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Tycs dcn sem 5 unit 1,2,3,4 (2017)


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Introduction - Data Communication, Networks, Internet, Intranet, Protocols,
OSI & TCP/IP Models, Addressing
Physical Layer - Signals, Analog, Digital, Analog VS Digital, Transmission
Impairment, Data Rate Limits, Performance
Digital Transmission - Line Coding (Unipolar, Polar, Biphase), Block
Coding(4B/5B Encoding), Analog to digital conversion, PCM, Transmission
Analog Transmission - Digital to analog conversion(ASK,FSK,PSK, QAM),
Analog to Analog conversion

Multiplexing - FDM, WDM, Synchronous TDM(time slots & frames,
interleaving, data rate management),
Spread Spectrum - FHSS, DSSS
Transmission Media - Guided & Unguided
Switching - Switching, Circuit-Switched Networks, Datagram networks,
Concept of Virtual circuit networks, structure of circuit switch & packet
switch, Concepts of DSL & ADSL

Data Link Layer -Error correction & detection, Types of errors, Detection VS
Correction, Block Coding, Hamming Distance, Linear Block codes(single parity
check, hamming codes), Cyclic codes, CRC Encoder & Decoder, CRC
Polynomial & its degree, Checksum
Data Link Control & Protocols - Framing, Flow & Error Control, Simplest,
Stop-N-Wait, Stop-N-Wait ARQ, Go Back N ARQ, Selective Repeat ARQ,
Piggybacking HDLC & PPP- HDLC Modes, HDLC Frames, PPP, PPP Transition

Multiple Access - Random(CSMA), Controlled(Reservation, Polling, Token
Passing), Channelization(FDMA, TDMA, CDMA)
Wired LAN - LLC, MAC, Ethernet, Ethernet frame, Addressing, Concept of
MBaseN Ethernet, Bridged, Switched, Full Duplex Ethernet, Concept of Fast &
Gigabit Ethernet
Wireless LAN - Introduction to WLAN(Architecture, Hidden, Exposed Station
Problem), Introduction to Bluetooth & Architecture, Cellular telephony,
Concept of 1G, 2G, 3G cellular telephony
Connecting Devices - Repeaters, Hubs, Bridges, Spanning tree algorithm, Two
& Three layer Switches, Routers, Gateways, Backbone networks, Concept of

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Tycs dcn sem 5 unit 1,2,3,4 (2017)

  2. 2. Course: USCS501 Data Communication and Networking Page no Unit I Introduction - Data Communication, Networks, Internet, Intranet, Protocols, OSI & TCP/IP Models, Addressing Physical Layer - Signals, Analog, Digital, Analog VS Digital, Transmission Impairment, Data Rate Limits, Performance Digital Transmission - Line Coding (Unipolar, Polar, Biphase), Block Coding(4B/5B Encoding), Analog to digital conversion, PCM, Transmission Modes, Analog Transmission - Digital to analog conversion(ASK,FSK,PSK, QAM), Analog to Analog conversion 1 Unit II Multiplexing - FDM, WDM, Synchronous TDM(time slots & frames, interleaving, data rate management), Spread Spectrum - FHSS, DSSS Transmission Media - Guided & Unguided Switching - Switching, Circuit-Switched Networks, Datagram networks, Concept of Virtual circuit networks, structure of circuit switch & packet switch, Concepts of DSL & ADSL 41 Unit III Data Link Layer -Error correction & detection, Types of errors, Detection VS Correction, Block Coding, Hamming Distance, Linear Block codes(single parity check, hamming codes), Cyclic codes, CRC Encoder & Decoder, CRC Polynomial & its degree, Checksum Data Link Control & Protocols - Framing, Flow & Error Control, Simplest, Stop-N-Wait, Stop-N-Wait ARQ, Go Back N ARQ, Selective Repeat ARQ, Piggybacking HDLC & PPP- HDLC Modes, HDLC Frames, PPP, PPP Transition states 71 Unit IV Multiple Access - Random(CSMA), Controlled(Reservation, Polling, Token Passing), Channelization(FDMA, TDMA, CDMA) Wired LAN - LLC, MAC, Ethernet, Ethernet frame, Addressing, Concept of MBaseN Ethernet, Bridged, Switched, Full Duplex Ethernet, Concept of Fast & Gigabit Ethernet Wireless LAN - Introduction to WLAN(Architecture, Hidden, Exposed Station Problem), Introduction to Bluetooth & Architecture, Cellular telephony, Concept of 1G, 2G, 3G cellular telephony Connecting Devices - Repeaters, Hubs, Bridges, Spanning tree algorithm, Two & Three layer Switches, Routers, Gateways, Backbone networks, Concept of VLAN 103
  3. 3. 1 UNIT 1 Introduction to Data Communication and Networking The word data refers to the information presented in whatever form is agreed upon by the parties creating and using the data. The word telecommunication means communication at a distance by the means of telephony, telegraphy and television. Q. Define Data Communication. Expalin its components and Characteristics Definition of Data Communication:- “Data Communication is the exchange of data between two devices via some form of transmission medium “ For data communication to occur, the communicating devices must be a part of a communication system made up of combination of hardware (physical equipment) and software (programs) Components of Data Communication:- 1.) Message: - the message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio and video. 2.) Sender: - the sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera and so on. 3.) Receiver: - the receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television and so on.
  4. 4. 2 4.) Transmission Medium: - the Transmission Medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted pair wire, coaxial cable, fiber-optic cable and radio waves. 5.) Protocol: - A protocol is a set of rules that govern data communication. It represents an agreement between the communication devices. Without a protocol, two devices may be connected but not communicate. Characteristics of Data Communication:- 1.) Delivery: - the system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. 2.) Accuracy: - the system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3.) Timeliness: - the system must deliver data in timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission. 4.) Jitter: - Jitters refers to the variations in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For e.g.: let us assume that video packets are sent every 30ms. If some of the packets arrive with 30 ms delay and others with 40 ms delay, and uneven quality in the video is the result. Q. Write a short note on types Data Flow 1.) Simplex: - in simplex mode, the communication is unidirectional, as on a one- way street. Only one of the two devices on a link can transmit; the other can only receive. The simplex mode can use the entire capacity of the channel to send data in one direction. Keyboards and traditional monitors are examples of simplex devices. 2.) Half-Duplex: - here, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive and vice versa. In half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time.
  5. 5. 3 3.) Full-Duplex: - in full-duplex mode, both the stations can transmit and receive simultaneously. In this mode, the signals going in one direction share the capacity of the link with signals going in the other direction. This sharing can occur in two ways: either the link must contain two physically separate transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals travelling in both directions. Q. Define Network and explain the Network Criteria. A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer or any other device capable of sending and/or receiving data generated by other nodes on the network. Most networks use distributed processing, in which a task is divided among multiple computers. Instead of one single large machine being responsible for all aspects of process, separate computers (usually a personal computer or workstation) handle a subset. Networks Criteria:- 1. Performance: - Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between and inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the
  6. 6. 4 type of transmission medium, the capabilities of the connected hardware and the efficiency of the software. 2. Reliability: in addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure and the network robustness in a catastrophe. 3. Security: Network Security issues include protecting data from unauthorized access, protecting data from damage and development, and implementing policies and procedures for recovery from breaches and data losses Networks Topologies:- The topology of a network is the geometric representation of the relationship of all the links and the linking devices (usually called as nodes) to one another. Q. List and Explain DifferentTypes of Topologies: 1.) Mesh Topology: in a mesh topology, every device has a dedicated point to point link to every other device. The term dedicated means that the link carries traffic only between the devices it connects. To find the number of physical link in a fully connected mesh network with n nodes, we calculate it as: Number of Links = n(n-1)/2 duplex mode links.
  7. 7. 5 Advantages:-  The use of dedicated links guarantees that each connection can carry its own data load.  Mesh topology is robust i.e. if one link becomes unstable, it does not incapacities the entire system  It provides privacy or security Disadvantages:  The more number of wires required  Possibility is that these wires may be expensive. 2.) Star Topology(short note on star topology April 2011 4m): in a star topology, each device has a dedicated point to point link only to the central controller, usually called a hub. The devices are not directly linked to one another instead the central controller acts as an exchanger who helps to exchange then data between the sender and the receiver. Advantages:  Reduced number of Links.  Less expensive than mesh topology  Star topology is robust. Disadvantages:  Dependency of the whole topology on a single point. If the hub goes down, the whole system is dead. 3.) Bus Topology: a bus topology is a multipoint link. One long cable acts as a backbone to link all the devices in a network. Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main
  8. 8. 6 cable or punctures the sheathing of a cable to create a contact with the metallic core. Advantages:-  Easy to install.  Uses less number of cables than mesh or star topology. Disadvantages:  A fault or break in the main cable may stop the transmission.  Adding new devices may require modification or replacement of the backbone. 4.) Ring Topology: in a ring topology, each device has a dedicated point to point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches the destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. Advantages:-
  9. 9. 7  To add or delete a device requires changing only two connections.  Generally, in a ring, a signal is circulating at all times. If one device does not receive a signal within specified period, it can issue an alarm. This alarm alerts the network operator of the problem and its location. Disadvantages:  The data flow is unidirectional, thus causing traffic in one direction.  A break in the ring can disable the entire network. However, this weakness can be solved by using a dual ring or a switch capable of closing off the break. 5.) Hybrid Topology: a Combination of more than one topology is called a hybrid topology. Q. Define Protocols and Explain its Key Elements In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. However, two entities cannot simply send bit streams to each other and expect to understand. For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines what is communicated, how it is communicated and when it is communicated. The key elements of a protocol are syntax, semantics and timing.  Syntax: the term syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.  Semantics: the term semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken based on that
  10. 10. 8 interpretation? For example, does an address identify the route to be taken or the final destination of the message  Timing: the term timing refers to two characteristics: when data should be sent and how fast they can be sent. For example: if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost. Q. Explain the Layers of OSI (Open System Interconnection) Model The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems. It consists of seven separate but related layers each of which defines a part of the process of moving information across a network. 1.) Physical Layer: the physical layer is responsible for the movement of individual bits from one node to another. The physical layer coordinates the function required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that
  11. 11. 9 physical devices and the interfaces have to perform for transmission to occur. The data here consists of a stream of bits (sequence of 0’s and 1’s). The physical Layer is also concerned with the following points: 1. Physical characteristics of interfaces and medium: It defines the characteristics of the interface between the devices and the transmission medium. 2. Representation of bits: It constist of stream of bits (0;s and 1’s) with no interpretation. The bits are represented in the form of signals ie in the encoded format. 3. Data Rate: The transmission rate ie the number of bits sent each second is also defined. 4. Synchronization of bits: the sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized. 5. Line Configuration: The physical Layer is concerned with the connection of devices to the media ie point to poin and multi poin configuration. 6. Physical topology: It defines how devices are connected to make a network. Devices can be connected by using a mesh topology, a star topology or any such kind of topologies. 7. Transmission Mode: It also defines the direction of transmission between devices : simplex, half duplex or full duplex. 2.) Data Link Layer: the data link layer is responsible for moving frames from one node to the other. The data link layer divides the stream of bits received from the network layer into manageable data units called frames. The data link layer makes the physical layer reliable by adding mechanisms to detect and retransmit damaged or lost frames. It also controls the flow (i.e. the rate at which the data is sent or received) of data. Responsibilities 1. Framing The data link layer divides the stream of bits received from the network layer into manageable data units called frames. 2. Physical addressing ..If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender of receiver of the frame. 3. Flow controlIf the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the
  12. 12. 10 sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver. 4. Error control The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames.It also uses a mechanism to recognize duplicate frames 5. Access Control When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over link ar any time. 3.) Network Layer: the network layer is responsible for the source to destination delivery of a packet, possibly across multiple networks (links). When the links are connected to the large network than there is a need to route a packet. This routing process is done by routers which exist at the network layer. Responsibilities 1. logical addressing: The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary we need another addressing system to help distinguish the source and Destination systems. The network layer adds a header to the packet coming from the upper layer that among other things, includes the logical address of the sender and receiver. 2. Routing. When independent networks or links are connected to create internetworks or large network , the connecting devices route or switch the packets to their final destination. 4.) Transport layer: the transport layer is responsible for process to process delivery of the entire message. A process is an application program running on a host. Like the data link layer, the transport layer is responsible for error control and flow control. A message is divided into segments, than it is the responsibility of the transport layer to reassemble the segments at the destination point and identify and replace the segments that were lost in transmission. This layer can either be connectionless or connection oriented. Responsibilities 1. Service point addressing: The transport layer header must include a type of address called service point address to check the network layer gets each
  13. 13. 11 packet to the correct computer and the transport layer gets the entire message to the correct process on that computer 2. Segmentation and reassembly: A message is divided into transmittable segments with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission. 3. Flow Control Like the data link layer , the transport layer is responsible for flow control. Howerver , flowcontrol at this layer is performed end to end rather than across a single link 4. Error Control : Here the sending transport layer makes sure that the entire message arrives at the receiving transport layer without error. Error correction is usually achieved through retransmission. 5.) Session Layer: the services provided by the first 3 layers (physical, data link, network) are sometimes not sufficient for some processes. At this point, the session layer acts as a dialog controller in the network. It establishes, maintains and synchronizes the interaction among communicating systems. The session layer allows a process to add checkpoints to a stream of data. Responsibilities 1. Dialog control: The session layer allows two systems to enter into dialog. It allows the communication between two processes to take place int either half duplex or full duplex. 2. Synchronization: The session Layer allows a process to add checkpoints or synchronization points to a stream of data. 6.) Presentation Layer: the presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. The presentation layer is responsible for translation, compression and encryption. Responsibilities 1. Translation: The processes in two systems are usually exchanging information in the form of character strings, numbers and so on.The information must be changed to bit streams before being transmitted. 2. Encryption To carry sensitive data, a system must be able to ensure privacy. Encryption means that the sender transforms the original information
  14. 14. 12 to another from and sends the resulting message out over the network. Decryption reverses the original process. 3. Compression- Data compression reduces the number of bits contained In the information. 7.) Application layer: the application layer enables the user, whether human or software to access the network. It provides user interfaces for services such as electronic mail, remote file access and transfer, shared database management and other types of distributed information services. SERVICES 1. Network virtual Terminal- is a software version of a physical terminal, and it allows a user to log on to a remote host. To do so the application creates a software emulation of a terminal at the remote host.The user computer talks to the software emulation of a terminal which in turn talks to the host and vice versa. 2. File transfer, access a nd management : The application allowas a user to access files in a remote host, to retrieve files from a remote computer for use in the local computer, and to manage a control files in a remote computer locally. 3. Mail service: The application provides the basis for email forwarding and storage. 4. Directory services: This application provides distributed database sources and access for global information about various objects and services. TCP/IP Protocol Suite TCP/IP is a hierarchical protocol made up of interactive modules. The modules are not necessarily interdependent. The term hierarchical means that each upper- level protocol is supported by one or more lower level protocols.
  15. 15. 13 Q. Explain the Layers of TCP/IP Model: 1.) Physical and Data Link Layers: at the physical and data link layers, TCP/IP does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork can be local- area network or wide-area network. 2.) Network Layer: At the Network layer, TCP/IP supports the Internetworking Protocol (IP). IP in turn uses some supporting protocols such as ARP, RARP, ICMP and IGMP. -IP (Internetworking Protocol) is an unreliable and connectionless protocol. It gives a best effort delivery service. -ARP (Address Resolution protocol) is used to get physical address when logical address is known. -RARP (Reverse Address Resolution protocol) is used to get logical address when physical address is known. -ICMP (Internet Control Message Protocol) sends query and error reporting messages. -IGMP (Internet Group Message Protocol) is used to facilitate the simultaneous transmission of a message to a group of recipients. 3.) Transport Layer: it has the same working like in OSI Model. This layer supports 2 protocols, i.e. TCP (Transmission Control Protocol) and UDP(
  16. 16. 14 User Datagram Protocol) and a new protocol for voice over internet i.e. SCTP( Stream Control Transmission Protocol) 4.) Application Layer: the application layer in TCP/IP Model is equivalent to the combined session, presentation and application layer in the OSI Model. Q List and Explain the Addressing in TCP/IP: Each address is related to a specific layer in the TCP/IP architecture, as shown in Fig. 1) Physical Address: - It is also known as link address. It is the address of a node as defined by its LAN or WAN. It is included in the frame used by the data link layer. It is the lowest level address. The size and the format of these addresses depend on the network.
  17. 17. 15 2) Logical Address: - (Also known as IP address). A universal addressing is needed because different networks have different address formats. The physical addresses will change from hop to hop, but the logical addresses usually remain the same. The logical address in the Internet uniquely defines a host connected to the Internet. No two publicly addresses and visible hosts on the Internet can have the same IP address. 3) Port Address:- Computers are the devices that can run multiple processes at the same time. For these processes to receive data simultaneously, we need a method to label the different processes. In the TCP/IP architecture, the label assigned to a process is called a port address. A port address in TCP/IP is 16 bits in length. A port address is a 16 bit address represented by one decimal number. 4) Specific Address: - Some applications have user- friendly addresses that are designed for that specific address. For e.g. email address, URL Physical Layer One of the major functions of physical layer is to move the data in the form of electromagnetic signals across a transmission medium. Transmission media work by conducting energy along a physical path. These electromagnetic signals can be either analog or digital in form. Both data and signals can be either analog or digital. Q. Explain the Difference Between Analog and Digital Data Analog Data and Digital Data:- Analog data refers to information that is continuous and digital data refers to information that has discrete states. Analog Signal and Digital Signal:- An analog signal has infinitely many levels of intensity over a period of time. As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path. A digital signal can have only a limited number of defined values. Although each value can be any number, it is often as simple as 1 and 0.
  18. 18. 16 Periodic Signal and Nonperiodic (aperiodic) Signal:- Both analog and digital can take form of periodic and nonperiodic signals. A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one full pattern is called as cycle. An aperiodic signal or a nonperiodic signal changes without exhibiting a pattern or cycle that repeats over time. Note: - Both analog and digital signals can be periodic or nonperiodic. In data communications, we commonly use periodic analog signals (because they need less bandwidth) and nonperiodic digital signals (because they can represent variation in data). Periodic Analog Signals: Periodic Analog Signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. Sine Wave: The sine wave is the most fundamental form of a periodic analog signal. When we visualize it as a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow. Each cycle consists of a single arc above the time axis followed by a single arc below it.
  19. 19. 17 A sine wave can be represented by three parameters: peak amplitude, frequency and phase.  Peak amplitude: the peak amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. For electric signals, peak amplitude is normally measured in volts.  Frequency: Frequency refers to the number of periods in 1 sec. Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. Frequency is formally expressed in Hertz (Hz), which is cycle per second.
  20. 20. 18  Phase: the term phase describes the position of the waveform relative to 0. Phase is measured in degrees or radians. Composite Signals: A signal made up of many sine waves is called as composite signal. Any composite signal is a combination of simple sine waves with different frequencies, amplitudes and phases. if the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies; if the composite signal is nonperiodic, the decomposition gives combination of sine waves with continuous frequencies Digital Signals Information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.
  21. 21. 19 Q. Define Transmission Impairment and explain how it is caused. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Causes of Impairments: 1.) Attenuation- Attenuation is the loss of energy. When a signal travels though a medium, it loses some of its energy in overcoming the resistance f the medium. That is why a wire carrying signals gets warm. To compensate that lost amplifiers are used. 2.) Distortion: - Distortion means a change in shape or form of a signal. Distortion can occur in a composite signal made of different frequencies. Signal components
  22. 22. 20 at the receiver have phases different from what they had at the sender thus arising a change in shape. 3.) Noise: Noise can be defined as any unwanted signal mixed with the transmitted signal. Noise can be of several types like thermal noise, induced noise, crosstalk and impulse noise. Thermal noise is the random motion of electrons in a wire which create and extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. Crosstalk is the effect of one wire on the other. Impulsive noise is a spike (a signal with high energy in a very short time) that comes from power lines, lighting and so on Q. What are Data Rate Limits: A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors:-  The bandwidth available  The levels of the signals we use  The quality of the channel (the level of noise)
  23. 23. 21 Two formulas were developed to calculate the data rate: Nyquist formula for noiseless channel and Shannon formula for noisy channel. Nyquist Formula:- Where, Bandwidth= bandwidth of the channel L= number of the signal levels used to represent data BitRate= Bits per second Shannon Formula: Capacity=bandwidth x log2 (1+ SNR) Where, Bandwidth=bandwidth of the channel SNR= signal -to-noise ratio Capacity= capacity of the channel in bits per second Note: the Shannon capacity gives us the upper limit, whereas Nyquist Formula tells us how many signals levels we need. Q. State the different types of Transmission modes. Explain any one of them (April- 2011 6m) The transmission of data from one device to another is done through the wiring. The question of concern here arises that do we send 1 bit at a time or do we group bits into larger groups and if so, how? Thus the answer to this question is transmission modes. LbandwidthBitRate 2log2  power power noise signal average average SNR 
  24. 24. 22 Types of transmission modes:-  Parallel Transmission:- By grouping, we can send data n bits at a time instead of 1. This is called parallel transmission. group can be transmitted with each clock tick from one device to another.  Serial Transmission:- In serial Transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices.
  25. 25. 23  Asynchronous Transmission:- It is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed upon patterns. As long as the patterns are followed, the receiving device can retrieve the information without regard to the rhythm in which it is sent.
  26. 26. 24  Synchronous Transmission:- nous transmission.
  27. 27. 25  Isochronous Transmission:- Q. Explain the Factors on which the Performance of the network depends. The performance of the network depends on following factors:- 1.) Bandwidth: Bandwidth is measured in two different values:  Bandwidth in Hertz: Bandwidth in Hertz is the range of frequencies contained in a composite signal or the range of frequencies a channel can pass.  Bandwidth in bits per second: it refers to the number of bits per second that a channel, a link, or even networks can transmit. 2.) Throughput: the throughput is a measure of how fast we can actually send data through a network. At first glance, the throughput and bandwidth (in bps) seems same but they are different .The throughput is always less than the bandwidth. 3.) Latency (Delay): the latency or delay defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source. We say that latency is made of four components: propagation time, transmission time, queuing time, and processing delay.  Propagation time: Measures the time required for a bit to travel from the source to destination.  Transmission Time: In data communication we just don’t send 1 bit, but we send the message. However there is a time between the first bit leave from
  28. 28. 26 the sender and the last bit arriving to the receiver. The first bit leaves earlier and arrives earlier, the last bit leaves later and arrives later. The time required for transmission of a message depends on the size of the message and the bandwidth of the channel.  Queuing Time: The time needed for each intermediate or end device to hold the message before it can be processed. The queuing time is not a fixed factor; it changes with the load imposed on the network. When there is heavy traffic on the network, the queuing time increases.  Processing Delay: the time taken to process all bits into message and send it to the upper layer. 4.) Bandwidth- delay Product: Bandwidth and Delay are two performances metrics of a link. The bandwidth delay product defines the number of bits that can fill the link. This measurement is important if we need to send data in burst and wait for the acknowledgement of each burst before sending the next one. Digital and Analog Transmission A computer network is designed to send information from one point to another. this information needs to be converted to either a digital signal or and analog signal for transmission. Digital Transmission:- 1.)Explain the techniques used in Digital to Digital Transmission: Here we represent the digital data by using digital signals. The conversion involves two techniques: line coding and block coding.
  29. 29. 27  Line Coding: Line coding is the process of converting digital data into digital signals. Line coding converts a sequence of bits to digital signals Unipolar: all the signals are on the same side of time axis, either below or above. NRZ (Non - Return- to- Zero): in this scheme, 1 bit represents a positive voltage and 0 bit represents zero voltage. It is called as NRZ because it does not return to zero in the middle of the bit. Polar Schemes: in this scheme the voltages are on both sides of the time axis. The voltage level for 0 can be positive and the voltage level 1 represents negative. NRZ (Non - Return- to- Zero): NRZ-L: the level of voltage determines the value of bit.
  30. 30. 28 NRZ- I: the inversion or the lack of inversion determines the value of bit. RZ (Returning to Zero): the main problem in NRZ encoding occurs when the sender and the receiver’s clocks are not synchronized. The receiver does not know when the bit started or ended. One solution is RZ which uses 3 values: positive, negative and zero. In RZ, the signal changes not between the bits but during the bits. Biphase: Manchester and Differential Manchester: the idea of RZ and NRZ-L are combined into Manchester. In Manchester, the duration is dividing into two halves. The voltage remains at one level during first half, and moves to the other level in the second half. Differential Manchester combines the idea of RZ and NRZ-I. There is always transition at the middle of the bit, but the bit values are determined at the beginning of the bit. If the next bit is 0, there is transition. if the next bit is 1, there is no transition.
  31. 31. 29 Bipolar: in bipolar encoding (sometimes called as multilevel binary), there are three voltage levels: positive, negative and zero. The voltage level for one data element is at zero, while the voltage level for 1 alternate between positive and negative. AMI and Pseudoternary: the AMI (Alternate Mark Inversion) alternates on 1 inversion. A neutral zero voltage represents binary 0. Binary 1 are represented by alternating positive and negative voltages. A variation of AMI is called as pseudoternary, in which the bit 1 is encoded as zero voltage and the bit 0 is encoded as alternating positive and negative voltages.  Block Coding: Block coding can give us the redundancy (to ensure synchronization) and improve the performance of line coding. Block coding changes a block of m bits into a block of n bits, where n is larger than m. Block Coding is referred to as an mB/nB encoding technique. Block coding usually
  32. 32. 30 involves 3 steps: division, substitution and combination. In the division step, a sequence of bits is divided into group of m bits. In substitution, we substitute and m-bit group for and n-bit group. Finally in combination, the n-bit group are combined together to form a stream. The new stream has more bits than the original bits. 4B/5B: the four binary/five binary coding scheme was designed to be used in combination with NRZ-I. NRZ-I has a good signal rate but it has a synchronized problem. A long sequence of 0’s can make the receiver clock lose synchronization. Thus the 4B/5B achieves this goal to change the bit stream prior to the encoding with NRZ-I. The block –coded stream does not have more than 3 consecutive 0’s. A 4 bit of data is substituted into 5 bit of block. At the receiver, the NRZ-I encoded digital signal is first decoded into a stream of bits and then decoded to remove the redundancy.
  33. 33. 31 Table below shows the corresponding pairs used in 4B/5B: 2.) ANALOG TO DIGITAL TRANSMISSION: There are two techniques to convert and analog signal to digital data. They are Pulse Code Modulation and Delta Modulation. I. Write a short note on Pulse Code Modulation (PCM): A PCM has three processes:-  The analog signal is sampled.  The sampled signal is quantized.  The quantized values are encoded as streams of bits.
  34. 34. 32  Sampling: the first step in PCM is sampling. The analog signal is sampled every Ts s where Ts is the sample interval or period. The inverse of the sampling interval is called sampling rate or sampling frequency. There are 3 sampling methods— ideal, natural and flat-top as shown in the diagram.
  35. 35. 33 The sampling process is also called as Pulse Amplitude Modulation (PAM). The sampling rate is decide by using the Nyquist Theoram , which says the sampling rate should be at least 2 times the highest frequency contained in the signal . quency, not the bandwidth.  Quantization: the result of sampling is a series of pulses with amplitude values between the maximum and minimum amplitudes of the signal. The set of amplitudes can be infinite with nonintegral values between two limits. Quantization is done in following steps:  We assume that the original analog signal has instantaneous amplitudes between Vmin and Vmax.  We divide the range into L zones, each of height delta (∆). so ∆= Vmin – Vmax / L.  We assign quantized values of 0 to L-1 to the midpoint of each zone.  We approximate the value of the sample amplitude to the quantized values  Encoding: after each sample is quantized and the number of bits per sample is decided, each sample can be changed to an nb-bit code word. The bit rate can be found from the formula: Bit Rate=sampling rate x number of bits per sample
  36. 36. 34 II. Write a Short note on Delta Modulation (DM): PCM is a very complex technique. Other techniques have been developed to reduce the complexity of PCM. The simplest is delta modulation. PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample. Note that there are no code words here; the bits are sent one after another. Delta Modulation Components: Modulator: The modulator is used at the sender site to create a stream of bits from an analog signal. The process records the small positive or negative changes, called delta. If the delta is positive, the process record 1; if it’s negative, the process records a 0. However the process needs a base against which the analog signal is compared. The modulation builds
  37. 37. 35 a second signal that resembles a staircase. Finding the change is then reduced to comparing the input signal with the gradually made staircase signal. Demodulator: The demodulator takes the digital data and, using the staircase maker and the delay unit, creates the analog signal. The created analog signal, however, needs to pass through a low-pass filter for smoothing. Analog Transmission: 1.) Digital to Analog Transmission: It is the process of changing one of the characteristics of and analog signal based on the information in digital data.
  38. 38. 36 Note:- Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.  Amplitude Shift Keying (ASK):  Frequency Shift Keying (FSK):
  39. 39. 37  Phase Shift Keying (PSK):  Quadrature Amplitude Modulation (QAM):
  40. 40. 38 1.) Analog to Analog Transmission Amplitude Modulation (AM): remain the same; only the amplitude changes to follow variations in the information
  41. 41. 39 Note:- The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B. The modulation creates a bandwidth that is twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency. However, the signal above and below the carrier frequency carry exactly the same information. Frequency Modulation (FM): changes, the frequency of the carries changes correspondingly.
  42. 42. 40 Note: The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B where β is a factor depends on modulation technique with a common value of 4. Phase Modulation (PM): changes, the phase of the carrier changes correspondingly. It can be proved mathematically that PM is the same as FM with one difference. in FM, the instantaneous change in the carrier frequency is proportional to the amplitude of the modulating signal; in PM the instantaneous change in the carrier frequency is proportional to the derivative of the amplitude3 of the modulating signal. NOTE:
  43. 43. 41 UNIT 2 Multiplexing and Spread Spectrum In real life, we have links with limited bandwidths. Sometimes we need to combine several low – bandwidth channels to make use of one channel with a larger bandwidth. Sometimes we need to expand the bandwidth of a channel to achieve goals such as privacy and antijamminmg. Thus we explore two categories of bandwidth utilization: multiplexing and spread spectrum Q. What is Multiplexing and why is this technique used. Whenever the bandwidth of a medium linking 2 devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. bandwidth of a link is greater than the bandwidth needs of the devices connected , it leads to a wastage of the bandwidth. An efficient system maximizes the utilization of all resources; bandwidth is one of the most precious resources we have in data communication. In a multiplexed system, n lines share the bandwidth of one link. Fig. below shows
  44. 44. 42 Q. Explain the Types of multiplexing:- techniques designed for analog signals , the third, for digital signals.  Frequency Division Multiplexing (FDM):
  45. 45. 43 Multiplexing Process in FDM: Each source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals modulate different carrier frequencies (f1, f2, f3). The resulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it. Demultiplexing Process in FDM: that separates them from their carriers and passes them to the output lines.
  46. 46. 44  Wavelength-Division Multiplexing (WDM):
  47. 47. 45 also be made to reverse the process.  Write a short note on Time Division Multiplexing (TDM): (April 2011 6m) FDM, time is shared. Each connection occupies a portion of time in the link.
  48. 48. 46 Time slots and Frames travels faster. Interleaving
  49. 49. 47 Data Rate Management: Multilevel Multiplexing: It is a technique used when the data rate of and input line is a multiple of others. As shown in the figure, we have 2 inputs of 20 kbps and 3 inputs of 40 kbps. The first two input lines can be multiplexed together to provide a data rate equal to the last three. A second level of multiplexing can create and output of 160 kbps. Multiple-Slot Allocation: Sometimes it is more efficient to allot more than one slot in a frame to a single input line. as shown in the fig. , the input line with a 50-kbps data rate can be given two slots in the output. We insert a serial –to- parallel converter in the line to make two inputs out of one.
  50. 50. 48 stuffing, bit padding, bit stuffing. As shown in the fig: the input with a data rate of 46 is pulse stuffed to increase the rate to 50 kbps. Now multiplexing can take place. Q Write a short note on Spread Spectrum. Q. Explain FHSS in brief. Q. Explain DSSS in brief. In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are somewhat different. SS is designed to be used in wireless applications (LANs and WANs). In wireless applications, all stations use air (or a vacuum) as the medium for communication. Stations must be able to share this medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder (For e.g. in military operations)
  51. 51. 49 Spread Spectrum achieves its goals through two principles : There are two techniques to spread the bandwidth: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS)  Frequency Hopping Spread Spectrum: Figure shows the general layout of FHSS:
  52. 52. 50 A pseudorandom code generator creates a k -bit pattern for every hopping period Th.  Direct Sequence Spread Spectrum: called chips, where the chip rate is n times that of the data bit. Q. Explain Transmission Media and its types. Q. Explain Guided Media in brief Q. Write a short note on Unguided Media. A transmission medium can be defined as anything that can carry information from a source to a destination. In data communication, the transmission medium is usually free space, metallic cable or fiber optic cable. The information is usually a signal that is the result of a conversion. Transmission media can be divided into two categories: guided and unguided.
  53. 53. 51 GUIDED MEDIA (WIRED MEDIA): Guided media are those that provide a channel from one device to another. It includes twisted-pair cable, coaxial cable and fiber optic cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium.  Twisted-pair Cable: A twisted –pair cable is a metallic copper conductor that accepts and transports signals in the form of electric current. A twisted pair consists of 2 conductors each with its own plastic insulation, twisted together. One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two. It may happen that noise or crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of the unwanted signals is not the same in both wires resulting in a difference at the receiver side. By twisting pairs, a balance is maintained and the difference between the unwanted signals is almost nullified. Thus, it is clear that the number of twists per unit of length (e.g. inch) has some effect on the quality of the cable. Twisted makes it probable that both wires are equally affected by external influences (noise or crosstalk). The twisted pair cable is again divided into two types: unshielded and shielded twisted pair cables.
  54. 54. 52 Applications: Twisted – pair cables are used in telephone lines to provide voice and data channels. The line that connects subscribers to the central telephone office usually consists of unshielded twisted-pair cables. Write a Short note on Coaxial Cable: (April 2011 4m) Coaxial cable (or coax) is also a metallic copper conductor that accepts and transports in the form of electric signals. It carries the signals of higher frequency ranges than those in twisted pair cable. Coax has a central core conductor of solid or stranded wire enclosed in an insulating sheath, which in turn encased in an outer conductor of metal foil, braid or a combination of two. The outer metallic wrapping serves as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
  55. 55. 53 Applications: Cable TV networks use coaxial cables. Another common application of coaxial cable is the Ethernet LAN’s.  Fiber Optic Cables: A fiber optic cable is made up of glass or plastic and transmits signals in the form of light. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. There are different types of propagation modes of the signals. There are: Multimode: multimode means multiple beams from a light source move through the core in different paths. Multimode Step index: the density of the core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. At the interface, there is an abrupt change due to lower density; this alters the angle of the beam. The term step index refers to the sudden change which contributes to the distortion of the signal as it passes through the fiber.
  56. 56. 54 Multimode Graded Index: it decreases the distortion of the signal through the cable. The word index is related to the index of refraction. The index of refraction is related to density. A graded index fiber is with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. Single Mode: Single mode uses step index and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. The decrease in the density results in a critical angle that is close enough to 900 to make the propagation of beams almost horizontal Advantages over metallic cables:  Supports higher bandwidth  Less signal attenuation  Immunity to electromagnetic interference  Resistance to corrosive material  Light weighted
  57. 57. 55 Disadvantages:  Installation and maintenance  Communication can be only be unidirectional, if bidirectional communication have to be used then two separate cables are needed  These cables are most expensive. UNGUIDED MEDIA: Unguided media transport electromagnetic waves without using a physical conductor. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Unguided signals can travel from the source to destination in several propagation types. These are:  Ground propagation: in ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. These low level frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet.  Sky propagation: higher frequency radio waves radiate upward into the ionosphere where they are reflected back to earth.  Line -of -sight propagation: very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by the curvature of the earth. Radio Waves: Electromagnetic waves ranging from 3 KHz and 1 GHz are called as radio waves. Radio waves are omnidirectional i.e. they are propagated in all directions. A sending antenna sends waves that can be received by any receiving antennas. Radio waves particularly those of low and medium frequencies can penetrate the walls. This has an advantage that it can receives the signals inside the building also. It has a disadvantage that we cannot
  58. 58. 56 isolate a communication to just inside or outside the building. Radio waves band is relatively narrow, if it is divided into sub bands, then it becomes narrower leading to a low data rate for digital communications. Applications: AM, FM radio and paging etc. Micro Waves: Electromagnetic waves ranging from 1GHz and 300 GHz are called as radio waves. Micro waves are unidirectional. Micro waves propagation is the line of sight propagation. Since the towers with the mounted antennas need to be in direct sight of each other. Towers that are far apart need to very tall. The curvature of the earth as well as other blocking obstacles does not allow too short towers to communicate by using micro waves. Very high frequencies microwaves cannot penetrate walls. Two types of antennas are used for micro waves communication.  Parabolic dish antenna: based on the geometry of a parabola. Every line parallel to the line of symmetry reflects off the curve at angles such that all the lines intersect in a common point called the focus. The parabolic dish works as a funnel catching a wide range of waves and directing them to a common point.  Horn antenna: it looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn.
  59. 59. 57 Applications: Cellular phones, satellite networks and wireless LANs. Infrared: Infrared waves have frequencies ranging from 300 GHz to 400 THz. can be used for short range communications. Infrared cannot penetrate walls. This advantage helps to prevents interference between one system and another thus used only for short range communication. However, this makes infrared signals useless for long distance communication. In addition, we cannot use infrared waves outside a building because the sun’s rays contain infrared waves that can interfere with the communication. Applications: Keyboards, mouse, PCs and printers. Switching Q. Define Switching and List its Types. between two or more devices linked to the switch. Fig. below shows a switched network
  60. 60. 58 Types of switched networks:  Circuit switched networks: mally divided into n channels by using FDM or TDM.
  61. 61. 59 Circuit Switching takes place at the physical layer. The actual communication in a circuit switched network requires three phases. They are:-  Setup Phase: before the two parties can communicate, a dedicated circuit needs to be established. Here the sender first needs to send a request to the receiver. This request must be accepted by the all the switches and the receiver also. This is called as setup phase. A circuit is reserved on each link, and the combination of circuits or channels defines the dedicated path.  Data transfer Phase: After the establishment of the dedicated circuit (channels), the two parties can transfer data.  Teardown phase: when one of the parties needs to disconnect, a signal is sent to each switch to release the resources. Note: - In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase. Packet Switched Networks: In packet switched networks, the data is divided into packets which are of fixed or variable length. The size of the packet is determined by the network and the governing protocol. In packet switching, there is no resource allocation for a packet. This means that there is no reserved bandwidth and there is no scheduled processing time for each packet.
  62. 62. 60 Resources are allocated on demand. The allocation is done on the first come first serve basis. Datagram Networks: in datagram networks, each packet is treated independently of all others. Here packets are known as datagram. Datagram switching is normally done at the network layer. The datagram networks are sometimes referred to as connectionless network. Connectionless means that packet does not maintain the information of connection i.e. there is no setup phase and teardown phase. A switch in this network maintains a routing table that is based on the destination address. The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet. Virtual Circuit Networks: A Virtual Circuit Networks is a combination of circuit switched and datagram switched networks. It has some characteristics of both as:-
  63. 63. 61 o As in a circuit switched network, there are 3 phases i.e. setup, data transfer and teardown phase. o Resources can be allocated using the setup phase or on demand. o As in datagram network, data is converted into packets and packet carries an address in the header. o As in circuit switched network, all the packets follow the same path established during the connection. o A virtual circuit network is normally implemented in the data link layer, while a circuit switched network is implemented in the physical layer and a datagram network in the network layer. But this may change in the future. In virtual circuit network, two types of addressing are used: global and local (also known as virtual- circuit identifier): --- A source or destination needs to have a global address which is unique in the scope of the network or internationally if the network is part of an international network. The virtual circuit identifier is a small number that has only switch scope is used between two switches. Three phases:  Setup phase: in the setup phase, a switch creates an entry for a virtual circuit. This is taken in two steps: the setup request and the acknowledgement.  Data transfer Phase: to transfer a frame from a source to its destination, all switches need to have a table entry for this virtual circuit.  Teardown Phase: after sending all frames to the destination, a special frame called as teardown request. Destination replies it back with teardown confirmation and all switches delete the entry from their table.
  64. 64. 62 Structure of a Switch Structure of Circuit Switches:  Space - Division Switch: Crossbar Switch: A Crossbar Switch connects n inputs to m outputs in a grid, using electronic micro switches (transistors) at each crosspoint. The major limitation of this design is the number of crosspoints required. To connect n input to m outputs using a crossbar switch requires n x m crosspoints. For example, to connect 1000 inputs to 1000 outputs requires a switch with 1,000,000 crosspoints. A crossbar with this number of crosspoints is impractical. Such a switch is also inefficient because only few of them are used, the rest are idle.
  65. 65. 63 Multistage Switch: The solution to the limitations of the crossbar switch is the multistage switch, which combines crossbar switches in several (normally 3) stages. In a single crossbar switch, only one row or column (one path) is active for any connection. So we need N x N crosspoints. If we can allow multiple paths inside the switch, we can decrease the number of crosspoints. Each crosspoint in the middle stage can be accessed by multiple crosspoints in the first or third stage. Thus in a three-stage switch, the total number of crosspoints is 2 2        n N kkN Multistage switch has one drawback--- blocking during heavy traffic. Blocking refers to times when one input cannot be connected to and output because there is no path available between them--- all possible intermediate switches are occupied.
  66. 66. 64 According to Clos criterion: n= (N/2)1/2 k >2n-1 Total number of crosspoints ≥ 4N [(2N)1/2 -1] Time - Division Switch Time Slot Interchange: Fig. shows a system connecting 4 input lines to 4 output lines. Imagine that each input line wants to send data to and output line according to following pattern: 1 3, 2 4, 3 1, 4 2
  67. 67. 65 Time and Space Division Switch Combinations The above TST switch consists of two time stages and one space stage and has 12 inputs and 12 outputs. Instead of one time division switch, it divides the inputs into 3 groups (of four inputs each) and directs them to 3 time slot interchanges. The result is that the average delay is one third of what would result from using one time slot interchange to handle all 12 inputs.
  68. 68. 66 Q. Explain the Structure of Packet Switch (April 2011 6m) in figure below:- sent on the line
  69. 69. 67
  70. 70. 68 added between the Batcher switch and the banyan switch. The trap module prevents the Batcher duplicate packets (packets with the same output destination) from passing to the banyan switch simultaneously. Only one packet for each destination is allowed at each tick; if there is more than one, they wait for the next tick. Concept of DSL and ADSL: Q. Write s Short note on DSL (Digital Subscriber Line): Benefits of DSL:- • High-speed data service • Always on connection • No need to ―dial-up‖
  71. 71. 69 • Uses existing copper wires • Reasonably priced today and • getting cheaper Q Write a Short note on ADSL (Asymmetric Digital Subscriber Line): ADSL provides higher speed (bit rate) in the downstream direction (From Internet to resident) than in the upstream direction (From resident to Internet). That is why it is called “asymmetric‖. "Digital" means that data or voice is converted into a binary format where the audio or video data is represented by a series of "1"s and "0"s. The term "subscriber line" simply refers to the copper pair telephone wires that are used for conventional phone service. ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. To provide ADSL broadband service, special equipment is required at the telephone exchange and an ADSL modem is needed at the residence or business. ADSL (Asymmetric Digital Subscriber Line) has existed for around ten years and was firstly developed to receive television via the standard telephone network. But the development of the Internet found another use for this technology, that of being able to surf the net quickly without occupying the telephone line. ADSL is also currently one of the only technologies available on the market which offers the transport of TV/video in digital format (MPEG1 or MPEG2) using a telephone connection. The modulation technique that has become standard for ADSL is called DMT (Discrete Multitone Technique) which combines QAM and FDM.
  72. 72. 70
  73. 73. 71 UNIT 3 Q Define Error. Explain its Types. ERROR DETECTION AND CORRECTION Errors are of two types…. NOTE
  74. 74. 72 In this case, NOTE Q. Q. Short note on BLOCK CODING in Error Detection. In block coding, we divide our message into blocks, each of k bits called datawords. We add r redundant bits to each block to make the length n=k+r. The resulting n bit blocks are called codewords. We have set of datawords, each of size k, and a set of codewords , each of size of n. With k bits, we can create a combination of 2k datawords; with n bits, we can create a combination of 2n codewords. Since n>k, the number of possible codewords is larger than the
  75. 75. 73 number of possible datawords. The block coding process is one-to-one; the same dataword is always encoded as the same codeword. This means that we have 2n – 2k codewords that are not used. We call this codewords invalid or illegal. The sender creates codewords out of datawords by using a generator that applies the rules and procedures of encoding. Each codeword sent to the receiver may change during transmission. If the received codeword is the same as one of the valid codewords, the word is accepted; the corresponding dataword is extracted for use. If the received codeword is not valid, it is discarded. However, if the codeword is corrupted during transmission but the received word still matches a
  76. 76. 74 valid codeword, the error remains undetected. This type of coding can detect only single errors. Two or more errors may remain undetected. NOTE:
  77. 77. 75 NOTE: LINEAR BLOCK CODES Almost all block codes used today belong to a subset called linear block codes. The use of non linear block codes for error detection and correction is not as widespread because their structure makes theoretical analysis and implementation difficult. NOTE: In a linear block code, the exclusive OR(XOR) of any two valid codewords creates another valid codeword.
  78. 78. 76 Encoder and decoder of simple parity check NOTE A Simple parity check code can detect an odd number of errors.
  79. 79. 77 A better approach is two-dimensional parity check. In this method, the dataword is organized in a table(rows and columns). The data to be sent, fare put in separate rows. For each row and each column, 1 parity-check bit is calculated. The whole table is sent to the receiver,which finds the syndrome for each row and each column.
  80. 80. 78 CYCLIC CODES Cyclic codes are special linear block codes with one extra property. In a cyclic code, if a codeword is cyclically shifted(rotated), the result is another codeword. For example if 1011000 is a codeword and we cyclically left-shift ,then 0110001 is also a codeword. CYCLIC REDUDANCY CHECK CRC ENCODER AND DECODER is a syndrome of n-k(3 here) bits, which is fed to the decision logic analyzer. The analyzer has a simple function. If the syndrome bits all 0s, the 4 leftmost bits of
  81. 81. 79 the codeword are accepted as the dataword; otherwise the 4 bits are discarded(error). ENCODER The encoder takes the dataword and augments it with n-k number of 0s. It then divides the augmented dataword by the divisor. (for figure refer classroom notes). We use the XOR operation for subtraction and addition which is done step by step. In each step a copy of thje divisor is XORed woth the 4 bits of the dividend. The result of the XOR operation(remainder) is 3 bits (in this case),which is used for the next step after 1 extra bit is pulled down to make it 4 bits long. Points to remember: 1. If the leftmost bit of the dividend is ), the step cammot use the regular divisor; we need to use an all 0s divisor. 2. Where there are no bits left to pull down , we have a result. The 3-bit remainder forms the check bits(r2,r2 and r0). The are appended to the dataword to create the codeword. DECODER The codeword can change during transmission. The decoder does the same division process as the encoder. The remainder of the division is the syndrome. If the syndrome is all 0s, there is no error; the dataword is separated from the received codeword and accepted. Otherwise every thing is discarded. (for figure refer classroom notes) POLYNOMIALS A better way to understand cyclic codes and how they can be analyzed is to reperesent them as polynomials. A pattern of 0s and 1s can be represented as a polynomial with coefficients of 0 and 1. The power of each term shows the position of the bit; the coefficient shows the value of the bit.
  82. 82. 80 Q Explain the method of CHECKSUM of error detection (APRIL 2011 4m) (For more detail about checksum refer classroom notes) Data Link Control and Protocols FRAMING Introduction::::::
  83. 83. 81 Framing are of two types::: 1. Fixed size – In this type of framing, there is no need for defininf the boundaries of the frames; the size itself can be used as a delimeter. 2. Variable – size – In this type we need a way to define the end of the frame and the beginning of the next.. two approaches were used for this purpose a. Character- oriented protocols – here data to be carried are 8-bit characters from a coding system such as ASCII. The header, which normally carries the source and destination addresses and other control information and the trailer, which carries error detection or error correction redundant bits, are also multiples of 8 bits. To separate one frame from the next, an 8-bit flag is added at the beginning and the end of the frame. The flag, composed of protocol- dependent special characters, signals the start or end of a frame.
  84. 84. 82 b. Bit-oriented protocols—Here, the data section of the frame is a sequence of bits to be interpreted by the upper layer as text, graphic, audio , video and so on. However, in addition to headers, we still need a delimeter to separate one frame from the other.
  85. 85. 83 FLOW AND ERROR CONTROL Q Write a short note on FLOW CONTROL NOTE:::::: ―Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgement.‖ ERROR CONTROL NOTE : ― Error control in the data link layer is based on automatic repeat request, which is the transmission of data‖ PROTOCOLS
  86. 86. 84 NOISLESS CHANNELS 1. Simplest Protocol This protocol has no flow or error control. It is a unidirectional protocol in which data frames are traveling in only one direction from the sender to receiver. We assume that the receiver can immediately handle any frame it receives with a processing time that is small enough to be negligible. The data link layer of the receiver immediately removes the header from the frame and hands the data packet to its network layer, which can also accept the packet immediately. In other words, the receiver can never be overwhelmed with incoming frames.
  87. 87. 85 Below is the design of simples protocol with no flow control and error control
  88. 88. 86 Design of stop and wait protocol NOISY CHANNELS To detect and correct corrupted frames, we need to add redundancy bits to our data frame. When the frame arrives at the receiver site, it is checked and if it is corrupted, it is silently discarded. The detection of errors in this protocol is manifested by the silence of the receiver. Lost frames are more difficult to handle than corrupted ones. On out previous protocols, there was no way to identify a frame. The received frame could be the correct one, or a duplicate, or a frame out of order. The solution is to number the frames. When the receiver receives a data frame that is out of order, this means that frames were either lost or duplicated. The corrupted and lost frames need to be resent in this protocol. If the receiver does not respond when there is an error, how can the sender know which frame to resend? To remedy this problem, the sender keeps a copy of the sent frame. At the same time, it starts a timer. If the Timer expires and there is no ACK for the sent frame, the frame is resent, the copy is held and the timer is restarted. Since the protocol uses the stop- and – wait
  89. 89. 87 mechanism there is only one specific frame that needs an ACK even though several copies of the same frame can be in the network. NOTE:: Error correction in stop- and –wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires. Means that the sequence is 0,1,0,1,0 and so on. Note::: ― in stop-and-wait ARQ, we use the sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic.
  90. 90. 88
  91. 91. 89 Q Write a short note on Sliding window Protocol. (APRIL 2011) In this protocol, the sliding window is an abstract concept that defines the range of sequence numbers that is the concern of the sender and receiver. In other words the sender and receiver need to deal with only part of the possible sequence numbers. The range which is the concern of the sender is called the send sliding window. The send window is an imaginary box covering the sequence numbers of the data frames which can be in transit. In each window position, some of these sequence numbers define the frames that have been sent; others define those that can be sent. The maximum size window is 2m -1. The window at any time divides the possible sequence numbers into four regions. The first region, from the far left of the left wall of the window, defines the sequence numbers belonging to frames that are already acknowledged. The sender does not worry about these frames and keeps no copies of them. The second region, colored in the figure defines the range of sequence numbers belonging to the frames that are sent and have an unknown status. The sender needs to wait to find out if these frames have been received or were lost.We call these outstanding frames. The third range, defines the range of sequence numbers for frames that can be sent; however , the corresponding data packets have not yet been received from the network layer. NOTE
  92. 92. 90 The send window can slide one or more slots when a valid acknowledgement arrives. The receive window makes sure that the correct data frames are received and that the correct acknowledgements are sent. The size of the receive window Is always 1. The receiver is always looking for the arrival of a specific frame. Any frame arriving out of order is discarded and needs to be resent.
  93. 93. 91 Window Size for go-Back-N ARQ
  94. 94. 92 Fig(Selective repeat ARQ, window size) An important point about piggybacking is that both sites must use the same
  95. 95. 93 algorithm. This algorithm is complicated because it needs to combine tow arrival events into one. (APRIL 2011) Q What is HDLC? State differenc types of frames of HDLC. (APRIL 2011 4m) High level Data Link Control(HDLC) is a bit-oriented protocol for communication over point –to-point and multipoint links. Normal Response Mode : :: Here the station Configuration is unbalanced. We have one primary station and multiple secondary stations. A primary station can send commands and a secondary station can only respond. The NRM is used for both point to point and multiple point links. Asynchronous Balanced Mode—Here the configuration is balanced. The link is point- to point, and each station can function as a primary and a secondary.
  96. 96. 94 Frame Format Each frame in HDLC can contain up to siz fields a beginning flag field, an address field, a control field, an information field, a frame check sequence(FCS) field, and an ending flag field. In multiple – frame transmissions, the ending flag of one frame can serve as the beginning flag of the next frame. TYPES OF HDLC FRAMES
  97. 97. 95 CONTROL field. The control field defines the type of frame and defines its functionality. Control field format for different frame types
  98. 98. 96
  99. 99. 97 Q Write a Short note on Point To Point Protocol Although HDLC is a general protocol that can be used for both point-to- point and multipoint configurations, one of the most common protocols for point-to-point access. Framing(Frame format of PPP)
  100. 100. 98 PPP Transition Phases- A PPP connection goes through phases which can be shown in a transition phase diagram (see Figure 11.33).  Dead. In the dead phase the link is not being used. There is no active carrier (at the physical layer) and the line is quiet.  Establish. When one of the nodes starts the communication, the connection goes into this phase. In this phase, options are negotiated between the two parties. If the negotiation
  101. 101. 99 is successful, the system goes to the authentication phase (if authentication is required) or directly to the networking phase.  Authenticate. The authentication phase is optional; the two nodes may decide, during the establishment phase, not to skip this phase. However, if they decide to proceed with authentication, they send several authentication packets, discussed later. If the result is successful, the connection goes to the networking phase; otherwise, it goes to the termination phase.  Network. In the network phase, negotiation for the network layer protocols takes place. PPP specifies that two nodes establish a network layer agreement before data at the network layer can be exchanged. The reason is that PPP supports multiple protocols at the network layer. If a node is running multiple protocols simultaneously at the network layer, the receiving node needs to know which protocol will receive the data.  Open. In the open phase, data transfer takes place. When a connection reaches this phase, the exchange of data packets can be started. The connection remains in this phase until one of the endpoints wants to terminate the connection.  Terminate. In the termination phase the connection is terminated. Several packets are exchanged between the two ends for house cleaning and closing the link. Link Control Protocol The Link Control Protocol (LCP) is responsible for establishing, maintaining, configuring, and terminating links. It also provides negotiation mechanisms to set options between the two endpoints. Both endpoints of the link must reach an agreement about the options before the link can be established. All LCP packets are carried in the payload field of the PPP frame with the protocol field set to C021 in hexadecimal. The code field defines the type of LCP packet. There are 11 types of packets as Shown- There are three categories of packets. The first category, comprising the first four packet types, is used for link configuration during the establish phase. The second category, comprising packet types 5 and 6, is used for link termination during the termination phase. The last five packets are used for link monitoring and debugging. The ID field holds a value that matches a request with a reply. One endpoint inserts a value in this field, which will be copied into the reply packet. The length field defines the length of the entire LCP packet. The information field contains information, such as options, needed for some LCP packets. There are many options that can be negotiated between the two endpoints.
  102. 102. 100 Options are inserted in the information field of the configuration packets. In this case, the information field is divided into three fields: option type, option length, and option data. Authentication Protocols Authentication plays a very important role in PPP because PPP is designed for use over dial-up links where verification of user identity is necessary. Authentication means validating the identity of a user who needs to access a set of resources. PPP has created two protocols for authentication: Password Authentication Protocol and Challenge Handshake Authentication Protocol. Note that these protocols are used during the authentication phase. PAP The Password Authentication Protocol (PAP) is a simple authentication procedure with a two-step process: 1. The user who wants to access a system sends an authentication identification (usually the user name) and a password. 2. The system checks the validity of the identification and password and either accepts or denies connection. The three types of packets used by PAP and how they are actually exchanged. When a PPP frame is carrying any PAP packets, the value of the protocol field is OxC023. The three PAP packets are authenticate-request, authenticate-ack, and authenticate-nak. The first packet is used by the user to send the user name and password. The second is used by the system to allow access. The third is used by the system to deny access.
  103. 103. 101 CHAP The Challenge Handshake Authentication Protocol (CHAP) is a three-way hand- shaking authentication protocol that provides greater security than PAP. In this method, the password is kept secret; it is never sent online. 1. The system sends the user a challenge packet containing a challenge value, usually a few bytes. 2. The user applies a predefined function that takes the challenge value and the user's own password and creates a result. The user sends the result in the response packet to the system. 3. The system does the same. It applies the same function to the password of the user (known to the system) and the challenge value to create a result. If the result created is the same as the result sent in the response packet, access is granted; otherwise, it is denied. CHAP is more secure than PAP, especially if the system continuously changes the challenge value. Even if the intruder learns the challenge value and the result, the password is still secret. Figure 11.37 shows the packets and how they are used. CHAP packets are encapsulated in the PPP frame with the protocol value C223 in hexadecimal. There are four CHAP packets: challenge, response, success, and failure. The first packet is used by the system to send the challenge value. The second is used by the user to return the result of the calculation. The third is used by the system to allow access to the system. The fourth is used by the system to deny access to the system.
  104. 104. 102 Network Control Protocols PPP is a multiple-network layer protocol. It can carry a network layer data packet from protocols defined by the Internet, OSI, Xerox, DECnet, AppleTalk, Novel, and so on. To do this, PPP has defined a specific Network Control Protocol for each network protocol. For example, IPCP (Internet Protocol Control Protocol) configures the link for carrying IP data packets. Xerox CP does the same for the Xerox protocol data packets, and so on. Note that none of the NCP packets carry network layer data; they just configure the link at the network layer for the incoming data. IPCP One NCP protocol is the Internet Protocol Control Protocol (IPCP). This protocol configures the link used to carry IP packets in the Internet. IPCP is especially of interest to us. The format of an IPCP packet is shown in Figure 11.38. Note that the value of the protocol field in hexadecimal is 8021. There are other NCP protocols for other network layer protocols. The OSI Network Layer Control Protocol has a protocol field value of 8023; the Xerox NS IDP Control Protocol has a protocol field value of 8025; and so on. After the network layer configuration is completed by one of the NCP protocols, the users can exchange data packets from the network layer. Here again, there are different protocol fields for different network layers. For example, if PPP is carrying data from the IP network layer, the field value is 0021 (note that the three rightmost digits are the same as for IPCP). If PPP is carrying data from the OSI network layer, the value of the protocol field is 0023, and so on. Figure 11.39 shows the frame for IP.
  105. 105. 103 UNIT 4 MULTIPLE ACCESS We consider the data link layer as two sublayers. The upper sublayer is responsible for data link control, and the lower sublayer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sublayer. Figure 12.1 shows these two sublayers in the data link layer. The upper sublayer that is responsible for flow and error control is called the logical link control (LLC) layer; the lower sublayer that is mostly responsible for multipleaccess resolution is called the media access control (MAC) layer. RANDOM ACCESS- In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send.
  106. 106. 104 Transmission is random among the stations. That is why these methods are called random access. No rules specify which station should send next. Stations compete with one another to access the medium. That is why these methods are also called contention methods. In a random access method, each station has the right to the medium without being controlled by any other station. However, if more than one station tries to send, there is an access conflict-collision-and the frames will be either destroyed or modified. To avoid access conflict or to resolve it when it happens, each station follows a procedure that answers the following questions: o When can the station access the medium? o What can the station do if the medium is busy? o How can the station determine the success or failure of the transmission? o What can the station do if there is an access conflict? The random access methods we study in this chapter have evolved from a very interesting protocol known as ALOHA, which used a very simple procedure called multiple access (MA). The method was improved with the addition of a procedure that forces the station to sense the medium before transmitting. This was called carrier sense multiple access. This method later evolved into two parallel methods: carrier sense multiple access with collision detection (CSMAlCD) and carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CD tells the station what to do when a collision is detected. CSMA/CA tries to avoid the collision. 1. ALOHA ALOHA, the earliest random access method, was developed at the University of Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is obvious that there are potential collisions in this arrangement. The medium is shared between the stations. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled. 2. Carrier Sense Multiple Access (CSMA) To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA) requires that each station first listen to the medium (or check the state of the medium) before sending. In other words, CSMA is based on the principle "sense before transmit" or "listen before talk." CSMA can reduce the possibility of collision, but it cannot eliminate it. Persistence Methods- What should a station do if the channel is busy? What should a station do if the channel is idle? Three methods have been devised to answer these questions: the I-persistent method, the nonpersistent method, and the p-persistent method. Figure 12.10 shows the
  107. 107. 105 behavior of three persistence methods when a station finds a channel busy. Figure 12.10 Behavior of three persistence methods I-Persistent The I-persistent method is simple and straightforward. In this method, after the station finds the line idle, it sends its frame immediately (with probability I). This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately. Nonpersistent In the nonpersistent method, a station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line again. The nonpersistent approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send
  108. 108. 106 simultaneously. However, this method reduces the efficiency of the network because the medium remains idle when there may be stations with frames to send. Flow Diagram of persistence methods- p-Persistent The p-persistent method is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time. The p-persistent approach combines the advantages of the other two strategies. It reduces the chance of collision and improves efficiency. In this method, after the station finds the line idle it follows these steps: 1. With probability p, the station sends its frame. 2. With probability q = 1 - p, the station waits for the beginning of the next time slot and checks the line again. a. If the line is idle, it goes to step 1. b. If the line is busy, it acts as though a collision has occurred and uses the backoff procedure. 3. Carrier Sense Multiple Access with Collision Detection (CSMA/CD)-
  109. 109. 107 The CSMA method does not specify the procedure following a collision. Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision.In this method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again. Figure 12.14 Flow diagramfor the CSMAlCD 4. Collision Avoidance (CSMA/CA)- The basic idea behind CSMA/CD is that a station needs to be able to receive while transmitting to detect a collision. When there is no collision, the station receives one signal: its own signal. When there is a collision, the station receives two signals: its own signal and the signal transmitted by a second station. To distinguish between these two cases, the received signals in these two cases must be significantly different. In other words, the signal from the second station needs to add a significant amount of energy to the one created by the first station. In a wired network, the received signal has almost the same energy as the sent signal because either the
  110. 110. 108 length of the cable is short or there are repeaters that amplify the energy between the sender and the receiver. This means that in a collision, the detected energy almost doubles. However, in a wireless network, much of the sent energy is lost in transmission. The received signal has very little energy. Therefore, a collision may add only 5 to 10 percent additional energy. This is not useful for effective collision detection. We need to avoid collisions on wireless networks because they cannot be detected. Carrier sense multiple access with collision avoidance (CSMAlCA) was invented for this network. Collisions are avoided through the use of CSMAICA's three strategies: the interframe space, the contention window, and acknowledgments, Interframe space First, collisions are avoided by deferring transmission even if the channel is found idle. When an idle channel is found, the station does not send immediately. It waits for a period of time called the interframe space or IFS. The IFS variable can also be used to prioritize stations or frame types. For example, a station that is assigned a shorter IFS has a higher priority. In CSMAlCA, the IFS can also be used to define the priority of a station or a frame. Contention Window The contention window is an amount of time divided into slots. A station that is ready to send chooses a random number of slots as its wait time. The number of slots in the window changes according to the binary exponential back-off strategy. However, if the station finds the channel busy, it does not restart the process; it just stops the timer and restarts it when the channel is sensed as idle. This gives priority to the station with the longest waiting time. Acknowledgment With all these precautions, there still may be a collision resulting in destroyed data. In addition, the data may be corrupted during the transmission. The positive acknowledgment and the time- out timer can help guarantee that the receiver has received the frame.
  111. 111. 109 Flow diagram for CSMAICA
  112. 112. 110 CONTROLLED ACCESS- In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. We discuss three popular controlled-access methods. 1. Reservation In the reservation method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation minislots in the reservation frame. Each minislot belongs to a station. When a station needs to send a data frame, it makes a reservation in its own minislot. The stations that have made reservations can send their data frames after the reservation frame. Figure shows a situation with five stations and a five-minislot reservation frame. In the first interval, only stations 1, 3, and 4 have made reservations. In the second interval, only station 1 has made a reservation. 2. Polling Polling works with topologies in which one device is designated as a primary station and the other devices are secondary stations. All data exchanges must be made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions. It is up to the primary device to determine which device is allowed to use the channel at a given time. The primary device, therefore, is always the initiator of a session.
  113. 113. 111 Figure 12.19 Select and poll functions in polling access method If the primary wants to receive data, it asks the secondaries if they have anything to send; this is called poll function. If the primary wants to send data, it tells the secondary to get ready to receive; this is called select function. Select- The select function is used whenever the primary device has something to send. Remember that the primary controls the link. If the primary is neither sending nor receiving data, it knows the link is available. If it has something to send, the primary device sends it. What it does not know, however, is whether the target device is prepared to receive. So the primary must alert the secondary to the upcoming transmission and wait for an acknowledgment of the secondary's ready status. Before sending data, the primary creates and transmits a select (SEL) frame, one field of which includes the address of the intended secondary. Poll- The poll function is used by the primary device to solicit transmissions from the secondary devices. When the primary is ready to receive data, it must ask (poll) each device in turn if it has anything to send. When the first secondary is approached, it responds either with a NAK frame if it has nothing to send or with data (in the form of a data frame) if it does. If the response is negative (a NAK frame), then the primary polls the next secondary in the same manner until it finds one with data to send. When the response is positive (a data frame), the primary reads the frame and returns an acknowledgment (ACK frame), verifying its receipt. 3. Token Passing In the token-passing method, the stations in a network are organized in a logical ring. In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. The current station is the one that is accessing the channel now. The right to
  114. 114. 112 this access has been passed from the predecessor to the current station. The right will be passed to the successor when the current station has no more data to send. When a station has some data to send, it waits until it receives the token from its predecessor. It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. The station cannot send data until it receives the token again in the next round. In this process, when a station receives the token and has no data to send, it just passes the data to the next station. Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. Logical Ring In a token-passing network, stations do not have to be physically connected in a ring; the ring can be a logical one. Figure 12.20 show four different physical topologies that can create a logical ring. Figure 12.20 Logical ring and physical topology in token-passing access method In the physical ring topology, when a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. This means that the token does not have to have the address of the next successor. The problem with this topology is that if one of the links-the medium between two adjacent stations fails, the whole system fails. The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is for emergencies only (such as a spare tire for a
  115. 115. 113 car). If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. After the failed link is restored, the auxiliary ring becomes idle again. Note that for this topology to work, each station needs to have two transmitter ports and two receiver ports. The high-speed Token Ring networks called FDDI (Fiber Distributed Data Interface) and CDDI (Copper Distributed Data Interface) use this topology. In the bus ring topology, also called a token bus, the stations are connected to a single cable called a bus. They, however, make a logical ring, because each station knows the address of its successor (and also predecessor for token management purposes). When a station has finished sending its data, it releases the token and inserts the address of its successor in the token. Only the station with the address matching the destination address of the token gets the token to access the shared media. The Token Bus LAN, standardized by IEEE, uses this topology. In a star ring topology, the physical topology is a star. There is a hub, however, that acts as the connector. The wiring inside the hub makes the ring; the stations are connected to this ring through the two wire connections. This topology makes the network less prone to failure because if a link goes down, it will be bypassed by the hub and the rest of the stations can operate. Also adding and removing stations from the ring is easier. This topology is still used in the Token Ring LAN designed by IBM. CHANNELIZATION- Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. In this section, we discuss three channelization protocols: FDMA, TDMA, and CDMA. 1. FDMA(Frequency Division Multiple Access) In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time. Each station also uses a bandpass filter to confine the transmitter frequencies. To prevent station interferences, the allocated bands are separated from one another by small guard bands. FDMA specifies a predetermined frequency band for the entire period of communication. This means that stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA. FDMA, on the other hand, is an access method in the data link layer. The data link layer in each station tells its physical layer to make a bandpass signal from the data passed to it. The signal must be created in the allocated band. There is no physical multiplexer at the physical layer. The signals created at each station are automatically bandpass-filtered. They are mixed when they are sent to the common channel.