Tech2301: Data Communications

1. Tech2301: Data Communications Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/TECH2301.html

2. Transmission Media Transmission rates • One of the more important media considerations is the supported data transmission rate or speed. • The different network media vary greatly in the transmission speeds they support. • Application-intensive networks require more than the 10Mbps. In some cases, even 100Mbps, which is found in many modern LANs. • Many organizations deploy 1Gbps networks, and some now even go for 10Gbps implementations.

3. Transmission Media • This is the physical path between a transmitter and a receiver in a data communication system. • They may be classified into two type: • Guided: waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable or fiber optic. • Unguided: provides a means for transmitting electromagnetic signals through air but do not guide them • This is also known as wireless transmission/ communication

4. Cont … Transmission Media Guided Media Unguided Media Twisted-pair Cable Coaxial Cable Fiber- optic Cable Air

5. Media Selection Criteria • Cost • For actual media and connecting devices such as NICs hubs etc • Installation • Difficulty to work with media • Special tools, training

6. Cont … • Capacity • The amount of information that can be transmitted in a giving period of time • Measured as • Bits per second bps (preferred) • Baud (discrete signals per second) • Bandwidth (range of frequencies)

7. Cont … • Node Capacity • Number of network devices that can be connected to the media • Attenuation • Weakening of the signal over distance

8. Cont … • Electromagnetic Interference (EMI) • Distortion of signal caused by outside electromagnetic fields • Caused by large motors, proximity to power sources • Other noise sources • Crosstalk • Echo

9. Twisted Pair Cable • The popularity can be attributed to the fact that it is lighter, more flexible, and easier to install than coaxial or fiber optic cable • It is also cheaper and can achieve greater speeds than its coaxial competition. • Ideal solution for most network environments. • Two main types of twisted-pair cabling are: • Unshielded Twisted Pair (UTP) • more commonplace than STP and is used for most networks • Shielded Twisted Pair (STP) • used in environments in which greater resistance to EMI and attenuation is required. • the greater resistance comes at a price.

10. Cont … • This extra protection increases the distances that data signals can travel over STP but also increases the cost of the cabling • UTP: one or more pairs of twisted copper wires insulated and contained in a plastic sheath • Uses RJ-45 telephone connector • STP: Same as UTP but with a aluminium/ polyester shield. • Connectors are more awkward to work with

11. Cont … • Twisted nature to reduce crosstalk. • Any interference from a physically adjacent channel that corrupts the signal and causes trans- mission errors is what is known as crosstalk. • UTP Categories: • Categories 1 and 2 (CAT 1 and CAT 2) • voice grade • low data rates up to 4 Mbps • Category 3 (CAT 3) • suitable for most LANs • up to 16 Mbps

12. Cont … • Category 4 • up to 20 Mbps • Category 5 • Supports Fast Ethernet • 100Mbps • more twists per foot • more stringent standards on connectors • Category 5e • up to 1000 Mbps • Category 6 • up to 1000 Mbps + • Data grade UTP cable usually consists of either 4 or 8 wires, two or four pair

13. Unshielded Twisted Pair (UTP)

14. Shielded Twisted Pair (STP)

15. Coaxial Cable • Commonly referred to as coax • Coax found success in both TV signal transmission as well as in network implementations. • Constructed with a copper core at the centre that carries the signal, plastic insulation, braided metal shielding, and an outer plastic covering • Constructed this way to avoid: • Attenuation • the loss of signal strength as it travels over distance • Crosstalk • the degradation of a signal caused by signals from other cables running close to it • EMI • Electromagnetic Interference

16. Cont … • Networks can use two types of coaxial cabling: thin coaxial and thick coaxial • Thin coax is only .25 inches in diameter, making it fairly easy to install • Disadvantages of all thin coax types are that they are prone to cable breaks

17. Cont …

18. Cont … • Size of Coax • RG-8, RG-11 • 50 ohm Thick Ethernet • RG-58 • 50 ohm Thin Ethernet • RG-59 • 75 ohm Cable T.V. • RG-62 • 93 ohm ARCnet

19. Fiber Optic • Addresses the shortcomings associated with copper-based media • Use light transmissions instead of electronic pulses • Advantages: • Threats such as EMI, crosstalk, and attenuation become a nonissue • Well suited for the transfer of data, video, and voice transmissions • It is the most secure of all cable media • Disadvantages: • difficult installation and maintenance procedures of fiber • often require skilled technicians with specialized tools • the cost of a fiber-based solution limits the number of organizations that can afford to implement it

20. Cont … • incompatible with most electronic network equipment; you have to purchase fiber-compatible network hardware. • Composed of a core glass fiber surrounded by cladding • An insulated covering then surrounds both of these within an outer protective sheath • Two types of fiber-optic cable are available: single and multimode fiber • multimode fiber, many beams of light travel through the cable bouncing off of the cable walls • weakens the signal, reducing the length and speed the data signal can travel • Single-mode fiber uses a single direct beam of light • allows for greater distances and increased transfer speeds

21. Cont …

22. Cont … • Common types of fiber-optic cable include the following: • 62.5 micron core/125 micron cladding multimode • 50 micron core/125 micron cladding multimode • 8.3 micron core/125 micron cladding single mode • The main advantage of optical fiber is the great bandwidth it can carry. • There are three main bands of wavelength used.

23. Characteristics of Cable Media

24. Media connectors • attach to the transmission media and allow the physical connection into the computing device BNC – Connector • Associated with coaxial media • Common BNC connectors include a barrel connector, T-connector, and terminators

25. BNC - connectors

26. RJ - 11 • RJ (Registered Jack) -11 connectors are small plastic connectors used on telephone cables • have capacity for six small pins • Not all the pins are used • a standard telephone connection only uses two pins • a cable used for a DSL modem connection uses four • Similar to RJ-45 connectors • Have small plastic flange on top of the connector to ensure a secure connection

27. Cont … RJ – 11 RJ – 45 • used with twisted-pair cabling • resemble the aforementioned RJ-11 phone jacks • support up to eight wires instead of the six

28. RJ - 45

29. Wire Propagation Effects • Propagation Effects • Signal changes as it travels • Receiver may not be able to recognise it Original Signal Final Signal Distance

30. Propagation Effects: Attenuation • Attenuation: signal gets weaker as it propagates • Attenuation becomes greater with distance • May become too weak to recognise Signal Strength Distance

31. Propagation Effects: Distortion • Distortion: signal changes shape as it propagates • Adjacent bits may overlap • May make recognition impossible for receiver Distance

32. Propagation Effects: Noise • Noise: thermal energy in wire adds to signal • Noise floor is average noise energy • Random signal, so spikes sometimes occur Spike Signal Signal Strength Noise Noise Floor Time

33. Propagation Effects: SNR • Want a high Signal-to-Noise Ratio (SNR) • Signal strength divided by average noise strength • As SNR falls, errors increase Signal Strength Signal SNR Noise Floor Distance

34. Propagation Effects: Interference • Interference: energy from outside the wire • Adds to signal, like noise • Often intermittent, so hard to diagnose Signal Signal Strength Interference Time

35. Propagation Effects: Termination • Interference can occur at cable terminator (connector, plug) • Often, multiple wires in a bundle • Each radiates some of its signal • Causes interference in nearby wires • Especially bad at termination, where wires are unwound and parallel Termination

36. Channel Types • A channel is any conduit for sending information between devices. • There are three basic types of channel: simplex, half-duplex and full-duplex. • A simplex channel is unidirectional, which means data can only be sent in one direction. For example, a TV channel only carries data from the transmitter to your TV set. Your TV set cannot send information back.

37. Channel Types • A half-duplex channel allows information to flow in either direction (but not simultaneously). • Devices at either end of the channel must take it in turns to transmit information whilst the other listens. For example, a walky-talky either transmits or receives but not both at the same time.

38. Channel Types • A full-duplex channel allows data to be sent in both directions simultaneously. • A full-duplex channel can be formed from two simplex channels carrying data in opposite directions. This may make it more expensive than a half-duplex channel. • There is no waiting for turns or for the devices swap roles, as is the case with a half-duplex channel. This means full-duplex can be faster and more efficient.

39. Unguided Media • Provides a means for transmitting electro-magnetic signals through air but do not guide them. • Also referred to as wireless transmission • Why is wireless communication so important? • Due to availability of mobile devices, people demand for communication from anywhere and to anyplace. • Mobility and flexibility are some of the reasons for having wireless communication • Wireless communications uses specific frequency bands which separates the ranges. • Main types, radio waves, microwaves, Bluetooth and Infrared.

40. Cont … • Transmission and reception are achieved by means of antennas • For transmission, an antenna radiates and electromagnetic radiation in the air • For reception, the antenna picks up electromagnetic waves from the surrounding medium • The antenna plays a key role; the characteristics of the antenna and the frequency that it receives • Basic types of antenna configuration • Point-to-point communications • Broadcast communications

41. Cont … Point-to-point communication • The transmitting antenna puts out a focused electromagnetic beams • The transmitter and receiver must be carefully aligned Broadcast communication • Transmitted signals spreads out in all directions • Transmissions can be received by many antennas

43. Cont … • The bands are the official ITU names and are based on the wavelengths The terms LF, MF, and HF refer to low, medium, and high frequency Politics of EM spectrum • There exist national and international agreements about who gets to use which frequencies • Allocate spectrum for AM and FM radio, television, and mobile phones • Ways of allocating frequencies • Beauty contest - proposal serves the public interest best • Lottery – interested company's • Auctioning off the bandwidth to the highest bidder

44. Cont … • A different approach is to use ISM (Industrial, Scientific Medical) bands for unlicensed usage • Garage door openers, cordless phones, radio-controlled toys, wireless mice use ISM bands • Spread spectrum techniques are used to minimize interferences Fig. 3.0: ISM Bands in the US ITU-T, “Whenever we exchange voice, data or video messages, communications cannot take place without standards linking the sender and the receiver”

45. Cont … • 900-MHz band works best, but it is crowded and not available worldwide. • 2.4-GHz band is available in most countries; subject to interference from microwave ovens and radar installations (Bluetooth and some of the 802.11 wireless LANs operate in this band) • 5.7-GHz band is new and relatively undeveloped; equipment relatively expensive

46. Radio Transmission • (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. • (b) In the HF band, they bounce off the ionosphere.

47. Radio Wave • Most radio frequencies are regulated • Must obtain a license from a regulatory board (CRTC, FCC) • A range of radio frequencies are unregulated • Low power single frequency • uses one frequency • limited range 20 to 30 meters • usually limited to short open environments • High power single frequency • long distance may use repeaters to increase distance • line of sight or bounced of the earth's atmosphere • uses a single frequency

48. Cont … • The spread spectrum maintains security of the radio transmission by: • Spreading the carrier signal frequency • Modulating the carrier frequency: • It employs two different schemes for using frequencies: • Frequency Hopping Spread Spectrum (FHSS) • Direct Sequence Spread Spectrum (DSSS

49. Cont … Frequency Hoping Spread Spectrum • The frequency spectrum is divided into channels. • Data packets are split up and transmitted on these channels in a random pattern known only to the transmitter and receiver • Multiple networks can operate in close proximity without interfering. • If interference is present on one channel, data transmission is blocked. • The transmitter and receiver ‘hop’ to the next channel in the hop table and the transmitter resends the data packet.

50. Cont … • Frequency hopping technology works best for small data packets in high interference environments