E&CE418 Topic 1: Introduction to communication networks 1
2 Organization of network operations 2.1 Why we need a network model 2.2 Layered architecture 2.3 Reference models 2.3.1 OSI model 2.3.2 TCP/IP model 2
2.1 Why need a network model Over the past couple of decades many of the networks that were built used different hardware and software implementations, as a result they were incompatible and it became difficult for networks using different specifications to communicate with each other. To address the problem of networks being incompatible and unable to communicate with each other, the International Organisation for Standardisation (ISO) recognised there was a need to create a NETWORK MODEL to help vendors create interoperable network implementations 3
2.2 Layered network model Generally, network operations involve several tasks which can be organized in layers 4
Layers, protocols, and interface Host A Layer N Host B Layer N Layer 2 Layer 2 Layer 1-2 service interface Layer 1 Layer 1 Physical medium Layer consist a group of related communication functions/protocols which perform particular tasks Protocol is the set of rules that govern communications in each layer 5
Each layer provides a service to the layer above through the interface defined between the layers 6
Benefits of layered architecture Decomposition of the overall problem into a set of layers results in a number of benefits: Layering simplifies design, implementation, and testing Layering provides flexibility in modifying protocols and services Layering promotes interoperability and compatibility by hiding internal implementations 7
2.3 Reference models The International Organisation for Standardisation (ISO) Open Systems Interconnection (OSI) model Internet Engineering Task Force (IETF) TCP/IP model 8
2.3.1 OSI reference model (7 layers) 9
Layer 7: Application The application layer is the OSI layer that is closest to the user. It provides network services that allow an application s processes, which run on different end hosts to communicate Example of application level protocols: HTTP, Telnet 10
Layer 6: Presentation Permits independence in presentation of data Code conversion Data encryption Data compression Examples: JPEG, MPEG, ASCII, EBCDIC, HTML. 11
Layer 5: Session The session layer defines how to start, control and end conversations (called sessions) between applications Examples: SQL, ASP(AppleTalk Session Protocol) 12
Layer 4: Transport Transfers data between processes in source and destination Message segmentation and reassembly Application multiplexing/demultiplexing End-to-end flow control Connection setup, maintenance, and release Examples: TCP, UDP 13
Layer 3: Network Guides packets from source to destination across multiple links routing Defines how routing works and how routes are learned so that the packets can be delivered Forwards packet across a node (hop by hop) Connection setup, maintenance and teardown for ATM Examples: IP, IPX, AppleTalk 14
Layer 2: Data Link Data transmission from node to node Error detection and correction Medium access Reliable delivery over physical medium Example: Ethernet, Frame relay, FDDI 15
Layer 1: Physical Transmit raw bit stream over channel Define and specify physical aspects of a communication link Example: EIA/TIA-232, RJ45, 1000BASE-T 16
Header and trailer 17
Summary The OSI reference model organizes network functions into seven numbered layers Each layer provides a service to the layer above it in the protocol specification and communicates with the same layer s software or hardware on other computers Layers 1-4 are concerned with the flow of data from end to end through the network and Layers 5-7 are concerned with services to the applications 18
2.2.2 TCP/IP (IETF) Model 19
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OSI and TCP/IP models More information are available at: http://en.wikipedia.org/wiki/osi_model http://en.wikipedia.org/wiki/tcp/ip_model 21