ECEN335-3-OSI Model.pdf
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Computer Networks OSI Model Prof. Nashwa Abdelbaki NewGiza University Network Layering Concept How many layers are needed? Physical and network access – Physical addressing, circuit switching, packet switching,... Networking and transport – Routing,...
Computer Networks OSI Model Prof. Nashwa Abdelbaki NewGiza University Network Layering Concept How many layers are needed? Physical and network access – Physical addressing, circuit switching, packet switching,... Networking and transport – Routing, reliability, resequencing,... Application – File transfer, remote login,... NewGiza University Prof. Nashwa Abdelbaki 2 Background Modern computer networks are designed in a highly structured form – hard to build networks In 1984, the International Standards Organization (ISO) proposed a model for networking – Open Systems Interconnection model NewGiza University Prof. Nashwa Abdelbaki 3 OSI Model Open Systems Interconnection model – is a framework for developing protocol standards – describes the communication between heterogeneous computers OSI Model is proposed to: – reduce the design complexity – make it possible to interconnect different kinds of network systems NewGiza University Prof. Nashwa Abdelbaki 4 Principles of OSI Model Each layer should perform a well-defined function A layer should be created where a different level of abstraction is needed The function of each layer should be chosen within internationally standardized protocols The layer boundaries should be chosen to minimize the information flow across the interfaces Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above NewGiza University Prof. Nashwa Abdelbaki 5 OSI Model Architecture Application Presentation High level protocols Session Transport Medium level protocols Network Data Link Low level protocols Physical NewGiza University Prof. Nashwa Abdelbaki 6 Protocol Stack The OSI model describes a conceptual stack of functional layers Each layer has a specific set of formal rules named as Protocols – to describe how to transmit data across a network The main terms of OSI Model: – Service – Interface – Protocol NewGiza University Prof. Nashwa Abdelbaki 7 OSI Model Sections The lower two layers (Physical and Data Link) deal with lower level functions including: – physical standards such as bit and byte ordering – transmission – error detection and correction of the bit stream The two medium level layers (Network and Transport) utilize internetwork protocols The upper three layers (Session, Presentation, Application) deal with higher layer functions such as – data formatting – terminal to computer dialogue – character sets – sequencing of messages NewGiza University Prof. Nashwa Abdelbaki 8 Physical Layer Controls the physical connection to the network Application Responsible for actual Presentation transmission and reception of Session bit stream Transport Deals with the electrical and Network mechanical charectaristics of Data Link the signal and signaling Physical methods – has no understanding of the meaning of bits NewGiza University Prof. Nashwa Abdelbaki 9 Data Link Layer controls communication between adjacent nodes Application detects, corrects errors and Presentation Session defines frames Transport has two sublayers: Network – Logical Link Control (LLC) Data Link LLC manages communication between MAC Physical devices over a single link of a network – Medium Access Control (MAC) uniquely identify devices NewGiza University Prof. Nashwa Abdelbaki 10 Network Layer establishes the route between the sender and receiver Application across switching points Presentation – which are typically routers Session static & dynamic routing Transport Network control Data Link packet segmentation Physical heterogeneous network control NewGiza University Prof. Nashwa Abdelbaki 11 Transport Layer splits up the data accepted from the Session Layer into smaller Application units Presentation – passes these to the Network Layer Session – ensures that all of the pieces arrive Transport correctly at the other end manages end-to-end Network communication Data Link regulates the data flow Physical establishes, maintains and terminates virtual circuits error detection and recovery NewGiza University Prof. Nashwa Abdelbaki 12 Session Layer allows users on different machines to establish Application sessions between them Presentation manages dialogue control Session Transport synchronizes data flow Network – by inserting checkpoints into the Data Link data stream Physical – after a crash, only the data after the last checkpoint must be retransmitted NewGiza University Prof. Nashwa Abdelbaki 13 Presentation Layer concerned with the syntax and semantics of the Application information transmitted Presentation information representation Session Transport data compression & Network decompression Data Link cryptography, encryption & Physical decryption NewGiza University Prof. Nashwa Abdelbaki 14 Application Layer defines a virtual terminal – editors and other programs can Application be written to these virtual Presentation terminals Session mapping of virtual world into Transport real world Network – file transferring Data Link between different file systems with Physical different file naming conventions – e-mail, telnet, remote job entry, directory lookup,... NewGiza University Prof. Nashwa Abdelbaki 15 OSI Model Communication Station A Station B message (APDU) 7 Application Application 7 message (PPDU) 6 Presentation Presentation 6 5 message (SPDU) 5 Session Session 4 message (TPDU) 4 Transport Transport 3 packets 3 Network Network 2 frames 2 Data Link Data Link 1 Physical bits Physical 1 PDU: Protocol Data Unit NewGiza University Prof. Nashwa Abdelbaki 16 Data Transmission Station A Station B AH data AT 7 Application Application 7 PH data PT 6 Presentation Presentation 6 5 SH data ST 5 Session Session TH data TT 4 Transport Transport 4 3 NH data NT 3 Network Network 2 DH data DT 2 Data Link Data Link 1 Stream of bits 1 Physical Physical NewGiza University Prof. Nashwa Abdelbaki 17 Network Architecture Host A Host B Application protocol 7 Application Application 7 Presentation protocol 6 Presentation Presentation 6 5 Session protocol 5 Session Session 4 Transport protocol 4 Transport Transport 3 Network Network Network Network 3 2 Data Link Data Link Data Link Data Link 2 1 Physical Physical Physical Physical 1 NewGiza University Prof. Nashwa Abdelbaki 18
