Mobile computing unit-I-notes 07.01.2020
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The document summarizes the key concepts from the course "Mobile Computing" taught in the sixth semester of the Bachelor of Engineering program in Computer Science and Engineering at RAMCO Institute of Technology. It covers the basics of mobile computing including definitions, applications, components of wireless communication systems, mobile communication architectures, and generations of mobile technologies from 1G to 5G. It also describes multiplexing techniques such as space division multiplexing, frequency division multiplexing, and time division multiplexing that allow multiple users to share communication channels and medium.
Mobile computing unit-I-notes 07.01.2020
- 1. RAMCO INSTITUTE OF TECHNOLOGY Department of Computer Science and Engineering Academic Year: 2019- 2020 (Even Semester) Degree, Semester & Branch: VI Semester B.E. CSE. Course Code & Title: CS8601 Mobile Computing. Name of the Faculty member: Dr.M.Kaliappan, Associate Professor/CSE ---------------------------------------------------------------------------------------------------------------- UNIT-I INTRODUCTION 9 Introduction to Mobile Computing – Applications of Mobile Computing- Generations of Mobile Communication Technologies- Multiplexing – Spread spectrum -MAC Protocols – SDMA- TDMA- FDMA- CDMA ---------------------------------------------------------------------------------------------------------------- Credit: Jochen Schiller, “Mobile Communication”, PHI, Prasant Kumar Pattnaik, Rajib Mall, “Fundamentals of Mobile Computing”, PHI --------------------------------------------------------------------------------------------------------------- Objective: To enable the students to understand the basic concepts of mobile computing. Outcome: Explain the basics of mobile computing and MAC protocols ---------------------------------------------------------------------------------------------------------------- 1. Mobile computing (2 marks) Mobile computing is a technology in which computation performed remotely while on move People can access information from anywhere at any time Mobility Change location while communicating to invoke computing service Computing Carryout process related to service invocations on a remote computer 1.1 Basics of Communication Technologies (4 marks) Mobile handset Individual users with handset directly communicate with each other over a radio link or wired link formed through intermediaries such as base station and fixed line. When all intermediaries are located in ground, the communication system is called terrestrial radio system. If at least one of the intermediaries is satellite, then it is referred as satellite radio system Cell phone system Provide data services like SMS,MMS, email and web browsing Current location of the user is maintained by mobile telecommunication networks while move Types of communication networks (2 marks) Voice networks Data networks Voice networks Analog signal is modulated on a carrier signal for transmission Ex: Traditional Telephonic network-Circuit switching
- 2. Data networks Data network is a telecommunications network that allows computers to exchange digital data. Data refers to any information like text, documents, picture, movie, sound Example: packet switching Computer networks Controller area networks(CAN) LAN Internet Controller area networks(CAN) (2 marks) Very small networks used to connect different components of embedded controller Less than 50 meters Ex : Automatic doors, coffee machines LAN (2 marks) Used to connect no. of computers connected within organization to share data and other resources like printers, fax. Typically deployed in a building or a campus Operate from 10mbps to 1Gbps Now 100Gigabit Ethernet
- 3. Internetworks Several LAN interconnected using switches Network of networks A node in LAN communicates with a node in another LAN using packet switching 1.2 Components of wireless communication system (4 marks) Transmitter It modulate or encode data, voice, video into higher frequency carrier signal, then it can be radiated and propagated use of radio frequency spectrum Receiver It receive the modulated signals and reverse the function of transmitter and thereby recover the transmitted signal Antenna It convert the electrical signal from transmitter to a electromagnetic RF wave or conversely, to convert RF wave to an electrical signals Filters It used to reject interfering signal lying outside the band of receiver and transmitter. Amplifier Amplifies the strength of a signal. It minimize the noise Mixer Used to achieve frequency conversion at the transmitters and receivers 1.3 Architecture of Mobile Telecommunication system WLAN Connectivity between computers over short distance using wireless medium. Educational institute, office building
- 4. 1.4 Mobile Computing vs Wireless networking (2 marks) Mobile Computing Wireless Networking It is a technology that access data through wireless network It is a network that Connectivity between computers over short distance using wireless medium It refers to a device performing computation that is not always connected to a central network It refers to the data communication without the use of a landline. Eg. Cellular Telephone, Two way radio, Satellite, Wireless Connection It denotes accessing information and remote computational services while on the move It provides the basic communication infrastructure necessary for mobile computing It refers to computing devices that are not restricted to a desktop. Eg: Smart Phone, PDA, Laptop etc It is a method/technology of transferring information between a computing devices such as PDA & data sources without a physical connection 1.5 Applications of mobile computing (2 marks) Emergency services Stock Broker Vehicles Estate Agents courts companies Stock Information Collection/Control Credit Card Verification Taxi/Truck Dispatch Electronic Mail/Paging 1.6 Disadvantages of Mobile computing (2 marks) Battery consumption hindrance Interference Inefficient bandwidth in transmission. Connection losses over entire network.
- 5. 1.7 Characteristics of Mobile computing (8 marks) • Ubiquity • Location awareness • Adaptation • Broadcast • Personalization Ubiquity • Ability of user to perform computation from everywhere at any place • Ex: Business executive receives notification and issue transaction in the coverage area Location awareness • Handheld device equipped with GPS that provide current location of the users. • Many application from strategic to personalized service require location based service • Ex: to locate the shop in nearby place • Traffic control • Fleet management • Emergency service Adaptation • Ability of system to adjust to bandwidth fluctuation without inconveniencing the user • Intermittent disconnection and bandwidth fluctuation occur due to handoff, obstacle and environment noise Broadcast • Delivery of data simultaneously to hundreds of mobile users • Ex: users at specific location near a railway station may ne sent advertising information of taxi Personalization • Services in mobile environment can be personalized according to user profile 1.8 Cellular Mobile communication (8 marks) • Partition the region into smaller regions called cells. • Each cell gets at least one base station or tower • Users within a cell talks to the tower • BS keep track of the calls Properties of Cell structure • Typical Cell sizes – tens of meters in buildings – some cites few hundred meters – country side few tens of kilometers
- 6. • Advantages of cell structures: – more capacity due to frequency reuse – less transmission power needed – more robust, tolerate failures – deals interference, transmission area locally • Problems: – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells Other common channel Channel Reuse • Cell structure can reuse frequency only when certain distance is maintained between cells that use the same channels. • Fixed Allocation schemes: – certain frequencies are assigned to a certain cell – problem: different traffic load in different cells • Dynamic Allocation schemes: – base station chooses frequencies depending on the frequencies already used in neighbor cells
- 7. 1.9. Generation of Mobile computing (8 marks) • 1G - First generation (Analog and FM) • 2G - Second generation (Digital, TDMA, CDMA) • 3G - Third generation (Multi-media) • 4G - Fourth generation (LTE) • 5G - Fifth Generation(WWWW) 1G • These phones were the first mobile phones to be used, which was introduced in 1982 and completed in early 1990. • It was used for voice services • It based on Advanced Mobile Phone System (AMPS) technology. • Analog signals are modulated on higher frequency • Signals transmitted by separate channels – Incoming signal-25MHz – Outgoing signal-25 MHz • Frequency spectrum was split into sub channels, each used by different users • 1G frequency band split into five band sub-band1, sub-band2, sub-band3, sub-band4, sub-band5 • Each channel to be adjacent to each other, and separated from adjacent channel about 30khz called guard band • Guard band reduce number of simultaneous calls in cell • 1G Standards – USA- AMPS(Advanced Mobile phone system) – Europe-NMT450(Nordic Mobile Telephone) – UK-TACS(Total access communication system) • 1G follows multiple access type , once a caller hanged up, another caller use the same frequency(FDMA) • Call was dropped during handoff when another call use the channel in another cell • Hand off or what happens when a user is mobile? (2 marks) Especially, when crossing a cell boundary while continuing the call. Handoff strategy is invoked. It finds a new base station and higher priority over new call invocation. – Hard handoff • Mobile user is passed between disjoint towers that assign different frequency or adapt different air-interface technology – Soft handoff – Mobile user communicates to two towers simultaneously and the signal is treated as a multipath signal Drawback of 1G • Poor voice quality and battery life • Large phone size • Poor handoff reliability • It has no security • Offered very low level of spectrum efficiency • The analogue signals are more to suffer interference problems 2G • It uses digital signals for voice transmission, and provides services to deliver text (SMS) and picture message at low speed (in kbps). • It use the bandwidth of 30 to 200 KHz
- 8. • It reduction in call cost • 2G follows some principles of 1G, • 2G use CDMA and TDMA modulation techniques as channel access technology • North America used IS-95 standard (CDMA), can multiplex up to 64calls per channel in 800MHZ • Europe used GSM, which used TDMA, can multiplex up to 8calls per channel in 900 and 800MHz • Advantages – Provides better quality and capacity – Provide higher voice quality – Provide data service like SMS and email – digital calls are free of static & background noise – 2G technology offers improved privacy • Disadvantages – Unable to handle complex data such as videos. – Required strong digital signals to help mobile phones work. 2.5G • It used GPRS(General packet Radio service) , extension of GSM • It based on packet switching technique • Deployment began in 2000 followed by EDGE(Enhanced Data GSM Environment) • Advantages and Disadvantages • User remains connected to the internet without additional cost. • Faster internet browsing • Use streaming application • Camera phones • Did not offer multi-megabit data rate • Cell towers had a limited coverage area. • Built mainly for voice services and slow data 3G • First 3G adopted in Japan in 2001 as DoCoMo • It referred as IMT-2000(International Mobile Tele communication) • It support purely data network, use packet switching • UMTS (Universal Mobile Telephone system) is one of the 3G mobile system for GSM networks. • UMTS support various cell size from Pico cell to global cell like satellite • Europe adopted UMTS, while the USA uses CDMA2000 • Advantages • Higher data transmission rate • Offered increased bandwidth • High quality traditional voice call • Provide cheaper calls • Support applications like email, live-video buffering location based service, video telephony, multimedia gaming 4G • It make mobile broadband internet access • It has high speed internet access from smart phone to laptop with USB wireless modem • It uses WiMAX and LTE standard • 4G including IP telephony, video conferencing and 3D television • Advantages
- 9. • Capable of provide 10Mbps-1Gbps speed • High quality streaming video • Combination of Wi-Fi and Wi-Max • High security • Provide any kind of service at any time as per user requirements anywhere • Expanded multimedia services • Low cost per-bit • Battery uses is more • Disadvantages • Hard to implement and Need complicated hardware • Expensive equipment required to implement next generation network Comparison of Generation of cell phones or mobile computing (2 marks) 5G • 5G will not only enable better usage of smartphones, but it will also start the era of driverless smart cars and smart homes. • In the smartphone sector, 5G is expected in the field of AR (Augmented Reality) and VR (Virtual Reality) • The main focus of 5G will be on world-Wireless World Wide Web (WWWW). • It is a complete wireless communication with no limitations. • Provides large broadcasting of data up to 20 Gbps. • Multi-media newspapers, watch TV programs with the clarity(HD Clarity) • Large phone memory, dialing speed, clarity in audio/video • T-Mobile's LTE/LAA network(License Assisted Access) Plan of 5G • Although 5G will first launch in the U.S. probably by the end of 2018 • China will start their pre-commercial use from 2019. • In India, we can expect the launch of 5G network by the end of 2019. • By 2020 ,we can use the network commercially
- 10. 1.10 Structure of Mobile Computing applications (8 Marks) Functionalities provided by each tier structure of a mobile computing application Presentation tier It describes the presentation of the information. Topmost level - Concerns the user interface Facilitates the users to issue requests and present the results to them meaningfully Programs run on the client‟s computer. Includes web browsers customized client programs for dissemination of information and For collection of data from users Application Tier It responsible for making logical decisions and performing computation Moves and processes data between the presentation and data tier “engine” of an automobile It processing of user input, obtaining information and then making decisions Implemented using technologies like Java, .NET services, cold fusion, etc. Data tier • Responsible for basic facilities of data storage, access and manipulation • Often contains a database • Information is stored and retrieved from this database (file system can also be used to store small amount of data)
- 11. • Implemented on a fixed server 1.10 Multiplexing (16 Marks) Multiplexing (2 Marks) • Multiplexing describes how several users can share a medium with minimum or no interference. • The task of multiplexing is to assign space, time, frequency, and code to each communication channel with a minimum of interference and a maximum of medium utilization. • Example, highways with several lanes. – Many users (car drivers) use the same medium (the highways) with hopefully no interference (i.e., accidents). – This is possible due to the provision of several lanes (space division multiplexing) separating the traffic. – In addition, different cars use the same medium (i.e., the same lane) at different points in time (time division multiplexing). 1.10.1 Types of Multiplexing • Space division multiplexing • Frequency division multiplexing • Time division multiplexing • Code division multiplexing Space division multiplexing (8 Marks) – The term communication channel refers to an association of sender(s) and receiver(s) who want to exchange data. – Figure shows six channels ki and introduces a three dimensional coordinate system such as code c, time t and frequency. • In space division multiplexing (SDM), the space si is represented as circles indicating the interference range • The channels k1to k3 can be mapped onto the three „spaces‟ s1 to s2 which clearly separate the channels and prevent the interference ranges from overlapping. • Guard space (2 Marks) – The space between the interference ranges is sometimes called guard space.
- 12. • For the remaining channels (k4 to k6), three additional spaces would be needed. • EX: Telephone Exchange system – Each subscriber is given a separate pair of copper wires to the local exchange • Disadvantages (2 Marks) – SDM implies a separate sender for each communication channel that clearly represents a waste of space Frequency division multiplexing (FDM) (8 Marks) • It subdivides the frequency dimension into several non-overlapping frequency bands. • Each channel ki is now allotted its own frequency band. • Senders using a certain frequency band continuously. • Guard spaces are needed to avoid frequency band overlapping or interference. • Ex: This scheme is used for radio stations – Within the same region, where each radio station has its own frequency. Advantages (2 Marks) • This scheme does not need coordination between sender and receiver: the receiver only has to tune in to the specific sender Disadvantages (2 Marks) • While radio stations broadcast 24 hours a day, mobile communication typically takes place for only a few minutes at a time. • Assigning a separate frequency for each communication scenario would be a waste of (scarce) frequency resources. • Additionally, the fixed assignment of a frequency to a sender inflexible and limits the number of senders. Time division multiplexing (8 Marks) In time division multiplexing (TDM), a channel ki is given the whole bandwidth for a certain amount of time, i.e., all senders use the same frequency but at different points in time. Guard spaces • Guard spaces represent time gaps, have to separate the different periods when the senders use the medium. • In our highway example, this would refer to the gap between two cars. If two transmissions overlap in time, this is called co-channel interference.
- 13. To avoid interference, precise synchronization between senders is necessary. Disadvantages (2 Marks) • It needs complex coordinating system to avoid interference. In which all senders need precise clocks or, distribute a synchronization signal to all senders. • A receiver listens at exactly the right point in time. Advantages (2 Marks) • This scheme is flexible as one can assign more sending time to senders with a heavy load and less to those with a light load Frequency and time division multiplexing (4 Marks) It can be combined, i.e., a channel ki can use a frequency band for a certain amount of time. Guard space • Guard spaces are needed both in the time and in the frequency dimension. Advantages • This scheme is more robust against frequency selective interference efficiently. Disadvantages (2 Marks) • Need necessary coordination mechanism between different senders. • Two senders will interfere when they select the same frequency at the same time. • Interference in a certain small frequency band. • A channel may use this band only for a short period of time. • EX: GSM • The mobile phone standard GSM uses this combination of frequency and time division multiplexing for transmission between a mobile phone and a base station
- 14. Code division multiplexing (8 Marks) Code division multiplexing (CDM) is a new scheme in commercial communication systems. First used in military applications due to its inherent security features it now features in many civil wireless transmission scenarios. All channels ki use the same frequency at the same time for transmission. Separation is achieved by assigning each channel its own „code‟, Guard spaces • Guard spaces are realized by using codes. e.g., orthogonal codes. • Advantages (2 Marks) It gives good protection against interference and tapping. Different codes have to be assigned. Assigning individual codes to each sender does not usually cause problems. • Disadvantages (2 Marks) • A receiver has to know the code and must separate the channel with user data from the background noise composed of other signals and environmental noise. • Additionally, a receiver must be synchronized with the transmitter to apply the decoding correctly. 1.11 Spread spectrum (16 marks) Spread spectrum (2 marks) Spread spectrum techniques involve spread across a wide bandwidth needed to transmit data i) The following figure shows an idealized narrowband signal from a sender of user data (here power density dp/df versus frequency f).
- 15. ii) The sender now spreads the signal in which, narrowband signal converts into a broadband signal iii) During transmission, narrowband and broadband interference add to the signal iv) The sum of interference and user signal is received. The receiver is converting the spread user signal into a narrowband signal again. v) The receiver applies a bandpass filter to cut off frequencies left and right of the narrowband signal. Finally, the receiver can reconstruct the original data • Example • Six different channels use FDM for multiplexing, which means that each channel has its own narrow frequency band for transmission. • Between each frequency band a guard space is needed to avoid adjacent channel interference. • Channels 1, 2, 5, and 6 could be received while the quality of channels 3 and 4 is too bad to reconstruct transmitted data. Narrowband interference destroys the transmission of channels 3 and 4.
- 16. • All narrowband signals are now spread into broadband signals using the same frequency range. • CDM is now used instead of FDM. Spreading of a narrowband signal is achieved using a special code • Each channel is allotted its own code, which the receivers have to apply to recover the signal. Without knowing the code, the signal cannot be recovered and behaves like background noise. • Spread spectrum now allows in license-free bands such as 850 MHz that use for TDM and FDM systems • Advantages of Spread Spectrum (2 marks) – Cross-talk elimination – Better output with data integrity – Better security – Reduction in noise – Not easy to demodulate/decode • Disadvantage (2 marks) • Increased complexity of receivers that have to despread a signal. • The large frequency band that is needed due to the spreading of the signal. • Spread signals appear more like noise; they still raise the background noise level. Types of spread spectrum (2 marks) • Frequency Hopping Spread Spectrum(FHSS) • Direct Sequencing Spread Spectrum(DSSS) Frequency Hopping Spread Spectrum (FHSS) (8 marks) o In frequency hopping spread spectrum (FHSS) systems, the total available bandwidth is split into many channels of smaller bandwidth plus guard spaces between the channels. o Transmitter and receiver stay on one of these channels for a certain time and then hop to another channel. This system implements FDM and TDM. o The pattern of channel usage in the frequency hopping spread spectrum is called the hopping sequence, o Dwell time (2 marks) The time spend on a channel with a certain frequency in the frequency hopping spread spectrum is called the dwell time
- 17. Hopping pattern: F2-F5-F3-F6-F1-F6-F4 Types of FHSS (2 marks) • Slow hopping • Fast hopping – Slow hopping – In slow hopping, the transmitter uses one frequency for several bit periods. – Figure shows five user bits with a bit period tb. – Performing slow hopping, the transmitter uses the frequency f2 for transmitting the first three bits during the dwell time td. Then, the transmitter hops to the next frequency f3. – Slow hopping systems are typically cheaper and have relaxed tolerances. It is an option for GSM – Fast hopping – For fast hopping systems, the transmitter changes the frequency several times during the transmission of a single bit. – In the example, the transmitter hops three times during a bit period. – Fast hopping systems are more complex to implement because the transmitter and receiver have to stay synchronized within same points of time. Example – FHSS system is used in Bluetooth
- 18. – Bluetooth performs 1,600 hops per second and uses 79 hop carriers equally spaced with 1 MHz in the 2.4 GHz ISM band Transmitter Receiver Applications of FHSP or spread spectrum (2 marks) – Military use – Bluetooth – Walkie-Talkies – Other radios • Ex YouTube Video: https://www.youtube.com/watch?v=CkhA7s5GIGc Direct sequence spread spectrum (8 Marks) • In Direct sequence spread spectrum (DSSS), a user bit stream XOR with a sequence pattern or spreading factor. It is called chipping sequence • While each user bit has a duration tb, the chipping sequence consists of smaller pulses, called chips, with a duration tc. • If the chipping sequence is generated properly that is called pseudo-noise sequence. • The spreading factor s = tb/tc determines the bandwidth of the resulting signal. • If the original signal needs a bandwidth w, the resulting signal needs s·w after spreading
- 19. • Civil applications use spreading factors between 10 and 100, military applications use factors of up to 10,000. • Wireless LANs standard IEEE 802.11 the sequence 10110111000 that is called Barker code. • The DSSS Barker codes exhibit a good robustness against interference and insensitivity to multi-path propagation. • Other known Barker codes are 11, 110, 1110, 11101, 1110010, and 1111100110101 DSSS transmitter Digital modulation (2 Marks) • The spreading of the user data with the chipping sequence. Radio modulation (2 Marks) • The spread signal is then modulated with a radio carrier • The receiver has to know the original chipping sequence generated from the transmitter. • The receiver only has to perform the inverse functions of the two transmitter modulation steps.
- 20. • However, noise and multi-path propagation require additional mechanisms to reconstruct the original data. • The first step in the receiver involves demodulating the received signal. This is achieved using the same carrier as the transmitter reversing the modulation. • Additional filtering can be applied to generate this signal. DSSS receiver • During a bit period, which also has to be derived via synchronization, an integrator adds all these products. Calculating the products of chips and signal, and adding the products in an integrator are also called correlation, the device a correlator. • Finally, in each bit period a decision unit samples the sums generated by the integrator and decides if this sum represents a binary 1 or a 0 Example • User data 01 • Applying the 11-chip Barker code 10110111000 • The results in the spread „signal‟ 1011011100001001000111. • On the receiver side, this „signal‟ is XORed bit-wise after demodulation with the same Barker code as chipping sequence. • This results in the sum of products equal to – 0 for the first bit – 11 for the second bit. • The decision unit map the first sum (=0) to a binary 0, the second sum (=11) to a binary 1. This constitutes the original user data What happens in case of multi-path propagation? (2 marks) • Several paths with different delays exist between a transmitter and a receiver. Additionally, the different paths may have different path losses. • In this case, rake receivers are used to provide a possible solution. It take advantage of the multi-path propagation by combining the different paths. • A rake receiver uses n correlators for the n strongest paths. Each correlator is synchronized to the transmitter plus the delay on that specific path. • As soon as the receiver detects a new path which is stronger than the currently weakest path, it assigns this new path to the correlator with the weakest path. – The output of the correlators are then combined and fed into the decision unit. Comparison between FHSS and DSSS (2 marks) FHSS DSSS Multiple frequencies are used Single frequency is used Frequency reuse is allowed Frequency reuse is not allowed
- 21. Sender need not wait Sender has to wait if the spectrum is busy Power strength of the signal is high Power strength of the signal is low It is never affected by interference It can be affected by interference It is cheaper It is expensive 1.12 MAC protocol (2 Marks) • It enforces the discipline in the access of shared medium when multiple user content to access that channel. • The objectives of MAC protocol are maximization of the utilization of the channel and minimization of the average latency of the transmission. • MAC (Medium access control) protocol is sub layer of data link layer, it directly invoke the physical layer. Properties of MAC protocol (2 Marks) • It helps maximize the utilization of channel • Channel allocation need to be fair • Support different types of traffic having different and average bitrate • It should be robust when equipment failure and change network condition • It enforce the discipline in the access of shared medium from multiple user • 802.11 is the standard for MAC protocol • –EX: 802.11 network card, router Functionalities of MAC protocol (4 Marks) • Roaming o •Provide uninterrupted service when user walk from one place to another within wireless station • Point Coordination function o •Provide guarantees on reduce access delay and minimum transmission bandwidth and other QoS • power conservation o •Provide power management o •Transceiver must be switched off whenever carrier sensing is not needed o •Two state : sleep and wake • Authentication o •recognizing a user‟s identity. It is the mechanism of associating an incoming request with a set of identifying credentials Wireless Networks • Infrastructure Based • Infrastructure less Infrastructure Based o Access points are connected to existing wired network o It interact with wireless node as well as existing wired node o It is bridge between other network and provide authentication mechanism
- 22. Infrastructure less network • The network set up on the fly at any place. Nodes networked among themselves. Nodes communicate directly each other, or forward message through intermediate nodes. It is called scatternet. • Difficulty in Collision detection and other Problems • Piconet : Set up personal area network using blutooth technology 1.8 MAC Protocol Issues (8 Marks) Identify the specific reasons as to why the MAC protocols designed for infrastructure-based wireless networks may not work satisfactorily in infrastructure less environment. Because, collisions detect scheme is difficult to implement in a wireless environment since collisions are hard to be detected by the transmitting nodes. Hidden and Exposed terminal problems makes MAC protocols inefficient. • Hidden terminal problem • Exposed terminal problem Hidden terminal problem (2 Marks) • Transmission of A reaches B, but not C • Transmission of C reaches B, but not A
- 23. • However, the radio signal of B reaches both A and C in the range of B • The effect is A can not detect C and C can not detect A • A starts sending to B, C does not receive this transmission, C also wants send to B and sense the medium. To C the medium appears to be free, Thus C starts sending to B that cause collision. • But now A cannot detect collision and continue with its transmission • A is hidden for C and vice versa • Exposed terminal problem (2 Marks) • The radio Transmission of A reaches B, and C • The radio Transmission of C reaches D, and A • A want to communicate B , A stars sending data to B • C wants to communicate with D, C sense the carrier and finds that A is talking to B • C has to wait till A finishes with B, However D is outside of A waiting is not necessary • In fact, A,B and C,D can communicate with each other in parallel with out any collision, but according to protocol that is not possible. • A and S are exposed terminals Solutions • Collision detection mechanism are good idea on wired LAN , but they cannot used in wireless LAN • Reason – CD mechanism requires the implementation of Full duplex technique for transmitting and receiving at the same time – In wireless LAN, we cannot assume that all the stations will be able to receive signal from each other(hidden terminal problem) – Collision Avoidance mechanism is a solution
- 24. 1.9 Taxonomy of MAC protocols (2 Marks) • Fixed assignment schemes – Circuit switched method – Resources are assigned for entire duration of the call • Random assignment schemes – Connection less Packet switched technique – The node start to transmit as soon as they have a packet to send • Reservation based assignment schemes – Connection based Packet switched technique – A node make a explicit reservation of the channel for a call before transmitting – Suitable for varying traffic 1.9.1 Fixed assignment schemes (16 Marks) – Frequency division multiple access(FDMA) – Time division multiple access(TDMA) – Code division multiple access(CDMA) 1.9.1.1 Frequency division multiple access(FDMA) (8 Marks) – The available bandwidth (frequency) is divided in to many frequency band called channel(coaxial cable or microwave beam) – Full duplex communication is take place, each user is allocated a forward channel for communication from handset to base station and reverse channel(Backward) for communicating from BS to hand set – Each user is allocated two unique frequency channel, one for transmitting , other for receiving – When a call is underway, no other user would be allocated the same frequency band – Unused transmission time occurs when the user pauses between the transmission, it does not achieve the high channel utilization – It is used in satellite communication systems and telephone trunklines.(First Generation) The two frequencies: uplink and Downlink – Uplink From mobile station to base station or from ground control to satellite – Downlink From base station to mobile station or from satellite to ground control
- 25. Advantages (2 Marks) • Reduces the bit rate information • It reduces the cost and lowers the inter symbol interference • An FDMA system can be easily implemented. • Since the transmission is continuous, less number of bits are required for synchronization and framing Disadvantages (2 Marks) • The maximum flow rate per channel is fixed and small • Guard bands lead to a waste of capacity. • Unused transmission time occurs when the user pauses between the transmission, it does not achieve the high channel utilization 1.9.1.2 Time division multiple access (8 Marks) • Multiple nodes are allotted different time slots to access the channel • Time line is divided into fixed time slot and these are allocated multiple users • All users use the same channel, but take turn in transmitting • Time slots are allocated in round robin manner • Each user assigned one time slot per frame, unused time slots goes ideal, leading low channel utilization • The fundamental unit of time is called a burst period and it lasts for approximately 0.577 ms. • Eight of these burst periods are grouped into what is known as a TDMA frame • A 25 MHz frequency range holds 124 single bandwidth of each kHz; each of these frequency channels contains 8 TDMA conversation channels Advantages of TDMA (2 Marks) – Permits flexible data rates. – No guard band required for the wideband system. Disadvantages (2 Marks) – Call time is needed in each slot to accommodate time to inaccuracies (due to clock instability). – Electronics operating at high bit rates increase energy consumption. – Complex signal processing is required to synchronize within short slots. 1.9.1.3 Code Division Multiple Access (CDMA) (8 Marks) • CDMA is a sort of multiplexing that facilitates multiple users to occupy a single transmission channel. It optimizes the use of available bandwidth. • Multiple users are allotted different code(sequence of 0 and 1) to access same channel • Special coding scheme is used that allows signal from multiple users to be multiplexed over the channel • Multiple users use the same frequency at the same time
- 26. • CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology • It used m-bit pseudo noise (PN) code sequence to generate different code. • Using m-bits, 2 m -1 different codes can be obtained. From this code , each user will use one code • CDMA used in second-generation (2G) and third-generation (3G) wireless communications • The technology is commonly used in ultra-high-frequency (UHF) cellular telephone systems, bands ranging between the 800-MHz and 1.9-GHz. CDMA code generation • A code for a user is orthogonal that refers the vector inner product is zero • Two vector: p=(2,5,0), q=(0,0,17) • Inner product p*q=(2*0 + 5*0 + 0*17) =0 • Bipolar notation: 0 represented as -1, 1 represented as +1 • Sender – Unique random number key – XOR the signal with random number key • Receiver – Knows the random number key – Demodulate the signal to get data • Sender – X_data = 1, X_key = 010011 – Bipolar representation of X_key = -1,+1,-1,-1,+1,+1 – Calculated signal at sender Xs= X_data * X_key – +1* X_key = +1 * (-1,+1,-1,-1,+1,+1)= (-1,+1,-1,-1,+1,+1) – Y_data = 0, Y_key = 110101 – Bipolar representation of X_key = +1,+1,-1,-1,+1 ,-1,+1 – Calculated signal at sender Ys= Y_data * Y_key – -1* Y_key = -1 * (+1,+1,-1,-1,+1 ,-1,+1)= (- 1,-1,+1,+1,-1 ,+1,-1) • Receiver – Xs+Ys= (-1,+1,-1,-1,+1,+1) + (- 1,-1,+1,+1,-1 ,+1,-1) – (-2, 0, 0, -2, +2, 0) • Z wants to get information of sender X and sender Y • Z * X_key = (-2, 0, 0, -2, +2, 0) * (-1,+1,-1,-1,+1,+1) = (2 +0+0+2+2+0)=6 > 0 (positive) X_data = 1 • Z * Y_key = (-2, 0, 0, -2, +2, 0) * (+1,+1,-1,-1,+1 ,-1,+1) = (-2 +0+0-2-2+0)=-6 < 0 (negative) Y_data = 0
- 27. Advantages (2 Marks) • CDMA provide a power control • Flexible transfer may be used. • Reduces interference. Disadvantages (2 Marks) • The code length must be carefully selected. A large code length can induce delay or may cause interference. • Time synchronization is required. Space Division Multiple Access (SDMA) (4 Marks) • It is used for allocating a separated space to users in wireless networks. • Application – Application involves assigning an optimal base station to a mobile phone user. The mobile phone receives several base stations with different quality. A MAC algorithm decide which base station is best, taking into account which frequencies (FDM), time slots (TDM) or code (CDM) are still available. – SDMA is always in combination with .one or more other schemes – The SDMA algorithm is formed by cells and antennas which constitute the infrastructure implementing space division multiplexing (SDM). – A new application of SDMA comes up with beam-forming antenna arrays. Single users are separated in space by individual beams. – This improves the overall capacity of a cell – e.g., measured in bit/s/m2 or voice calls/m2. 1.9.2 Random assignment schemes (16 Marks) • Pure ALOHA • Slotted ALOHA • CSMA • CSMA/CD • CSMA/CA Pure ALOHA • Developed at university of Hawai • If a node has data to send , it begins to transmit • It does not check whether the channel is busy before transmitting • If the frame successfully reaches the destination , it send next frame • If the frame fails at destination , it is resent after a random amount of time to reduce the probability of re-collision • Suitable for small network, not suitable for large network. It leads collision.
- 28. Slotted ALOHA • The time divided into equal size slots in which a packet can be sent, the size of the packet is restricted • A node wants to send a packet, start transmission at beginning of the slot • The slotted ALOHA employs beacon signal that are sent at precise interval that mark the beginning of the slot at which point the nodes can start to transmit • Transmission time is broken into slots equivalent to the transmission time of a single packet. Stations are only allowed to transmit at slot boundaries • Does not work well in large network Problem of Slotted ALOHA (2 Marks) – The random retransmission of the packet will have an effect on the delay associated with successful packet delivery. – If the limit is too short, the probability of re-collision is high. – If the limit is too long the probability of re-collision less but there is unnecessary delay in the retransmission. CSMA (Carrier sense Multiple Access) • A node senses the medium before transmit, It the medium is busy, wait before transmitting – CSMA/CD – CSMA/CA – CSMA/CD does not work well in wireless network, it difficult for a transmitting node to detect collision since received signal from other node would be too feeble – Only destination node notice the corrupted frame after it computes checksum. This leads to retransmission and wastage of channel utilization – In contrast, in wired network when node detects a collision , it immediately stop transmission, thereby minimize the channel wastage Do you agree with the following statement: “In CSMA/CD protocol, when two nodes transmit on a shared medium, a collision can occur only when two nodes start transmitting exactly at the same time instant”, Explain your answer. • Yes. A collision may occur only when two nodes start transmitting exactly at the same time instant because a node senses the medium before transmit, It the medium is free, it sends data immediately Why are collision detection based MAC protocols not suitable for wireless networks? (2 Marks) CD mechanism requires the implementation of Full duplex technique for transmitting and receiving at the same time In wireless LAN, we cannot assume that all the stations will be able to receive signal from each other(hidden terminal problem)
- 29. Collision Avoidance mechanism is a solution CSMA/CA (8 Marks) • Idea is prevent collision • When node is transmitting on the channel, other nodes might be waiting to transmit until channel become free • When transmitting node complete its transmission , the waiting nodes start transmitting at a same time, • To overcome collision, all the nodes are forced to wait for a random time and then sense the medium again • The chances of two nodes starting to transmit at the same time is greatly reduced • Non persistent • 1-persistent • P-Persistent • Non persistent • Channel transmit frame if it is ideal, otherwise it wait a random time and again sense line 1-persistent • When the station detects an idle-channel, it immediately transmits the frame with probability 1 P-Persistent • If time slot <= propagation time slot, send data if channel is ideal • With the probability q=l-p, the station then waits for the beginning of the next time slot
- 30. 1.9.3 Reservation based scheme (8 Marks) • It is the RTS/CTS scheme • A sender transmits an RTS(Request to send) packet to receiver before transmission • The receiver sends a CTS (clear to send) packet, the data transmission commences only after that. • When the other node senses the CTS packet, they wait until the transmission is complete. • Reservation is achieved by sending RTS/CTS packets between sender and receiver, other nodes defer its transmission in specified period to avoid collision. • RTS/CTS based MAC protocol – MACA, MACAW, MACA-BI, PAMAS,DBTMA,MARCH,S-MAC – designed for wireless sensor networks • MACA – Multiple access collision avoidance – MACA solves the hidden/exposed terminal problem – A node requests to use the medium by sending RTS to receiver. Since radio signals propagate Omni directional , every node within the senders range will hear RTS and then refrain from transmitting – The receiver is ready to receive data, it sends CTS Solution for Hidden terminal problem (2 Marks) • RTS contain sender‟s name and receiver‟s name as well as length of the future transmission • CTS contain sender‟s name and receiver‟s name as well as length of the planned transmission • CTS hear by C also, the medium is reserved by A with specified time, C refrains from transmitting for time indicated in CTS • The collision cannot occur at B during data transmission • Issues
- 31. • Collision can occur during the sending of RTS. A and C could send RTS at the same time. But an RTS occurs over a very small time compared to data transmission. The probability of collision occur less Solution for exposed terminal problem (2 Marks) Issues • Collision can occur during the sending of RTS. A and C could send RTS at the same time. But an RTS occurs over a compared to data transmission. Thevery small time probability of collision occur less Application oriented questions 1 Consider a user data 101. What is the spreading signal when applying the 11-chip Barker code 11110111001?