Lecture 7 Communications Basics (Part I)

1. Lecture 7 Communications Basics (Part I)

2. Introduction • When transmitting any type of electrical signal over a transmission line, the signal is attenuated (decreased in amplitude) and distorted by the transmission medium • Present with all types of transmission medium is an electrical signal known as noise • The amplitude of the noise signal varies randomly with time and adds itself to the electrical signal being transmitted over the line • Due to this noise effect at some stage the receiver is unable to determine from the attenuated received signal whether the transmitted signal was a binary 1 or 0

3. Example – Effect of attenuation, distortion, and noise on transmitted signal • If the amplitude of the received signal falls below the noise signal level, then the received signal may be incorrectly interpreted and a transmission/bit error will result

4. Introduction • The level of signal impairment is determined by: - the type of transmission medium - the length of the transmission medium - the bandwidth of the medium - the bit rate of the data being transmitted

5. Transmission media • The type of transmission medium is important, since various types of media have different bandwidth associated with them • The associated bandwidth will determine the maximum bit rate that can be used • The common types of transmission media are: - two-wire open lines - twisted-pair lines - Coaxial cable - Optical Fibre - Satellites - Terrestrial microwave - Radio

6. Two-wire open lines • In two-wire open lines each wire is insulated from the other and both are open to free space • This type of line is adequate for connecting equipment that is up to 50m apart using moderate bit rates (less than, say, 19.2kbps) • Two-wire open lines are used normally to connect a DTE to local data circuit-terminating equipment (DCE) – for example modem • With this type of line, care must be taken to avoid cross-coupling of electrical signals between adjacent wires in the same cable. This is known as crosstalk

7. Transmission Media – Copper Wire • Is the simplest transmission medium; each wire is insulated from the other and both are open to free space. This type of line is adequate for connecting equipment that is up to 50 m apart using moderate bit rates (less than 19.2kbps)

8. Transmission Media – Unshielded twisted pair

9. Twisted-pair lines • Much better immunity to spurious noise signals can be achieved by using the twisted-pair line • In this a pair of wires are twisted together • The proximity of the signal and ground reference wires means that any interference signal is picked up by both the signal and reference wires • If multiple twisted pairs are enclosed within the same cable, the twisting of each pair within the cable reduces crosstalk • Twisted pairs are suitable for bit rates in the order of 1Mbps over short distances (less than 100m) and lower bit rates over long distances

10. Transmission Media – Unshielded and Shielded twisted pair

11. Twisted-pair lines • The limiting factor of a twisted-pair line are its capacity and a phenomenon known as the skin effect • As the bit rate of the transmitted signal increases, the current flowing in the wires tends to flow only on the outer surface of the wire, thus using less of the available cross section • This increases the electrical resistance of the wires for higher frequency signals, leading to higher attenuation • In addition at higher frequencies more signal power is lost as a result of radiation effects • Hence for applications that demand a high bit rate over long distances, coaxial cable is often used as the transmission medium

12. Coaxial cable minimizes both these effects • In this type of cable the signal and ground reference wires run concentrically (coaxially) inside a solid ( or braided) outer circular conductor • Ideally the space between the two conductors should be filled with air, but in practice it is filed with a dielectric insulating material with a solid or honeycomb structure • The centre conductor is effectively shielded from external interference signals by the outer conductor • Only minimal losses occur as a result of the electromagnetic radiation and the skin effect because of the presence of the outer conductor • These cables can be uses with either baseband or modulated transmission, but typically 10Mbps over several hundred meters Coaxial cable

13. Optical Fibre • Although coaxial cable significantly reduces the various limiting effects, the maximum frequency, and hence the bit rate that can be transmitted using a solid conductor, although very high, is limited • The optical fibre cable differs from both these transmission media in that it carries the transmitted bit stream in the form of a fluctuating beam of light in a glass fibre • Light waves have a much wider bandwidth than electrical waves, enabling optical fibre cable to achieve transmission rates of hundreds of Mbps • It is used in the core transmission network of PSTNs and LANs and also CATV networks

14. Optical Fibre • Light waves are also immune to electromagnetic interference and crosstalk • Hence optical fibre cable is extremely useful for the transmission of lower bit rate signals in electrically noisy environments, e.g steel plants • It is also being used increasingly where security is important, since it is difficult physically to tap • The light signal is generated by the transmitter which uses a light-emitting diode (LED) or laser diode (LD) • The receiver uses a light-sensitive photodiode

15. Optical fibre • The fibre consists of two parts: an optical core and an optical cladding with a lower refractive index • Light propagates along the optical fibre core in one of three ways depending on the type and width of the core material used • In a multimode stepped index fibre the cladding and core material each has a different but uniform refractive index • All the light emitted by the diode at an angle less than the critical angle is reflected at the cladding interface and propagates along the core by means of multiple reflections • Depending on the angle at which it is emitted by the diode, the light will take a variable amount of time to propagate along the cable

16. Therefore the received signal has a wider pulse width than the input signal with the decrease in the maximum permissible bit rate • This effect is known as dispersion • Dispersion can be reduced by using a core material that has a variable refractive index • This type of fibre is known as the multimode graded index fibre • In this fibre the light is refracted by an increasing amount as it moves away from the core • This has the effect of narrowing the pulse width of the received signal compared to the stepped index fibre Optical fibre

17. Optical fibre • Further improvement can be obtained by reducing the core diameter to that of a single wavelength (3-10 m) so that all the emitted light propagates along a single path • Hence the received signal will be of comparable width to the input signal and is called the monomode fibre • Alternatively multiple high bit rate transmission channels can be derived from the same fibre by using different proportions of the optical bandwidth for each channel • This mode of operation is known as wavelength-division multiplexing (WDM) • Using this bit rates in excess of tens of Gbps can be achieved

18. Transmission Media – Optical fibre transmission media

19. Satellites • In satellite systems the data is transmitted using electromagnetic (radio) waves through free space • A collimated microwave beam, onto which the data is modulated, is transmitted to the satellite from the ground • This beam is received and retransmitted to the predetermined destinations using an on board circuit known as a transponder • A single satellite has many transponders, each covering a particular band of frequencies • A typical satellite has a bandwidth of 500MHz and can provide many high bit rate data links using TDM

20. Satellites are widely used for data transmission applications ranging from interconnecting different national computer communication networks to providing high bit rate paths to link communication networks in different parts of the same country • In data communication applications, a more common configuration involving a central hubground station that communicates with a number of ground stations distributed around the country is used • Each ground station has a small antenna associated with it – typically 1 metre in diameter which receives and transmits signals • Typically the central site broadcasts to all VSAt’s at a bit rate of 0.5 – 2 Mbps while in the reverse direction each VSAT transmits as a lower bit rate of up to 64 kbps Satellites

21. To communicate with a particular VSAT (very small aperture terminal), the central site broadcasts the message with the identity of the intended VSAT at the head of the message • In the next generation of satellites direct VSAT-to-VSAT communication is possible Satellites

22. Transmission Media

23. Transmission Media – Satellite System: data communications

24. Radio transmission using lower-frequency radio waves is also used for the digital information transmission in place of the fixed-wire links • Example applications include mobile telephony and more general mobile data applications • A radio transmitter (base station (BS)) is located at a fixed-wire termination point • This provides a cordless link to the fixed-wire termination point for any handset/terminal that is within the (radio) field of coverage of the base station • Multiple base stations must be used for wider mobile coverage • The coverage area of the BS is restricted hence wider coverage is achieved by arranging multiple base stations in a cell structure Radio

25. Transmission Media • Radio transmission using a lower-frequency radio waves is also used for the transmission of digital information in place of fixed-wire links over distances up to several kilometers

26. Transmission Media • Wider coverage is achieved by arranging multiple base stations in a cell structure; the size of each cell varies and is determined by such factors as the handset/terminal density and local terrain

27. Radio • Each base station operate using a different band of frequencies from its neighbours • Since the field of coverage is limited it is possible to use its frequency band in other parts of the network • All base station within a region are connected by fixed-wire lines using the mobile switching centres (MSCs) • This is connected to other MSCs in other regions and to the fixed telephone network

28. As a signal propagates along a transmission medium its amplitude decreases known as signal attenuation • Normally a limit is set on the length of the cable that can be used to ensure that the receiver circuitry can reliably detect and interpret the received attenuated signal • If the cable is longer then one or more amplifiers (repeaters) are inserted at intervals along the cable to restore the received signal to its original level • Signal attenuation increases as a function of frequency • To overcome this effect the amplifiers are designed to amplify different frequency signals by varying amounts • Devices such as equalizers are used to equalize the attenuation across a defined band of frequencies Attenuation

29. The rate of propagation of the sinusoidal signal along a transmission line varies with the frequency of the signal • Consequently, when we transmit a digital signal the various frequency components making up the signal arrive at the receiver with varying delays resulting in delay distortion of the received signal • As the bit rate increases, some of the frequency components associated with each bit transition are delayed and start to interfere with the frequency components associated with a later bit • Delay distortion is also known as intersymbol interference (ISI) • The level of intersymbol interference associated with a transmission channel can be observed by means of an eye diagram Delay distortion

30. Delay distortion - ISI

31. Delay distortion • The eye diagram is obtained by displaying the signal received from the channel on an oscilloscope which is triggered by the transitions in the signal • Assuming the received signal contains random binary 1 and 0 signal transitions, the oscilloscope will display all the possible signals superimposed on one another • Higher the level of interference, the smaller the central section – known as the eye

32. Asynchronous transmission • With asynchronous transmission each character or byte that makes up a block/message is treated independently for transmission • This can be used for transfer of simple characters entered at a keyboard, or for the transfer of blocks of characters/ bytes across a low bit rate transmission line/channel • Since all transfers that are external to the system are carried out bit-serially, the transmission control circuit on the network interface card (NIC) must do the following • - parallel-to-serial conversion of each character or byte in preparation for its transmission on the line • - serial-to parallel conversion of each received character or byte in preparation for its storage and processing in the received end system

33. Asynchronous transmission • - a means of the receiver to achieve bit, character, and frame synchronization • - the generation of suitable error check digits for error detection and the detection of such errors at the receiver should they occur • Using the PISO shift register a full character can be loaded in parallel and shifted out bit-serially • Serial to parallel conversion is carried out by the SIPO shift register

34. Asynchronous Transmission – principle of operation

35. Frame synchronization • When messages comprising blocks of characters or bytes – normally referred to as information frames - are being transmitted, in addition to bit and character synchronization, the receiver must be able to determine the start and end of each frame • This is known as frame synchronization • The simplest method of transmitting blocks of printable characters is to encapsulate the complete block between two special transmission control characters: STX (start-of-text) which indicates the start of a new frame after an idle period and ETX (end-of-text)

36. Frame synchronization

37. Frame synchronization • Although the above scheme is satisfactory for the transmission of block of characters, when transmitting blocks of strings of bytes, the use of ETX to indicate the end is not sufficient • In the case of string bytes one of the string bytes may be same as the ETX character, which would cause the receiver to terminate the reception process abnormally • To overcome this problem, when transmitting this type of data the two transmission control characters STX and ETX are each preceded by a third transmission control character known as data link escape (DLE) • After transmitting the start-of-frame sequence (DLE-STX) the transmitter inspects each byte in the frame prior to transmission to determine if it is the same as the DLE character. If it is, irrespective of the next byte, a second DLE character is transmitted before the next byte • This process is known as character or byte stuffing

38. Frame synchronization

39. Smart Home Technologies

40. Smart Homes • Imagine a completely networked home – in which every appliance has its own Internet address and can be remotely managed from anywhere on the Internet with a simple Web browser • The general goal of the smart-home movement is to use networking technology to integrate the devices, appliances and services found in homes so that the entire domestic living space can be controlled centrally or remotely

41. What are smart homes? • A house with intelligent devices that can obey the orders of humans and serves as a reliable monitor of home activities • Smart home networks will bring the integration of sophisticated alarm systems, network of sensors and small microelectronic devices able to start home electrical devices remotely • The recent emergence in communication technologies, especially wireless and fixed small range networks, enabled extended use of smart homes and their remote control

42. What are smart homes? • In the current smart homes, a large number of sensors observe the environment and regulate certain parameters of the environment, such as temperature • The next generation of smart homes are emerging as a result of sophisticated wireless networking technologies that have become reality in recent years • However, the important issue for this smart home network of next generation is that it has to reach the same level of reliability and user availability as previous generations • User interface is one of the key issues for a successful design of a smart home.

43. South Korea’s LG Electronics made Internet ready refrigerator designed to give users the ability to surf the Web from the kitchen • It had two prominent 15-inch LCD displays on the front panels, one on each of the two main doors • Through these panels you can access real-time grocery prices, health and nutrition tips, cooking information also if the users first tell the refrigerator what goods are being stored inside, alerts about expiration dates • Based on the ingredients inside it can also provide few recipes for you!! Currently the cost is £5000. Internet Refrigerator?

44. Smart home Technologies and issues • Smart home systems must run sophisticated algorithms which have to be trained to adapt to the users • Most important issues: • - User interface design • - communication technology • - location identification • - automatic decision making • - On-demand action

45. Tracking and location systems are one of the most important services in smart home networking. • These systems allow users to feel more independent and secure since they can be informed by the network about possible obstacles and about their precise location • Key issues in the development of the tracking system and more generally of the whole smart networking systems include: • - Cost • - Accuracy of the location algorithm • - size and weight of the devices and power consumption Smart home Technologies and issues

46. UPnP (Universal Plug and Play) • UPP is a standard that uses Internet and Web protocols to enable devices such as PCs, peripherals, intelligent appliances, and wireless devices to be plugged into a network and automatically know about each other • With UPnP, when a user plugs a device into the network, the device will configure itself, acquire a TCP/IP address, and use a discovery protocol based on HTTP to announce its presence on the network to other devices • e.g a camera connected to the network will look for a printer to print a colour photo Technologies for smart homes

47. UPnP (Universal Plug and Play) • The camera and printer will use XML to establish a common language, or “protocol negotiation”, to talk to each other and determine capabilities • Microsoft one of the 29 companies sponsoring UPnP hopes that UPnP will make it as easy to plug a device or appliance into a home or small business data network as it is to plug a lamp into an electrical outlet Technologies for smart homes

48. Bluetooth • Bluetooth is a telecom standard that describes how mobile phones, computers, and PDAs can be easily interconnected using a short-range wireless connection • Bluetooth requires that a low-cost transceiver chip be included in each device • Each device has a 48-bit address from the IEEE 802 standard • Maximum range is 10 meters • A frequency hop scheme allows devices to communicate even in areas with a great deal of electromagnetic interference • Built-in encryption and verification provided

49. Jini • Jini is a new idea that Sun Microsystems calls “spontaneous networking”. • Using Jini architecture, users will be able to plug printers, storage devices, speakers, and any kind of device directly on to the network. • And the user on the network will know that a new device has been added • Each pluggable device will define itself immediately to a network device registry • When someone wants to use or access the resource, their computer will be able to download the necessary programming from it to communicate with it.

50. No longer will the special device support software known as the device driver need to be present in an operating system • The operating system will know about all accessible devices through the network registry • Jini can be viewed as the next step after the Java programming language to make the network look like one large computer • A printer could be added to the network with a microchip-embedded operating system and that can be shared by users of a mix of computers: Windows, Macintosh, UNIX etc.