1. Data Communications Architecture Models

2. What is a Protocol? For two entities to communicate successfully, they must “speak the same language”. What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable conventions. These conventions are referred to as a protocol.

3. Key Elements of a Protocol Syntax –Data formats –Signal levels Semantics –Control information for coordination –Error handling Timing –Speed matching –Sequencing

4. Protocol Architecture Task of communication broken up into modules For example file transfer could use three modules –File transfer application –Communication service module –Network access module

5. Simplified File Transfer Architecture

6. A Three Layer Model Application Layer Transport Layer Network Access Layer

7. Application Layer Support for different user applications e.g. e-mail, file transfer

8. Transport Layer Reliable data exchange Independent of network being used Independent of application

9. Network Access Layer Exchange of data between the computer and the network Sending computer provides address of destination May invoke levels of service Dependent on type of network used (LAN, packet switched etc.)

10. Addressing Requirements Two levels of addressing required Each computer needs unique network address Each application on a (multi-tasking) computer needs a unique address within the computer –The service access point or SAP

11. Protocols in Simplified Architecture

12. Protocol Data Units (PDU) At each layer, protocols are used to communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added –Destination SAP –Sequence number –Error detection code This gives a transport protocol data unit

13. Network PDU Adds network header –network address for destination computer –Facilities requests

14. Operation of a Protocol Architecture

15. TCP/IP Protocol Architecture Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) Used by the global Internet No official model but a working one. –Application layer –Host to host or transport layer –Internet layer –Network access layer –Physical layer

16. TCP/IP Protocol Suite Dominant commercial protocol architecture Specified and extensively used before OSI Developed by research funded US Department of Defense Used by the Internet

17. Physical Layer Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates etc.

18. Network Access Layer Exchange of data between end system and network Destination address provision Invoking services like priority

19. Internet Layer (IP) Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers

20. Transport Layer (TCP) Reliable delivery of data Ordering of delivery

21. Application Layer Support for user applications e.g. http, SMTP

22. TCP/IP Protocol Architecture Model

23. Some Protocols in TCP/IP Suite

24. OSI Model Open Systems Interconnection Developed by the International Organization for Standardization (ISO) Seven layers A theoretical system delivered too late! TCP/IP is the de facto standard

25. OSI - The Model A layer model Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers

26. OSI as Framework for Standardization

28. OSI Layers Application Presentation Session Transport Network Data Link Physical

29. The OSI Environment

30. OSI v TCP/IP

31. Standards Required to allow for interoperability between equipment Advantages –Ensures a large market for equipment and software –Allows products from different vendors to communicate Disadvantages –Freeze technology –May be multiple standards for the same thing

32. Standards Organizations Internet Society ISO ITU-T (formally CCITT) IEEE ANSI

33. Functions of Standards Encapsulation Segmentation and reassembly Connection control Ordered delivery Flow control Error control Addressing Multiplexing Transmission services

34. Encapsulation Addition of control information to data –Address information –Error-detecting code –Protocol control

35. Segmentation (Fragmentation) Data blocks are of bounded size Application layer messages may be large Network packets may be smaller Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP) –ATM blocks (cells) are 53 octets long –Ethernet blocks (frames) are up to 1526 octets long Checkpoints and restart/recovery

36. Why Fragment? Advantages –More efficient error control –More equitable access to network facilities –Shorter delays –Smaller buffers needed Disadvantages –Overheads –Increased interrupts at receiver –More processing time

37. Connection Control Connection Establishment Data transfer Connection termination May be connection interruption and recovery Sequence numbers used for –Ordered delivery –Flow control –Error control

38. Connection Oriented Data Transfer

39. Ordered Delivery PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering

40. Flow Control Done by receiving entity Limit amount or rate of data Stop and wait Credit systems –Sliding window Needed at application as well as network layers

41. Error Control Guard against loss or damage Error detection –Sender inserts error detecting bits –Receiver checks these bits –If OK, acknowledge –If error, discard packet Retransmission –If no acknowledge in given time, re-transmit Performed at various levels

42. Addressing Addressing level Addressing scope Connection identifiers Addressing mode Let’s look at two of these in more detail

43. Addressing level Level in architecture at which entity is named Unique address for each end system (computer) and router Network level address –IP or internet address (TCP/IP) –Network service access point or NSAP (OSI) Process within the system –Port number (TCP/IP) –Service access point or SAP (OSI)

44. Address Concepts

45. Addressing Mode Usually an address refers to a single system –Unicast address –Sent to one machine or person May address all entities within a domain –Broadcast –Sent to all machines or users May address a subset of the entities in a domain –Multicast –Sent to some machines or a group of users

46. Multiplexing Supporting multiple connections on one machine Mapping of multiple connections at one level to a single connection at another –Carrying a number of connections on one fiber optic cable –Aggregating or bonding ISDN lines to gain bandwidth

47. Transmission Services Priority –e.g. control messages Quality of service –Minimum acceptable throughput –Maximum acceptable delay Security –Access restrictions

48. Review Questions What are the layers of the TCP/IP model? What are the layers of the OSI model? What is meant by encapsulation? Trace an FTP command as it moves down through the layers, across the medium, and up the layers on the receiving side.