Networks 2 Network Layer Models Dr John Cowell phones off (please) CSCI1412 Lecture 16 Networks 2 Network Layer Models Dr John Cowell

Overview Protocol layer models The OSI seven layer model 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

Layer Models

Protocol Layer Models Head of State Formulate message in country’s interest Receive message and decide upon response High-level communication no common language Ambassador/Diplomats Rephrase in diplomatic language Rephrase using appropriate language Communicate ideas no generation or change of meaning Translator Communicate, but no concept of meaning Translate to intermediate language Translate into national language

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

Agencies Protocols often written/regulated by agencies, e.g. ANSI American National Standards Institute FDDI, ASCII CCITT Comité Consultatif Internationale 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

Who is ISO? International Organisation for Standardisation ISO’s name 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”

ISO’s OSI Model Open Systems Interconnection The seven layer model 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

The OSI 7 Layer Model

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.

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

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

Information Processing Process sending information Logical communication between layers 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

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

Application (Layer 7) The application layer Provides services such as 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

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

Session (Layer 5) The session layer The 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

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

Network (Layer 3) The network layer It provides functions such as 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

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)

Physical (Layer 1) The physical layer This layer describes 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

The Real World

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

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

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)

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

Summary Protocol layer models The OSI seven layer model The real world 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