CSCI1412 Lecture 16

1. phones off(please) CSCI1412Lecture 16 Networks 2Network Layer Models Dr John Cowell

2. Overview • Protocol layer models • what is a layer model and why use it? • standards and standards agencies • The OSI seven layer model • overview of the layer model • the seven layers explained • The real world • how the theoretical model applies to the Internet

3. Layer Models

4. Head of State High-level communicationno common language Ambassador/Diplomats Communicate ideasno generation or change of meaning Translator Communicate, but no concept of meaning Protocol Layer Models Formulate messagein country’s interest Receive message and decide uponresponse Rephrase in diplomatic language Rephrase usingappropriate language Translate to intermediate language Translate into national language

5. Protocol Standards • A protocol is a set of rules governing communication issues - a standard • diplomats have protocols • so do networks • so do computer networks! • Two types of standards • de-facto - devolved standards • e.g. many IBM products, Microsoft Office, TCP/IP • designed - written standards • submitted to a standards agency for approval • agreed by a group of companies or organisations

6. Agencies • Protocols often written/regulated by agencies, e.g. • ANSI • American National Standards Institute • FDDI, ASCII • CCITT • ComitéConsultatifInternationale de Télégraphic et Téléphonique • V series for telephone communications (modems) • X series for network interfaces & public Networks • IEEE • Institute of Electrical & Electronic Engineers • Project 802 - LAN standards • IAB • Internet Activities Board • set and document internet policies and standards, e.g. TCP/IP

7. Who is ISO? • International Organisation for Standardisation • a worldwide federation of national standards bodies from 130 countries, one from each country • a non-governmental organization established in 1947 • to promote the development of standardization • ISO’s work results in international agreements which are published as International Standards • ISO’s name • shouldn’t the acronym be “IOS”? • yes, if it were an acronym; but it is not • “ISO” is a word, from the Greek isos, meaning “equal”

8. ISO’s OSI Model • Open Systems Interconnection • a standard description or “reference model” for how messages should be transmitted between any two points in a telecommunication network • any two computers can communicate when connected • regardless of underlying architecture • The seven layer model • the process of communication between two users in a comms. network can be divided into layers, with each layer adding its own set of special, related functions • written standard • not implemented

9. The OSI 7 Layer Model

10. Origins of the Model Late 1970’s 2 separate projects by • ISO • CCITT • To standardise a framework for emerging network technologies. • 1984 published as ISO 7498 and CCITT as X.200 • The Internet and growth of TCP/IP (Transmission Control Protocol/ Internet Protocol) became the de-facto standard.

11. User services, application, activities Actual data transmissioncommunications subnet The Seven Layers Application layer Presentation layer Session layer Transport layer Network layer Data Link layer Physical layer

12. What They Do • The layers are in two groups • the upper four layers are used whenever a message passes from or to a user • within the same computer or to another computer • lower three layers used when any message passes through the host computer to another • messages destined for some other host are not passed up to the upper layers but are forwarded to another host • Purpose is to separate specific functions • makes implementation transparent to other components • allows independent testing & maintenance of each layer

13. Information Processing Process sending information Logicalcommunicationbetweenlayers Process receiving information Application layer Application layer Presentation layer Presentation layer Session layer Session layer same computer Transport layer Transport layer Network layer Network layer Data Link layer Data Link layer Physical layer Physical layer other computers

14. Data Unit at Different Layers Each layer deals with different grouping of data • Data - Application, Presentation, Session • Segment – Transport • Packet – Network • Frame – Data Link • Bit - Physical

15. Application (Layer 7) • The application layer • interacts with applications that offer some form of communication component • not an application program • Provides services such as • identifying communicating partners • determining the identity and availability of partners • determining resource availability • decide whether sufficient networking resources are available • synchronising communication • all communication between applications requires cooperation

16. Presentation (Layer 6) • The presentation layer • presents data in user understandable format • allows user to see information, not data • translates between formats • Provides a variety of coding and conversion functions that are applied to application layer data • data representation • e.g. JPEG, GIF, TIFF formats for graphics • character representation • e.g. ASCII, EBCDIC encodings for characters • data compression schemes • data encryption schemes

17. Session (Layer 5) • The session layer • responsible for managing connections between two machines during the course of communication • determines whether all pertinent information for the session has been transmitted and/or received • The layer • establishes logical connection • controls synchronisation between two ends • manages acknowledgements and retransmissions • reconnects on premature termination (at lower level) • releases logical connection

18. Transport (Layer 4) • The transport layer • end-to-end communications • decides upon network to use • according to data type, e.g. packets or continuous bit stream • Transport layer functions typically include • flow control • ensuring there is enough ‘space’ to send/receive messages • multiplexing • combining more than one message on the same connection • packeting • breaking larger units down into small pieces to be transferred • error checking and recovery

19. Network (Layer 3) • The network layer • provides routing and addressing functions that enable multiple data links to be combined into a network • sending it in the right direction to the right destination on outgoing transmissions • receiving incoming transmissions at the packet level • It provides functions such as • routing strategies • deciding which route to use and why • forwarding • sending on packets destined for other hosts • controlling communications subnet

20. Data Link (Layer 2) • The data link layer • provides the first level of organization of the data bits into a rudimentary structure called a datalink frame • this is organized into fields of information that convey • the beginning and ending of the frame • the address of the sender • the address of the receiver • a method to ensure that the frame does not contain errors • an area to provide some basic administrative function • It is sometimes broken down in two sub layers • logical link control layer (LLC) • media access control layer (MAC)

21. Physical (Layer 1) • The physical layer • defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems • transmits/receives bits as electrical impulses • This layer describes • the network topology (shape, structure) • the transmission media (cables, broadcasts) • the transmission devices (network cards, modems) • the electrical or optical signalling characteristics

22. The Real World

23. OSI The Real World 7 application layer 5 world wide web (HTML/HTTP) email (POP / SMTP) file transfers (FTP) remote sessions (TELNET) 6 presentation layer 5 session layer 4 transport layer 4 transmission control protocol (TCP) 3 network layer 3 internet protocol (IP) 2 data link layer 2 ethernet PPP 1 physical layer 1 (physical level) modem The Real World! • Loosely, a five layer model

24. Application Layer • Top layer (5) comprises other protocols such as • HTML / HTTP (the World Wide Web) • POP / SMTP (email) • FTP (file transfer) • TELNET (remote sessions) • DNS (Domain Name Server) • maps domain names to IP addresses • 146.227.102.2  http://www.cms.dmu.ac.uk/ • This application layer roughly corresponds to layers 5, 6, and 7 of the OSI model

25. TCP/IP • TCP/IP is the basic communication protocol (language) of the Internet • Transmission Control Protocol • manages assembling a message or file into smaller parts (packets) that will be transmitted over a network • reassembles packets when received • TCP  OSI layer 4 (transport) • Internet Protocol • handles the address part of each packet to ensure that it gets to the correct destination • IP  OSI layer 3 (network)

26. Lower Levels • Three of the most common methods of connecting to the Internet are • Ethernet • LAN protocol developed by Xerox, DEC & Intel in 1976 • specifies the way data is transmitted as well as the physical connection options • incorporates layers 1 and 2 of the OSI model • PPP over a modem link • point-to-point protocol controls how the data is sent (level 2) • now included in Windows and other operating systems • modem handles transmission of data over telephone line (level 1) • DSL (Broadband) • utilises unused bandwidth on standard telephone wires • need frequency divider (DSL ‘modem’) to separate voice call from data signal

27. Summary • Protocol layer models • what is a layer model and why use it? • standards and standards agencies • The OSI seven layer model • overview of the layer model • the seven layers explained • The real world • how the theoretical model applies to the Internet