1 OSI Reference Model

2 Agenda The Layered Model Layers 1 & 2: Physical & Data Link Layers Layer 3: Network Layer Layers 4–7: Transport, Session, Presentation, and Application Layers

3 The Layered Model

4 Layered Communication Source: Tanenbaum, 1996 I like rabbits L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Message Information for the Remote Translator Information for the Remote Secretary Location A

5 Layered Communication I like rabbits L: Dutch Ik hou van konijnen L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen J’aime les lapins Information for the Remote Translator Information for the Remote Secretary Location A Location B Message

6 Layered Communication I like rabbits L: Dutch Ik hou van konijnen L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen Fax #:--- L: Dutch Ik hou van konijnen J’aime les lapins Information for the remote translator Information for the remote secretary Location A Location B Layers Message

7 Why a Layered Network Model? 7Application 6Presentation 5Session 4Transport 3Network 2Data Link 1Physical Reduces complexity (one big problem to seven smaller ones) Standardizes interfaces Facilitates modular engineering Assures interoperable technology Accelerates evolution Simplifies teaching and learning

8 Devices Function at Layers 7Application 6Presentation 5Session 4Transport 3Network 2Data Link 1Physical NIC Card

9 Host & Media Layers 7Application 6Presentation 5Session 4Transport 3Network 2Data Link 1Physical Host layers: Host layers: Provide accurate data delivery between computers Media layers: Media layers: Control physical delivery of messages over the network } }

10 Layer Functions Provides network services to application processes (such as electronic mail, file transfer, and terminal emulation) 7 Application

11 Layer Functions Network services to applications Ensures data is readable by receiving system Format of data Data structures Negotiates data transfer syntax for application layer 7 Application 6 Presentation Data representation

12 Layer Functions Inter-host communication Establishes, manages, and terminates sessions between applications 7 Application 6 Presentation 5 Session Network services to applications Data representation

13 Layer Functions 7 Application 6 Presentation 5 Session Transport4 Inter-host communication Network services to applications Data representation End-to-end connection reliability Concerned with data transport issues between hosts Data transport reliability Establishes, maintains, and terminates virtual circuits Fault detection and recovery Information flow control

14 Layer Functions 7 Application 6 Presentation 5 Session Transport4 Network 3 Inter-host communication Network services to applications Data representation End-to-end connection reliability Addresses and best path Provides connectivity and path selection between two end systems Domain of routing

15 Layer Functions 7 Application 6 Presentation 5 Session Transport4 Network 3 Data Link 2 Inter-host communication Network services to applications Data representation End-to-end connection reliability Addresses and best path Access to media Provides reliable transfer of data across media Physical addressing, network topology, error notification, flow control

16 Layer Functions 7 Application 6 Presentation 5 Session Transport4 Network 3 Data Link 2 Physical1 Inter-host communication Network services to applications Data representation End-to-end connection reliability Addresses and best path Access to media Binary transmission Wires, connectors, voltages, data rates

17 7Application 6Presentation 5Session 4Transport 3Network 2Data Link 1Physical Host A Peer-to-Peer Communications Application Presentation Session Transport Network Data Link Physical Bits Frames Packets Segments Host B

18 Data Encapsulation Application Presentation Session Transport Network Physical Data Link Application Presentation Session Transport Network Physical Data Link Data } { Network Header Frame Header Frame Trailer Data Network Header Host A Host B

19 Layers 1 & 2: Physical & Data Link Layers

20 Physical and Logical Addressing c

21 MAC Address 24 bits ROM RAM 24 bits c Serial Number Vendor Code MAC address is burned into ROM on a network interface card

22 Layer 3: Network Layer

23 Network Layer: Path Determination Which Path? Layer 3 functions to find the best path through the internetwork

24 Network Layer: Communicate Path Addresses represent the path of media connections Routing helps contain broadcasts

25 Addressing— Network and Node Network Node Network address—Path part used by the router Node address—Specific port or device on the network

26 Protocol Addressing Variations Network Node 1 1 General Example Network Host TCP/IP Example (Mask ) NetworkNode 1aceb0b c00.6e25 Novell IPX Example

27 Network Layer Protocol Operations Each router provides its services to support upper layer functions XY A A C C

28 Network Layer Protocol Operations Each router provides its services to support upper layer functions B B XY A A C C Presentation Data Link Physical Data Link Physical Router A Router B Router C Data Link Physical Data Link Network Transport Session Presentation Application Physical Host X Host Y Data Link Network Transport Session Application Physical Network

29 Routed Versus Routing Protocol Routed protocol used between routers to direct user traffic Destination Network Protocol Protocol Name Examples: IP, IPX, AppleTalk, DECnet Exit Port to Use

30 Routed Versus Routing Protocol Routed protocol used between routers to direct user traffic Examples: IP, IPX, AppleTalk, DECnet Routing protocol used only between routers to maintain routing tables Examples: RIP, IGRP, OSPF

31 Static Versus Dynamic Routes Uses a protocol route that a network administrator enters into the router Static Route Uses a route that a network protocol adjusts automatically for topology or traffic changes Dynamic Route

32 Static Route Example Point-to-point or circuit-switched connection “Stub” network Only a single network connection with no need for routing updates A A B B Fixed route to address reflects administrator’s knowledge

33 Adapting to Topology Change A A B B C C D D Can an alternate route substitute for a failed route?

34 Adapting to Topology Change A A B B C C D D X X Can an alternate route substitute for a failed route? Yes Yes—With dynamic routing enabled

35 LAN-to-LAN Routing Example Network 1 Network 3 E0 E1 To0 Host 5 Host 4 Network 2 Token Ring Net 2, Host 5 Routing Table Destination Network Outgoing Interface E0 To0 E1

36 LAN-to-LAN Routing Network 1 Network 3 E0 E1 To0 Host 5 Host 4 Network 2 Token Ring Net 2, Host 5 Routing Table Net 2, Host From LAN to LAN Destination Network Outgoing Interface E0 To0 E1

37 LAN-to-WAN Routing Data Data A A Token Ring 2.4 Token Ring Data B B 1.3 From LAN Frame Relay Data Frame Relay Data To WAN Ethernet Data Data Data To LAN Data

38 Layers 4–7: Transport, Session, Presentation, and Application Layers

39 Transport Layer Segments upper-layer applications Establishes an end-to-end connection Sends segments from one end host to another Optionally, ensures data reliability

40 Transport Layer— Segments Upper-Layer Applications Electronic Mail File Transfer Application Presentation Session Segments Data Application Port Transport Data Application Port Terminal Session

41 Transport Layer— Establishes Connection Synchronize Acknowledge Negotiate Connection Receiver Data Transfer Connection Established (Send Segments) Sender

42 Transport Layer— Sends Segments with Flow Control Transmit Buffer Full Not Ready Stop Process Segments Buffer OK Ready Go Resume Transmission Receiver Sender

43 Transport Layer— Reliability with Windowing Window Size = 1 Sender Send 1 Receive 1 Receiver Ack 2 Send 2 Receive 2 Ack 3 Sender Send 1 Send 2 Receive 1 Receive 2 Receiver Window Size = 3 Send 3 Receive 3 Ack 4 Send 4

44 Transport Layer— An Acknowledgement Technique Sender Receiver Send 2 Send 1 Send 3 Ack 4 Send 5 Send 4 Send 6 Ack 5 Send 5 Ack

45 Transport to Network Layer End-to-End Segments Routed Packets

46 Session Layer Network File System (NFS) Structured Query Language (SQL) Remote-Procedure Call (RPC) X Window System AppleTalk Session Protocol (ASP) DEC Session Control Protocol (SCP) Service Request Service Reply Coordinates applications as they interact on different hosts

47 Presentation Layer ASCII EBCDIC Encrypted Text Data login: PICT TIFF JPEG GIF Graphics Visual images Sound Video MPEG QuickTime MIDI Provides code formatting and conversion for applications

48 Application Layer Word Processor Presentation Graphics Spreadsheet Database Design/Manufacturing Project Planning Others COMPUTER APPLICATIONS Electronic Mail File Transfer Remote Access Client-Server Process Information Location Network Management Others NETWORK APPLICATIONS INTERNETWORK APPLICATIONS Electronic Data Interchange World Wide Web Gateways Special-Interest Bulletin Boards Financial Transaction Services Internet Navigation Utilities Conferencing (Voice, Video, Data) Others

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