CHAPTER 3: PACKET SWITCHING

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CHAPTER 3PACKET SWITCHING
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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

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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.
• Thats why data networks use a modified form of
  message switching called PACKET SWITCHING

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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

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A format of a typical packet shown below

• 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.

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• 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.

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• 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.

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• 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.

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4.1 Packet Switching Protocols

• Packet switching offers users an open structure
  communications environment, which allows
  flexibility in the choice of terminal and
  computing equipment combined with growth
  capability.
• The network makes cost effective use of line
  bandwidth for data transmission 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.

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• 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.

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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

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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

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Electrical/optical mechanical requirements to permit a continuous path for data transfer to be provided over physical medium.
Data frame composition including header, control error detection/correction to enable data transfer over the path provided by layer 1.
Interconnection of paths provided by layer 1 carrying the data composed by layer 2, including the requirements to deliver the data in correct sequence.
Matching between data users of any multiplexing requirements as predetermined by, for e.g. packet-switched methods of communications.
Matching between data users of data-transfer modes(2 way simultaneous/alternate or one way only), sync requirements etc.
Matching data structures and coding method between data users.
It concerns the relationship between a data users computer program and the communication environments.
DATA LINK
SESSION
APPLICATION
NETWORK
PHYSICAL
PRESENTATION
TRANSPORT
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4.2 - The OSI 7-Layers
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LAYER NAME
7 APPLICATION All
6 PRESENTATION People
5 SESSION Seem
4 TRANSPORT To
3 NETWORK Need
2 DATA LINK Data
1 PHYSICAL Processing
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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.

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4.3 - Some of the CITT/ITU Protocol
X.3 X.28 X.29 Asynchronous telex terminal to the Packet Switched Data Network (PSDN), Packet Assembler/Disassembler (PAD)
X.25 Computer/Intelligent terminals (users) to the PSDN.
X.75 PSDN to PSDN
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4.4 - CITT X.25 three layer protocol
Layer 1 Synchronization
Layer 2 Error detection/correction by retransmission Network Transparency
Layer 3 Sequencing Flow control Multiplexing Call setup Clearing Network Inter-working Logical In/Out Band Signalling
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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

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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

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4.6 - MODEM

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

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Do we need modem for DATAPLUS?

• Yes. Even two modem is needed.
• One at exchange and another at subscriber
  premises.

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

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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

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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.

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• 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)

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