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CHAPTER 3: PACKET SWITCHING

CHAPTER 3: PACKET SWITCHING. Objectives. Introduction on Packet Switching 4 methods of accessing the service DATAPLUS packet switching DATAPLUS overseas destinations MODEM Rates and packet traffic charges of DATAPLUS. Introduction on Packet Switching.

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CHAPTER 3: PACKET SWITCHING

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  1. CHAPTER 3:PACKET SWITCHING

  2. Objectives • Introduction on Packet Switching • 4 methods of accessing the service • DATAPLUS packet switching • DATAPLUS overseas destinations • MODEM • Rates and packet traffic charges of DATAPLUS

  3. Introduction on Packet Switching • In message switching, each switching centre stores incoming messages until required outgoing circuit becomes free and then re-transmits them. • Result in delays. • Not suitable for data. • That’s why data networks use a modified form of message switching called PACKET SWITCHING

  4. How packets are transferred? Data sent out of sequence Long messages are split into number of PACKETS Small chunks (packets) of data sent at a time Packets passed from node to node between source and destination

  5. A format of a typical packet shown below: HEADER TEXT DATA CRC • Data must be preceded by a HEADER. • Then there is data. • Packet ends with bits added for error detection and correction. • The most known is CRC-Cyclic Redundancy Check code.

  6. HEADER TEXT DATA CRC The header should consists of: • Destination address. • Sequence number for reassembling in correct order at destination. • Sending address, for request retransmission. • The length of the packet, so that the receiver knows it has completely received the data. Some systems use a FLAG, to predetermine unique combination of digits to indicate the end of the packet. Destination Address Op Code Source address Seq Byte count

  7. The data network and terminals, DTE (Data Terminal Equipment) handle the packets by procedures known as PROTOCOL. • This PROTOCOL is defined in terms of the OSI 7-Layers Model.

  8. Packet switching was developed firstly for private data networks. • It is widely used in LAN – Local Area Network. • Now is also used in WAN – Wide Area Network. • Linking two networks that uses same protocol, a simple equipment is used called BRIDGE. • If they uses different protocol, a complex equipment is use called GATEWAY.

  9. 4.1 – Packet Switching Protocols • Packet switching offers users an open structure communications environment, which allows flexibilityin the choice of terminal and computing equipment combined with growth capability. • The network makes cost effectiveuse of line bandwidth for datatransmission in computer to computer transactions, maximising data integrity and network availability while minimising the cost. • Packet switch uses the multi layer CCITT X.25 Protocol to establish switched virtual circuits between any points in the data networks.

  10. Packet switching is becoming more established around the world and this is possible by the use of CCITT X.25. • One greatest attraction of CCITT X.25 is FLEXIBLE ROUTING. • The network can be left to decide its path to reach its destination, hence it prevent calls from being blocked, known as BOTTLENECKS.

  11. Advantages of Packet Switching • Handling large data volumes • Security, reliability & speed which is not possible on PSTN • World wide access • On-line information retrieval to get most up-to-date information quickly and efficiently • Access to public data to have news, publications, technical research data, portfolios, statistics etc • And lots more…

  12. Disadvantages of Packet Switching • Each packet switching node introduces a delay • Overall packet delay can vary substantially • This is referred to as jitter • Caused by differing packet sizes, routes taken and varying delay in the switches • Each packet requires overhead information • Includes destination and sequencing information • Reduces communication capacity • More processing required at each node

  13. 4.2 - The OSI 7-Layers

  14. 4.3 CITT/ITU Protocol • Stands for Consultative Committee for International Telephone & Telegraphy • An agency of UN that develops recommended world-wide standards & protocol for telecommunications industry. • Now known as ITU-TS (stands for International Telecommunication Union-Telecommunication Standardization.

  15. 4.3 - Some of the CITT/ITU Protocol

  16. 4.4 - CITT X.25 three layer protocol

  17. 4.5 - Four ways of accessing service in Brunei • By using a dedicated/leased line circuit • By using the PSTN through dial-up circuit • By using an electronic telex/teletax access • By using paging circuit

  18. What is DATAPLUS? • Stands for Data Packet Link Usage Services • Designed to provide common public network simultaneous use by number of varies data terminals and computer users • To facilitate the interconnection equipment of various origin

  19. 4.6 - MODEM • It is a MOdulator & DEModulator • Used to convert digital signal into analogue signal suitable for telephone lines and vice versa.

  20. Do we need modem for DATAPLUS? • Yes. Even two modem is needed. • One at exchange and another at subscriber premises.

  21. Dedicated subscriber uses 4 wires connection and require dedicated modem • Dial-up subscriber uses 2 wires connection and require dial-up modem

  22. 4.8 – DATAPLUS National Services • Two types of circuit: • Dedicated circuit (4 wires) • BSP • Great Eastern Life • Tractor Malaysia • Police • Dial up circuit (2 wires) • Brunei Investment Agency • Slumberger

  23. 4.10 – EWSP system in Brunei • EWSP is not used for telephony but for packet switching. • This system is a packet switching with modular system architecture based on a multiprocessor concept which enables optimum network configurations to be set up to suit different network requirements • It is used for transmitting large quantities of data, such as for telex, teletext, video text and data processing, by packet switching.

  24. EWSP permits interworking with existing switching and transmission facilities. • Network interworking is possible from packet switching network to: • PSTN • Circuit switched public data network • Another Packet Switched Public Data Network (PSPDN) • TELEX network • ISDN via a Packet Server Module (PSM)

  25. PSPDN TELEX NETWORK X.75 X.28 PSPDN + EWSP X.75 Async DTE X.28 PSM X.32 Packet DTE PSTN ISDN

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