Circuit and Packet Switching Methods Presentation
- 1. Switching Networks • Long distance 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 by being switched from node to node
- 2. Nodes • Nodes may connect 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 • Two different switching technologies —Circuit switching —Packet switching
- 4. Circuit Switching • Dedicated communication 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
- 5. Circuit Switching - Applications • Inefficient —Channel capacity dedicated for duration of connection —If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone)
- 7. Telecomms Components • Subscriber — Devices attached to network • Subscriber line — Local Loop — Subscriber loop — Connection to network — Few km up to few tens of km • Exchange — Switching centers — End office - supports subscribers • Trunks — Branches between exchanges — Multiplexed
- 9. 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
- 11. Packet Switching Principles • 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
- 12. Basic Operation • 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 —Store and forward
- 13. Use of Packets
- 14. Advantages • Line efficiency — Single node 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
- 15. Switching Technique • Station breaks long message into packets • Packets sent one at a time to the network • Packets handled in two ways —Datagram —Virtual circuit
- 16. Datagram • Each packet 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
- 17. Datagram Diagram
- 18. Virtual Circuit • 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
- 20. Virtual Circuits v Datagram • 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
- 21. Packet Size
- 22. Circuit v Packet Switching • Performance —Propagation delay —Transmission time —Node delay
- 23. Event Timing
- 24. Virtual Circuits
- 25. Virtual Circuit Service • Logical connection between two stations —External virtual circuit • Specific preplanned route through network —Internal virtual circuit • Typically one to one relationship between external and internal virtual circuits • Can employ X.25 with datagram style network • External virtual circuits require logical channel —All data considered part of stream