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Computer Networks-Switching( Circuit, Packet)
- 1. 8.1 Module 2 Circuit &Packet Switching Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 2. Switched Network • Anetwork is a set of connected devices. • Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. • One solution is to make a point-to-point connection between each pair of devices (mesh or star topology). — impractical and wasteful when applied to very large networks. • Multipoint connection –bus topology - ruled out because the distances between devices and the total number of devices increases beyond the capacities of the
- 3. • A bettersolution is switching. A switched network consists of a series of interlinked nodes, called switches. • Switches are devices capable of creating temporary connections between two or more devices linked to the switch. • In a switched network, some of these nodes are connected to the end systems (computers or telephones, for example). Others are used only for routing.
- 4. Simple Switched Network •End systems (communicating systems) – A, B, C, D, E, F, G, H, I, J. • Switches – I,II,III,IV,V
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- 6. Switching Networks • Longdistance transmission is typically done over a network of switched nodes • Nodes not concerned with content of data • End devices are stations —Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data routed being switched from node to node
- 7. Nodes • Nodes mayconnect to other nodes only, or to stations and other nodes • Node to node links usually multiplexed • Network is usually partially connected —Some redundant connections are desirable for reliability
- 8. Taxonomy of switchednetworks
- 9. Circuit Switching • Dedicatedcommunication path between two stations • Three phases —Establish —Transfer —Disconnect • Must have switching capacity and channel capacity to establish connection • Must have intelligence to work out routing
- 10. Circuit Switching • Inefficient —Channelcapacity dedicated for duration of connection —If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Circuit switching designed for voice —Resources dedicated to a particular call —Much of the time a data connection is idle —Data rate is fixed • Both ends must operate at the same rate
- 11. Public Circuit SwitchedNetwork
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- 14. Circuit Switching Concepts •Digital Switch —Provide transparent signal path between devices • Network Interface • Control Unit —Establish connections • Generally on demand • Handle and acknowledge requests • Determine if destination is free • construct path —Maintain connection —Disconnect
- 15. Blocking or Non-blocking •Blocking —A network is unable to connect stations because all paths are in use —Used on voice systems • Short duration calls • Non-blocking —Permits all stations to connect (in pairs) at once —Used for some data connections
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- 17. 8.18 As a trivialexample, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4-kHz voice channels. We assume that each link uses FDM to connect a maximum of two voice channels. The bandwidth of each link is then 8 kHz. Figure 8.4 shows the situation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course the situation may change when new connections are made. The switch controls the connections. Example
- 18. 8.19 Circuit-switched network usedin Example
- 19. Packet Switching Principles •In a packet-switched network, there is no resource reservation; resources are allocated on demand. • Data transmitted in small packets —Typically 1000 octets —Longer messages split into series of packets —Each packet contains a portion of user data plus some control info • Control info —Routing (addressing) info • Packets are received, stored briefly (buffered) and past on to the next node
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- 21. Advantages • Line efficiency —Singlenode to node link can be shared by many packets over time —Packets queued and transmitted as fast as possible • Data rate conversion —Each station connects to the local node at its own speed —Nodes buffer data if required to equalize rates • Packets are accepted even when network is busy —Delivery may slow down • Priorities can be used
- 22. Packet Switching Technique •Station breaks long message into packets • Packets sent one at a time to the network • Packets handled in two ways —Datagram Approach —Virtual circuit Approach
- 23. Datagram Approach • Eachpacket treated independently • Packets can take any practical route • Packets may arrive out of order • Packets may go missing • Up to receiver to re-order packets and recover from missing packets. • A switch in a datagram network uses a routing table that is based on the destination address.
- 24. A datagram networkwith four switches
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- 26. If there areno setup or teardown phases, how are the packets routed to their destinations in a datagram network? • Routing Table • Each switch has a routing table which is based on the destination address. • The routing tables are dynamic and are updated periodically. • The destination addresses corresponding forwarding output recorded in the tables. and the ports are
- 27. Virtual-Circuit Networks • Avirtual-circuit network is a cross between a circuit-switched network and a datagram network 1. As in a circuit-switched network, there are setup and teardown phases in addition to the data transfer phase. 2. Resources can be allocated during the setup phase, as in a circuit-switched network, or on demand, as in a datagram network. 3. As in a datagram network, data are packetized and each packet carries an address in the header. • address has local jurisdiction (it defines what the next switch should be and the channel on which the packet is being carried) 4. As in a circuit-switched network, all packets follow the same path established during the connection (normally implemented in the data-link layer)
- 28. Addressing in Virtualcircuit Networks • Global -A source or a destination needs to have a global address— an address that can be unique in the scope of the network • Local (virtual-circuit identifier) or label-used for data transfer is a small number that has only switch scope; it is used by a frame between two switches • Each switch can use its own unique set of VCIs.
- 29. Phases in Virtualcircuit network • Setup Phase: • the source and destination use their global addresses to help switches make table entries for the connection • Data Transfer Phase: • Teardown Phase: • the source and destination inform the switches to delete the corresponding entry
- 30. Switch and tablesin a virtual-circuit network
- 31. Source-to-destination data transferin a virtual- circuit network
- 32. Virtual Circuit Approach •Preplanned route established before any packets sent • Call request and call accept packets establish connection (handshake) • Each packet contains a virtual circuit identifier instead of destination address • No routing decisions required for each packet • Clear request to drop circuit • Not a dedicated path
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- 34. Virtual Circuits vDatagram • Virtual circuits —Network can provide sequencing and error control —Packets are forwarded more quickly • No routing decisions to make —Less reliable • Loss of a node looses all circuits through that node • Datagram —No call setup phase • Better if few packets —More flexible • Routing can be used to avoid congested parts of the network
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- 38. Cell Switching • CellSwitching is a connection oriented switching technique in which message is transmitted in the form of packets of same size. • Cell switching is similar to packet switching as it operates in a similar way to packet switching but uses small fixed length cells for data transport. • Used in Asynchronous Transfer Mode networks. • Cell switching can handle multiple data types, i.e. voice, video and data. Moreover, cell switching is typically a high bandwidth and high speed technology. • Cell switching is essentially an attempt to combine the best of circuit switching and packet switching.