Module 1 Computer Communications and Networking
Topic Covered 1. 7 layer model for computer communications 2. Network topologies 3. Network bearers 4. LANs and WANs 5. CSMA and token protocols 6. Networking protocol performance 7. Bridges and routing 8. Internet protocols 9. Network security CCN: Lecture Notes Computer Comm. & Networking 2
Computer communications and Networking Computer communication is a special case of digital communications Interconnectivity is a major issue Traffic is very variable Speed is of utmost importance Error free communication is vital Installation costs need to be low due to vast installations Compatibility must be maintained CCN: Lecture Notes Computer Comm. & Networking 3
How do we do all of this? Consider a simple connection between computers CCN: Lecture Notes Computer Comm. & Networking 4
Do they have the same plug? Are the voltages the same? Is the communication speed the same? What about the communication protocol? Parity Block size Error correction Handshaking Will the program talk? Hey, I want to connect 2 more computers to yours as well AAARRRGGGHHH se... This is too hard! There must be a better way than this CCN: Lecture Notes Computer Comm. & Networking 5
Specialised computer networks As long as each computer supports the correct hardware and software to converse with the local network, then all computers should be able to communicate. CCN: Lecture Notes Computer Comm. & Networking 6
Q: How can all of these networks, computers, modems, lines, all made by different manufacturers be interconnected successfully? Answer: International standard CCN: Lecture Notes Computer Comm. & Networking 7
Originally computer manufacturers all devised their own interconnection system Closed System No compatibility Now they have seen the light and have adopted recommended standards Firstly need an overall framework to allow any device or process to be able to communicate with any other. ISO Reference Model for Open Systems Interconnection (OSI) ISO Reference Model Model is quite complex Needs some form structure Makes writing code easier and debugging simpler Firstly break model into 3 environments CCN: Lecture Notes Computer Comm. & Networking 8
3 Environments Network environment: Concerned with protocols an standards relating to underlying data communication network. OSI environment: network environment plus application oriented protocols and standards to allow end systems (computers) to communicate with one another in an open way. Real systems environment: adds to OSI environment the functions required for the manufacturers proprietary software and services. CCN: Lecture Notes Computer Comm. & Networking 9
The ISO reference model environments may be broken down into 7 clearly defined layers When two computers/processes communicate, the layers in each computer appear to communicate directly with each other. CCN: Lecture Notes Computer Comm. & Networking 10
Application Layer Provides primitives for the application process to access distributed information services, i.e., remote file access, e-mail, etc. This layer also provides Identification of the communication partners by name or address Information about availability of intended communication partner Establishment of authority to communicate Agreement on privacy (encryption) Authentication of intended communication partner Selection of dialogue discipline, initiation and release procedures Agreement on responsibility for error recovery Identification of constraints on data syntax CCN: Lecture Notes Computer Comm. & Networking 11
Presentation Layer This layer is concerned with the syntax of data exchanged between application layers. Local syntaxes mush be translated to a common transfer syntax. This layer also handles Data encryption and compression Virtual terminal translation CCN: Lecture Notes Computer Comm. & Networking 12
Session Layer Allows to application layers to set up, maintain and synchronise, and clear their data exchange or session. This layer also provides Interaction management, i.e., full or half duplex Synchronisation, i.e., inset synchronisation points and retransmit error data Exception reporting, i.e., errors reported to application CCN: Lecture Notes Computer Comm. & Networking 13
Transport Layer This layer ensures that data units are delivered error free, in sequence, with no losses or duplications. This layer also Optimises use of network services Provides a requested Quality of Service (QoS) to session layer CCN: Lecture Notes Computer Comm. & Networking 14
Network Layer This layer provides transparent transfer between transport layers. Transport layer need not know anything about data transmission Responsible for establishing, maintaining and terminating connections CCN: Lecture Notes Computer Comm. & Networking 15
Data Link Layer This layer attempts to make the communication link reliable. Activates, maintains and deactivates the data link Provides error correction and control to higher layers Handles synchronisation and flow control CCN: Lecture Notes Computer Comm. & Networking 16
Physical Layer This is the physical interface between devices, purely concerned with bit transfer. Mechanical Plug type Electrical signalling Voltage levels Timing Codes Functional Data Control Timing Ground Procedural Sequence of events CCN: Lecture Notes Computer Comm. & Networking 17
ISO Reference Model is used as a template Standards organisations build from this template ISO and IEEE produce standards for computer manufacturers CCITT produce standards for connecting equipment Prior to ISO US Dept of Defence developed ARPANET Expanded into Internet Transmission Control Protocol/Internet Protocol (TCP/IP) adopted TCP/IP has now become the pseudo standard for computer communications It performs all the functions of the OSI but has divided them in a slightly different way (example) CCN: Lecture Notes Computer Comm. & Networking 18
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Compare ISO and CCITT standards CCN: Lecture Notes Computer Comm. & Networking 20