Introduction to Networks, Reference Models

Computer Networks VR-10, CS6004

Introduction, Reference Models The Physical Layer

UNIT I Introduction : Uses of Computer Networks, Network Hardware, LANs, MANs, WANs, Network Software. Reference M odels: The OSI Reference Model, TCP/IP Reference Model, the comparison of OSI, The and TCP/IP reference models. Th e Physical Physical L aye ayer r : Guided transmission media: Magnetic Media, Twisted Pair, Coaxial Cable, and Fiber Optics.

A computer network is an interconnected collection of autonomous computers able to exchange information. A computer network usually require users to explicitly login onto one machine, explicitly submit jobs remotely,explicitly move files/data around the network. In a Distributed system, the existence of multiple autonomous computers in a computer network is transparent to the user. The operating system automatically allocates jobs to processors, moves files among various computers without explicit user intervention. Def: “ A network is simply a collection of computers or other hardware devices that are

connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate. Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise otherwise working with networks and network technologies.

”

Networking has become become an indispensable part of modern society in every aspect of life.

Mukesh Chinta Asst Prof, CSE, VRSEC

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Some of the important properties of the networks are: 

Scope: A network should provide services to several applications

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Scalability: A network should should operate efficiently when deployed deployed on a small-scale as well as on a large-scale

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Robustness: A network should operate in spite of of failures or lost data

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Self-Stabilization: A network, after a failure or other problem, problem, should return to normal (or near normal) without human intervention

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Autoconfigurability: A network should optimize optimize its own parameters in order to achieve better performance

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Safety: A network should prevent failures as well as prevent failures from affecting other areas of the network

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Configurability:

A network’s parameters should be configurable to improve

performance 

Determinism: Two networks networks with identical conditions conditions should yield identical results

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Migration: It should be possible to add new new features to a network without without disruption of network service

Business network applications 

Resource sharing: Data, programs, equipment are available to users regardless of their physical location.

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High reliability: Files and databases could be duplicated on multiple machines. Multiple CPUs prevent total system loss.

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Economically Economically sound: Networked micro computers using the client-server model offer better price/performance price/performance ratio than mainframes.

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Communication Medium: Networks provide powerful, effiencient and fast communication among the employees via electronic mail (email), IP telephony or VOIP, Desktop sharing etc.

Home (personal) network applications 

Access to remote information: Financial information, database access, the Web, newsgroups, Wikipedia etc.

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Person to person communication: Email, voice, videoconferencing, instant messaging, social networking etc.

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Interactive entertainment: Video on demand, interactive TV (IPTV), networked games.


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Computer Networks VR-10, CS6004 


Electronic commerce commerce (e-commerce): Many forms such as home home shopping, finance finance and bills management are getting common. Some common forms forms of e-commerce e -commerce are given below:

Mobile Users 

Connectivity to the internet: Smart phones and other mobile devices are dependent on internet for providing advacned services like GPS etc to the users. Wireless Hotspots are found everywhere now-a-days which enable people to connect to the internet on their devices

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M-commerce: Short text messages from the mobile are used to authorize payments for food in vending machines, movie tickets, and other small items instead of cash and credit cards.

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NFC (Near Field Communication): Mobile can act as an RFID smartcard and interact with a nearby reader for payment.

Computer Networks can be categorized based on many dimensions, but two dimensions standout namely as transmission technology and scale. Data communications networks can be generally categorized as either point-to-point or multipoint. A point-to-point configuration involves only two locations or stations, whereas a multipoint configuration involves three or more stations.


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A two-point circuit involves the transfer of digital information between a mainframe computer and a personal computer, two mainframe ma inframe computers or two data communications networks. A multi-point network is generally used to interconnect a single mainframe computer (host) to many personal computers or to interconnect many personal computers and capacity of the channel is either Spatially shared : Devices can use the link simultaneously or Timeshared : Users take turns Transmission Modes

There are four modes m odes of transmission for data communications communications circuits:

In simplex mode(SX), mode(SX) , 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. Commercial radio broadcasting is an example. Simplex lines are also called receive-only, transmit-only or oneway-only lines. half-duplex(HDX) mode, each station can both transmit and receive, but not at the In half-duplex(HDX) mode, same time. When one device is sending, the other can only receive, and vice versa. The halfduplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction. Citizens band (CB) radio is an example where push to talk (PTT) is to be pressed or depressed while sending and transmitting. mode(FDX) (called duplex), both stations can transmit and receive In full-duplex mode(FDX) simultaneously. One common example of full-duplex communication is the telephone network. The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel must be divided between the two directions.


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transmission , computer networks are divided into point-to-point Based on the mode of transmission, networks or broadcast networks. Broadcast networks use networks use a single communication channel shared by all computers in the network. Short messages (packets) are sent by any machine and received by all other computers on the network. An address field within the packet specifies the intended recipient. Others receiving this packet simply ignore it. Broadcast systems generally also allow the possibility of addressing a packet to all destinations by using a special code in the address field. Most localized networks are broadcast networks. A variant of broadcasting called multicasting in which transmission is done to a subset of machines. One possible scheme is to reserve one bit to indicate multicasting. The remaining n - 1 address bits can hold a group number. Each machine can ''subscribe'' ''subscribe'' to any or all of the grou ps. When a packet is sent to a certain group, it is delivered to all machines subscribing to that group. networks consist of many connections between individual In contrast, point-to-point networks pairs of machines. To go from the source to the destination, a packet on this type of network may have to first visit one or more intermediate machines. Point-to-point transmission with unicasting. Generally, smaller, geographically one sender and one receiver is sometimes called unicasting. Generally, localized networks tend to use broadcasting, whereas larger networks usually are point-topoint. Networks are also classified according to their scale. scale. Distance is an important metric for classification because different techniques are used at different scales. Three main categories of networks are LAN{Local Area Networks}, MAN{Metropolitan Area Networks} and WAN{Wide Area Networks}.

Other categories

of networks are SAN{storage SAN{storage area network or sever area network},

PAN{Personal PAN{Personal Area Network}, CAN{Cluster CAN{Cluster Area Networks or Campus area networks} etc


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LAN {Local Area Networks} A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. LANs use a network operating system to provide two-way communications at bit rates in the range of 10 Mbps to 100 Mbps. In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring.

LANs are distinguished from other kinds of networks by three characteristics: (1) their size, (2) their transmission technology, and (3) their topology. LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance, which simplifies network management. LAN’s often use a transmission technology consisting of a single cable to which all the machines are attached. Traditional LAN’s

run at speeds of 10 to 100 Mbps, have very low delay and make few transmission errors. {A megabit is 1,000,000 bits and Mbps means Megabits per second. Megabytes mean 1,048,076 bits}. Network topology topology refers to the way a network is laid out either physically or logically. A topology describes the configuration of a network and influences the networks cost and broadcast LAN’s. performance. Various topologies are possible for broadcast

Five basic topologies are bus, ring, star, tree and mesh.


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: A star topology is designed with each node (file server, workstations, and

peripherals) connected directly to a central network hub, switch, or concentrator. Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow.

Bus networks use a common backbone to connect all devices. A single cable, (the backbone) functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. The bus topology is the simplest and most common method of interconnecting computers. The two ends of the transmission line never touch to form a complete loop. A bus topology is also known as multidrop or linear bus or a horizontal bus.

In a ring network (sometimes called a loop), every device has exactly two neighbours for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counter clockwise"). All the stations are interconnected in tandem (series) to form a closed loop or circle. Transmissions are unidirectional and must propagate through all the stations in the loop. Each computer acts like a repeater and the ring topology is similar to bus or star topologies.


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The mesh mesh topology incorporates a unique network design in which each computer on the network connects to every other, creating a point-to-point connection between every device on the network. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. A mesh network in which every device connects to every other is called a full mesh. A disadvantage is that, a mesh network with n nodes must have n(n-1)/2 links n(n-1)/2 links and each node must have n-1 I/O n-1 I/O ports (links).

This topology (sometimes called mixed topology) is simply combining two or more of the traditional topologies to form a larger, more complex topology. Main aim is being able to share the advantages of different topologies.


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MAN {Metropolitan Area Network} A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. Its geographic scope scope falls between a WAN and LAN. A MAN might be a single network like the cable television network or it usually interconnects a number of local area networks (LANs) using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the Internet.

A metropolitan area network based on cable TV

MANs typically operate at speeds of 1.5 Mbps to 10 Mbps and range from five miles to a few hundred miles in length. A MAN (like a WAN) is not generally owned by a single organization. The MAN, its communications links and equipment are generally owned by either a consortium of users or by a single network provider who sells the service to the users. DQDB {Distributed Queue Dual Bus}, Bus}, is the metropolitan area network standard for data communication. It is specified in the IEEE 802.6 standard. Using DQDB, networks can be up to 20 miles (30 km) long and operate at speeds of 34 to 155 Mbit/s.

It consits of two unidirectional buses (cables) to which all the computers are connected. Each bus has a head-end, which initiates transmission activity. Traffic destined for a computer to the right of the sender uses the upper bus and to the left uses the lower one.


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WAN {Wide Area Networks} Wide area networks are the oldest type of data communications network that provide relatively slow-speed, long-distance transmission of data, voice and video information over relatively large and widely dispersed geographical areas, such as country or entire continent. WAN contains a collection of machines (hosts) intended for running user (i.e., application) programs. programs. The hosts are connected by a communication subnet, or just subnet. The hosts are owned by the customers, whereas the communication subnet is typically owned and operated by a telephone company or Internet service provider. The job of the subnet is to carry messages from host to host. In most wide area networks, the subnet consists of two distinct components: transmission lines and switching elements . Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or even radio links. Switching elements are specialized computers that connect three or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line on which to forward them. Router is the common name given for a s witching element.

If two routers that do not share a transmission line wish to communicate, they must do this indirectly, via other routers. When a packet is sent from one router to another via one or more intermediate routers, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and then forwarded. A subnet organized according to this principle is called a store-and-forward or packet-switched subnet . When a process on some host has a message to be sent to a process on some other host, the sending host first cuts the message into packets, each one bearing its number in the sequence. These packets are then injected into the network one at a time in quick succession. The packets are transported individually over the network and deposited at the receiving host, where they are reassembled into the original message and delivered to the receiving process. Routing decisions are made algorithm . locally according to a routing algorithm.


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Two types of WAN’s are VPN {Virtual Private Network} and ISP {Internet Service Provider}

network. Compared to the dedi-cated arrangement, a VPN has the usual advantage of virtualization, which is that it provides flexible reuse of aresource (Internetconnectivity). Some WAN’s use wireless technologies and examples are satellite systems and cellular telephone

network.

PAN {Personal Area Networks} PANs(Personal Area Networks) Networks) let devices communicate over the range of a person. A common example is awireless network thatconnects a computer with its peripherals. companies got together to design a short-range wireless Bluetooth to connect network called Bluetooth these

components

without

wires.

Bluetooth networks use the master-slave paradigm. The system unit (the PC) is normally the master, talking to the mouse,

keyboard,

Themaster addresses

tells to

use,

etc., the

as

slaves.

slaves

when

they

what can

broadcast, how long they can transmit, what frequencies they can use, and so on. Bluetooth can be used inother settings like connecting a headset to a mobile phone, linking up a digital music player or mobile phone to a car stereo etc. PANs can also be built with othertechnologies that communicate over short ranges, such as RFID on RFID on smartcards and library books.

Internetworks People on one network want to communicate with other network’s necessiating the need for

connection of different and probably incompatible networks. A collection of inter connected networks is called an internetwork or internet. internet. The Internet uses ISP networksto connect enterprise networks, home networks, and many other networks. A network is formed by the combination of a subnet and its hosts. Gateway is Gateway is the common name given to the machine that makes a connection between two or more networks and provides the necessary translation, both in terms of hardware and software. Gateways are distinguished by the layer at which they operate in the protocol hierarchy


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Circuit Switched vs Packet switched networks In ciruit switched netwroks, netwroks , a dedicated circuit is established across a set of links. Example is the telephone network. An end-to-end permanent connection is maintained. Once the communication communication is complete, the connection is ended and the li nks are released. Advantages: Guaranteed bandwidth, reliable communication, simple data routing, low perpacket overhead. Disadvantages: wasted bandwidth, bandwidth, blocked connections, connection set-up delay.


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networks , data is split into blcoks called packets and each packet contains In packet In packet switched networks, indentification information. Packets traverrse the network individually. Destination address is used to forward packets. Advantages: Multiplexing, service, adaption to congestion and failures Disadvantages: No guaranteed bandwidth, per packet overhead, complex end-to-end control, delay and congestion. An intranet is a private network that is contained within an enterprise. It may consist of many interlinked local area networks and also use leased lines in the wide area network. An intranet uses TCP/IP, HTTP, and other Internet protocols and in general looks like a private version of the Internet. With tunneling, companies can send private messages through the public network, using the public network with special encryption/decryption and other security safeguards to connect.

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. The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented. Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n protocol. A protocol is an agreement between the communicating parties on how communication communication is to proceed. A five layer network model is shown below.


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A communication protocol is protocol is a set of rules allowing end users (computers) and network devices to send and receive data in an orderly and structured manner. A protocol consists of three key rules: syntax syntax (format (format of the data), semantics semantics (interpretation (interpretation of data), and timing timing (when (when to send and when to respond, with what speed). The entities comprising the corresponding layers on different machines are called peers called peers.. The peers may be processes, hardware devices, or even human beings. Peers communicate using the protocols. No data are directly transferred from layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. Below layer 1 is the physical medium through which actual communication occurs. Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes architecture . A list available to the upper one. A set of layers and protocols is called a network architecture. of protocols used by a certain system, one protocol per layer, is called a protocol a protocol stack. A message, M, is produced by an application process running in layer 5 and given to layer 4 for transmission. Layer 4 puts a header in front of the message to identify the message and passes the result to layer 3.

Information flow in a five layer model

As layer 3 imposes a limit on the size of the message transmitted, it must break up incoming messages into smaller units, packets, prepending a layer 3 header to each packet. In this example, M is split into two parts, M 1 and M2. Layer 3 decides which of the outgoing lines to


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use and passes the packets to layer 2. Layer 2 adds not only a header to each piece, but also a trailer, and gives the resulting unit to layer 1 for physical transmission. At At the receiving machine the message moves upward, from layer to layer, with headers being stripped off as it progresses. None of the headers for layers below n are passed up to layer n. Advantages of Layering architecture 

Interoperability Interoperability - Layering promotes greater interoperability between devices from different manufacturers manufacturers and even between different generations of the same type of device from the same manufacturer.

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Reduction of the Domino Effect - Another very important advantage of a layered protocol system is that it helps to prevent changes in one layer from affecting other layers. This helps to expedite technology development.

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Modularity –

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Task Segmentation Segmentation - Breaking a large complex system into smaller more manageable subcomponents subcomponents allows for easier development and implementation of new technologies; as well as facilitating human comprehension of what may be very diverse and complex systems.

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Enhanced Troubleshooting and Fault Identification - Troubleshooting and fault identification are made considerably easier thus resolution times are greatly reduced. Layering allows for examination in isolation of subcomponents subcomponents as well as the whole.

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Rapid Application Development (RAD) - Work loads can be evenly distributed which means that multiple activities can be conducted in parallel thereby reducing the time taken to develop, debug, optimize and package new technologies ready for production implementation.

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Promotion of Multi-Vendor Development Development - Layering promotes multi-vendor development through the standardization of networking components at both the hardware and software levels because of the clear and precise delineation of responsibilities that layering brings to the developers' table.

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Standardization and Certification Certification - The layered approach to networking protocol specifications facilitates a more streamlined and simplified standardization and certification process.

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Portability Portability - Layered networking protocols are much easier to port from one system or architecture architecture to another


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Design Issues for the Layers 1. Addressing: Mechanism for identifying senders and receivers, on the network need some form of addressing. There are multiple processes running on one machine. Some means is needed for a process on one machine to specify with whom it wants to communicate. 2. Error Control: There may be erroneous transmission due to several problems during communication. These are due to problem in communication circuits, physical medium, due to thermal noise and interference. Many error detecting and error correcting codes are known, but both ends of the connection must agree on which one being used. In addition, the receiver must have some mechanism of telling the sender which messages have been received correctly and which has not. 3. Flow control: control : If there is a fast sender at one end sending data to a slow receiver, then there must be flow control mechanism to control the loss of data by slow receivers. There are several mechanisms used for flow control such as increasing buffer size at receivers, slow down the fast sender, and so on. Some process will not be in position to accept

arbitrarily

long

messages.

Then,

there

must

be

some

mechanism

to disassembling, transmitting and then reassembling reassembling messages. 4. Multiplexing / demultiplexing: demultiplexing : If the data has to be transmitted on transmission media separately, it is inconvenient or expensive to setup separate connection for each pair of communicating processes. So, multiplexing is needed in the physical layer at sender end and demultiplexing is need at the receiver end. 5. Routing: When data has to be transmitted from source to destination, there may be multiple paths between them. An optimized (shortest) route must be chosen. This decision is made on the basis of several routing algorithms, which chooses optimized route to the destination.

Layers can offer two types of services namely connection oriented service and connectionless service. Connection oriented service: The service user first establishes a connection, uses the connection and then releases the connection. Once the connection is established between source and destination, the path is fixed. The data transmission takes place through this path established. The order of the messages sent will be same at the receiver end. Services are reliable and there is no loss of data. Most of the time, reliable service provides acknowledgement is an overhead and adds delay.


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Connectionless Services: In this type of services, no connection is established between source and destination. Here there is no fixed path. Therefore, the messages must carry full destination address and each one of these messages are sent independent independent of each other. Messages sent will not be delivered at the destination in the same order. Thus, grouping and ordering is required at the receiver end, and the services are not reliable. There is no acknowledgement confirmation from the receiver. Unreliable connectionless service is often called datagram service, which does not return an acknowledgement to the sender. In some cases, establishing a connection to send one short messages is needed. But service can be used for these reliability is required, and then acknowledgement datagram service applications.

service. In this type of service, the sender transmits a Another service is the request-reply service. single datagram containing a request from the client side. Then at the other end, server reply will contain the answer. Request-reply is commonly used to implement communication in the client-server model. Service Primitives A service is formally specified by a set of primitives (operations) available to a user process to access the service. These primitives tell the service to perform some action or report on an action taken by a peer entity.


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The set of primitives available depends on the nature of the service being provided. The primitives for connection-oriented service are different from those of connectionless service. The Relationship of Services to Protocols A service is a set of primitives (operations) that a layer provides to the layer above it. The service defines what operations the layer is prepared to perform on behalf of its users, but it says nothing at all about how these operations are implemented. A service relates to an interface between two layers, with the lower layer being the service provider and the upper layer being the service user. A protocol , in contrast, is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols to implement their service definitions. They are free to change their protocols at will, provided they do not change the service visible to their users. Services relate to the interfaces between layers, whereas protocols relate to the packets sent between peer entities on different machines. entities, which can either be a software The active elements in each layer are often called entities, s oftware entity or hardware entity. Entities in the same layer on different machines are called peer entities.

Services are available at SAP’s which have an unique address for identification. Layer n SAP’s

are the places where layer N+1 can access the services offered.


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Reference Ref erence Model A reference model is a conceptual layout that describes how communication between devices should occur. A reference model has many advantages such as it defines standards for building network components thereby permitting multiple-vendor development and also defines which functions should be performed at each layer of the model thereby promoting the standardization standardization of network.

The OSI Reference Model International standard organization (ISO) established a committee in 1977 to develop architecture architecture for computer communication and the OSI model is the result of this effort. In 1983, ISO

published

a

document

called

‘The Basic Reference Model for Open Systems

Interconnection’, which visualizes network protocols as a seven-layered model. In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. OSI is a standard reference model for communication between end users in a network. The term Open system means a set of protocols using which a system can communicate with any other system irrespective of the differences in their underlying hardware and software. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller SEVEN smaller and more manageable problems In 1983, Day and Zimmerman laid down certain principles that were applied to arrive at the seven layers can be briefly summarized as follows: 1. A layer should be created where a different abstraction is needed. 2. Each layer should perform a well-defined function. 3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. 4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy


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. OSI model consists of a seven separate but related layers namely, physical, data link,

network, transport, session, presentation and application layers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network. The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications. Data is encapsulated with the necessary protocol information as it moves down the layers before network transit. Physical Layer {the physical layer is responsible for transmitting individual bits from one node to the next} The physical layer is the lowest layer of the OSI hierarchy and coordinates the functions required to transmit a bit stream over a physical medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission occur. The physical layer specifies the type of transmission medium and the transmission mode (simplex, half duplex or full duplex) and the physical, electrical, functional and procedural standards for accessing data communication networks.

Transmission media defined by the physical layer include metallic cable, optical fiber cable or wireless radio-wave radio-wave propagation. propagation. The physical layer also includes the carrier system system used to propagate the data signals between points in the network. The carrier systems are simply


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communication systems that carry data through a system using either metallic or optical fiber cables or wireless arrangements such as microwave, satellites and cellular radio systems. Data-link Layer { {the data link layer is responsible for transmitting frames from one node to the next} The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery. It makes the physical layer appear error free to the upper layer (network layer).

The data link layer packages data from the physical layer into groups called blocks, frames or packets. 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 physical address of the sender (source address) and/or receiver (destination address) of the frame. The data-link layer provides flow-control, accesscontrol, and error-control. Network Layer {is responsible for the delivery of individual packets from the source host to the destination host} The network layer provides details that enable data to be routed between devices in an environment using multiple networks, subnetworks or both. This is responsible for addressing messages and data so they are sent to the correct destination, and for translating logical addresses and names (like a machine name FLAME) into physical addresses. This layer is also responsible for finding a path through the network to the destination computer.


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The network layer provides the upper layers of the hierarchy with independence from the data transmission and switching technologies used to interconnect systems. Networking components components that operate at the network layer include routers and their software. Transport Layer { {is responsible for delivery of a message from one process to another} The transport layer controls and ensures the end-to-end integrity of the data message propagated through the network between two devices, providing the reliable, transparent transfer of data between two endpoints.

Transport layer responsibilites includes message routing, segmenting, error recovery and two types of basic services to an upper-layer protocol: connection oriented and connectionless. The transport layer is the highest layer in the OSI hierarchy in terms of communicatons and may provide data tracking, connection flow control, sequencing of data, error checking, and application addressing and identification. Session Layer { Layer {responsible responsible for dialog control and synchronization} synchronization } Session layer, some times called the dialog controller provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations conversations (called sessions) s essions) between applications.

Session layer protocols provide the logical connection entities at the application layer. These applications include file transfer protocols and sending email. Session responsibilities include


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network log-on and log-off procedures and user authentication. Session layer characteristics include virtual connections between applications, entities, synchronization of data flow for recovery purposes, creation of dialogue units and activity units, connection parameter negotiation, and partitioning services into functional groups. compression, and encryption} Presentation Layer { Layer {responsible for translation, compression,

The presentation layer provides independence to the application processes by addressing any code or syntax conversion necessary to present the data to the network in a common communications format. It specifies how end-user applications should format the data.

The presentation layer translated between different data formats and protocols. Presentation functions include data file formatting, encoding, encryption and decryption of data messages, dialogue procedures, data compression algorithms, synchronization, interruption, and termination. Application Layer { Layer {responsible for providing services to the u ser} The application layer is the highest layer in the hierarchy and is analogous to the general manager of the network by providing p roviding access to the OSI environment.

The applications layer provides distributed information services and controls the sequence of activities within and application and also the sequence of events between the computer application and the user of another application. The application layer communicates directly


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with the user’s application program. User application processes require application layer

service elements to access the networking environment. The service elements are of two ty pes: CASEs (common application service elements) elements) satisfying particular needs of application processes like association control, concurrence and recovery. The second type is SASE (specific (specific application service elements) which elements) which include TCP/IP stack, FTP, SNMP, Telnet and SMTP.

OSI OSI M ode odel D ata F low The sending process passes data to the application layer. The application layer attaches an application header and then passes the frame to the presentation layer.


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The presentation layer can transform data in various ways, if necessary, such as by translating it and adding a header. It gives the result to the session layer. The presentation layer is not aware of which portion (if any) of the data received from the application layer is the application header and which portion is actually user data, because that information is irrelevant to the presentation layer's role. The process of adding headers is repeated from layer to layer until the frame reaches the data link layer. There, in addition to a data-link header, a data-link trailer is added. The data-link trailer contains a checksum and padding if needed. This aids in frame synchronization. The frame is passed down to the physical layer, where it is transmitted to the receiving computer. On the receiving computer, the various headers and the data trailer are stripped off one by one as the frame ascends the layers and finally reaches the receiving process. Although the actual data transmission is vertical, each layer is programmed as if the transmission were horizontal. For example, when a sending transport layer gets a message from the session layer, it attaches a transport header and sends it to the receiving transport layer. The fact that the message actually passes through the network layer on its own computer is u nimportant.

The TCP/IP Reference Model suite is the set of communications protocols used for the Internet and The Internet protocol suite is similar networks, and generally the most popular protocol stack for wide area networks. It is TCP/IP, because of its most important protocols: Transmission Control commonly known as TCP/IP, Protocol (TCP) and Internet Protocol (IP), which were the first networking protocols defined in this standard. It is occasionally known as the DoD model. model. TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination. It has four abstraction layers which are used to sort all Internet protocols according to the scope of networking involved.


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TCP/IP model has four different layers.

1. The internet layer is the linchpin that holds the whole architecture together. Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a completely different order than they were sent, in which case it is the job of highe r layers to rearrange them, if in-order delivery is desired. Internet Protocol (IP) is the most important protocol in this layer. It is a connectionless protocol and does not provide reliability, flow control, or error recovery. P provides a routing function that attempts to deliver transmitted messages to their destination. A message unit in unit in an IP network is called an IP datagram. datagram. This is the basic unit of information information transmitted across TCP/IP networks. Other internetwork-layer protocols are IP, ICMP {Internet Control Message Protocol}, IGMP {Internet Group Management Protocol}, ARP {Address Resolution Protocol}, and RARP {Reverse ARP}.

2. The layer above the internet layer in the TCP/IP model is the transport layer and its designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols namely TCP {Transmission Control Protocol} and UDP {User Datagram Protocol} have Protocol} have been defined. TCP is a reliable connection-oriented protocol that permits a byte stream originating on one machine to be transported without error on any machine in the internet. It divides the incoming byte stream into discrete message and passes each one onto the internet layer. At the destination, the receiving TCP process collects the received message into the output stream. TCP deals with flow control to make sure a fast sender cannot swamp a slow receiver with more message than it can handle.


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3. In TCP/IP model, session or presentation layer are not present. Application layer is present on the top of the Transport layer. It includes all the higher-level protocols which are virtual terminal (TELNET), file transfer (FTP) and electronic mail (SMTP) . The virtual terminal protocol permits a user on one machine to log into a distant machine and work there. The file transfer protocol offers a way to move data efficiently from one machine to another. Electronic mail was used for file transfer purpose but later a specialized protocol SMTP, was developed for it. FTP was designed to permit reliable transfer of files over different platforms and it uses TCP to ensure reliability. HTTP permits applications such as browsers to upload and download web pages. It makes use of TCP at the transport layer again to check reliability. HTTP (Hyper Text Transfer Protocol) is Protocol) is a connectionless protocol that sends a request, receives a response and then disconnects the connection. HTTP delivers HTML documents plus all the other components components supported within HTML such as JavaScript, Visual script and applets. By using TCP, SMTP sends email to other computers that support the TCP/IP protocol suite. SMTP (Simple (Simple Mail Transfer Protocol) provides an extension to the local mail services that existed in the early years of LANs. It supervises the email sending from the local mail host to a remote mail host. It is not reliable for accepting mail from local users or distributing received mail to recipients this is the responsibility of the local mail system. For the transport of network management information, SNMP (Simple Network Management Protocol) is used as standardized protocol. Managed network devices can be cross-examined by a computer running to return details about their status and level of activity. To reduce the overhead of network traffic, SNMP uses UDP at the transport transport layer.

4. The network interface layer, also called the link layer or the data-link layer, is the interface to the actual network hardware. This interface may or may not provide reliable delivery, and may be packet or stream oriented. In fact, TCP/IP does not specify any protocol here, but can use almost any network interface available, which illustrates the flexibility of the IP layer. Examples are IEEE 802.2, X.25,ATM, FDDI, and even SNA.TCP/IP specifications do not describe or standardize any network-layer protocols, they only standardize ways of accessing those protocols from the internetwork layer.


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A Comparison of the OSI and TCP/IP Reference Models OSI and TCP/IP models are based on the concept of a stack of independent protocols. protocols. Functions of the layers are more or less similar. Three concepts are central to OSI Model: Model: Services, Interfaces, and Protocols. Services: This definition tells what the layer does. It defines the layers semantics. Interface: It tells the process above it how to access it. Peer Protocols: Protocols used in a layer are the lay er’s own business.

TCP/IP model did not clearly distinguish between service, interface and protocol. The protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes. It is learnt that the OSI model was devised before the protocols were invented. This ordering means that the model was not biased toward one particular set of protocols. Downside of this ordering is that the designers did not have much experience with the subject and did not have good idea of which functionality to put in which layer. The committee originally expected that each country would have one network ,run by the government and using the OSI Protocols, so no thought was given to internetworking. With TCP/IP, the protocols came first and the model was really just a description of existing protocols. Protocols fit the model perfectly. The only trouble was that the model did not fit any other protocols stacks. The difference between two models is the number of layers: the OSI model has seven layers and the TCP/IP has four layers .Both have (inter)network, transport and Application layers, but the other layers are different. Another difference is in the area of connectionless and connection oriented communication. OSI Model supports connection oriented communication in transport layer, whereas in network layer it supports both connectionless and connection oriented. The TCP/IP model has only one mode in the network layer but supports both modes in transport layer.


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Physical Layer The physical layer is the first layer of the Open System Interconnection Model (OSI Model). The physical layer deals with bit-level transmission between different devices and supports electrical or mechanical interfaces connecting to the physical medium for synchronized communication. Analog data is defined as the data having continuous states and digital data is defined as the data having discrete states. Analog signal is a signal that passes through and includes a wide range of varying values of intensity over a period of time, whereas a signal that has only a finite range of values is called as a digital signal.


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Periodic signal exhibits a specific signal pattern that repeats over time, whereas non-periodic (aperiodic) does not repeat any specific signal pattern. A composite signal is a collection of one or more signals having different frequencies, amplitude and phases. It can be periodic or aperiodic signal. The time required to transmit a character depends on both the encoding method and the signaling speed (i.e., the modulation rate - the number of times/sec the signal changes its voltage) •

Baud (D) - the number of changes per second

•

Bandwidth (H) - the range of frequencies that is passed by a channel. The transmitted signal is constrained by the transmitter and the nature of the transmission medium in cycles/sec (hertz)

•

Channel capacity (C) – the rate at which data can be transmitted over a given channel (R) } under given conditions.{This is also referred to as data rate (R)}

Nyquist theorem: To theorem: To transmit at a transmission rate of Hmin = fb/2 Hz

→

fb Hz

requires a minimum bandwidth of

This specifies the maximum data rate for the noiseless case as:

Shannon’s theorem :

If information rate does not not exceed channel capacity, capacity, there exists a

coding technique such that information can be transmitted over a noisy channel, error free. The channel capacity capacity provides the maximum possible data rate for the general noisy case as:

Network data rate is limited by the choice of medium and communication technology Some of the advantages of digital transmission are: 1. Repeaters are used for long distance communication does not induce noise as with the case of amplifiers in analog transmission. 2. Digital data & digitalized analog data can be easily encrypted and decrypted. 3. With LSI & VLSI, size and cost of digital eq uipment has been significantly reduced. 4. Multiplexing can be done easily in order to increase the channel capacity.


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Transmission Media medium is the physical path between transmitter and receiver in a data The transmission medium transmission system. It is included in the physical layer of the OSI protocol hierarchy. 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 of data from another form. Transmission media can be generally categorized as either unguided either unguided or guided. Guided guided. Guided Transmission Media uses a "cabling" system (or some sort of conductor) that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. The conductor directs the signal propagating down it. Only devices physically connected to the medium can receive signals propagating down a guided transmission medium. Examples of guided transmission media are copper wire and optical fiber.

Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media. Media. Unguided transmission media are wireless systems. Signals propagating down an unguided transmission medium are available to anyone who has a device capable of receiving them. A physical facility is one that occupies space and has weight as opposed to wireless media such as earth’s atmosphe re or a vacuum and includes metallic cables and optical cables.

Metallic transmission lines includes open-wire, twin-lead, and twisted-pair copper wire as well as coaxial cable, and optical fibers include plastic- and glass-core fibers encapsulated in various kinds of cladding materials.


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Guided Media Various types of physical medium exist and each one has its own niche in terms of bandwidth, delay, cost, ease of installation and maintenance. For guided transmission media, the transmission capacity, in terms of either data rate or bandwidth, depends critically on the distance and on whether the medium is point-to-point or multipoint.

Magnetic Media One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or removable media (e.g., recordable DVDs), physically transport the tape or disks to the destination machine, machine, and read them back in again. Though this is more cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor, the delay characteristics are very poor.

Twisted pair Twisted pair is the simplest, oldest and low priced cable medium. It is made up of two insulated copper wires about 1mm thick, twisted around each other in a continuous spiral. The purpose of twisting the wires is to reduce electrical interference (or noise) from similar pairs close by. The most common application of the twisted pair is the telephone system. Twisted pairs can run several kilometers without amplification, but for longer distances, repeaters are needed.

Twisted pairs can be used for transmitting either analog or digital signals. The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases. Two basic types of twisted-pair cable exist: Unshielded twisted pair (UTP) and shielded twisted pair (STP). Unshielded Twisted Pair (UTP)

An UTP cable (category 5) is one of the most popular LAN cables. This cable consists of 4 twisted pairs of metal wires (that means there are 8 wires in the cable). Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. Each twisted pair consists of two metal conductors that are insulated separately with their own coloured plastic insulation.


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UTP cable relies solely on the cancellation effect produced by twisted wire pairs to limit the signal degradation caused by electromagnetic interference and radio frequency interference. To further reduce crosstalk, the number of twists in the wire pairs varies. UTP cable is often installed using a Registered Jack 45 (RJ- 45) connector. The RJ-45 is an eight-wire e ight-wire connector used commonly to connect connect computers onto a local area network network (LAN), (LAN), especially Ethernet. UTP cables are suited for both data and voice transmissions; hence are commonly used in telephone systems. They are also widely used in DSL lines, 10Base-T and 100Base-T local area networks. UTP has several advantages as it is the cheapest media, easy to install and maintain. It also occupies less space. It is the fastest copper-based medium today. Different categories of UTP are:


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Shielded Twisted Pair (STP)

This cable has a metal foil or braided-mesh covering that covers each pair of insulated conductors. The metal foil is used to prevent infiltration of electromagnetic noise. This shield also helps to eliminate crosstalk.

STP reduces electrical noise both within the cable and from outside the cable. STP is suited for environments with electrical interference and also provides better performance at higher data rates. But the extra shielding makes makes the STP cables quite quite bulky and more expensive expensive that UTP cables. Also, the metallic shielding must be ground at both ends. Or else it acts as an antenna picking up unwanted signals. Coaxial cable

It is also one of the common transmission medium (called as coax) in current day data communications. They are also relatively inexpensive, but most costly than UTP on a per-unit length. A coaxial cable consists of a stiff central conductor (copper wire) as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely-woven braided mesh. The outer conductor is covered in a protective plastic plastic sheath sheath known known as jacket.

Although

coaxial cabling is difficult to install, it is highly resistant to


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signal interference. It can support greater cable lengths between network devices and greater bandwidth than than twisted-pair cable. Coaxial cables are capable of transmitting data at a fast rate of 10Mbps. Thicknet and Thinnet are two varieties of coaxial cable, but rarely used. Ethernet can run approx 100mts (328 feet) with UTP, while coaxial cable increases this distance to 500mts (1640 feet). The RG numbering system used with coaxial cables refers to cables approved by U.S. Department of Defense (DoD).

Categories of coaxial cables

To connect coaxial cable to devices, it is necessary to use coaxial connectors. The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. BNC connectors are sometimes referred to as bayonet mount, as they can be easily twisted on o r off.

There are three types: the BNC connector, the BNC T connector, the BNC terminator. Coaxial cable applications include analog and digital telephone networks, cable TV networks, Ethernet LANs and short range connections. Fiber-optic Cable

Fiber-optic cable or optical fiber consists of thin glass fibers that can carry information in the form of visible light. The typical optical fiber consists of a very narrow strand of glass or plastic called the core . Around the core is a concentric layer of less dense glass or plastic called the cladding , whose refractive index is less than that of the core. The outer most layer of the cable

is known as the jacket, which shields the cladding and the core from moisture, crushing and abrasion. Optical fibers transmit a beam of light by means of total internal reflection. When When a light beam from a source enters the core, the core refracts the light and guides the light along


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its path. The cladding reflects the light back into the core and prevents it from escaping through the medium.

Fiber optic cables support two modes of propagating light, which are multimode and single mode. In multimode, many beams from a light source traverse along multiple paths and at multiple angles. In single mode, the beams propagate almost horizontally. horizontally.

LED or LASER (Light Amplification by Stimulated Emission of Radiation) acts as the source converting electric pulse to light pulses and photodiode acts as receiver doing viceversa. Fiber optic cables uses 3 types of connectors, which are : SC (Subscriber Connector)- used to connect cable TV ST (Straight Tip)- to connect network devices MT-RJ (Mechanical Transfer-Registered Jack)- for network applications. applications.


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Introduction to Networks, Reference Models

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