1. Protocol Architectures

2. Simple Protocol Architecture Not an actual architecture, but a model for how they work Similar to “pseudocode,” used for teaching programming Once we understand the building blocks, we can look at specific examples  Open Systems Interconnection (OSI) 7-Layer Model  TCP/IP

3. Why Use Protocol Architecture? Data communications requires complex procedures  Sender identifies data path/receiver  Systems negotiate preparedness  Applications negotiate preparedness  Translation of file formats For all tasks to occur, high level of cooperation is required

4. Modular Approach Break tasks into subtasks Each module handles specific subset of tasks Communication occurs  between different modules on the same system  between similar modules on different systems

5. Simple Modular Example File transfer facility Three modules  File transfer module could handle translation and inter-application communication  Communication service module could handle negotiation of preparedness, data flow  Network access module could handle data path

6. Advantages of Modularity Easier application development Network can change without all programs being modified

7. Three-Layer Model Distributed data communications involves three primary components:  Applications  Computers  Networks Three corresponding layers  Network access layer  Transport layer  Application layer

8. Network Access Layer Concerned with exchange of data between computer and network Includes addressing, routing, prioritizing, etc Different networks require different software at this layer

9. Transport Layer Concerned with reliable transfer of information between applications Independent of the nature of the application Includes aspects like flow control and error checking

10. Application Layer Logic needed to support various applications Each type of application (file transfer, remote access) requires different software on this layer

11. Addressing Each computer on a network requires a unique address on that network Each application on the computer must have a unique address within the computer to allow the transport layer to support multiple applications Data units must include network and application addresses

12. Standardized Protocol Architectures Vendors like standards because they make their products more marketable Customers like standards because they enable products from different vendors to interoperate Two protocol standards are well-known:  TCP/IP: widely implemented  OSI: well-known, less used, still useful for modeling/conceptualizing

13. OSI Open Systems Interconnection Developed by ISO Contains seven layers Application Presentation Session Transport Network Data Link Physical

14. OSI Lower Layers Physical Data Link Network

15. OSI Physical Layer Responsible for transmission of bits Always implemented through hardware Encompasses mechanical, electrical, and functional interfaces e.g. RS-232

16. OSI Data Link Layer Responsible for error-free, reliable transmission of data Flow control, error correction e.g. HDLC

17. OSI Network Layer Responsible for routing of messages through network Concerned with type of switching used Handles routing between networks, as well as through packet-switching networks

18. OSI Upper Layers Transport Session Presentation Application

19. OSI Transport Layer Isolates messages from lower and upper layers Breaks down message size Monitors quality of communications channel Selects most efficient communication service necessary for a given transmission

20. OSI Session Layer Establishes connections between systems Manages log-ons, password exchange, log-offs Tracks physical location of files on both sides of a transfer

21. OSI Presentation Layer Provides format and code conversion services Examples  File conversion from ASCII to EBDIC  Invoking character sequences to generate bold, italics, etc on a printer

22. OSI Application Layer Provides access to network for end-user User’s capabilities are determined by what items are available on this layer

23. OSI in Action: Outgoing File Transfer FTP program issues command to Application Layer Application passes it to Presentation, which may reformat, passes to Session Session requests a connection, passes to Transport Transport breaks file into chunks, passes to Network Network selects the data’s route, passes to Data Link Data Link adds error- checking info, passes to Physical Physical transmits data, which includes information added by each layer

24. OSI in Action: Incoming File Transfer Physical receives bits, passes to Data Link Data Link checks for errors, passes to Network Network verifies routing, passes to Transport Transport reassembles data, passes to Session Session determines if transfer is complete, may end session, passes to Presentation Presentation may reformat, perform conversions, pass to Application layer Application presents results to user (e.g. updates FTP program display)

25. TCP/IP v. OSI See diagram page 359 for relationships Most “production software” uses TCP/IP rather than OSI Why has OSI “lost the war”? Two primary reasons:  Not as mature as TCP/IP  Unnecessarily complex (seven layers rather than five)

26. Why Study OSI? Still the best model for conceptualizing and understanding protocol architectures Later ICSA networking classes expect you to know and understand this model Key points:  Modular  Hierarchical  Boundaries between layers=interfaces

27. TCP/IP Transmission Control Protocol/Internet Protocol Developed by DARPA No official protocol standard Can identify five layers Application Host-to-Host (transport) Internet Network Access Physical

28. TCP/IP Physical Layer Physical interface between a DTE (e.g. computer or terminal) and a transmission medium Specifies:  Characteristics of medium  Nature of signals  Data rate  Similar to mechanical aspects of RS-232

29. TCP/IP Network Access Exchange of data between end system and network Address of host and destination Prioritization of transmission Software at this layer depends on network (e.g. packet-switching vs. Ethernet) Segregation means that no other software needs to be concerned about net specifics

30. TCP/IP Internet Layer An Internet is an interconnection of two or more networks Internet layer handles tasks similar to network access layer, but between networks rather than between nodes on a network Uses IP for addressing Implemented in workstations and routers

31. TCP/IP Transport Layer Also called host-to-host layer Reliable exchange of data between applications Uses TCP protocols for transmission

32. TCP/IP Application Layer Logic needed to support variety of applications Separate module supports each type of application (e.g. file transfer)