TM255 Lecture Notes PDF

## محاضرات الزبدة ### م. محمد موسى **TM255** ### Definitions | Term | Meaning | |---|---| | wavelength | the distance between two consecutive peaks of a wave | | Attenuation | the gradual loss of signal power as it travels through a medium | | Absorption | one of the main causes of attenuation i...

## محاضرات الزبدة ### م. محمد موسى **TM255** ### Definitions | Term | Meaning | |---|---| | wavelength | the distance between two consecutive peaks of a wave | | Attenuation | the gradual loss of signal power as it travels through a medium | | Absorption | one of the main causes of attenuation in radio signals. | | Antenna | conductor of a specific shape that can convert an electrical signal in a wire to an electromagnetic wave outside the wire or vice versa | | bandwidth | maximum data rate of a digital channel, and in that case is quoted in bits per second (bps) | | capacity | refers to the theoretical maximum data rate of a channel, in bits per second (bit/s or bps) | | channel | A is usually specified in terms of the central frequency but will include a range of frequencies below and above this. | | Throughput | is the available capacity for user data (expressed in bit/s or bps, or sometimes as a percentage of the maximum data rate) | | Carrier | term used to describe a radio wave that will carry information from source to destination | | modulation | At the sending end, the message is superimposed upon a radio carrier wave | | demodulation | The message is extracted from the carrier wave at the receiving end. | | Multiplexing | is the process that enables multiple data signals to be transmitted simultaneously over the same channel | | multipath | when Reflected signals reach the receiver at a slightly different time to the dominant line-of-sight signal and can act as interference, which attenuates (weakens) the signal | | fading | | | Unauthorized use of bandwidth | Finding and connecting to a wireless AP without the permission of the owner is sometimes known as war driving | | Eavesdropping | the intercepting and reading of information by someone who is not the intended recipient | | Threats to network availability | prevents legitimate users of a network from accessing services or information by flooding the network with bogus messages so that genuine ones are delayed or do not get through at all | | Roaming | is the ability of a mobile user to use networks other than their 'home' network when travelling outside the geographical coverage area of their home network - like when travelling to other countries | | Internet of Things (IoT) | is a concept that describes the connection of physical things to enable the collection and exchange of data | | sensor | is something that measures or monitors a property of the physical environment, such as temperature or movement, and delivers its readings in the form of a displayed output or a transmittable signal. | | A LAN (Local Area Network) | is a term usually used to describe a network that provides access to services and applications within a common organisational structure or a limited geographical area. | | WLAN | Wireless Local Area Networks also known as Wi-Fi, are one type of wireless network that use the radio signals as the communication medium instead of wires. | | wireless NIC | End-user devices access a wireless network using small transmitter/ receiver units described as wireless LAN adapters or sometimes wireless network interface cards (wireless NICs) They provide the interface between the device and the network | ### Questions **1. List four factors that can influence the performance of a radio system:** * The design of the equipment, as this determines the initial transmission. * power - that is the power in the signal to begin with, and the receiver. * sensitivity - the minimum signal power that can be received for the desired. * output signal to be recovered accurately. * The nature of radio waves * the path between transmitter and receiver. * The frequency of the wave, * The distance between transmitter, receiver and objects in between them. **2. The effect of the multipath environment depends on** * the number of reflective surfaces, * the distance from the transmitter to the receiver, * the equipment designs. * and the radio technologies used. **3. The capacity is dependent on many factors, list them** * Signal power relative to the noise power * The bandwidth of the channel: the wider the bandwidth, the higher the capacity. * Standards usually refer to capacity. **4. How is Interference created, show with diagram the difference between it's types?** It occurs when delayed versions of a radio wave interfere with the wave itself. When two separate radio waves combine, they can either reinforce each other or cancel each other out. - **Constructive Interference** the delay between the two original radio signals is relatively small and the waves tend to reinforce each other. The combination has a greater value than the original sinewaves - **destructive Interference** some values of delay the two original waves cancel each other out. .**5. Properties of carrier that can be altered or for example: )أنواع التعديل( ** * Turn a radio signal on or off * The amplitude * The frequency **6. Each asset is vulnerable to different threats.** * **Hardware:** can be stolen, physically damaged or tampered with so that normal service or access is restricted or denied. * **Software:** can be deleted or modified so that it no longer performs in the way required. This may occur through the use of malicious software such as viruses. * **Data:** can be deleted, stolen or compromised in terms of its privacy or integrity. **7. Computer networks make use of many security measures:** * Physical security measures: such as security labelling of network equipment and restricted access to buildings and rooms where equipment is located. * Software security measures: include, for example, antivirus software and patches, or updates that fix flaws in current versions of software as they are identified. * Procedural measures: ensure rules are followed so that things are done in a systematic way and can define how the other measures are implemented; for example, installation of software updates, and change of passwords. * Technical measures: involve the use of technologies such as authentication and encryption, as well as many more. **8. Give examples for some situations or applications where a wireless LAN could be more effective than a wired LAN?** * computing devices around the home to be connected quickly and conveniently * greater flexibility in an office environment, where costs associated with office changes and furniture reorganizations can be minimized * medical staff with handheld update central patient information instantly * schools, colleges and universities to provide easy access to information and learning material in a variety of learning environments * People in meetings to remain connected to information needed to make support decisions. **9. What are similarities between the 802.11 and 802.3 families of standards?** * Both based on a layered architecture model. * Both designed to perform a broadly similar task: getting data across a network. **10. In the context of network communications, what is a standard? Why standards are important in communications networks?** A standard is a formal document containing an agreed norm or requirement. It establishes methods, criteria, processes and practices relating to a specified technology. Standards are important because they enable different companies to produce items that will work in conjunction with other items following the same standard. Therefore, the items are interoperable. **11. The Wi-Fi Alliance** describes itself as a global, non-profit industry association with over 300 companies. It was formed to address the problem of incompatibility between products produced by different companies, some of which did not fully implement 802.11. **12. What is the differences between the two types of media access control are: Control-based, Contention-based?** - **control-based media access** Each device is given its own scheduled time slot for placing data on the network, so all the network devices take turns. This is inefficient because some devices may not have data to transmit, while those that do have to wait for their turn - **contention-based media access** any device can attempt to place data on the network at any time. This is more efficient. there is the possibility that two or more devices attempt to place data on the network at the same time in which case a 'collision' occurs. **13. The AP forms part of the wired network infrastructure and is not mobile. List AP purposes** * receive * buffer (store for a short time) * transmit data between one wireless device and another **14. There are two basic configurations for a wireless network. Briefly explain these configurations and then give proper diagrams for each one of them.** - **Ad hoc or peer-to-peer network** Wireless nodes are within range, they can set up an ad hoc network. Connecting to another person's notebook computer to swap files for example can be useful. Doesn't need any administration or pre configuration. - **Infrastructure network** Main component of this type of network is a transmitter/receiver unit called access point (AP) or sometimes a wireless access point (WAP). The AP connects to a router, which in turn connects to a wired LAN or to a broadband connection. For small office and home networks it's common for the AP and the router to be housed together in a single unit. **15. What does the CSMA/CD stand for? Explain how it works.** CSMA/CD: carrier sense multiple access with collision detection. In networks using the CSMA/CD protocol, a node continues to sense the medium while it is transmitting, and it immediately stops transmitting and sends a jamming signal if it detects signals from another node. **16. In the context of WLAN, CSMA/CA is a media access protocol that defines how WLAN users can share the same wireless transmission channel. Give a detailed explanation of the CSMA/CA process.** (CSMA/CA): carrier sense multiple access with collision avoidance. When a node wants to send a packet, it checks to see if the channel is clear; If the channel is clear, the data frame is sent immediately. If the channel is not clear, the sending node waits for a random time called the backoff and then checks again to see if the channel is clear. If the channel is clear after the backoff, the device transmits the data packet. If the channel is still not clear, the backoff random delay is set again and the process is repeated until the channel is clear. the receiving device sends an acknowledge (ACK) frame if the data is received without error. If the sending device fails to receive the ACK frame, then the whole process is repeated. **17. In these cases 802.11 defines an optional extra stage to the CSMA/CA protocol:** When this option is implemented, the node sends a small information frame called a request to send (RTS). This contains information about the data to be sent, such as its source and destination. The AP responds with a clear to send (CTS) frame if the channel is clear. On receipt of the CTS frame, the node sends the data packet. When this is received the AP sends an ACK frame as confirmation. If the sending device fails to receive an ACK frame, it repeats the process until an ACK is received. **18. Describe the process of connecting to an Access Point -AP?** When a device is switched on it scans the area at regular set intervals to look for an AP to connect to. It does this by sending out special packets of information called probe request frames. It then waits for a response to its 'probe'. This is returned in the form of a probe response frame. If more than one AP responds, the device will connect to the one with the strongest signal. **19. Describe Segmentation of data, and its benefit** Segments: break the message up into multiple small chunks and to send each segment separately. Benefit: Segmentation also increases the reliability of network communication. **20. 802.11 increases the level of management and control of the network by specifying three frame types. Explain, briefly these frame types.** * **Data frames:** carry application-level data. * **Management frames:** manage access to the wired network. * **Control frames:** assist the delivery of data frames. **21. Earlier in this section you met different frame types for a probe request, a probe response, RTS, CTS and ACK. Place each of these in the category of either management frame or control frame.** Probe request and probe response frames are related to access to the wireless network and are therefore management frames. RTS, CTS and ACK frames are related to the delivery of data frames and are therefore control frames. **22. The 802.11 frame provides four address fields which can contain up to four of the five address types, list them** * Basic service set identifier (BSSID) * Destination node address * Source node address * Receiver AP address * Transmitter AP address **23. The main reason that wireless networks are more vulnerable than wired networks is the nature of the transmission channel they use.** In a wired LAN, access to the network is restricted to those with physical access to a port into which they can plug a network-enabled device. Wireless networks have no defined boundary, as the radio waves used to carry data travel varying distances. The radio transmissions used in wireless LANs are relatively low in power in comparison with those in other wireless networks, such as mobile phone networks. **24. The main risks and concerns that wireless networks face due to their inherent characteristics are:** * Unauthorized use of bandwidth * Eavesdropping * Threats to network availability **25. However, security protocols designed for wired networks may not be best suited to a wireless environment. Why?** Wireless devices are often battery powered, and the use of complex protocols is more draining on battery life. Security controls should not interfere with the way in which a network is intended to operate and the services it is intended to offer. **26. Network security can be improved through using “authentication” and “encryption”. Explain briefly these terms.** - **Authentication:** is a process where upon a person must prove they are legitimate users of that certain resource. Authentication tends to involve the user presenting something or some information that proves their identity, for example, a password, token or fingerprint. - **Encryption:** is the process of transforming a message so that it is unintelligible to anyone except the intended recipient who will know how to reverse the transformation. Encryption is performed by 'scrambling' the message in a systematic way, using an often complex mathematical procedure. **27. Measures have to be carefully designed to consider:** * User experience * Limitations of the devices * Transmission channel * Intended use of the network whilst ensuring adequate levels of security are provided. **28. WEP is part of the original 802.11 standard and provides an optional encryption procedure. What is the main aim of WEP? Explain how WEP works and how its encryption scheme works.** Wired Equivalent Privacy The main aim of WEP was: * provide protection against casual eavesdropping * make a wireless LAN equivalently secure to a wired LAN. it allows access control to be set up so that only legitimate users can gain access. **29. What does WPA stand for? For which standard WPA has been defined? What type of cipher does the WPA use?** * WPA stands for Wi-Fi Protected Access. WPA is part of the 802.11 standard. It uses the same stream cipher used by WEP with a 128-bit encryption key. It uses the same stream cipher used by WEP with a 128-bit encryption key, but the known flaws that existed with WEP's implementation have been addressed. **30. What is WAP2 means?** WPA2, required new hardware due to the use of a completely different encryption mechanism and, therefore, took longer than WPA to integrate into products. It develops and extends the security offerings of WPA and provides an even higher level of security, including a special ‘government-grade' encryption algorithm. **31. What is the features of WPA/WPA2-personal options than enterprise?** * Easier to set Up * Used in most home or simple Wi-Fi networks. * The SSID is the network name and can be changed. **32. What is the 802.1x, then list parties of 802.1x?** IEEE 802.1x, defines a centralised authentication protocol for both LANs and WLANS 802.1x defines three parties * Supplicant - the client or device wishing to access the network * Authenticator – in the case of Wi-Fi this is the wireless access point * Authentication server- known as a Remote Authentication Dial-in User Service (RADIUS) server, which supports the necessary authentication protocols. **33. What has enabled 802.11n to improve on 802.11g's maximum data rate in the 2.4 GHz band so significantly?** * transmit multiple data streams simultaneously using a single channel * Use the available bandwidth more efficiently * Reduce the management overheads. **34. MIMO is a technology used in 802.11n standard to increase the channel capacity of WLANs. It is based on what is known as spatial diversityWhat does MIMO stand for? Explain the principle of MIMO technique.** MIMO: Multiple Input Multiple Output. MIMO use of multiple transmitters and multiple receivers, each slightly separated from the other. The transmitter divides the data stream between the antennas and transmits them simultaneously over the same channel. Each stream takes a slightly different path to the receivers, so that each fluctuates and fades quite independently from the others. This uniqueness enables each stream to be independently identified by the receivers (Spatial Multiplexing) **35. WLAN standards use two techniques to enhance the efficiency of bandwidth use which are: "Block acknowledgment” and “frame aggregation”. Briefly explain these techniques.** * Block acknowledgment: Protocol reduces the bandwidth consumption by acknowledging several frames in a single ACK frame * Frame aggregation: 802.11n addresses this by specifying options for putting multiple frames together as a single transmission with a single overhead. a block of frames destined for a single address must be collected. Other Techniques: Reduce the size of the ACK frame, and MIMO technique. **36. List recent developments in the 802.11 standard and its services** * Internet of Things: amendment 802.11ah and Provides a communication infrastructure * White-Fi and Super Wi-Fi: amendment 802.11af and define the use of Wi-Fi within TV white space (TVWS), which is the unused TV spectrum in the VHF and UHF bands. * WiGig: 802.11ad it Operates in the 60 GHz frequency range (the millimeter band) for different applications to those in 802.11ac or 802.11ax. **37. What is the essential difference between?** 1. an analogue signal and a digital signal? An analogue signal can take on any value, whereas a digital signal is restricted to a finite set of values. 2. a 'binary signal' and 'digital signal'? A binary signal is restricted to two values; a digital signal can take a finite set of values (not necessarily just two). 3. an analogue signal bandwidth and a digital signal bandwidth? Bandwidth of an analogue signal: is the difference between the upper and lower cut-off frequencies present in the signal. Bandwidth of a digital signal: normally taken to be the same as the bit rate; that is, the number of bits transmitted per second **38. What does the term FDMA stands for? Explain briefly this concept.** FDMA: frequency-division multiple access frequency-division: because it uses frequency-division multiplexing for the various channels multiple access: because each channel is accessed by different users as calls are set up and then closed down. it allocates a channel to each mobile phone user for only as long as that user's call lasted. As soon as a phone call finished, the channel could be allocated to another user. **39. When a user carrying a mobile phone move from one cell to another in a cellular system, a handover process is carried out by the two base stations of two cells. Explain how the handover process is carried out.** The base station of the cell the mobile phone is in will have identified that the signal strength from that phone is falling, indicating the phone is approaching the cell edge. The mobile phone, meanwhile, will be detecting stronger and stronger signals from one of the adjacent cells and will relay this information to the base station of its current cell. This base station alerts the new base station. This second base station assigns the phone a pair of channels from among its unused ones. The mobile phone is then sent signals to tell it to change the frequencies on which it is sending and receiving. **40. FDMA requires more capability in the mobile phone itself:** * it must be able to send and receive signals on any pair of a number of pairs of channels, and to respond to a control signal telling it which pair to use in any particular call. **41. There is no need for cell size to be the same in urban area when compared with open countryside. Discuss this issue with explaining its advantages as well as its trade-offs.** Cells can be made smaller in urban areas, and much larger in the open countryside. There is an added advantage to varying the cell size in this way. If each cell offers the same number of channels and there are more cells per square kilometer, then more calls per square kilometer can be made., where there will be more mobile phone users. * a trade-off is that more base stations will be needed, and so there is a cost to the system provider. A second, the base station will need to emit a weaker signal, and a weaker signal means more work for the mobile terminal and hence higher battery consumption **42. Discuss femtocells applying its advantages and disadvantages** femtocells (small cells) have emerged for use within a building - for example, for home or small business use. Provided via small, low-power base stations. Advantage: can provide improved indoor coverage for the user and a cost-effective solution for mobile operators. Disadvantage: they are using licensed spectrum and therefore regulation and careful planning is necessary to avoid interference. **43. The placement of the base station within the cell is a consideration in the design of networks.** * In earlier systems, such as the 1G analogue networks: the base station would typically be at the centre of the cell use omnidirectional antenna that transmitted signals equally in all directions. More recently: * base stations have been located at the edges or corners of cells use a directional antenna to cover sections of different cells and increase the chances of receiving a strong signal. **44. List three problems of cellular phone systems:** * eavesdropping. * multipath propagation or fading * Interference of various types * Many countries were using different systems. little scope for roaming **45. How do the various companies distinguish between the calls being made by their users and those being made by other users?** The radio spectrum available for GSM telephony has been divided up into various bands, and each company has been allocated just some of these bands. Thus, the companies distinguish between their subscribers and those of other companies by using different sets of frequencies for the calls. **46. Explain, briefly, three of the functions of the mobile switching centers in the GSM mobile network system.** * Acting as telephone exchange. * Keeping a record of calls and their duration for billing purposes. * Keeping track of where any particular mobile terminal is in the network so that an incoming call can be correctly directed to it. **47. Explain how the synchronization problem was solved in GSM digital mobile system as explained in this module?** * by exploiting the potential of digital processing as follows: Under software control, the base station monitors the time taken for the radio signal to make a round trip to the mobile terminal. This information is periodically transmitted to mobile as it moves around. The mobile then 'resets' its internal clock so as to ensure that the bits it transmits arrive at the base station in exactly the correct time slot. **48. The GSM system has introduced techniques to combat the fading problem. Mention and explain two of these techniques.** * The use of a training sequence: A sequence of bits called training sequence is transmitted as the central portion of each time slot is used. It allows the digital circuitry to be 'trained' to its best behaviour. * frequency hopping: The fading phenomenon is frequency dependent, and transmission on one frequency can suffer degradation at a particular location while another frequency can be comparatively free from problems. GSM allows for the rapid switching from one carrier frequency to another **49. The problems of eavesdropping were addressed in 2G by the use of subscriber identity modules (SIMs). A SIM is a small card in the mobile terminal on which electronic data is stored, mention five examples of that data?** * The SIM Serial Number (SSN), also known as the Integrated Circuit Card ID (ICC-ID). * The International Mobile Subscriber Identity (IMSI). * An authentication key assigned by the mobile service provider. * The identity of the MSC in whose area the mobile terminal is currently located. * user's personal contacts and other similar data. **50. What is the important difference between a circuit-switched network and a packet-switched network? Provide your explanation with an example for each network.** In a circuit-switched network, such as a telephone network, a circuit is set up between the sender and the recipient and is maintained for the duration of the phone call, even if no one is speaking. The circuit cannot be used for any other call during this time. In a packet-switched network, such as the internet, a message is split up into packets which are sent one after the other from the sender to the recipient. Although the path taken by the packets can be the same for all packets, it does not have to be. In addition, packets from other messages can use the path whenever the original message has no packets to send. **51. List the problems of 2G? and what is the solution** GSM was designed as a circuit-switched system. This was appropriate for voice calls, and acceptable for the sort of short text messages sent via SMS. But the limitations of a circuit-switched system quickly became apparent when an attempt was made to offer a more sophisticated data transfer service, including web browsing, via GSM. service providers, it was an inefficient use of the time slots; a pair of slots **52. Explain features of GPRS** * Each base station is connected to both a MSC and a GSN. * single GSN will probably control data traffic in several cells. Whereas voice calls are charged based on time, data transfers are often charged on the basis of the quantity of data transferred. To achieve higher data rates, time slots are allocated differently in GPRS from GSM. They are allocated on a need's basis, rather than on the basis of being fixed for the duration of the call. So any time slots not currently being used for voice calls can be used for packets of data, and more than one time slot per TDMA frame can be allocated to a single user **53. How can Edge achieve high data rate than GPRS** EDGE could achieve approximately three times the maximum data rate of GPRS, while still using the same infrastructure as GSM and GPRS. EDGE used a completely different modulation technique used to superimpose the data onto the carrier prior to transmission to or from the mobile devices: it is a 'simple software upgrade' GPRS and EDGE are sometimes described as 2.5G systems **54. What is the difference between TDMA and FDMA?** * **TDMA:** time-division multiple access * **FDMA:** frequency-division multiple access In a TDMA system, therefore, both the time slots for individual telephone signals and the carrier frequencies have to be allocated to individual users as needed and then reallocated to different users when the original users have terminated their calls. FDMA system: The frequency band was divided into individual channels, each carrying one analogue telephone signal at any one time. **55. What is the aims of 3G** * to achieve more efficient use of the spectrum than 1G and 2G * to achieve higher maximum data rates than 2G * to support equally well all of voice, text, pictures, audio and video. **56. UMTS uses radio network controllers (RNCs) to control both voice and data., explain its function** * RNC controls several base stations (Node B is the UMTS terminology for a base station possibly a hundred or more; and as in GSM/GPRS, the RNCs are networked together. * The RNC sends voice data to a circuit-switching network. * The RNC basically provides the same functions as the MSC in GSM * The nodes dealing with data traffic in UMTS are called serving GPRS support nodes (SGSN). * Gateway SGSNs enable connections from SGSNs to other IP networks, such as the internet. **57. In UMTS, quality of service - QoS refers to an aggregation of system performance metrics, list the most five important of these metrics?** * Availability * Throughput * Packet loss * Latency * Jitter **58. To allow for different QoS requirements, UMTS defined four classes of service. Mention and briefly explain each of these classes.** * Conversational class: requires low latency and low jitter, but is not very stringent on its throughput requirements, example: voice communication * Streaming class: requires high throughput, low packet loss and low jitter, but is not so stringent on latency, example: video streaming * Interactive class:, example: video streaming * Varies as to its throughput requirements because the data tends to be 'bursty'. * It can have high demands, but only for short periods. * It requires low packet loss because missing or incorrect items on the screen would be annoying. * Background class: has no particular QoS requirement., example: Email **59. What is CDMA?** Code-division multiple access (CDMA) is a digital message which is about to be transmitted that is first coded. In this process, every individual bit in the message is replaced by a binary code word, known as a code sequence. The individual bits in this code sequence are called chips. Although the method of generating code sequences is similar for all messages, each code sequence in any specific message is unique to the message of which it is a part. The coded message is then sent as a signal in its allocated channel. In that channel, many other signals, all with their own unique coding, will also be in the process of being sent. **60. what is the advantage of increasing the bandwidth of individual signals, given that bandwidth is limited?** The point is that once several messages have been coded (using different spreading codes) they can all be transmitted together at exactly the same time and at exactly the same frequency. A decoding process at the receiver simply sorts them out again. So the increased bandwidth does not matter very much; it has just been a means of getting many signals to share one channel. Other advantages: it offers greater immunity to fading and resistance to interference than do other multiple-access schemes. **61. Explain with drawing the multiple Access Schemes?** * In FDMA users use different frequencies at the same time and the diagram is showing 2 users using the same time, but different frequencies. * In TDMA up to 8 users use the same frequency, but at slightly different times. The diagram shows 2 users using the same frequency at different times. * In CDMA users use the same frequency at the same time, but each sends a different code. **62. What is WCDMA or wideband CDMA?** The term means 'with a large bandwidth' (the nature of WCDMA). Type of CDMA used in connection with message signals whose data rate is very high. all cells can and do use the same channel frequencies. This is because the various signals being conveyed in a UMTS system are distinguished by their spreading codes, not by the frequencies at which they are transmitted. **63. What are the trade-offs that designers must make in the design of wireless communication system?** * a service over the greatest possible range, * for the largest number of the most mobile of users, * at the highest conceivable data rates, * all with minimum power consumption **64. What are three approaches of convergence?** * Content: how text, graphics, video and audio can all be represented digitally to store and transmitted * Platform: computers, televisions, mobile phones, cameras, etc. are no longer necessarily separate devices. * Distribution how the content gets to the platform, such as via wires or radio waves, via a telephone company, mobile network operator, broadcasting company, etc. **65. What is the difference between Triple Play and Quadruple Play?** - **Commercially** * **Triple play** provision of TV, broadband internet access and telephone services to a user by a single company * **Quadruple play** triple play with mobility added - **Technically** * **provision of video, voice and broadband internet access by means of the same transmission medium** * **provision of video, voice and broadband internet access by means of a wireless mobile system** **66. List the 4G technologies** * LTE-Advanced, an upgrade from LTE. * Mobile WiMAX 2.0. **67. What is the WiMAX stands for, how it used?** * Worldwide Interoperability For Microwave Access: IEEE 802.16 Standard * conveying broadband wireless signals over relatively long distances (up to 50 km) between fixed sites (WWANs). * A more recent development known as mobile WiMAX 2.0 (specified in IEEE 802.16m). * IEEE 802.16m is based on similar technologies to LTE - it uses both OFDM and OFDMA - but it implements them differently. **68. What is the initial aim for Long-Term Evolution (LTE) ?** * To achieve maximum data rates of 100 mbps downlink and 50 mbps for the uplink * Maximum data rates should be achieved throughout a cell of 5 km radius * They assume that a 20 MHz frequency band is available. If the available band is narrower than this then the peak data rates are reduced accordingly. * coexist in the same geographical area with any or all of GSM, plain UMTS and HSPA. **69. What is the difference between FDM and OFDM, and what is the reasons for this technique ?** * **OFDM:** orthogonal frequency-division multiplexing - used in LTE * The big difference in OFDM from ordinary FDM is that instead of a user being allocated a single frequency channel, and hence a single carrier frequency, each user is allocated to a number of channels, and hence a number of subcarrier frequencies. * There are two reasons for this: * using several channels makes the message more resistant to interference * When the signal from a single user is shared among several channels, the data rate on each of these channels is reduced. **70. What is the main aims of LTE-A according to IMT-Advanced?** * The peak downlink data rate for stationary users was to be increased from the 100 Mbps of LTE to 1 Gbps the peak uplink data rate from 50 Mbps to 500 Mbps. * Achieve a peak spectral efficiency in the downlink of 30 bps per Hz. * Support an increased number of users per cell, * improve the performance at the cell edges. **71. What are the key 5G services or use cases** * Enhanced mobile broadband: for example, higher data rates for mobile users * Massive machine type communications: for examples the connection of devices and gadgets to the internet and each other for Internet of Things applications * Ultra-reliable and low latency communications: for example, connected and driverless cars. **72. How GPS works? And how it is possible to fix the position of a receiver using the Global Positioning System (GPS)?** GPS satellites transmit radio signals that are picked up by the GPS receiver. The GPS receiver can calculate its distance from each of three satellites based on the speed of radio signals and the times taken for the signals from each satellite to reach the receiver. the GPS receiver must be located somewhere on the surface of a sphere centered on the satellite, and whose radius is the calculated distance from the satellite. The intersection of the spheres from two satellites restricts the possible location of the receiver to a circle. The surface of the third sphere intersects this circle at just two points, and only one of these will be on the Earth's surface. fourth satellite is used in practice to provide the required level of measurement accuracy. **73. One important requirement in global positioning system (GPS) is that a GPS receiver needs to be able to locate the GPS satellites in the sky in order to lock on to the signals they are transmitting. Explain how this can be done.** the US Department of Defense, which is responsible for the satellites, publishes tables of the satellites' positions at any given time, and these tables are programmed into each GPS receiver. the processes involved in the measurement of the propagation time will show that the location obtained from the simple three-satellite triangulation is not **74. Explain briefly three of these errors erors can occur in GPS position measurements.** * Inaccurate clocks in GPS receivers. * The radio signals from the satellite do not travel at a constant speed. * some of the signals will travel through more of the Earth's atmosphere than others. * The satellite may not be exactly where the stored tables say it should be. * multipath propagation. This problem