mobile computing - Unit 1 Material.pdf
- 1. Prepared By, Mrs. V. Deepa Priya AP/IT Kamaraj College of Engineering and Technology, VIrudhunagar
- 2. Mobile Computing – Mobile Computing Vs wireless Networking – Mobile Computing Applications – Characteristics of Mobile computing – Structure of Mobile Computing Application. MAC Protocols – Wireless MAC Issues – Fixed Assignment Schemes – Random Assignment Schemes – Reservation Based Schemes.
- 3. Mobile Computing refers a technology that allows transmission of data, voice and video via a computer or any other wireless enabled device. It is free from having a connection with a fixed physical link. It facilitates the users to move from one physical location to another during communication. People to access information from anywhere and at anytime.
- 4. The concept of Mobile Computing can be divided into three parts: ◦ Mobile Communication ◦ Mobile Hardware ◦ Mobile Software
- 5. Mobile Communication specifies a framework that is responsible for the working of mobile computing technology. In this case, mobile communication refers to an infrastructure that ensures seamless and reliable communication among wireless devices. This framework ensures the consistency and reliability of communication between wireless devices. The mobile communication framework consists of communication devices such as protocols, services, bandwidth, and portals necessary to facilitate and support the stated services. These devices are responsible for delivering a smooth communication process.
- 7. Fixed and Wired: In Fixed and Wired configuration, the devices are fixed at a position, and they are connected through a physical link to communicate with other devices. ◦ For Example, Desktop Computer. Fixed and Wireless: In Fixed and Wireless configuration, the devices are fixed at a position, and they are connected through a wireless link to make communication with other devices. ◦ For Example, Communication Towers, WiFi router
- 8. Mobile and Wired: In Mobile and Wired configuration, some devices are wired, and some are mobile. They altogether make communication with other devices. ◦ For Example, Laptops. Mobile and Wireless: In Mobile and Wireless configuration, the devices can communicate with each other irrespective of their position. They can also connect to any network without the use of any wired device. ◦ For Example, WiFi Dongle.
- 9. Mobile hardware consists of mobile devices or device components that can be used to receive or access the service of mobility. Examples of mobile hardware can be smartphones, laptops, portable PCs, tablet PCs, Personal Digital Assistants, etc.
- 10. Mobile software is a program that runs on mobile hardware. This is designed to deal capably with the characteristics and requirements of mobile applications. This is the operating system for the appliance of mobile devices. In other words, you can say it the heart of the mobile systems. This is an essential component that operates the mobile device.
- 11. Web or Internet access. Global Position System (GPS). Emergency services. Entertainment services. Educational services.
- 12. Mobile computing denotes accessing information and remote computational services while on the move Wireless networking provides the basic communication infrastructure necessary to make this possible. So, Mobile computing is based on wireless networking and helps one to invoke computing services on remote servers while on the move: be it be office, home, conference, hotel and so on.
- 13. Wireless networking is important ingredient of mobile computing Mobile computing also requires the application themselves-their design, development and the hardware at the client and server sides. Wireless networking is increasingly replacing traditional networks because of the low setup time and low initial setup time and low initial investment required to set up the wireless network.
- 14. Wireless networking is divided into two basic types : 1) Extension of wired network 2)Adhoc Network ◦ Extension of wired network – uses fixed infrastructure such as base stations to provide single hop wireless communication with a wired network
- 15. ◦ Ad-hoc Network It does not uses any fixed infrastructure and it is based on multi-hop wireless communication
- 16. One popular example of a fixed infrastructure is wireless LAN(WLAN) that implements the IEEE802.11 protocol. In this diagram, the access point (AP) provides the last hop connectivity of the mobile nodes to a wired network AP’s – Bridging between wireless and wired mediums.
- 17. A recent development in wireless networking of various types of devices using Bluetooth technology It is the direct wireless connection of a cell phone with devices such as printers, cameras, scanners, laptop and desktop computers. Bluetooth replaces the cables and infrared for exchanging information between devices. Bluetooth enables the users to easily connect to a wide range of personal computing and telecommunication devices without the need of buy, carry or lay out cables. Bluetooth technology setting up Personal Area Networks(PAN) known as piconets and ad hoc nework known as scatternet.
- 19. Ad-hoc network is also known as Mobile Ad- hoc Network (MANET). It is the collection of mobile nodes that form a network on the fly without requiring the support of any fixed infrastructure. Wireless sensor network are a special type of wireless ad hoc networks.
- 20. Business: ◦ Managers can use mobile computers say, critical presentations to major customers. They can access the latest market share information. To enable the company to keep track of all activities of their travelling employees, to keep databases consistent etc. With wireless access, the laptop can be turned into a true mobile office, but efficient and powerful synchronization mechanisms are needed to ensure data consistency. Infotainment: ◦ Now a days there is a huge market of Entertainment for humans while they are on move. Watching movies, listening music and playing a game is become a part of life for entertainment. In this case mobile computing performs a major role to provide uninterrupted internet connection to digital devices.
- 21. Replacement of Wired Networks: ◦ wireless networks can also be used to replace wired networks. e.g., remote sensors, for tradeshows, or in historic buildings. Due to economic reasons, it is often impossible to wire remote sensors for weather forecasts, earthquake detection, or to provide environmental information. Credit Card Verification ◦ At Point of Sale (POS) terminals in shops and supermarkets, when customers use credit cards for transactions, the intercommunication required between the bank central computer and the POS terminal, in order to effect verification of the card usage, can take place quickly and securely over cellular channels using a mobile computer unit. This can speed up the transaction process and relieve congestion at the POS terminals.
- 22. Emergencies: ◦ An ambulance with a high-quality wireless connection to a hospital can carry vital information about injured persons to the hospital from the scene of the accident. All the necessary steps for this particular type of accident can be prepared and specialists can be consulted for an early diagnosis. Tourism: ◦ Tourism is the largest industry for all the countries. Mostly tourism places are at remote location from the developed cities. In this case wireless communication performs a vital role in connectivity for people who are enjoying their tour. They are continuously in contact with the family and friends and searching for travel services, hotel services food services etc.
- 23. E-Governance: ◦ Governments are using many communication services to update their rural areas. To provide health, education, safety , farming , weather forecast and many other related information to governance, governments are connecting rural areas with head quarter offices for monitoring. Education: ◦ 2020 COVID Pandemic teach us a great lesson to provide distance education to all who can not reach to the college or schools. Wireless communication and digital devices are the backbone for such distance learning concept. Now all private and public industries are moving towards the online education with help of good wireless connectivity.
- 24. Manage Personal Records : ◦ Some mobile applications allows user to manage their personal records such as day to day activities, some useful notes, etc… Social Media and Group Message : ◦ Some mobile applications allows user to keep in touch with their friends and relatives by sending messages, images, audio and video clips. Transaction : ◦ Some mobile applications allow the facility of transaction such as recharge mobile, pay bills etc…
- 25. 1. Portability - The Ability to move a device within a learning environment or to different environments with ease. 2. Social Interactivity - The ability to share data and collaboration between users. 3. Context Sensitivity - The ability to gather and respond to real or simulated data unique to a current location, environment, or time. 4. Connectivity - The ability to be digitally connected for the purpose of communication of data in any environment. 5. Individual - The ability to use the technology to provide scaffolding on difficult activities and lesson customization for individual learners. 6. Small Size - Mobile devices are also known as handhelds, palmtops and smart phones due to their roughly phone-like dimensions. A typical mobile device will fit in the average adult's hand or pocket. Some mobile devices may fold or slide from a compact, portable mode to a slightly larger size, revealing built-in keyboards or larger screens. Mobile devices make use of touch screens and small keypads to receive input, maintaining their small size and independence from external interface devices. The standard form of a mobile device allows the user to operate it with one hand, holding the device in the palm or fingers while executing its functions with the thumb. 7. Wireless Communication - Mobile devices are typically capable of communication with other similar devices, with stationary computers and systems, with networks and portable phones. Base mobile devices are capable of accessing the Internet through Bluetooth or Wi-Fi networks, and many models are equipped to access cell phone and wireless data networks as well. Email and texting are standard ways of communicating with mobile devices, although many are also capable of telephony, and some specialized mobile devices, such as RFID and barcode.
- 26. Ubiquity: ◦ Ubiquity is present everywhere. ◦ The ability of a user to perform computations from anywhere and at anytime. ◦ Example : A business executive can receive business notifications and issue business transactions as long he is in the wireless coverage area Location Awareness ◦ A hand held device equipment with GPS can transparently provide information about the current location of a user to a tracking station ◦ Example: a person travelling by road in a car, may need to find out a car maintenance service that may be available nearby. ◦ Through mobile communication he easily track the near by car maintenance service. ◦ Other examples: traffic control, fleet management and emergency services.
- 27. Adaptation ◦ Ability of a system to adjust to bandwidth fluctuation without inconveniencing the user. ◦ It is crucial, because of intermittent disconnections and bandwidth fluctuations that can arise due to a number of factors such as handoff, obstacles, environmental noise etc., Broadcast ◦ Efficient delivery of data can be made simultaneously to hundreds of mobile users. ◦ Example: all users at a specific location such as those near a railway station, may be sent advertising information by a taxi service operator Personalization ◦ It is easily personalized according to a user’s profile. ◦ Example – Mobile user may need only a certain types of information from specific sources.
- 30. Presentation Tier ◦ It is the user interface ◦ The user has to issue requests and to present the results to them meaningfully ◦ Programs at this layer run on the client’s machine ◦ It includes web browser and customized client programs for dissemination of information and for collection of data from the user.
- 31. Application Tier ◦ This layer makes logical decisions and performing calculation ◦ It moves and process data between the presentation and data layers. ◦ It performs the processing of user input, obtaining information and then making decisions. ◦ It can be implemented by using Java, .NET services, cold fusion etc., ◦ It is independent of the database design ◦ This layer of functionalities is usually implemented on a fixed server.
- 32. Data Tier ◦ It provides the basic facilities of data storage, access, and manipulation. ◦ This layer contains a database. ◦ The information is stored and retrieved from this database. ◦ This layer is also implemented on a fixed server.
- 33. Media access control (MAC) protocols enforce a methodology to allow multiple devices access to a shared media network. Before LANs, communication between computing devices had been point-to-point. That is, two devices were connected by a dedicated channel. LANs are shared media networks, in which all devices attached to the network receive each transmission and must recognize which frames they should accept. Media sharing reduced the cost of the network Objective of MAC protocol are maximization of the utilization of the channel and minimization of average latency of transmission.
- 34. MAC protocol should implement some rules – it helps to enforce discipline when multiple nodes connect for a shared channel. It should help maximize the utilization of the channel. Channel allocation needs to be fair. No node should be discriminated against at any time and made to wait for a long time for transmission It should be capable of supporting several types of traffic having different maximum and average bit rates. It should be robust in the face of equipment failures and changing network conditions
- 35. Bandwidth Efficiency – The shortage of data transfer capacity assets in these networks requires its proficient use. To evaluate this, we could state that bandwidth capacity is the proportion of the bandwidth used for data transmission to the complete accessible bandwidth capacity. Quality of Service Support – Quality of service support is difficult due to the mobility of the nodes. Once a node moves out of reach, the reservation in it is lost. In these networks, QoS is extremely important because if it is being used in military environments, the service support needed time to time.
- 36. Synchronization – Some instruments must be found so as to give synchronization among the nodes. Synchronization is significant for directing the bandwidth reservation. Hidden Terminal Problem – When there are two nodes, both are outside of each other’s range and try to communicate with same node within their range at the same time, then there must be packet collision.
- 37. Hidden Terminal Problem (Contd) ◦ Node A is in the radio range of the node B, and B is also within the radio range of C. ◦ Node A and Node C are not in the radio range of each other. ◦ If both A and C start to transmit to B at the same time, the data received at node B would get garbled. ◦ This situation can arise because A and C are “hidden” from each other because they are outside each other’s network range.
- 38. Exposed Terminal Problem: ◦ In wireless LANs (wireless local area networks), the exposed terminal problem is a transmission problem that arises when a transmitting station is prevented from sending frames due to interference with another transmitting station
- 39. Exposed Terminal Problem: (Contd..) ◦ Node B is within the transmission range of A and C ◦ Node D is within the transmission range of C ◦ Node A is within the transmission range of B ◦ Node A will not be able to transmit to any node when B is transmitting to C. ◦ The problem occurs because A is exposed to B’s transmission.
- 40. The MAC protocols can be broadly divided into the following three categories. ◦ Fixed assignment schemes ◦ Random assignment schemes ◦ Demand based Scheme.
- 41. It is usually called circuit-switched schemes. In fixed assignment schemes, the resources required for a call are assigned for the entire duration of the call. On the other hand, the random assignment schemes and the reservation schemes are called packet switched schemes The random assignment schemes are connection less and packet switching schemes. In this no resource reservations are made, the nodes simply start to transmit as soon as they have packet to send In reservation schemes, a node makes explicit reservation to a connection based packet switching scheme. The reservation based MAC schemes are suitable to handle calls with widely varying traffic characteristics.
- 42. It is categorized into three parts ◦ Frequency Division Multiple Access (FDMA) ◦ Time Division Multiple Access (TDMA) ◦ Code Division Multiple Access (CDMA) Example: ◦ Consider a student’s common room (Channel) in which many students want to communicate with each other. ◦ If the student want to avoid cross-talk in the on going process, then either the students could take turns in speaking(time division) or they could speak in different pitches(frequency division) or they could speak in different language (Code Division) ◦ In CDMA, when the students who are speaking the same language understand each other, but the rest of the students cannot. ◦ In this case many codes occupy the same channel, but only the users who share a specific code will be able to understand each other
- 43. In FDMA, the available bandwidth is divided into many narrower frequency bands called channels.
- 44. In the previous example shows a division of the existing bandwidth into many channels (channel1, channel2, etc.,) For full duplex communication to take place, each user is allocated a forward link for communicating from it (Mobile handset) to base station (BS) Reverse channel for communicating from the BS to it. Each user making a call is allocated two unique frequency bands (Channels) one for transmitting and other for receiving signals during the call. When call is underway, no other user would be allocated the same frequency band to make a call. Unused transmission happens when the allocated caller pauses between transmission or when no user is allocated a band, goes idle and wasted. FDMA does not achieve a high channel utilization.
- 45. Forward link and reverse link
- 46. TDMA is an access method in which multiple nodes are allotted different time slots to access the same physical channel. The time line is divided into fixed sized time slots. These are divided among multiple nodes who can transmit. In this case, all sources use the same channel, but take turns in transmitting
- 47. Time slots are allocated to the user in the round robin manner In TDMA, each user of the channel owns the channel for exclusive use for one time slot at a time in a round robin fashion. Ch1 Ch2 … ChN Ch1 Ch2 … ChN
- 48. Multiple users are allotted different codes that consist of sequences of 0 and 1 to access the same channel. A special coding scheme is used that allows signals from multiple users to be multiplexed over the same physical channel. Three different users who have been assigned separate codes are multiplexed on the same physical channel. In CDMA, multiple users use the same frequency at the same time and no time scheduling is applied. All the senders send signals simultaneously through a common medium
- 50. How to distinguish transmission from different nodes ◦ Two vectors are said to be orthogonal if their inner product = 0 Let p and q be two vectors and suppose p = (2,5,0) and q = (0,0,17) then the inner product of p*q = (2*0+5*0+0*17) = 0 ◦ For good autocorrelation, binary 0 is represented as -1 and binary 1 is represented as +1. ◦ Let the binary sequence be 1001, then the representation sequence is +1-1-1+1.
- 51. All nodes transmit on the same frequency at the same time using the entire bandwidth of the transmission channel. Each sender has unique random number key, and the send XORs the signal with this random number key. The receiver can “tune” into his signal if it knows the pseudorandom number. Consider an example, where X,Y are the transmitters and Z is a receiver.
- 52. Sender X_data = 1 and X_Key = (010011). Its autocorrelation representation is (-1, +1, -1, -1, +1, +1) The signal is calculated at sender X is Xs=X_data*X_key = +1*X_key = (-1, +1, -1, - 1,+1,+1) Similary Sender Y_data = 0 and Y_key = (110101) Its autocorrelation representation is (+1, +1, -1, +1, -1,+1) Ys = Y_data*Y_Key = -1*X_key = (-1, -1, +1, -1, +1, -1) Xs+Ys = (-2,0,0,-2,+2,0)
- 53. At the receiver, in order to receive the data sent by send X, the signal Z is dispread. If Z wants to get information of send X data, then Z*X_key = (-2,0,0,-2,+2,0)*(-1, +1,-1,- 1,+1,+1) = 2+0+0+2+2+0 = 6>0 (positive) that is the original bit was a 1. Similarly the information of send Y data may be obtained Z*Y_key = (-2,0,0,-2,+2,0) * (+1, +1, -1, +1, -1, +1) = -2+0+0-2-2+0 = -6 < 0 (negative) so the Y data original bit was a 0.
- 54. There are number of random assignment schemes are used in MAC protocol. They are ◦ ALOHA ◦ Slotted ALOHA ◦ CSMA CSMA/CD ◦ CSMA/CA
- 55. ALOHA ◦ ALOHA is a simple protocol. ◦ If a node has data to send, it begins to transmit. ◦ PURE ALOHA does not check whether the channel is busy before transmitting. ◦ If the frame successfully reaches the destination the next frame is sent. ◦ If the frame fails to be received at the destination, it is sent again. ◦ If the transmission is high, the collision also high in simple ALOHA
- 56. ALOHA
- 57. Slotted ALOHA ◦ An improvement in pure ALOHA scheme is the slotted ALOHA scheme. ◦ In slotted aloha scheme the chances of collision are attempted to be reduced by enforcing the following restrictions. ◦ The time is divided into equal sized slots in which a packet can be sent. ◦ The size of the packet is restricted. ◦ A node wants to send a packet, can start to do so only at the beginning of the slot. ◦ The slotted ALOHA system employs beacon signals that are sent at precise intervals that mark the beginning of a slot. ◦ The protocol does not work well if the number of stations contending to send data is high. ◦ CSMA schemes works better than ALOHA scheme.
- 59. CSMA ◦ Popular MAC arbitration technique is the Carrier Sense Multiple Access (CSMA) ◦ A node sense the medium before starting to transmit. ◦ If it senses that some transmission is already underway, it defers its transmission. ◦ Two popular extensions of the basic CSMA technique. CSMA/CD – Collision Detection CSMA/CA – Collision Avoidance
- 60. CSMA/CD ◦ In wireless network the CSMA/CD technique does not work very well. ◦ In CSMA/CD technique, the sender starts to transmit if it senses the channel to be free. ◦ If it senses the channel to be free, there can be collision (Why?) during transmission. ◦ In wired network, collision detection technique is simple. ◦ In wireless network, it is very difficult for a transmitting node to detect a collision, since any received signal from other nodes would be too feeble compared to its own signal and can easily be masked by noise.
- 61. CSMA/CD ◦ As a result, a transmitting node would continue to transmit the frame and only the destination node would notice the corrupted frame after it computes the checksum ◦ This leads to retransmission and severe wastage of channel utilization. ◦ In wired network when a node detects a collision, it immediately stops transmitting, thereby minimizing channel wastage.
- 62. CSMA/CA ◦ In wireless network, a collision avoidance scheme works much better compared to a collision detection based scheme. ◦ Collision Avoidance scheme is based on the idea that it is necessary to prevent collision at the moment they are most likely to occur, that is when the bus is released after a packet transmission. ◦ During the time of transmission on the channel, several nodes might be wanting to transmit. ◦ These nodes would be monitoring the channel and waiting for it to become free. ◦ The moment the transmitting node completes its transmission, these waiting nodes would sense the channel to be free and would all start transmitting at the same time. ◦ To overcome such collisions, all nodes are forced to wait for a random time and then sense the medium again, before starting their transmission. ◦ If the medium is still busy, the node has to wait for another random amount of time and sense the medium again.
- 63. CSMA/CD
- 64. The basic form of reservation scheme is RTS/CTS scheme A sender transmits the RTS(Ready to Send) packet to the receiver before the actual data transmission. In receiving end, the receiver sends a CTS (Clear to Send) packet, and the actual data transfer commences only after that. When other nodes sharing the medium sense the CTS packet, they refrain from transmitting until the transmission from sending node is complete.
- 65. In contention based MAC protocol, the node wants to reserve the medium by using an appropriate control message. A reservation of the medium can be achieved by RTS/CTS scheme. A sender transmits RTS (Ready to Send) packet to the receiver before the actual data transmission. On receiving side, the receiver sends CTS(Clear to Send) packet, and the actual data transfer commences only after that. If the other nodes sharing the medium, it sense the CTS packet they refrain from transmitting until the transmission from the sending node is complete. A few examples of RTS-CTS based MAC protocols are MACA, MACAW, MACA-BI, PAMAS, DBTMA, MARCH, S- MAC.
- 66. It solves the hidden/exposed terminal problems by regulating the transmitter power. A node sends the RTS to the receiver. Radio signal is omini-directional, so every node within the sender’s radio range will hear this and then refrain from transmitting The receiver is ready to receive data, it responds with a CTS.
- 68. MACA is a non-persistent slotted protocol. This implies that if the medium is detected as busy, a station waits for a random time period after the beginning of a time slot and then it sends an RTS. This assures fair access to the medium.
- 70. The whole process will work as follows: A will send RTS frame to the B Then b will send CTS frame to A When CTS frame is received by A then it will start sending data frame to B Upon receiving data successfully it will send acknowledgment frame(ACK)
- 71. Solution to Hidden/Exposed Terminal Problem: ◦ MACA protocol uses RTS and CTS to avoid hidden and exposed terminal problem. ◦ In hidden terminal problem two nodes try to contact same node at a same time which can create collision to combat this if two nodes send RTS to same node then the node which receives CTS will send the data not the other one which will avoid the collision.