Data Communications

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.