rates.

13. Network Technology CSE3020 - 2006 13 Optical Fiber Greater capacity - Data rates of hundreds of Gbps. Smaller size & lighter weight. Lower attenuation. Electromagnetic isolation. Greater repeater spacing - tens of km at least. Transmits a signal-encoded beam of light by means of total internal reflection. Act as wave guide for 10 14 to 10 15 Hz. –Portions of infrared and visible spectrum.

14. Network Technology CSE3020 - 2006 14 Optical Fiber TIR – Total Internal Reflection –100% of light that strikes a surface is reflected –By comparison a mirror reflects 90% Optical fibers should guide light waves with minimal loss Angle of Incidence I – the angle the ray makes with the line perpendicular (90 o ) to the interface between two substances Critical angle Refraction - I <= critical angle Reflection – I > critical angle

15. Network Technology CSE3020 - 2006 15 Optical Fiber - Applications Long-haul trunks - about 1500 km in length & 20,000-60,000 voice channels. - undersea optical fiber Metropolitan trunks. - about 12 km in length & 100,000 voice channels. - underground conduits joining telephone exchanges. Rural exchange trunks. - about 40 – 160 km in length & less than 5000 voice channels. Subscriber loops. - handling voice, data, image and video. LANs. - Capacity of 100 Mbps to 1 Gbps.

16. Network Technology CSE3020 - 2006 16 Optical Fiber - Transmission Characteristics Three types of transmission modes. –Step-index multimode Rays are reflected, absorbed and propagated along the fiber. Light pulses spread out in time. –Single mode Single transmission path. Long distance application (telephone & cable TV). –Graded-index multimode Intermediate between single mode and step-index multimode. Used in LAN.

17. Network Technology CSE3020 - 2006 17 Optical Fiber Transmission Characteristics

18. Network Technology CSE3020 - 2006 18 Optical Fiber - Transmission Characteristics Light sources: semiconductor devices that emit a beam of light when a voltage is applied. –Light Emitting Diode (LED): Cheaper. Wider operating temperature range. Last longer. –Injection Laser Diode (ILD): More efficient. Greater data rate. Wavelength Division Multiplexing (FDM) – multiple beams of light at different frequencies (or wavelengths) are transmitted on the same fiber. –Commercial systems – 80 channels of 10 Gbps

19. Network Technology CSE3020 - 2006 19 Wireless Transmission Unguided media (Transmission and reception via antenna). Directional (Higher Frequencies): –Focused beam. Omnidirectional (Lower Frequencies): –Signal spreads in all directions. Propagation in free space always like light (straight line). Receiving power proportional to 1/d². (d = distance between sender and receiver). Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction.

20. Network Technology CSE3020 - 2006 20 Channel characteristics change over time and location. Influences on the received signal power: –Fading: variation of signal strength with time. –shadowing –reflection at large obstacles –scattering at small obstacles –diffraction at edges reflection scattering shadowing Wireless Transmission diffraction

21. Network Technology CSE3020 - 2006 21 Wireless Transmission signal at sender signal at receiver XXX Delay Transmitted signal Delay Received signal Multipath channel:

22. Network Technology CSE3020 - 2006 22 Wireless Transmission Frequencies: –2GHz to 40GHz Microwave. Highly directional beam. Point-to-point transmission. Satellite. –30MHz to 1GHz Broadcast radio. Omnidirectional transmission. –3 x 10 11 to 2 x 10 14 Infrared. Local point-to-point and multipoint application. To reduce channel effects: –Error correction codes: remove bit errors introduced in transmission. –Equalizations: remove channel attenuation & delay (multipath effects). –Diversity techniques: use of more than one signal.

23. Network Technology CSE3020 - 2006 23 Terrestrial Microwave Parabolic dish antenna ( typical size of 3 m in diameter ). Focused beam to achieve line of sight transmission. Located at substantial heights above ground level. Microwave relay towers used to achieve long distance. Applications: –Long haul telecommunications service. –Short point-to-point links between buildings. Higher frequencies give higher data rates. Attenuation and interference. –Attenuation is increased with rainfall. –With growing applications, transmission areas overlap and resulting in interference.

24. Network Technology CSE3020 - 2006 24 Satellite Microwave Satellite is a microwave relay station. Satellite receives on one frequency band (uplink) and amplifies or repeats signal and transmits on another frequency band (downlink). May requires geo-stationary orbit: –To maintain line of sight. –Height of 35,784km. Applications. –Television distribution. –Long-distance telephone transmission. –Private business networks. –VSAT systems. High propagation delay (about 0.25 seconds).

25. Network Technology CSE3020 - 2006 25 Broadcast Radio Omnidirectional. Does not require dish-shaped antenna. Applications: –FM radio –UHF and VHF television Limited by line of sight. Less sensitive to attenuation from rainfall. Suffers from multipath interference (reflections).

26. Network Technology CSE3020 - 2006 26 Infrared Modulate noncoherent infrared light. Transceivers (transmitters/receivers) must be within the line of sight. Does not penetrate walls. E.g.: TV remote control. No frequency allocation & no licensing. Required Reading W. Stallings, “Data and Computer Communications (6th edition),” Prentice-Hall, 2000. >> Chapter 4

27. Network Technology CSE3020 - 2006 27 Asynchronous and Synchronous Transmission Transmission requires cooperation and agreement between the two sides. Fundamental requirement is Synchronization: Receiver must know the beginning and end of a bit/rate at which bits are received. –Asynchronous transmission –Synchronous transmission Asynchronous transmission: –Each character treated independently and begins with a start bit. –Not good for long block of data. Synchronous transmission: –Block of data is formatted as a frame with a starting and an ending flag. –Good for block of data.

28. Network Technology CSE3020 - 2006 28 Asynchronous Transmission Avoid sending long, uninterrupted streams of bits. Data transmitted on character at a time (5 to 8 bits). Timing only needs to be maintained within each character. Receiver can resynchronize with each new character. Operation: In idle state (binary 1), receiver looks for transition from 1 to 0 (start bit). Then samples next seven intervals (char length). May add a parity bit (odd or even). The final signal element is the stop bit (binary 1). The stop bit is same as idle state. Then looks for next 1 to 0 transition for next character. Advantages: Simple and Cheap. Disadvantage: Overhead of 2 or 3 bits per char (~20%).

29. Network Technology CSE3020 - 2006 29 Asynchronous Transmission

30. Network Technology CSE3020 - 2006 30 Synchronous Transmission Block of data transmitted without start or stop bits. Clocks must be synchronized. Can use separate clock line. –Good over short distances. –Subject to impairments. Embed clock signal in data. –Manchester encoding (digital). –Carrier frequency (analog)

31. Network Technology CSE3020 - 2006 31 Synchronous Transmission Block Level: –Need to indicate start and end of block –Use preamble and postamble. e.g. series of SYN (hex 16) characters e.g. block of 11111111 patterns ending in 11111110 –More efficient (lower overhead) than asynchronous.

32. Network Technology CSE3020 - 2006 32 Line Configuration Topology: physical arrangement of stations on a transmission medium. –Point to point –Multi point. Half duplex –Only one station at a time. –Requires one data path. Full duplex –Simultaneous transmission. –Requires two data paths.

33. Network Technology CSE3020 - 2006 33 Interfacing Data processing devices or data terminal equipments (DTE) do not (usually) include data transmission facilities. Need an interface called data circuit terminating equipment (DCE).

34. Network Technology CSE3020 - 2006 34 Interfacing DCE transmits bits on medium. DCE communicates data and control information with DTE. –Done over interchange circuits. –Clear interface standards required. Characteristics of Interface: –Mechanical: Connection plugs. –Electrical: Voltage levels, timing of voltage changes, encoding Determines the data rate and the distance. –Functional: Data, control, timing, grounding. –Procedural: Sequence of events for transmitting data.

35. Network Technology CSE3020 - 2006 35 V.24/EIA-232-F Most widely used interface (e.g.: PC serial port) ITU-T standard: V.24 –Only specifies functional and procedural aspects. –References other standards for electrical and mechanical. Virtually identical standard EIA-232-F (USA) covers all aspects. –Mechanical: ISO 2110 –Electrical: V.28 –Functional: V.24 –Procedural: V.24 EIA-232 was first issued by Electronic Industries Alliance in 1962, as RS-232. Used in modem and other interconnection applications.

36. Network Technology CSE3020 - 2006 36 Mechanical Specification

37. Network Technology CSE3020 - 2006 37 Electrical Specification Digital signals on all interchange circuits. Values interpreted as data or control, depending on circuit. Less than -3v is binary 1, more than +3v is binary 0 (NRZ-L). Signal rate < 20kbps. Distance <15m. For control, less than -3v is off, and more than +3v is on. Functional Specification Refer table 6.1 in Stallings chapter 6 for functional specifications. Loopback control is a useful fault isolation tool. - Make use of functional specifications.

38. Network Technology CSE3020 - 2006 38 Local and Remote Loopback

39. Network Technology CSE3020 - 2006 39 Procedural Specification Defines the sequence in which the various circuits are used. E.g. Asynchronous private line modem: –When turned on and ready, modem (DCE) asserts DCE ready. –When DTE ready to send data, it asserts Request to Send. –Modem responds when ready by asserting Clear to send. –DTE sends data. –When data arrives, local modem asserts Receive Line Signal Detector and delivers data.

40. Network Technology CSE3020 - 2006 40 Dial Up Operation

41. Network Technology CSE3020 - 2006 41 Null Modem

42. Network Technology CSE3020 - 2006 42 ISDN Physical Interface Connection between terminal equipment (correspond to DTE) and network terminating equipment (correspond to DCE). Defined in ISO 8877. Cables terminate in matching connectors with 8 contacts. Transmit/receive carry both data and control

43. Network Technology CSE3020 - 2006 43 ISDN Electrical Specification Balanced transmission: –Carried on two lines. e.g., twisted pair –Signals as currents down one conductor and up the other. –Differential signaling –Value depends on direction of voltage. –Tolerates more noise and generates less (Unbalanced, e.g. RS- 232 uses single signal line and ground). –Data encoding depends on data rate. –Basic rate 192kbps uses pseudoternary. –Primary rate uses alternative mark inversion (AMI) and B8ZS or HDB3.

44. Network Technology CSE3020 - 2006 44 Required Reading W. Stallings, “Data and Computer Communications (6th edition),” Prentice-Hall, 2000. >> Chapter 6