- Slides: 38
Chapter 14 Switching • Circuit Switching • Packet Switching • Message Switching
SWITCHING • When the distance between devices and the total number of devices increase beyond the capacities of the media and equipment, mesh, bus, star, and ring topologies cannot be readily and feasibly used. • Solution: Switching
• A switched network consists of a series of interlinked nodes called switches. • Switches are devices that are capable of creating temporary connections between two or more devices linked to the switch but not to each other.
Switching Methods Circuit Switching Packet Switching Message Switching
Circuit Switching • Creates a direct physical connection between two devices. • A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and an output link. • Switches can use either space-division or time-division.
Circuit Switching Space-Division Switching Time-Division Switching
Space-Division Switches • The paths are separated from each other spatially. • The most commonly used is crossbar switch. • It uses electronic switching device ate each crosspoint. • Limitation: number of crosspoints is n m • Solution: Multistage Switches. • Problem: Blocking: an input cannot be connected to an output because there is no path available between them – all other paths are busy.
Time-Division Switches • Slots are divided by time instead of space (TDM). • Time-Slot Interchange (TSI) is a device that changes the ordering of slots based on the desired connection. • Space-division switch: Pros: switching is instantaneous. Cons: number of crosspoints • Time-division switch: Pros: No crosspoints are needed. Cons: Delays. • Combine both in a multistage switch: time-spacetime (TST), STTS, TSST, … etc.
TDM without TSI
TDM with TSI
PACKET SWITCHING • Circuit switching wastes the data link; data transmission tends to be bursty. • In circuit switching, both source and destination must have the same data rate. • Circuit switching is inflexible: Once the circuit is established, all parts of the transmission take the same path. • Circuit switching sees all transmissions as equal, no priority.
• Packet Switching: Data is transmitted in discrete units of potentially variable length blocks called packets. • Each packet contains a header that carries control information. • At each node, the packet is stored and then routed according to the information in its header.
Packet Switching Datagram Approach Virtual circuit Approach
Datagram Approach • Each packet is treated (routed) independently from all others. • Packets are referred to as datagrams. • Datagrams may arrive at destination out of order. • It is the responsibility of the transport layer to reorder them. • A link can contain multiple channels using TDM or FDM.
Datagram Approach, Multiple Channels
Virtual Circuit Approach • All packets that belonging to the same message follow the same path (route). • In virtual circuit approach multiplexing is done at the switches. • In circuit switching no multiplexing is done at the switches. • There are two formats for VC: Switched Virtual Circuit (SVC) and Permanent Virtual Circuit (PVC).
Switched Virtual Circuit (SVC) • A virtual circuit is created whenever it is needed and exists only for the duration of the transfer. • Three phases: Connection establishment, Data transfer, and Connection release. • Each time a station wishes to communicate with another station, a VC is established with similar or different routes. • SVC provides the reliability of circuit switching with the flexibility of datagram.
Switched Virtual Circuit
Switched Virtual Circuit
Switched Virtual Circuit
Permanent Virtual Circuit (PVC) • Comparable to leased lines. • The same virtual circuit is provided between two users on a continuous basis. • Dedicated to a specific user. • No connection establishment and connection termination is needed.
Message Switching • A node receives a message, stores it until the appropriate route is free, then sends it along (store and forward). • In message switching, the message is stored in a secondary storage (disk), while in packet switching, packets are stored and forwarded from RAM. • Was common in 60 s and 70 s.
Network Layer • Responsible for establishing, managing, and terminating connections between networks. • Protocols at layer provide transparent routing and relaying services between networks. • Network layer services: connection-oriented network services (CONS) and connectionless network services (CLNS).
Connection-Oriented Network Services (CONS) • Establishes a virtual circuit for the transmission of data for the entire transmission. • Process: - Sender transmit a connection-request packet. - Receiver acknowledges with a connection- confirm packet. - Sender transmit data (repeatedly) - Sender transmit a disconnect-request packet. - Receiver acknowledges with a disconnect-confirm packet.
• Advantages: - Allows a comprehensive sequence, error, and flow control. - Needs less protocol control information (PCI) on packets. (packets follow the same path). • Disadvantages: - No routing flexibility after connection. - Packets must be checked and acknowledged or retransmitted.
Connectionless Network Service • • Each packet is routed independently. No connection establishment or termination. Advantages: Speed and cost. Disadvantages: - If reliability is an issue or sequencing connectionless routing is inadequate. - Each packet must carry complete PCI (overhead). • Connectionless or connection-oriented transmission can occur at three layers (Data link, Network, and Transport).