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Information Technology Management
ITM 301
Franklyn Prescod

Chapter 1 – Uses of Computer Networks Computer Networks – is a computer in which a large number of computers of separate but interconnected computers do the job - They can be connected through fibre optics, cooper cable, microwaves, infrared, and communication satellites Distributed System – are computer that are collection of independent computers but appear to users as a single coherent system - A layer of software on top of the operating system, middleware, is responsible for implementing this model - Example of the distributed system is the World Wide Web Business Applications Ways computer networking help companies 1. Resource sharing – is the make all the business applications, data and equipment available for everyone on the network - Such as printer, applications, customer records, inventories, and any information available VPN (Virtual Private Networks) – are used to join the individuals networks at different sites into one extended network - Allows individual to access information/data that is 15000 km away, it ends the “tranny of geography” Servers – are powerful computed that store data and are maintained by system administrative Client – are simpler computer, which are operated by individuals to access remote data in the server Client-server model - The clients are connect to a network which is connected to a server - The client-server model is applicable when they are in the same building or not - The request from the client goes through the network to the server, which perform the requested data and send it back 2. Communication Medium – are ways employees communicate with other employee - Email, VoIP (Voice over internet protocol) Skype, desktop sharing (allow two or more individuals to see and interact with a graphical computer screen) 3. E-commerce – is ability to doing business electronically with customers and suppliers - Book stores, retail stores, movies stores, air lines Home Applications - Like companies, homes have the ability to connect to the internet, communicate with people in the house, - Peer- to-peer communication – are individuals who form a loose group can communicate with other in the group, there is no fixed division into clients and servers o Are used to share music videos, photos, videos, o Instant message (MSN, facebook, twitter) o Group of people can work to together to create an content (Wikipedia) - Access to e-commerce because peer-to-peer in a sense that consumers act as buyers and sellers o B2B – Business to Business – Microsoft to IBM o B2C – Business to Customer – Future Shop to Ragu o G2C – Government to Corporations – distributing taxes o C2C – Corporation to Corporation – auctioning second hand products o P2P – Peer to peer – selling, sharing stuff - Access to Entertainment though IPTV ( IP television) o Access to movies, radio and shows through media streaming applications - Ubiquitous computing – is embedding computers into everyday life o Security systems, sensors, water, electricity and gas to the computer - RFID (Radio Frequency IDentification) o Are chips the size of stamp and has information that the RFID meter needs  Example. Credit card, has information able limits, pin # and user Mobile Users - People can send emails, surf the web, tweet, download songs, search for information - Hotspot – always laptops and mobile computer to be connected to the internet - Text messaging or text is very popular, allow people to send a short message to another mobile subscriber - Smart Phones, Blackberry and IPhone, allows for fast data services and handling phone calls - GPS (Global Positioning System) – help users have a better idea where they are, search for stores nearby and the local weather - M-commence - short text message from mobile devices are used to authorize payments for books, movies, and other small items, and charge on the monthly bill - Sensor network – are made up of nodes that gather and wirelessly relay information they sense about the state of the physical world - Wearable computers, smart watches with radio and insulin pumps Network Hardware - There are two types of transmission technology: o Broadcast links  Is a communication channel is shared by all the machines in the network  A packet sent by one user and sent to all machines and after each machines check if it is the right address field.  An address field within each packet specifies the intended recipient o Point-to-point link  They connect individual pair of machines  Packets (envelope with a message), are used to move around the network, it can be in different routes and go through different machines  Point to point transmission with exactly one sender and one receiver are called unicasting  When a packet has a special code in the address field, so that it is processed by all machines, this is called broadcasting Personal Area Network - Lets devices communicate over the range of a person o Ex. Connect the computer to other devices, like printer, mouse, monitor o These can be connect by wires or by Bluetooth (a short range wireless network) o They can also be used with RFID on smartcards and library books Local Area Network - Is a privately owned network that operates within and nearby a single building like a home, office or a factory - It can connect personal computer and customer product to share information - Companies using LANs called enterprise networking - Wireless LANs are very popular, because of the trouble of install cable, all you need is a (AP) Access Point, wireless router or base station o Which relay packets between computers and the internet o This standard wireless LAN is called IEEE 802.11 or WiFi, runs speed from 11 to 100 mbps o Wired LAN are faster 100 mbps to 1 gbps, fewer errors, low delays, easier to send signal - The topology of many wired LANs is built from point-to-point links, called Ethernet, 802.3. o Each computer is wired to a port, which is connected to a switch, where the switch sending the address packet the correct computer o Dividing one physical LAN into to two small LAN  When the switch getting the packet from a computer, it will attach the packet to the right port, say the marketing port - Both wireless and wired broadcast networks are divided into static and dynamic designs o Static allocation, divide time into discrete intervals and use round robin algorithm, allowing each machine to broadcast only when its time slot come, which waste channel capacity when a machine doing nothing o Dynamic allocation for a common channel is centralized or decentralized  Centralized: there is a single entity (base) that determines who goes next  Decentralized: each machine must decide for itself where to transmit Metropolitan Area Network - MAN covers a city o How it works is that all the computer in the city are connected to a junction box, which is connected to a head end, which is connect to the Internet/Antenna(for cable) Wide Area Network - Spans a large geographical area, often a country or continent - Host (Computers) are connected by communication subnet or just subnet , which is responsible for carrying the messages from one computer to another o The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. o The subnet consist of two distinct component:  Transmission lines  Move the bits through copper wire, optical fibre or even radio links  Switching elements  Are specialized computers, known as routers that connect two or more transmission line, which take the data and send it on a outgoing lines on which to forward them o A subset is a collection of routers that are connected to each other and allow packets to move though them, from the source to the destination - Host is connect to the router, which is connect to transmission lines and help by subnet - Two other types of WAN o Virtual Private Network: instead of transmission lines and subnet, it uses the internet o The subnet is run by another companies, it is a Internet service provider and it use the internet to connect with other networks –pg 26 Internetworks - A collection of interconnected networks are called an internetwork or internet - A common form of internet is a collection of LANs connected by a WAN. - The only real technical distinction between a subnet and a WAN in this case is whether hosts are present. If the system within the gray area contains only routers, it is a subnet; if it contains both routers and hosts, it is a WAN. The real differences relate to ownership and use. o The Internet uses ISP networks to connect to other networks - Subnet - is the collection of router and communication lines owned by the network operator - A network is formed by the subnet and its hosts - An internetwork is formed when distinct networks are interconnected. In our view, connecting a LAN and a WAN or connecting two LANs forms an internetwork, but there is little agreement in the industry over terminology in this area. Network Software - Protocol Hierarchies o *Most networks are organized as a stack in layers or levels and each level or layer is different in name, function, content and each layer offer certain services to the higher layer o When a layer communicates with each other, they have rules and convention used in the conversation, which is called a protocol.  Violating the protocol makes the communication more difficult o The entities comprising the corresponding layers on different machines are called peers o *Data is not transfer from layer to layer directly because the data is passes down to the lowest layers and then; it is physical medium where the actual communication takes place o *Between each pair of adjacent layers is a interface, which primitive operations and services the lower layer make available to the upper one o *A set of layers and protocols is called a network architecture o *A list of the protocol used by a certain system, one protocol per layer is called protocol stock o Pg 31-32 good examples Design Issues for the Layers 1) Reliability – is a design issue of making a network that operates correctly even though it is made up of a collection of components that are themselves unreliable o Finding error  Uses codes for error detection, information that is received can then be retransmitted until it is received correctly  More powerful codes allow for error correction where the correct message is recovered from the possibly incorrect bits that were originally received o Finding the working path through a network  Since there are multiple pathways in a larger network, there are some links or routers that are broken  *If message is post to be sent from London to Germany to Paris, but it will not go through, the network should automatically make this decision, this is called routing 2) Evolution of the network o As the network grows larger and connects with other networks, it gets complex  The key structuring mechanism used to support change by dividing the overall problem and hiding implementation detail: protocol layering  Every layer needs a mechanism for identifying the senders and receiver that are involve in the message. The mechanism is called addressing or naming, in the lower and higher layers 3) Resource allocation o They need mechanisms that divide resources so that one host don’t interface with others o Statistical multiplexing – sharing based on the statistics of demand o How to keep fast sender from swamping a slower receiver  Flow control – feedback from the receiver to the send is used o When there are too much information being sent from computers, which is overloading the network is called congestion 4) Securing the Network o Use confidentially, o Use authentication, prevent someone from impersonating someone else o Use integrity, prevent surreptitious changes to messages, like “debit my account 10” to “debit my account to 1000” Connection Oriented vs Connectionless Service - Layers are offer two different types of service to the layers o Connection oriented – service is modeled after the telephone system  First they establishes a connections, uses it and release the connection  The sender pushes objects (bits) from one end to the other end of the receiver  When the connection is established, they negotiate about the parameters  A circuit is another for a connection with associated resources, such as a fixed bandwidth o Connectionless – is modeled after the postal system  Each message has a address and go through intermediate nodes  A packet is a message at the network layer  When the intermediate nodes receive a message in full before sending is called store –and forward switching  When the onward transmission of a message at a node starts before it is completely received by the node is called cut-through switching o Read pg 35-40 *Reference Models Open Systems Interconnection Reference Model (OSI Model) - The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows: 1. A layer should be created where a different abstraction is needed. 2. Each layer should perform a well-defined function. 3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. 4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy. - Physical Layer o Primarily used to transmit data bits (0 s and 1s) over a communication channel o It defines rules, while 0s and 1s are transmitted, voltage to electricity, # of bit/s and the physical format of the cables and connectors used o When one side sends 1 bit to the other side, it should receive a 1 bit o Design issues here largely deal with mechanical, electrical, and timing interfaces, and the physical transmission medium, which lies below the physical layer. - Data Link Layer o Is to transform a raw into a line that appear free of error  It does it by masking the real errors so the network layer does not see them o After it is break up the input data into data frames (typically hundred of bits) and transmit them sequentially o Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast transmitter from drowning a slow receiver in data. - Network Layer o Controls the operation of the subnet o When a packet travels from one network to another network, problems may rise  The addressing used in on network may be different on the other  May not accept a packet because it is too large and so on  It is up to the network layer to overcome all the problems, to allow heterogeneous networks to be interconnected - Transport Layer o Accept data from above it, break them into smaller part if need, pass them to the network layer, and ensure that all the pieces have arrived at the other side o It determines what type of service to provide to the session layer  The most popular one is an error free point to point channel that delivers messages or byte in order in which they were sent o The transport layer is a true end to end layer  It carries the data from the source to the destination - Session Layer o Allows users on different machines to establish session between them  Dialog control – keeping track on whose turn it is to transmit  Token management – prevents two parties from attempting the same critical operation at the same time - Presentation Layer o Is concerned with the syntax and semantics of the information transmitted o Manages abstract data structures and allows high level data structures to be defined and exchange - Application Layer o Contains a variety of protocols that are commonly needed by users, like HTTP TCP/IP Reference Model o The Data Link Layer  Describes what links such as serial lines and classic Ethernet must do to meet the needs of this connectionless internet layer o The Internet Layer  Is the linchpin that holds the whole architecture together  Its job is to permit hosts to inject packets into any network and have travel to the destination, from the same order to arrived  The internet layer defines an official packet format and protocol called IP (Internet Protocol). The job of the internet layer is to deliver IP packets where they are supposed to go. o Transport Layer  It is designed to allows peer entities on the source and destination hosts to carry on a conversion  Two end to end protocols have been defined  TCP (Transmission Control Protocol) o Is a reliable connection oriented protocol that allows a byte stream originating on one machine to be delivered without error on another machine o Is responsible for separate the message pass it to the internet layer and responsible for reassembles the message into the output streaming and handles flow control  UDP(User Datagram Protocol) o Is an unreliable connectionless protocol for applications that do not want TCP’s sequencing or flow control and wish to provide their own o Application Layer  It contains all the higher level protocols  TELNET,FTP, SMTP, DNS o The Host-to-Network Layer  Below the internet layer is a great void. The TCP/IP reference model does not really say much about what happens here, except to point out that the host has to connect to the network using some protocol so it can send IP packets to it. This protocol is not defined and varies from host to host and network to network. The Model Used in This Book - The Hybrid Model o Physical - Specifies how to transmit the bits across different media as electrical; signals o Data Link – is concerned with how to send finite length message between directly connected computers with specified levels of reliability o Network - deal with how to combine multiple links into networks and networks of networks into internetworks so we can send packets between distant computers o Transport – strengthens the delivery guarantees of the Network layers, and increasing reliability o Application – contain programs that make use of the network  The use of HTTP protocol in the case Web Browser A compansion of the OSI and TCP/IP Reference Model - Similar: Transport layer, based on the concept of a stack of independent protocols - Three concepts are central to the OSI Model o Service - Help the layers above, tell what the layer does , and the layers semantics o Interface – tells the processes above how to access it and specifies parameters and what result to expect o Protocols – used in a layer are the layers own business, use any protocols - The obvious difference is the number of layers between the two models and how the OSI model support both connectionless and connection-oriented communication in the network layer and the TCP/IP only supports connectionless in the network layer and both in the transport layer A Critique of the OSI Model and Protocols - Bad Timing o The apocalypse of the two elephant  There was a new subject and burst of research happened, but many companies invested and people were in a hurry to get stated  The OSI is crushed but not the TCP/IP, which was widespread, many companies offer TCP/IP product, when OSI came around, no company offered to help - Bad Technology o OSI and TCP/IP, their protocols are flawed  The choice of seven layers was more political than technical and the two layers session and presentation were empty and the data link and network was overfull - Bad Implementation o The initially implementations were huge, unwieldy and slow, poor quality  Where the TCP/IP was goods and free - Bad Politics o It was widely thought to be the creature of the European telecommunication ministrations , where they were trying the slave a technically interior standard down the throats of poor researchers and programmes down in the trenches actually developing computers network that did not aid OSI’s cause A Critique if the TCP/IP Model **** IMPORTANT ************ - It does not clearly distinguish the concept of services, interfaces and protocols - The model is not at all general and is poorly suited to describing any protocol stake - The distinction between an interface and a layer is crucial, and one should not be sloppy about it. - The model does not distinguish between the physical and data links layer o Physical – transmits data through copper wires, fibre optics and wireless communication o Data Link – its job is to delimit the start and ends of frames and them from one side to the other with the desired degree of reliability Example Networks The Internet - Is a collection of networks, with common protocols and provide certain common services - It was a unusual system, not planned by anyone and it not controlled by anyone The ARPANET - Structure of the telephone system o There are thousands of telephones, which are connected to switching office, which are connected to high level toll offices - Baran’s proposed distributed switching system o Since the paths between any two switching offices were now much longer than analog signals could travel without distortion, he proposed using digital packet switching technology - The original design of ARPANET o The subnet consists of minicomputer called IMP (Interface Message Protocols), connected with 56 kbps transmission lines and each IMP was connected at least two IMP’s o The subnet was a datagram subnet where if some lines and IMPs were destroyed , the message with be rerouted automatically o Each nodes of the network was to consist of a host and IMP connected by a short line o The host send the message to the IMP, where the message is broken up into packets and forward them to their destination o The subnet software consisted o f the IMP end of the host-IMP connection, IMP-IMP protocol, and the source IMP to destination IMP protocol designed to improve reliability - Growth of the ARPANET – pg 58 o December 1969.  The network went online with four nodes at UCLA, UCSB, SRI and the University of Utal, they were chose because they had larger number if ARPA contracts and had different host computers, in two month the first message was sent o In the span of three year, it rapidly grow, as more IMP’s were delivered and installed o In 1974, the invention of the TCP/IP model and protocols were made, it was design to handle communication over internetworks o As the scale increased, finding host was hard, so DNS (Domain Name System) was created to organize machines into domains and map host names onto IP addresses NSFNET - NSF decide to build a backbone network to connect its six super computers centers, each supercomputer was given a little brother, consisting of an LSI-11 microcomputer called a fuzzball o They were connected with 56 kbps leased lines and formed a subnet  And the first NSFNET was backbones in 1988 - NSF was taken over by ANS (Advanced Network and Services) and upgraded the 1.5 mbps to 45 mbps to form ANSNET and later in 5 years it was sold to America Online Architecture of the Internet – pg 62 - It all start with a computer at home and it was connected to the Internet, through a Internet Service Provider. This allows to exchange packets with other host connect to the internet. They can surf the Web or do the other thousand thing possible - People can connect to an ISP through the phone line; the DSL (Digital Subscriber Line) uses the telephone line that connects to your house for digital data transmission. - *The computer is connected to the DSL Modem, which converts between digital and analog signals that pass through the telephone line and at the other end a device called DSLAM ( Digital Subscriber Line Access Multiplexer) converts between signals and packet - *Also we can use cable tv systems, where the device at the home end is called the cable modem and device at the cable headend is called CMTS (Cable Modem Termination System) - *Broadband - is internet access at much greater than dial-up speed - By using optical fibre, it can provide rates of 10 to 100 mbps, known as FTTH (Fibre to the Home) - *The location were the packets enter the ISP network is the ISPs POP(Point of Presence) - The DSLAM is connect to routers, which are on backbone of the ISP, which connect their network to exchange at IXPs (Internet Exchange Points) o The IXP are rooms full of routers at least one per ISP - *Tier 1 – are ISPs that do not pay for transit and it is said to form the backbone of the Internet, since everyone else must connect to then to be able to reach the entire internet - *Data centers – are the computers from companies like Google and Yahoo, that provide lots of content - *Racks and racks machines that are filled in the data center is called server farm Third Generation Mobile Phone Network - First generation system: AMPS (Advanced Mobile Phone System) - Second generation mobile system: GSM (Global Systems for Mobile Communication) - Third Generation Mobile System: UMTS (Universal Mobile Telecommunication System) also called WCDMA (Wideband Code Division Multiple Access) - The cellular network uses a scarcity of spectrums for mobile phone networks o The coverage area is divided into cells so that there isn’t any radio interference  Within the cell, users are assigned channels so that is no interference  This allows for good reuse of the spectrum or frequency reuse o The modern 3G system allow each cell to use all frequencies, but results in the interference of neighbouring cells Architecture of the UMTS 3G mobile phone network 1) There is the air interface, it is the radio communication protocol that is used over the air between the mobile device and the cellular base station ( A fixed station in a cellular wireless network, used for communicating with mobile terminals (phones) 2) The cellular base stations together with its controller forms the radio access network 3) The RNC (Radio Network Controller) controls how the spectrum is used 4) The rest of the mobile network carries the traffic for the radio access network, called core network 5) In the core network there is a both packet and circuit switched equipment 6) The MSC (Mobile Switching Center), GMSC (Gateway Mobile Switching Centre) and MGW (Media Gateway) elements that set up connections over the circuit-switched core network such as the PSTN (Public Switched Telephone Networking) – telephone calls 7) The SGSN(Serving GPRS Support Node) and GGSN (Gateway GPRS Support Node) deliver data packets to and from mobiles - Each mobile phone network has a HSS (Home Subscriber Server) in the core network that knows the location of each subscriber, as well as other profile information that is used for authentication and authorization - A removable chip containing the subscriber identity and account information Wireless LANs: 802.11 - A Wireless LAN, is called Wi-Fi but the proper name is 802.11 - The Wi-Fi network is made of clients and infrastructure called APs (Access Point) that is installed in buildings and access points are sometimes called base stations o *The access points connect to the wired network, and all communication between clients goes through an access point o *But clients can talk directly, such as three computers in an office without an access point, which is called ad hoc network - At the frequencies used for 802.11, radio signals can be reflected off solid objects so that multiple echoes of a transmission may reach a receiver along different along, the echoes cancel or reinforce each other, causing the received signal to fluctuate greatly, this is called multipath fading - Path diversity – the ability to send information along multiple, independent paths, so if the problems come, another path can be taken - When multiple transmissions that are sent at the same will collide, to avoid this problem it is called ALOHA o The computers waits for a short random interval before transmitting and defer their transmission if they hear someone is already transmitting RFID and Sensor Networks - RFID are used in everyday products and things from books, cards, supply, books and more - They are stamp like tag that consists of a small microchip with a unique identifier and an antenna that receives radio transmission o As the chip pass a RFID Reader, the information on the tag can be read o *If a RFID Reader uses radio waves to operate them, is it called passive RFID o *If a RFID Reader uses power source to operate them, is it called active RFID - *One common form of RFID is UHF RFID (Ultra-High Frequency RFID), are used on shipping pallets and some drivers licenses. o *They can be read at distances of several meters by changing the way they reflect the reader signal, this way of operating is called backscatter - *Another common RFID is the HF RFID (High Frequency RFID) are used in passports, credit cards, books o *HF RFID have a short range typically a meter or less - RFID chips are rapidly changing to computers because they have memory that can be used and later used - Sensor network – are deployed to monitor aspects of the physical world o Sensor nodes are small computers, they have batteries and they must communicate carefully to delivers the sensor information to an external collection point o Multihop network – is when nodes self-organize to relay messages for each other Network Standardization - Standards fall under two categories: de facto and de jure - De facto - They are standards that have just happened, without any formal plan (HTTP) - De jure – standards that are adopted through rules of some formal standardization body o Internalization standardization authorities are generally divided into two classes:  Those established by treaty among national government  Those comprising voluntary, nontreaty organization Metric Units - 1kb = 1024 bits 1000kbs = 1 mb 1000 Mb = 1 gb Chapter 2 – Physical Layer The Theoretical Basis for Data Communication - Information can be transmitted on wires by varying some physical property such as voltage or current o And it is represented by a single function of time o Fourier Analysis: Equation  It is called Fouriers series Bandwidth –Limited Signals Bandwidth – the width of the frequency range transmitted with being strongly attenuated - Is quoted that the bandwidth is from 0 to the frequency at which the received power has fallen by half - The bandwidth is a physical property of the transmission medium that depend on the construction, thickness and the length of a wire or fibre o Filters are used to limit the bandwidth o The filter lets more signals share a given region of spectrum, which improves the overall efficiency of the system - Signals that run from 0 up to a maximum frequency are called baseband - Signals that are shifted to occupy a higher range of frequencies, as is the case for all wireless transmission, are called passband signals - An ordinary telephone line called voice-grade line has an artificially introduced cut-off frequency just above 300Hz - P9 92-93 The Maximum Data Rates of a Channel - Maximum data rate = 2B log V2bits/sec - The amount of thermal noise present is measured by the ratio of the signal power to the noise power is called the SNR (Signal to Noise Ratio) o If we denote the signal power by S and the noise by N, the signal to noise ratio is S/N o An ration of 10 is 10dB, 100 is 20dB, 1000 is 30dB - The units of this log scale is called decibels (dB) - Shannons major result is that the maximum data rate or capacity of a noisy channel whose bandwidth is B Hz and signal-to-noise ratio is S/N o Maximum data rate = B log (2 + S/N) GUIDED TRANSMISSION MEDIA Magnetic Media - Are used to transport data from one computer to another (removable disks or magnetic tape) - The delay characteristic are poor Twisted Pairs - For many online applications is needed, it is one of the oldest and still most common transmission is twisted pair o It consists of two copper wires, about 1 mm thick and they are twisted together o The most common applications of the twisted pair is the telephone system  All telephone are connected to the telephone company by twisted pairs, o Are used to transmit both analog and digital information o The bandwidth depends on the thickness of the view and the distance traveled, but several megabits/sec can be achieved for a few kilometres in many cases o Twisted-pair cabling comes in several varieties  A category 5 twisted pair consists of two insulated wires gently twisted together  Four such pairs are typically grouped in a plastic sheath to protect the wires and keep them together o Full-duplex links – links can be used in both directions at the same time, like a two-line road o Half duplex links– links can be used in either direction, but one way at a time, like a single-track railroad line o Simplex links – links that allow traffic in only one direction, like a one-way street - Category 6, is referred as UTP (Unshielded Twisted Pair), as they consist simply of wires and insulators - Category 7 cables have shielding on the individual twisted pairs o The shielding reduces the susceptibility to external interference and crosswalk with other nearby cables to meet demanding performance specifications Coaxial Cable - It is better shielding and greater bandwidth than unshielded twisted pairs, so it can span longer distance and higher speeds o There are a two kinds of coaxial cable  A 50 ohm, used for digital transmission from the start  A 75 ohm, used for analog transmission and cable television - The coaxial cable consists of a stiff copper wire as the core, surround by an insulating material, the insulator is covered by a cylindrical conductor, often as closely woven braided mesh, and that is covered by a protective plastic sheath - They have a good combinations of high bandwidth and excellent noise immunity o Are being current replaced by fibre optic s on long haul routes Power Lines - Power lines deliver electric power to houses and electrical wiring within houses distributes the power to electrical outlets o In high-rate communication over these lines, both inside the home as a LAN and outside the home for broadband Internet access o Simply plug a TV and a receiver into the Wall, and they send and receives movies over the electrical wiring  The data signals is superimposed of low frequency power signal as both signals use the wiring at the same time  It is very difficult to use electrical wiring for network o Electrical signals are sent at 50-60 Hz and the wiring attenuates the much higher frequency signals needed for high-rate data communication Fibre Optics - Moore’s Law – the # of transistors per chip are double every two years - Fiber Optics are used for long haul transmissions in network backbones, high speed LAN’s and high speed Internet access such as FTTH (Fiber to the Home) o It has three components  The Light Source  A pulse of light, indicates 1 bit and the absence of light indicates 0 bits  The Transmission Medium  Is an ultra thin fiber of glass  The Detector  Generates an electrical purse when the light falls on it o By attaching a light source to one end of an optical fiber and a detector to the other end, converts and transmits it by light pulses and then reconverts the output to an electrical signal at the receiving end o Each ray is said to have a different mode, so a fiber is called multimode fiber o If the fiber diameter is reduced to a few wavelengths of lights, the fiber acts like a wave guide and the light can travel only in a straight line, without bouncing, yielding a single- mode fiber  Very expensive, long distance, transmits data at 100 gb for 100 km Transmission of Light through Fiber - The attenuation of light through glass depends on the wavelength of the light - Three wavelength bands are most commonly used at present for optical communication o The centred at 0.85, 1.30 and 1.55 microns  The 0.85 is used first and its used for shorter distance - Light pulses sent down a fiber spread out in length as they propagate, it’s called chromatic dispersion - Making the pulses in a special shape related to a reciprocal of the hyperbolic cosine causes nearly all the dispersion affects cancel out, so it is possible to send pulses for thousands of kilometres without appreciable shape distortion, these pulses are called solitons Fiber Cable - In multimode fiber, the core is typically 50 microns in diameter and the single mode fiber, the core is 8 to 10 microns - The core is surround with cladding glass and next comes the thin plastic jacket to the protect the cladding and the fiber cables are bundled together and protected by an outer sheath - The fibers can be connected in three different ways o They terminate in connectors and plugged into fiber sockets o They can be spliced mechanically o Two pieces of fiber can be fused (melted) to form a solid connection - There are two kinds of light sources, which are used to do the signalling Item LED Semiconductor laser Data rate Low High Fibre Type Multi-mode Multi-mode or single-mode Distance Short Long Lifetime Long life Short life Temperature Minor Substantial Cost Low Expensive Comparison of Fibre Optics and Copper Wire - Fibre Optic is better because it can handle much higher bandwidths, repeaters are only about every 50 km, not effected by power failures or surges, lightweight, lower installation cost, - Downside: is a less familiar technology, can be damaged easily WIRELESS TRANSMISSION The Electromagnetic System - The # of oscillations per second of a wave is called its frequency (f )and measured in Hz (Hertz) - The distance between two consecutive maxima or minima is called a wavelength, while is represented by lambda( λ) 8 - Speed of light, in a vacuum, 3 x 10 m/sec or about 1 foot per nanosecond - λ f = c where c is the speed of light, f is the frequency and λ is the wave length - *The radio, microwave, infrared, and visible light portions of the spectrum can all be used for transmitting information by modulating the amplitude, frequency, or phase of the waves. - People like fibre optic because there are many GHz of bandwidth available to tap for data communication in the microwave band - Most transmission use a relatively narrow frequency band o They concentrate their signals in this narrow bands to use the spectrum efficiently and obtain reasonable data rates by transmitting with enough power o A wider band is used, in three variations  Frequency hopping spread spectrum  The transmission hops from frequency to frequency hundreds of times per second  Direct sequence spread spectrum  Uses a code sequence to spread the data signal over a wider frequency band, these signals are given different codes called CDMA (Code Division Multiple Access)  UWB (Ultra Wide Band)  It sends a series of rapid pulses, varying their position to communicate information o Leads to a signal that spreads thinly over a very wide frequency band Radio Transmission - Radio Frequency (RF) waves are easy to generate, travels in all directions from the source and the transmitter and receiver can be anywhere (omnidirectional), widely used for communication and travel long distances o At low frequencies, radio waves pass through obstacles well, but the power falls off sharply from the source as the signal energy is spread more thinly over a larger surface, this is called the path loss  The Path loss reduces power o At high frequencies radio waves are also absorbed by rain and other obstacles to a larger extent than are low-frequency ones - In the VLF, LF and MF bands, radio waves follow the ground o These waves can be detected for perhaps 1000 km at the lower frequencies, less at the higher ones o AM Radio uses MF bands, that why Boston AM is not heard in New York o HF and VHF are ground waves that tends to be absorbed by the earth  Used by the military and the Amateur radio operators Microwave Transmission - Microwaves travel in a straight line, so if the tower are too far apart, repeaters are needed periodically o The higher the towel, to the farther apart they can be, for 100 meter-high towers, repeaters can be 80 km apart o Microwaves do not pass through buildings well o The delayed waves may arrive out of phase with the direct wave, thus the signal is cancelled, this effect is called multipath fading o They are used for long distance calls, mobile phones, television distribution and other purposes that a severe shortage of spectrum has developed, it is very cheap The Politics of the Electromagnetic Spectrum - There are three algorithms were used when a piece of spectrum was allocated 1) Beauty Contest – requires the carrier to explain why its proposes serve the public interest best  Then the government decide which carrier to give it to. 2) Lottery – is holding a lottery among the interested companies 3) Auction – the highest bidder win the spectrum - Most governments have set aside some frequency bands called the ISM(Industrial Scientific Method), used for garage doors, cordless phones, wireless mice and more o ISM bands limit their transmit power and use other techniques to spread their signals over a range of frequencies Infrared Transmission - Unguided infrared waves are used for short-range communications, ex. Remotes for televisions, VCR, DVD o They are cheap, easy to build, relatively directional, but don’t pass though solid objects o It will not interfere, with your neighbours television, no government license is needed to operate a infrared system o Infrared communication are somewhat used on computers used to connect notebooks and printers with IrDA (Infrared Data Association) Light Transmission - Unguided optical signalling or free-space optics has been in use for centuries - A more modern application is to connect the LANs in two buildings using lasers mounted on their rooftops o Optical signalling using lasers is inherently unidirectional, so each ends needs a laser and its own photo detector  Very high bandwidth at very low cost and quires not FFC license o Wind and temperature changes can distort the beam and laser beams also cannot penetrate rain or thick fog COMMUNICATION SATELLITES - They are thought of as a big microwave repeater in the sky, it includes several transponders, each transponders is listening to some portion of the spectrum, amplifies the incoming signal and the rebroadcasts it at another frequency to avoid interference with incoming signals, o This mode of operation is bent pipe Geostationary Satellites - Is located 35,800 km from earth - These are high-flying satellites are often called GEO (Geography Earth Orbit) - The effects of solar, lunar and planetary gravity tend to move them away from their assigned orbits slots and orientations, an effect countered by on-board rocket motors, this fine-tuning activity is called station keeping The Principle Satellite bands Band Downlink Uplink Bandwidth Problems L 1.5 GHz 1.6 GHz 15 MHz Low Bandwidth: crowded S 1.9 GHz 2.2 GHz 70 MHz Low Bandwidth: crowded C 4.0 GHz 6.0 GHz 500 MHz Terrestrial interference Ku 11 GHz 14 GHz 500 MHz Rain Ka 20 GHz 30 GHz 3500 MHz Rain, equipment cost - The first geostationary satellites had a single spatial beam that illuminated at 1/3 of the earth surface called its footprint - A recent development in the communication satellite world is the development of low-cost micro stations, sometimes called WSATs (Very Small Aperture Terminals) o They are tiny terminals that have 1-meter or small antennas and can put out about 1 watt of power o Uplink = 1 Mbps and Downlink = Several Mbps o Since micro stations don’t have enough power to communicate with each other, so the hub, which has a large high-speed antenna is needed for the VSATs to communicate with each other o Even though signals travel at the speed of light, the long round-trip distance introduces a substantial delay for GEO satellites, it may take about 270 msec from the user to the ground station Medium – Earth Orbit Satellites (MEO) - Takes about 6 hours to circle the earth, they must be tracked as they move through the sky, has a smaller footprint on the ground and require and require less powerful transmitter and used for navigation rather than communication, 20200 km attitude Low Earth Orbit Satellites - Due to their rapid motion, large number of them are needed for a complete system - They are close to earth, therefore only a few millisecond, cheap to launch - Iridium are positioned at an altitude of 750 km, with one satellite every 32 degrees of latitude, each satellite has a max of 48 cells and capacity of 3840 channels and each satellite has 4 neighbours and the satellites relay the call across this grid until it is finally sent down to the callee at the South Pole - Globalstar o Uses a traditional bent-pipe design, the call originating at the North Pole is sent back to earth and picked up by the large ground station at Santa’s Workshop  Most of the complexity is on ground, where it is easy to manage Digital Modulation and Multiplexing - To send digital information, we must make analog signals to represent bits, the process of converting between bits and signals that represent them is called Digital Modulation o The process of converting bits into signals are called baseband transmission, in which the frequencies occupies from 0 up to a maximum that depends on the signalling rate - Schemes that result that control the amplitude, phase, or frequency of a carrier signal to convey bits and called passband transmission, in which the signal occupies a band of frequencies around the frequencies of the carrier signal - Multiplexing – is when a single wire carries several signals than one wire per signal Baseband Transmission - In digital modulation, to use a positive voltage to represent a 1 and a negative voltage to represent a 0, this scheme is called NRZ (Non-Return to Zero) - When the NRZ signal is sent, the receiver convert it into bits by sampling that signals at regular intervals of time, then attenuated and distorted by the channel o To decode the bits, the receiver maps the signal samples to the closest symbols Bandwidth Efficiency - With NRZ, the signal may cycle between the positive and negative levels up to every 2 bots, therefore the bandwidth has to be at least B/2 Hz when the bit rate is B bits/sec - We call the rate at which the signal changes the symbol rate to distinguish it from the bit rate o The bit rate is the symbol rate multiplied by the number of bits per symbol Clock Recovery (pg 128-129) - Read - To encode bits into symbols, we have the know where the start and ends are - Accurate clocks help identifying the number of zeros and ones - One strategy is to send a separate clock to the receive, mix the clock signal with the data signal by XORing them - When it is XORed with the 1 level it is inverted and makes a high to low transaction, the transaction is a logical 1. This scheme is called Manchester o It is requires twice as much bandwidth as the NRZ signal Balanced Signals - Signals that have as much as positive voltage as negative voltage even over short period of time are called balanced signals - One method of connecting the receiver to the channel called capacitive coupling pass only the AC of a signal - Read Pg 129-138 Passband Transmission - We can take a baseband signal that occupies 0 to B Hz and shift it up to occupy a passband of S to S+B Hz without change the amount of information - Digital modulation is accomplished with passband transmission by regulating or modulating a carrier signal that sits in the passband - We can modulate the amplitude, frequency or phase of the carrier signal o ASK (Amplitude Shift Keying)  Two different amplitudes are used to represent 0 and 1 o FSK (Frequency Shift Keying)  Two or more different tones are used o PSK(Phase Shift Keying)  The carrier wave systematically shifted 0 or 180 degrees at each symbol period o BPSK (Binary Phase Shift Keying)  Refers to the two symbols, not that the symbols represent 2 bits - Usually, Amplitude and phase are modulated in combinations o This diagram is called constellation diagram, it has 4 parts  QPSK, has dots at 45, 135, 225, 315  QAM-16 (Quadrature Amplitude Modulation), has 16 combinations of amplitudes and phase, 4 bits per symbol  QAM-64, scheme with 64 combinations and 6 bits per symbols Frequency Division Multiplexing (Ask) - The modulation scheme let us send one signal to convey bits along a wired or wireless link - Multiplexing scheme have been developed to share lines among many signals o The FDM take advantage of passband transmission to share a channel o It divides the spectrum into frequency band with each user having exclusive possession of some band in which to send their signal o When many channels are multiplexed together 4000 Hz is allocated per channel, which is known as guard band Time Division Multiplexing - TDM, the users take turns, each one periodically getting the entire bandwidth for a short of time o Bits from the input are taken in a fixed time slot and output to the aggregate stream  This steam runs at the sum rate of the individual streams, the streams must be synchronized in time  Small intervals of guard time analogous to a frequency guard band may be added to accommodate small timing variations o TDM is used in telephone and cellular networks Code Division Multiplexing (Ask) - CDM is a form of spread spectrum communication in which a narrowband signal is spread out over a wider frequency band o There is more tolerant of interference, as well as allowing multiple signals for different users to share the same frequency band o It is commonly called Code Division Multiple Access  It allows each stations to transmit over the entire spectrum  Each bit time is subdivided into m short intervals called chips  There are 64 or 128 chips per bit  Each station is assigned a unique m-bit code called chip sequence THE PUBLIC SWITCHED TELEPHONE NETWORK Structure of the Telephone System - It all started as a telephone was connected another telephone through cables, it run over houses, roads and it was all over the place - By 1890, the three major parts of the telephone system were in place: o The switching office (responsible for connecting calls between people) for long distance and local calls - Each telephone has two copper wires coming out of it that go directly to the telephone company nearest end office (local central office), which is about 1 to 10 km - The two wire connection between the subscriber and the end office are known in the trade as the local loop - The wires connecting the end offices and toll offices are called the toll connecting truck - Each end office has a # of outgoing lines to one or more nearby switching center, called toll offices (and if they are within the same area is called, tandem offices) - If the caller and callee don’t have a toll office in common, a path is established between them by high-bandwidth intertoll trucks, which are connected to the Intermediate switching centre - Local loops consist of Category 3 twisted pairs, with fiber just starting to appear Telephone→ Local loop →End Office →Toll connecting truck→ Toll office→ Very high Bandwidth intertoll trucks→ INTERMEDIATE SWITCHING OFFICES → toll offices→ end offices →Telephone In Summary, the telephone system consists of three major components: 1) Local Loops(analog twisted pairs going to houses and business) 2) Trucks (digital fiber optic links connecting switching offices) 3) Switching offices(where calls are moved from one truck to another) The Politics of Telephones - After the MFJ (Modified Final Judgement), resulted to increased competition, lower rates and better service - Local number portability – customers are switch telephone companies without having to get a new telephone number - The United States was divided up into 164 LATA (Local Access and Transport Access) o Within each LATA, there is a LEC (Local Exchange Carrier) o All inter-LATA traffics were handled by a different kind of company IXC (IntereXchange Carrier  Any IXC that wishes to handle calls originating on a LATA can build a switching office called POP(Point of Presence) o The LEC is required to connect each IXC with every end office The Local Loop: Modems, ADSL and Fiber Telephone Modem - To send bits over the local loop, it must be converted into analog signals and in the end of the channel it is converted back to bits - A device that converts between a steam of digital bits and an analog signal that represent the bits is called a modem - Logically, the modem is inserted between the (digital) computer and (analog) telephone system - Telephone modems are used to send bits between two computers over a voice-grade telephone line, in place of the conversation that usually fills the line Digital Subscriber - There was many overlapping high-speed offering, many Digital Subscriber Lines - Services with more bandwidth than standard telephone service was called broadband - The reason for the modems for being slow is that they were going through a filter - A telephone technician must install a NID (Network Interface Device) on the customer’s premises o This is a plastic box marks the end of the telephone company’s property and the start of the customers property o The Splitter is an analog filter that separates 0-4000 Hz band used by POTS from the data o Pg 149-150 Fiber to the home - Is using fibre optic wires to the home, for services like television and cable - Copper provides fast enough speed over the last short distance - The fiber simply carries the signal between the home and the end office, which reduces the cost and improves reliability o The optical splitter, splits the signal from the end office to all the house o The optical combiner, joins the fibers from 100’s of homes so that only a single fiber reaches the end office per 100 houses o This architecture is called a PON (Passive Optical Network)  There are two types of PON’s  GPONs (Gigabit-capable PONs) come from the world of telecommunications  EPON (Ethernet PONs) are more in tune with the world of networking Trucks and Multiplexing - Trucks in the telephone network are faster than the local loops - The core telephone network carriers digital information that are bits not voice - The trucks has the ability to carry thousands and millions of calls simultaneously Digitizing Voice Signals - In the early times, voice calls were handled as analog information - 12 calls in the 60 to 180 Hz known as a group and five groups are known as supergroup - Since TDM can only be used for digital data and the local loops produced analog signals and they were converted at the end office, the device for digitizing is called codec Time Division Multiplexing - TDM carries multiple voice calls over trucks by sending a sample from each call every 125 usec o The method used in North America and Japan was T1 carrier, it consists of 24 channels and each channel, get in insert 8 bits into the output stream, there 193 bits/usec  The group of 24 channels is a called extended super frame o T1 format has several variation  Earlier versions sent signalling information in band, meaning in the same channel as the data, by using some of the data bits  This is known as channel associated signalling  The older T1 carry data carried data, only 7 or 8 bits or 56 kbps can be used in each of the 24 channels  The new T1 provides clear channels in which all of the bits may be used to send data  Signalling for any voice calls is then handled out of band SONET/SDH - The ITU recommendations are called SDH (Synchronous Digital Hierarchy) - Virtually all the long distance telephone traffic now uses truck running SONET (Synchronous Optical NETwork) o SONET has four major goals 1. It had to make it possible for different carriers into interwork 2. Means was need to unify the US, European and Japanese digital systems, all of which were based on 64 kbps PCM channel 3. SONET had to provide a way to multiplex multiple digital channels 4. SONET had to provide support for operations, administration and maintenance Wavelength Division Multiplexing - Four fiber wires come together at an optical combiner, each with its energy present at a different wavelength, the four beams are combined into one fiber for transmission to a distant destination and at the end they are split into many fibers - The reason WDM is popular is that the energy on a single channel is typically only a few gigahertz wide because that is current limit of how fast we can convert between electrical and optical channels Switching - There are two different types of switching o Circuit Switching  The switching equipment within the telephone system seeks out a physical path all the way from your telephone to the receiver system  The telephone is connected to each of the six rectangles, which represents a carrier switching office  When a call passes through a switching office, a physical connection is established between the line on which the call came in and one of the output lines  The circuit-switching equipment used worldwide was known as Strowger gear o Packet Switching  Packets are sent as soon as they are available and there is no need to set up a dedicated path in advance  It is up to routers to use store-and-forward transmission to send each packet on its way to the destination on its own  The connection setup is the reservation of bandwidth all the way from the sender to the receiver  In packet switching there is no fixed path ,different packet follow different paths  Queuing delay – occurs when there are too many packets sent at the same time The Mobile Telephone System First Generation (1G) Mobile Phones: Analog Voice Push to talk system – to talk the user has to push a button on the transmitter and disabled the receiver - It had a single channel, used for both sending and receiving - After the IMTS (Improved Mobile Telephone System), was installed , which supported 23 channels o Users often had to wait a long time before getting a dial tone Advanced Mobile Phone System The AMPS (Advanced Mobile Phone System) is geographic regions is divided up into cells and they are 10 to 20 km across and each cells uses a frequency that the different from the neighbour - It was able to have 10 to 15 calls on each frequency and they reuse transmission frequencies - The cells are grouped in 7s, many base station antennas are alliance with the Roman Catholic Church - When the number of users increase the system is overload, and the power can be reduced, the overload cells are split into smaller microcells - At the center of each cell there is a base station to which all the telephone in the cell transmit o The base stations has a computer, transmitter/receiver connected to an antenna o In a smaller system, all the base stations are connected to MSC (Mobile Switching Center) or MTSO (Mobile Telephone Switching Office)  In larger system, more MSC are needed  It is essentially ends offices as in the telephone system Channels AMPS uses FDD (Frequency Division Duplex) uses 832 full duplex channels, which are from 824 to 849 MHZ are used for mobile to base station transmission and 832 simplex channels from 869 to 894 MHz are used for base station to mobile transmission - The 832 channels are divided into four categories o Controls Channels (base to mobile) are used to manage the system o Paging channels (base to mobile) alert mobile users to calls for them o Access Channels (bidirectional) are used for call setup and channel assignment o Data channels (bidirectional) carry’s voice, fax or data Call Management - When the phone is switched on, it scans a pre-programmed list of 21 channels to find the most powerful signal, then broadcasts its 32 bits serial numbers and 34 bit telephone number and then goes to the MSC SECOND GENERATION MOBILE PHONES: DIGITAL VOICE - Switching to digital has many advantages o Allows voice signals to be digitalized and compressed, improve security by allowing voice and control signals to be encrypted, and allows for text messaging o There are three system widely deployed  DAMPS (Digital Advanced Mobile Phone System) is a digital version of AMPS and allow multiple calls on the same frequency  GSM (Global System of Mobile Communication) it dominated the 2G System  CDMA (Code Division Multiple Access ) was the basis for the 3G systems  PCS (Personal Communication Services) GSM – Global System Mobile Communication - A 2G mobile itself is now divided into a handset, SIN (Subscriber Identity Module), with the subscriber and account information - The mobile talk to the cell case stations, which are connected to a BSC (Base Station Controller) that controls the radio resources of cells and handles handoff, which is connected to an MSC and connects to the PSTN - VLR (Visitor Location Register) is a database that maintains the nearby mobiles that are associated with the cells it manages - HLR(Home Location Register)is a database in the mobile network that gives the last known location of each mobile - GSM runs on a range of frequencies worldwide including 900, 1800m and 1900 MHz - GSM is a frequency division duplex cellular system and transmits on one frequency and receives on another frequency Third Generation Mobile Phones: Digital Voice and Data - It is all about the digital voice and data - First, data traffic already exceeds voice traffic on the fixed network and is growing exponentially, where as the voice traffic is flat - Second the telephone entertainment and computer industries have all gone digital and are rapidly converging - Apple IPhone is a good example of a 3G device - The basic services that the IMT(International Mobile Telecommunications) 2000 network was supposed to provide to its users are: o High quality voice transmission o Messaging (replacing email, fax, SMS, chat) o Multimedia (playing music, video, films, television) o Internet access (Web surfing, including pages with audio and video) - Several proposals were made, and after some winnowing, it came down to two main ones. o The first one, W-CDMA (Wideband CDMA), was proposed by Ericsson.  This system uses direct sequence spread spectrum of the type we described above. o It runs in a 5 MHz bandwidth and has been designed to interwork with GSM networks although it is not backward compatible with GSM. o It does, however, have the property that a caller can leave a W-CDMA cell and enter a GSM cell without losing the call. This system was pushed hard by the European Union, which called it UMTS (Universal Mobile Telecommunications System). - Advantages of the CDMA o It can improve the capacity by taking advantage of small periods who some transmitters are silent o With CDMA each cells uses the same frequencies, unlike GSM, AMPS and FDM is not needed to separate the transmission of different users, o It also makes it easy for a base station to use multiple directional antennas, or sectored antennas, instead of omnidirectional antenna o CFMA facilities soft se station before the handoff, in which the mobile is acquired by the new base station before the previous one signs off, the alternative is a hard handoff, in which the old base station drops the calls before the new one acquires it CABLE TELEVISION Community Antenna Television - Each house uses a drop cable that is connected to a coaxial cable, with a tap and which is connected to the headend, where there is a big anteena on top of a hill to pluck the television signals out of the air, an amplifier, called the headend, which sent back through the coaxial cable to the house CHAPTER 3 DATA LINK LAYER DESIGN ISSUES Main Functions of the Data-Link Layer: 1. Provide a service to the network layer 2. Deal with transmission errors 3. Regulate the flow of data, so the slow receivers are not swamped by fast receivers - To accomplish this goals, the data link layer takes the packets it get from network layer and make then into frames for transmission o Each frame contains a frame header, a payhold field for holding the packet and a frame trailer  *the frame management forms the heart in what the data link layer does - All the layers in the network, are dependent on each other and have the same goal Services Provided to the Network Layer - The principal service is transferring data from the network layer to the network layer on the destination machine - The job of the DLL is to transmit the bits to the destination machine, so they can be handed over to the network layer there The DLL can be designed to offer various services: 1. Unacknowledged connectionless service - There is when the source machine is sending independent frames to the destination machine without their acknowledge them o Good example is Ethernet o There is no logical connection established so if a frame is lost, it wont be detected o This service can be used only when the error rate is low 2. Acknowledged connectionless service - There is no logical connection but each frame sent is individually acknowledged - The sender knows whether or not the frame has been received or not o If not received within a time interval, the frames is sent again - This service is useful over unreliable channels, such as wireless system, WiFi o Errors can be corrected more directly and more quickly - The network layer doesn’t know the protocol for the DLL 3. Acknowledged connection-oriented service - “The most sophisticated service” - The source and the destination establish a connection before any data is transmitted, each frame is numbered and when the frame is received, the data link layer guarantees that each frame is indeed received - It is appropriate over long, unreliable links such as satellite channel or a long- distance telephone circuit - There are three phases o The connection is established btw the source and destination and counter needed to keep track of which frames have been received or not o The frames are transferred o The connection is released, freeing up the variable s, buffers, and other resources used to maintain the connection Framing - The physical layer accepts the raw bit stream and attempt to deliver it to the destination - When the bits are received by the DDL from the PL, it is the DDL job to detect and correct any errors - When the bits are broken into frames, they are given checksum’s, so the when the frame arrives this checksum is recomputed Framing Methods: 1. Byte count - It uses a field in the header to specify the number of bytes in the frame - The problem with this algorithm is that the count can be garbled by a transmission error o Eg if the byte count of 5 in the second frame of becomes a 7 due to a single bit flip o Therefore the byte count method is rarely used 2. Flag bytes with byte stuffing - The second framing method gets around the problem of resynchronization after an error by having each frame start and end with special bytes - The flag byte is used as both starting and ending delimiter, there are two consecutive flag bytes indicate the end on one frame and the start of the next - So if the receiver ever loses sync it can just search for two flag bytes to the find the end of the current frame and the start of the next frame - The Problem: o The flag byte error may occur in the data, when the binary data such as photographs or songs are being transmitted, this situation would interfere with the framing  Solution: the sender can have a ESC, it can distinguish from one in the data by the absence or presence of an escape byte before it o The DLL on the receiving end removes the escape bytes before giving the data to the network layer, which is called byte stuffing o The byte stuffing scheme is a slight simplification of one used in PPP (Point to Point Protocol), which carries packets over communication links 3. Flag bits with the bit stuffing - The third method of delimiting the bit stream get around a disadvantage of byte stuffing and use a 8 bit bytes, which is developed for the once very popular HDLC (High Level Data Link Protocol) o Each frame begins and ends with a special bit pattern, 0111110, this pattern is a flag byte, when the sender DDL, encounters 5 1’s , it automatically stuffs a 0 bit into the outgoing system, pg 200  The receiver see the 5 1’s , followed by a 0 and dyestuffs the 0 bit and that is bit stuffing 4. Physical layer coding violation - It is to use a shortcut from the physical layer, the encoding of bits as signals often includes redundancy to help the receiver, this redundancy means that some signals will not occur in regular data o Ex. 4B/5B line code 4 data bits are mapped to 5 signal bits to ensure sufficient bit transaction, this means that 16 out of 32 signals possibilities are not used delivery Error Control - The usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at the other end and the receiver sends back special control frames bearing positive or negative acknowledgement about the incoming frames - Another problems is from hardware troubles may cause a frame to vanish o The receiver will not react, because he doesn’t know about the frame, the waiting for acknowledgement waits forever o Solution:  Introducing timers, if the acknowledgment is not received in a time interval, the frame is sent again Flow Control - The situation can occur is the sender is running on powerful computer and the receiver is running on a slow computer o Ex. Smart Phone requests a Web Page for a powerful server - There are two approaches o Feedback based flow Control  The receiver sends back information to the sender giving it permission to send more data or at least telling the sender how the receiver is doing o Rate-Based flow control  The protocol has a built in mechanism that limits the rate at which senders may transmit data, without using feedback from the receiver ERROR DETECTION AND CORRECTION - Optical fiber communication have tiny error rates and wireless links and aging loops have high error rates - There are two strategies for dealing with errors o Include enough redundant information to enable the receiver to deduce what the transmitted data must have been (error-correcting codes) o Include only enough redundancy to allow the receiver to deduce that an error has occurred (error-detecting codes)  This use EDC is referred to as FEC (Forward Error Correction) Error- Correcting Codes All these codes add redundancy to the information that is sent - Block code o The redundant check bits are computed solely as a function of the message data bits - Systematic Code o The message bits are sent directly along with the redundant bits, rather than being encoded themselves before they are sent - Linear code o The redundant bits are computed as a linear function of the message bits 1. Hamming codes - The bits of the codeword are numbered, starting with bit 1 at the left end, bit 2 to its immediate right and so on - The bits that are powers of 2 are check (redundant) bits, the rest (3, 5, 7...) are filled up with m data bits - The construction gives a code with a Hamming distance of 3, which means that it can correct single errors, the reason for the very careful numbering of messages and check bits become apparent in the decoding process o When the codeword arrives, the receiver redoes the check bit computations, called the check results  If the all the check results are 0, its valid  If the results are not 0, an error has been detected o The set of check results forms the error syndrome that is used to pinpoint and correct the error 2. Binary convolutional codes - An encoder processes a sequence of input bits and generates a sequence of output bits , there is no natural message size or encoding boundary as in a block code - The output depends on the current and previous input o Each input produces 2 output, the code rate is 1/2 o The number of previous bits on which the output depends is called the constraint length - Convolutional codes are used in deployed networks, GMS Mobile phone system, satellite communication and WiFi - The convolutional code is decoded by finding the sequence of input bits that is mostly likely to have produced the observed sequence of output bits - The convolutional code is very popular in practise because it is easy to factor the uncertainty of a bit being a 0 or a 1 into the decoding 3. Reed Solomon codes - They are linear block codes and they are often systematic too - Unlike hamming codes which operate on individual bits, RSC operates on message bits - RSC are based on the fact that every n degree polynomial is uniquely determined by n+1 points - RSC are widely used in practise because of their strong error-correction properties, particularly for burst errors o They are used for DSL, data over cable. Satellite communication, CDs, DVDs and Blue Ray Discs - RSC are often used in combination with other codes such as the convolutional code 4. Low Density Parity Check codes - LDFC codes are linear blocks codes, each output bit is formed from only a fraction of the input bit, this leads to a matrix representation of the code that has a low density of 1s - LDFC are practical for large block sizes and have excellent error-correction abilities that outperform many other codes Error Detecting - There are three different error-detecting codes o Parity  The parity bit is chosen so that the number of 1 bits in the codeword in even (or odd)  For example 1011010 to 10110100  With odd parity 1011010 becomes 10110101  One difficulty with this scheme is that a single parity bit can only reliably detect a single-bit error in the block  If the block is badly grabbed by a long burst error, the probability that the error will detected is only 0.5  Advantage: Interleaving  A parity bit for each of the n columns as and send all the data bits as k rows o The technique is to convert a code that detects isolated errors into a code that detects burst error  Which cause parity errors (Figure 3.8) o Checksums  Is often used to a group of check bits associated with a message, regardless of how are calculated  A group of parity bits is one example of a checksum  There checksum is usually placed at the end of the message  Errors may be detected by summing the entire received codeword, if the result is zero, no errors have been found o Cyclic Redundancy Checks (Figure 3.9)  The stronger kind of error detecting code is in wide-spread use at the data link layer: the CRC (Cyclic Redundancy Check), also known as polynomial code  Polynomial codes are based upon treating bit strings as representations of polynomial with coefficient of 0 and 1 only  A k-bit frame is regarded as the coefficient list for a polynomial with k terms, ranging from x and x 0  For example 110001 has 6 bits  THE use of addition, subtractions and long division ELEMENTARY DATA LINK PROTOCOLS - The physical layer process and some of the data link layer process rim on dedicate hardware called the NIC (Network Interface Card) - The rest of the link layer and the network layer process run on the main CPU as part of the operating system, with the software for the link layer process often taking the form of a device driver - The data link layer is concerned, the packet passed across the interface to it from the network layer is pure data, whose every bit is to be delivered to the destination network layer - When the packet is received by the DDL, it adds a header, trailer and a frame o Frame has the embedded packet, some control information is in the header and the checksum is in the trailer - When the frame arrives at the receiver , the checksum is computed o If the frame is incorrect, it is reports to the DDL o If the frame is correct, the DDL is informed so that it can acquire the frame  After passes the packet to the network layer - Boolean, take value of true or false - A frame contained 4 fields (kind, seq, ack, and info), the first three of which contain control information and the last one contains the actual data, these control A Utopian Simplex Protocol - Is a protocol that is as simple as it can be because it does not worry about the possibility of anything going wrong - Since data are transmitted in one direction only, therefore the processing time can be ignored - There is never a damage or loss of frames - The protocol consists of two distinct procedures, a sender and a receiver o The sender runs in the data link layer of the source machine, and the receiver runs in the data link layer of the destination machine A Simplex Stop and Wait Protocol for a Error Free Channel - This will prevent the sender from flooding the receiver with frames faster than the latter is able to process them o One solution is to build the receiver to be powerful enough to process a continuous stream of back to back frames  Which requires hardware and can be wasteful of resources of the utilization of the link is mostly low o Another solution, would be having passed a packet to its network layer, it send a dummy frame back to the sender, for permission to send the next frame  The process of which the sender sends one frame and wait for the permission of the receiver is called stop-and-wait A Simplex Stop-and-Wait Protocol for a Noisy Channel - Frames may be damaged or lost completely, we assume that if a frame is damaged in transit, the receiver hardware will detect this when it computes the checksum - The obvious way to achieve when a frame is seen for the first time from the retransmission, is to have the sender put a sequence number of each arriving frame to see if it is a new frame or a duplicate to be discarded - A frame can be damaged, lost and late (before the timer ends) - After transmitting a frame, the sender starts the timer running. If it was already running, it will be reset to allow another full timer interval. The time interval should
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