Data Communications Network Model PDF

Data Communications Prakash Poudyal Department of Computer Science and Engineering Kathmandu University Data Communication Are the exchange of the data between two devices through some form of transform medium such as a wire cable. Depend upon four fundamental char...

Data Communications Prakash Poudyal Department of Computer Science and Engineering Kathmandu University Data Communication Are the exchange of the data between two devices through some form of transform medium such as a wire cable. Depend upon four fundamental characteristics: Delivery : Deliver data to the correct destination. Accuracy : The system must deliver the data accurately Timeliness : time is matter Jitter : Jitter refers to the variation in the packet arrival time. Eg : let us assume that video packets are sent every 30ms Others are sent 30-ms delay and other 40ms delay than quality of video decreases Data Communication Systems Components Message : Message is the information (text), numbers, pictures, audio and video Sender The sender is the device that sends the data message. Eg computer, workstation, camera Receiver The receiver is the device that receives the message eg computer, television Transmission medium Physical path : eg twisted-pair wire, coaxial cable, fiber-optic cable Protocol A protocol is a set of rules that governs data communication. It represents an agreement between the communication devices. Without protocol, two devices may be connected but not communicating. Eg. Person speaking French cannot understood by a person who speaks Japanese only Data Representation Text Represented through a bit pattern, a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent texts symbols. Today, the coding system is called Unicode, which uses 32 bits to represent a symbol or characters. American Standard Code for Information Interchange (ASCII) 127 characters in Unicode. Numbers Numbers are also representation by bits patterns. Images Represented by bit patterns Imaged is composed of a matrix of pixels, where each pixel is a small dot. Black pixel is 00, dark gray pixel 01, light gray pixel by 10 and white pixel by 11 Audio Recoding or broadcasting or sound or music Video Recoding or broadcasting of pictures or movie. Data Flow Simplex Simplex mode, the communication is unidirectional , as a one-way street. Only one of the two devices on a link can transmit; other can only receive Example : keywords and traditional monitors  Half-Duplex Each station can both transmit and receive, but not at the same time. Example : crossing road by the car Example : Walkie-talkies  Full-Duplex Called also duplex Both stations can transmit and receive simultaneously Example : telephone network Network A network is the interconnection of a set of devices capable of communication. A device can be a host(e.g computer , desktop, laptop, cellular phone) A network must be able to meet certain number of criteria. The most important of these are : Performance : Measured in many ways, including transit time and response time. Reliability Is measured in frequency of failure, the time it takes a link to recover from a failure Security Network security issues include protecting data from unauthorized access Protecting data from damage and development Physical Structures Types of connection A network is two or more devices connected through the links. Point to Point Provides a dedicated link between two devices The entire capacity of the link is reserved for transmission between these two devices. Wire or cable Example : while changing the channel from remote to TV, satellite link MultiPoint Is one in which more than one specific devices share the single link LAN Topologies(Physical) Physical Describes the geometric arrangement of components that make up the LAN 1) Mesh Topology 2) Bus Topology 3) Star Topology 4) Ring Topology 10  Mesh Topology Every device has dedicated point to point link to every other Dedicated means link carries traffic only between the two devices it connects. Find the physical links in a fully connected mesh networks with n nodes Node 1 must connected to n-1 nodes Node 2 must be connected to n-1 nodes Node n must be connected to n-1 nodes We need n(n-1) physical links. If both directions duplex mode than n(n-1)/2 Advantage Dedicated link guarantees that each connection can carry its own data load. Helps to eliminating the traffic problems If one link becomes unusable, it does not incapacitate the entire system Advantage of having privacy or security Helps to find fault identification and fault isolation easy. Traffic can be routed to avoid links Disadvantage The main disadvantage of a mesh are related to the amount of cabling and number of I/O ports Every device need to connected Sheer bulk of wiring can be greater than the available space can accommodate Hardware required to connect I/O Therefore mesh topology is usually implemented in a limited fasion. Star topology Have connections to networked devices that “radiate” out form a common point Each networked device in star topology can access the media independently Have become the dominant topology type in contemporary LANs Stars have made buses and rings obsolete in LAN topologies 14 Advantages of star topology 1) Compared to Bus topology it gives far much better performance 2) Easy to connect new nodes or devices 3) Centralized management. It helps in monitoring the network 4) Failure of one node or link doesn’t affect the rest of network 15 Disadvantages of star topology 1) If central device fails whole network goes down 2) The use of hub, a router or a switch as central device increases the overall cost of the network 3) Performance and as well number of nodes which can be added in such topology is depended on capacity of central device 16 Bus topology All networked nodes are interconnected, peer to peer, using a single, open-ended cable Both ends of the bus must be terminated with a terminating resistor to prevent signal bounce 17 Advantages of Bus topology 1) Easy to implement and extend 2) Well suited for temporary networks that must be set up in a hurry 3) Typically the least cheapest topology to implement 4) Failure of one station does not affect others 18 Disadvantages of Bus topology 1) Difficult to administer/troubleshoot 2) Limited cable length and number of stations 3) A cable break can disable the entire network; no redundancy 4) Maintenance costs may be higher in the long run 5) Performance degrades as additional computers are added 19 Ring topology started out as a simple peer-to-peer LAN topology Data was transmitted unidirectionally around the ring Sending and receiving of data takes place by the help of TOKEN Token contains a piece of information which along with data is sent by the source computer This token then passes to next node, which checks if the signal is intended to it If yes, it receives it and passes the empty to into the network otherwise passes token along with the data to next node 20 Advantages of Ring topology 1) This type of network topology is very organized 2) No need for network server to control the connectivity between workstations 3) Additional components do not affect the performance of network 4) Each computer has equal access to resources 21 Disadvantages of Ring topology 1) Each packet of data must pass through all the computers between source and destination, slower than star topology 2) If one workstation or port goes down, the entire network gets affected 3) Network is highly dependent on the wire which connects different components 22  Network types : Local Area Network (LAN) Less than 1 km in range 10 mbps – 1 Gbps – data flow capacity Within Room Building Campus LAN or Local Area Network connects network devices in such a way that personal computer and workstations can share data, tools and programs. The group of computers and devices are connected together by a switch, or stack of switches, using a private addressing scheme as defined by the TCP/IP protocol. Private addresses are unique in relation to other computers on the local network. Routers are found at the boundary of a LAN, connecting them to the larger WAN  Metropolitan Area Networks (MAN) Size between a LAN and WAN (Within city area) 1-10 kms in range Data flow capacity is less than LAN Combination of Multiple LAN’s High Speed Wireless Internet access has been standardized Example Cable TV network MAN or Metropolitan area Network covers a larger area than that of a LAN and smaller area as compared to WAN. It connects two or more computers that are apart but resides in the same or different cities. It covers a large geographical area and may serve as an ISP (Internet Service Provider). WAN 100 Km -1000 km Data flow less than MAN Combination of multiple LAN’s or MAN’s Hosts are connected by a communication subnet. Subnets consist of many routers Images, audio and video information over the large geographic areas WAN or Wide Area Network is a computer network that extends over a large geographical area, although it might be confined within the bounds of a state or country. A WAN could be a connection of LAN connecting to other LAN’s via telephone lines and radio waves and may be limited to an enterprise (a corporation or an organization) or accessible to the public. The technology is high speed and relatively expensive.  Two example of WANs today Point to point WAN A point –to point WAN is a network that connects two communicating devices through the mission media (Cable, air) Network connecting in different network.  Switched WAN A switched WAN is a network with more than two ends Used as a backbone of global communications todays Switched WAN is a combination of several point-to-point WANs that are connected by switches. Wireless Network System Interconnection : Interconnecting the components of a compute using short range radio. Wireless mouse , keyword Wireless LAN’s Wireless WAN’s Interconnection of Networks : Internet Work It is very rare to see a LAN, MAN or a LAN in isolation, they are connected to one another. When two or more networks are connected then we say it internet work or Internet. The internet is a structured , organized system. Protocols and Standards Protocols : In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. Two entities cannot simply send bit streams to each other and expect to be understand for communication to occur the entities must agree on a protocol which is a set of rules that govern data communication.  Key elements of protocols are: Syntax: It refers to the structure or format of data, meaning the order in which they are presented. Semantics : The word semantics refers to the meaning of each section of bits. The pattern of bits must be correct. Timing : It refers to the two characteristics : When data should be sent and how fast they can be sent. Standards Standard provide guidelines to manufactures, venders, government agencies and other service providers to ensure the kind of interconnectivity necessary is today’s marketplace and international communications. Some organizations that set standards are: International Organization for Standardization (ISO) International Telecommunication Union - Telecommunication Standard Sector (ITU-T) American National Standards Institute (ANSI) Institute of Electrical and Electronics Engineer (IEEE) Network Models Protocol Layering First Scenario Single-layer Protocol Layered Tasks Layered task, play vital role to accomplished the task successfully. Lets take an example, Each layer at the site uses the services of the layer immediately below. A Three- layer protocol  A network is a combination of hardware and software that sends data from one location to another. Hardware consists of physical equipments that carries signals from one point of the network to another. Software consists of instruction sets that make possible the services that is expected from networks  Protocol Hierarchies : To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. No data are directly transferred from the layer “n” or one machine to layer “n” of other machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.  Protocol Hierarchies : Interface Defines which primitive operations and services the lower layer makes available to the upper level. A set of layers and protocols is called network architecture. Design Issues for the layers Addressing : Every layer needs a mechanism for identifying senders and receivers. Since a network has many computers, it needs some sort of addressing for the accurate delivery of the message. Error Control: There must be some kind of error control mechanism. Receivers must have some way of telling the sender which message have been correctly received and which have not. Flow Control : There must be some kind of mechanism for flow control in order to guarantee that sent message has been received. There might be a case like the sender in fast and the receiver in slow. Multiplexing: Using multiple channel in a single link in order to maximum utilization of available bandwidth of the medium. Routing: When there are multiple paths between sender and receiver, then a route must be chosen for the fast and efficient transmission. Connection Oriented and Connection less Services: Connection Oriented: It is modeled after telephone system Service user first established and connection, use the connection and then release the connection. It acts like a tube Connectionless : It is modeled after postal system. Each message carries the full destination address and each one is routed through the system independent of all the others. Reference Model It is a standard reference model for communication between two end users in a network. It deals with connecting open system ie, system that are open for communication with other system. Open Systems Interconnection (OSI) The OSI Model The purpose of the OSI model to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and design of network. 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 has responsibility for the process of moving information across a network. The OSI Reference Model  Protocol Reference Model of OSI Layer abstraction and the path of the message Layered Architecture Peer to Peer Processes Between machines, layer “x” on one machine communicates with layer “x” on another machine. This communication is governed by an agreed-upon series of rules and conventions called protocols. The processes on each machine that communicate at a given layer are called peer-to-peer processes. Interface Defines which primitive operations and services the lower layer makes available to the upper level. A set of layers and protocols is called Network Architecture Organization of Layers The seven layers can be thought of as belonging to three subgroups. Layer 1,2,3 are the network support layers ; deal with physical aspects of moving data from one device to another (such as electrical specifications, physical connections, physical addressing and transport timing and reliability) Layers 5,6,7 are the user support layers: Allows interoperability among unrelated software systems Layer 4 links these two subgroups and ensure that what the lower layers have transmitted is in a form that the upper layers can use. Application Layer The application layer enables the user, whether human or software, to access the network. It Provides user interfaces and support for services such as electronic mail, remote file access and transfer. Applications on that layer (E-mail clients, web browsers, Chats, etc.) – top-stack applications (As people are on the top of the stack) Presentation Layer : Presentation layer is also called the Translation layer. The data from the application layer is extracted here and manipulated as per the required format to transmit over the network. The functions of the presentation layer are : Translation : For example, ASCII to EBCDIC (Extended Binary Coded Decimal Interchange Code). Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data. Compression: Reduces the number of bits that need to be transmitted on the network. Session Layer This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security. The functions of the session layer are : Session establishment, maintenance and termination: The layer allows the two processes to establish, use and terminate a connection. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the data. These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided. Dialog Controller : The session layer allows two systems to start communication with each other in half-duplex or full-duplex. All the below 3 layers(including Session Layer) are integrated as a single layer in the TCP/IP model as “Application Layer”.  Figure 2.12 Session layer 2.52 Session Layer (Cont..) Lets suppose, a system sends 1000 page of file. It is advisable to insert checkpoints after every 100 pages to ensure that each 100 pages unit is received and acknowledges independently. In this case if a crash happens during the transmission of page after 535. The only pages that need to be resent after system recovery are pages 501 to be resend. Transport Layer: Transport layer provides services to session layer and takes services from network layer. The data in the transport layer is referred to as Segments. At sender’s side: Transport layer receives the formatted data from the upper layers, performs Segmentation and also implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination port number in its header and forwards the segmented data to the Network Layer. Note: The sender need to know the port number associated with the receiver’s application. Generally, this destination port number is configured, either by default or manually. For example, when a web application makes a request to a web server, it typically uses port number 80, because this is the default port assigned to web applications. Many applications have default port assigned. At receiver’s side: Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.  The functions of the transport layer are : Segmentation and Reassembly: This layer accepts the message from the (session) layer , breaks the message into smaller units. Each of the segment produced has a header associated with it. The transport layer at the destination station reassembles the message. Service Point Addressing: In order to deliver the message to correct process, transport layer header includes a type of address called service point address or port address. Thus by specifying this address, transport layer makes sure that the message is delivered to the correct process.  The services provided by the transport layer : Connection Oriented Service: It is a three-phase process which include Connection Establishment Data Transfer Termination / disconnection In this type of transmission, the receiving device sends an acknowledgement, back to the source after a packet or group of packet is received. This type of transmission is reliable and secure. Connection less service: It is a one-phase process and includes Data Transfer. In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service. Data in the Transport Layer is called as Segments. Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls. Network Layer: The network layer is responsible for creating a connection between the source computer and the destination computer. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP address are placed in the header by the network layer. The functions of the Network layer are : Routing: The network layer protocols determine which route is suitable from source to destination. This function of network layer is known as routing. Logical Addressing: In order to identify each device on internetwork uniquely, network layer defines an addressing scheme. The sender & receiver’s IP address are placed in the header by network layer. Such an address distinguishes each device uniquely and universally. Segment in Network layer is referred as datagram. Network layer is implemented by networking devices such as routers.  Data Link Layer The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address. The packet received from Network layer is further divided into frames depending on the frame size of NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header. The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.  Data Link Layer The functions of the data Link layer are : Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame. Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of sender and/or receiver in the header of each frame. Error control: Data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames. Flow Control: The data rate must be constant on both sides else the data may get corrupted thus , flow control coordinates that amount of data that can be sent before receiving acknowledgement. Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time. Packet in Data Link layer is referred as Frame. Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines. Eg. Switch & Bridge are Data Link Layer devices. Physical Layer: The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together. Physical Layer: The functions of the physical layer are : Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at bit level. Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits sent per second. Transmission mode: Physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex. Hub, Repeater, Modem, Cables are Physical Layer devices. TCP/IP Protocol Suite This protocol was developed prior to OSI layer It has five layers TCP/IP is a protocol suite used in the Internet today. Missing session and presentation  Figure 2.15 Addresses in the TCP/IP protocol suite Application layer -> someorg.com or email address Transport layer  port numbers port numbers are local addresses Network-layer address  addresses are global, with the whole Internet as the scope Data-link layer address  MAC addresses are locally address, each of which defines a specific host or routerTCP/IP in aProtocol network Suite (LAN or WAN) 63 OSI model vs TCP/IP TCP/IP Protocol Suite 64  Figure 2.18 Relationship of layers and addresses in TCP/IP 2.65 OSI model vs TCP/IP The OSI model appeared after the TCP/IP protocol suite. Most experts thought that the TCP/IP protocol would be fully replaced by the OSI model. This didn’t happen There were three reasons (agreed by experts) OSI was completed when TCP/IP was fully in place and a lot of time and money has been spent which results changing would cost a lot of money Some layers were not fully defined such as presentation and session layer. When OSI was implemented by an organization in a different application, it didn’t show a high enough level of performance to entice the Internet authority to switch from the TCP/IP protocol suite to the OSI model Encapsulation and Decapsulation Encapsulation at the Source Host At the application layer : message (does not contain any header or trailer) Transport layer : takes message as the payload Adds as transport layer header to the payload which contains the identifiers of the source and destination application programs Transport-layer packet is called segment in (TCP) and the user datagram (in UDP) The transport layer then passes the packet to the network layer. Network Layer : takes the transport – layer packet as data or payload. Adds its own header to the payload. The header contains the addresses of the source and destination hosts and some more information used for error checking for the header. The result is the network-layer packet called as datagram. The network layer packets transfer to the data link layer.  Data-link layer takes the network layer packet as data and its own header which contains link layer address of the host or next hob (router). The link layer packet is called as frame. The frame is passed to the physical layer for the transmission. Decapsulation and Encapsulation at the Router Router has encapsulation as well as decapsulation. After sets of bits are delivered to the data-link layer , this layer decapsulates the datagram from the frame and passes it to the network layer. Network layer just inspect the source and destination address in the datagram header and consults its table to forward to the respective hob. The datagram should not be changed by the network layer in the router unless there is a need to fragment the datagram if it is too big. Decapsulation at the Destination Host At the destination host, each layer only decapsulates the packet received, removes the payload, and delivers the payload to the next-higher layer protocol until the message reaches the application layer. It is necessary to say that decapsulation in the host error checking.