CS408/533 Computer Networks

1. CS408/533 Computer Networks Text: William StallingsData and Computer Communications, 6th edition Chapter 1 - Introduction

2. A Communications Model • Source • generates data to be transmitted • Transmitter • Converts data into transmittable EM signals • Transmission System • Carries data • Receiver • Converts received signal into data • Destination • Takes incoming data

3. Simplified Communications Model - Diagram

4. Key Communications Tasks (also an overview of the course) • Transmission System Utilization - efficient use of resources (e.g. Multiplexing, Congestion Control) • Interfacing and Signal Generation - hook up and generate transmittable and interpretable signals • Synchronization - to know beginning and end of data • Error detection and correction • Flow control - do not overwhelm the destination • Addressing and routing - destination identification and path selection • Security - only the intended recipient must read • Network Management – Complex System, need to be managed (planning, configuration, status monitoring)

5. Networking • Point to point communication not usually practical • Devices are too far apart • Large set of devices would need impractical number of connections • Solution is a communications network

6. Simplified Network Models

7. Wide Area Networks • Large geographical area • Resemble “highways” • interconnected switching nodes • Data are switched from one node to another towards the destination • These nodes are not interested in the data • Alternative technologies • Circuit switching • Packet switching • Frame relay • Asynchronous Transfer Mode (ATM)

8. Circuit Switching • Dedicated communications path established for the duration of the connection • Channels are pre-allocated • Minimal (if not none) delay at each node • e.g. telephone network

9. Packet Switching • No dedicated channels • Data sent out in a sequence • Small chunks (packets) of data at a time • Packets passed from node to node between source and destination • Packets are • received • stored briefly (necessary for packet processing) • forwarded to the next node

10. Frame Relay • Packet switching systems have large overheads to compensate for errors • Modern transmission systems are more reliable • The ideas behind frame relay • Errors can be caught at the end points • Let’s take the advantage of high data rates and low error rates • Most overhead for error control is stripped out • Data Rate • 2 Mbps as compared to 64Kbps of X.25 (packet switching)

11. Asynchronous Transfer Mode • ATM • Evolution from frame relay • Little overhead (even less than frame relay) for error control • higher layers of end users are responsible for error check • Fixed packet (called cell) length • 48 bytes of data + 5 bytes of header • Anything from 10Mbps to Gbps range

12. ISDN • Integrated Services Digital Network • Voice, Data and other traffic has a single interface • network devices will understand the required service and process accordingly • Designed to replace Public Telephone Network • could not do it • Narrowband ISDN (or just ISDN) • Broadband ISDN (B-ISDN)

13. Local Area Networks • Smaller scope • Building or small campus • Usually owned by same organization as attached devices • requires set up and maintenance • Data rates much higher than WANs • Usually broadcast systems • Now some switched systems and ATM are being introduced

14. Protocols • Cooperative action is necessary • computer communications is not only to exchange bytes • huge system with several utilities and functions. For examples • error detection • encryption • For proper communication, entities in different systems must speak the same language • Mutually acceptable conventions • what is communicated? • when is communicated? • how is communicated? • Those conventions and associated rules are referred as “PROTOCOLS”

15. 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

16. Simplified File Transfer Architecture File Transfer Application Layer: Application specific commands, passwords and the file(s) transmission – high level data Communications Service Module: reliable transfer of those data – error detection, ordered delivery of data packets, etc. Network Module: actual transfer of data and dealing with the network – if the network changes, only this module is affected, not the whole system

17. A General Three Layer Model • Generalize the previous example for a generic application • we can have different applications (e-mail, file transfer, …) • Network Access Layer • Transport Layer • Application Layer

18. Network Access Layer • Exchange of data between the computer and the network • Sending computer provides address of destination • so that network can route • Software is dependent on type of network used (LAN, packet switched etc.) • Independent of upper layers

19. Transport Layer • Reliable data exchange • To make sure that all the data packets arrived in the same order in which they are sent out • Independent of network being used • Independent of application

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

21. 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

22. Protocol Architectures and Networks X Z Y

23. Protocol Data Units (PDU) • User data is passed from layer to layer • Control information is added/removed to/from user data at each layer • header • each layer has a different header • Data + header = PDU (Protocol Data Unit) • each layer has a different PDU

24. Transport PDU • Transport layer may fragment user data • Each fragment has a transport header added • Destination SAP • Sequence number • Error detection code • This is transport protocol data unit

25. Network PDU • Adds network header • network address for destination computer • optional facilities from network (e.g. priority level)

26. Operation of a Protocol Architecture