1. Networking

2.  Purpose of the OSI model and its seven layers  Function of each layer  Process for communication between devices  TCP/IP Model

3.  O pen S ystems I nterconnection model is fundamental to all communications between network devices.  Developed in 1974 by ISO after the American Department of Defence began using the TCP/IP suite of protocols.  Finally adopted in 1977. It is now the theoretical model for how communication takes place between network devices.

4. Session Physical Transport Network Data Link Presentation Application Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1

5.  In the sense of purpose and responsibility, each layer is separate and independent  Each has its own function, but also provides a service to those layers above and below itself  The model should be considered an aid to understanding the nature of communication on the network – and useful in sorting out troubles that might occur on a network  By providing, it allows both software engineers and hardware manufacturers ensure their products work together.

6. As the next slide shows:  When communicating, each OSI layer talks with the same layer in the other device  E.g. the Application Layer of Device A communicates with the Application Layer of Device B, by passing the data through the other layers  The Application Layer of each device is not concerned with how the other layers are functioning, but it does rely on them to do their job

8. A pplication A ll P resentation P eople S ession S eem T ransport T o N etwork N eed D ata Link D ata P hysical P rocessing

9. When data is sent from the application on the source computer the following happens  Data in the form of a packet moves down through the layers  When it reaches the Physical Layer it is ready to be sent along the cable  At the Physical Layer the bits may be analogue or digital, in the form of electrical, light or radio waves

10.  The data is transmitted to the destination device  It travels up through the layers of the OSI model, reaching the user.  As data moves down through the layers it is encapsulated – ie additional information is added as headers or trailers  The data in the packet does not change  See the following diagram

12.  ‘Closest’ layer to the user  Works with the applications you use to communicate over the network  E.g.. Services include SMTP, HTTP and FTP  Clicking on a link on a web page issues a command for the browser to retrieve the relevant information from the Internet  In this example your computer is the source, and the host of the web site information is the destination  The application completes your request and delivers the information to your computer

13.  The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions: Resource sharing and device redirection  Remote file access  Remote printer access  Inter-process communication  Network management  Directory services  Electronic messaging (such as mail)  Network virtual terminals

14.  The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station. The presentation layer provides: Character code translation: for example, ASCII to EBCDIC (Extended Binary Coded Decimal Interchange Code).  Data conversion: bit order, CR-CR/LF (CR=Carriage return, LF=Line Feed), integer-floating point, and so on.  Data compression: reduces the number of bits that need to be transmitted on the network.  Data encryption: encrypt data for security purposes. For example, password encryption.

15.  The session layer allows session establishment between processes running on different stations. It provides: Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.  Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

16. The following services are provided:  Establishing a Connection  Maintaining the Session  Ending the Connection  Dialogue Control  Dialogue Separation

17.  Handshaking – SYN and ACK packets  ‘Keep alive messages’  Session must be terminated (otherwise one device will be still transmitting without any device actually listening)  Dialogue Control (simplex, half-duplex, full- duplex)  Dialogue Separation – checkpoints within the transmission which allow the detection of lost packets, and subsequent re-transmission

18.  Ensures reliable transport of packets from source to destination  Also manages the speed of transmission – flow control  There are two types of transmission (Connection-Oriented Transmissions and Connectionless Transmissions) – see next slide

19.  Also known as ‘ Reliable Transport Method’ – uses acknowledgement (ack) packets on successful receipt of data  Extra packets slows down communication  Features are › Reliability › Slower Communication › Packets are re-transmitted if unrecognisable or not received  Once all the data is received successfully, the packet is re-assembled and the Transport Layer passes it to the Session Layer

20.  In this mode the transmitting device does not require acknowledgements from the receiver, and continues to transmit on the assumption that the data was received  Features are: › Little or No Reliability › Faster Transmission › Packets are not Re-transmitted

21.  Establishes the maximum speed at which both sender and receiver can communicate at  Transport Layer determines largest packet size which can be sent  Packets are numbered – to allow re-assembly in the correct order

22.  Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.  Message acknowledgment: provides reliable end-to- end message delivery with acknowledgments.  Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.  Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

23.  Responsible for the correct addressing and delivery of packets of data  These are known as datagrams  Uses the network address ( this is a logical address – and does not depend upon any hardware in the device, or the device’s physical location)

24. The Network Layer does the following:  Adds the address to the packet (encapsulation)  Maps the network address to the devices physical address  Determines the best path for the packet (routing)  Ensures that the packet is in the correct format for the destination See Diagram = 

25.  Encapsulation at the Transport Layer involves adding the address of the sender to the datagram  The destination address is now added. Both addresses are logical.  Both addresses are necessary for packets to move between end systems.  If a packet must move to another network, a routing protocol is required  If different packet lengths are used on the different networks, the Network Layer formats the data accordingly  The primary piece of hardware which works on this layer is the router.

26.  Routing: routes frames among networks.  Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.  Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.  Logical-physical address mapping: translates logical addresses, or names, into physical addresses.  Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

27.  Has two sub layers of its own: › Logical Link Control (LLC) › Media Access Control (MAC)  LLC acts between protocols such as Internet Protocol (IP) and the MAC method.  MAC is responsible for the connection to the physical media (eg cable)

28.  Each NIC has a unique number hard coded in to the card – its physical address  The first 6 digits denote the manufacturer, the next six are unique) – type “winipcfg” on your PC  When the MAC address is added to the packet it is now known as a frame (IEEE 802.3)  It now has all the information required to travel from the source to the destination

29.  Link establishment and termination: establishes and terminates the logical link between two nodes.  Frame traffic control: tells the transmitting node to "back- off" when no frame buffers are available.  Frame sequencing: transmits/receives frames sequentially.  Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non- acknowledged frames and handling duplicate frame receipt.  Frame delimiting: creates and recognizes frame boundaries.  Frame error checking: checks received frames for integrity.  Media access management: determines when the node "has the right" to use the physical medium.

30.  The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.  It provides: Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines: › What signal state represents a binary 1 › How the receiving station knows when a "bit-time" starts › How the receiving station delimits a frame

31.  Physical medium attachment, accommodating various possibilities in the medium: › Will an external transceiver (MAU) be used to connect to the medium? › How many pins do the connectors have and what is each pin used for?  Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.  Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines: › What physical medium options can be used › How many volts/db should be used to represent a given signal state, using a given physical medium

32. OSI Layers Common TCP/IP protocols 7ApplicationFTPHTTPPOP3SMTP 6Presentation 5SessionDNS 4TransportTCPUDP 3NetworkIPICPMDHCP 2DatalinkARPRARP 1Physical FTP – File Transfer Protocol TCP – Transfer Control Protocol IP – Internet Protocol ARP – Address Resolution Protocol UDP – User Datagram Protocol DHCP – Dynamic Host Configuration HTTP – HyperText Transfer Protocol POP3 - Post Office Protocol SMTP – Simple Mail Transfer Protocol DNS – Domain Name System ICPM – Internet Control Messaging Protocol RARP – Reverse Address Resolution Protocol

33.  OSI model is an important concept  Protocol most in use on modern networks is TCP/IP  TCP/IP does not map its layers precisely to OSI model  OSI = 7 layers, TCP/IP = 4 layers (sometimes a 5 th physical layer is referred to)

34. Session Physical Transport Network Data Link Presentation Application Application or Process Layer Transport Data Link Internet

35. Sr. No.TCP/IP Reference ModelOSI Reference Model 1Defined after the advent of Internet.Defined before advent of internet. 2 Service interface and protocols were not clearly distinguished before Service interface and protocols are clearly distinguished 3TCP/IP supports Internet workingInternet working not supported 4Loosely layeredStrict layering 5Protocol Dependant standardProtocol independent standard 6More CredibleLess Credible 7 TCP reliably delivers packets, IP does not reliably deliver packets All packets are reliably delivered

36.  Application or Process Layer – concerned with how data at both ends is handled.  Transport Layer – manages flow of data  Internet Layer – consists of several protocols, primary protocol is IP (providing hierarchical addressing scheme  Data Link (or Network Interface) Layer – manages transmission of data within the network  Physical Layer – not really defined, TCP/IP leaves the physical connection to manage itself

37.  Purpose of the OSI model and its seven layers  Function of each layer  Process for communication between devices  TCP/IP Model  ADDITIONAL READING IS STRONGLY RECOMMENDED