Circuit Switching and Packet Switching Switching Networks Long

Circuit Switching and Packet Switching

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. • Each station attaches to a node, and collection of nodes is a communications network

Switched network

Switched networks

CIRCUIT-SWITCHED NETWORKS It consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

Figure 8. 3 A trivial circuit-switched network

Three phases • Setup phase • Data transfer • Teardown phase

Three phases End system A needs to communicate with M, A needs to request a connection to M that must be accepted by M, is called setup phase The dedicated path made of connected circuits is established, data transfer can take place Teardown phase: When one of the parties needs to disconnected, a signal is sent to each switch to release the resources

Note In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase.

Example 8. 1 As a trivial example, let us use a circuit-switched network to connect eight telephones in a small area. Communication is through 4 -k. Hz 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 k. Hz. 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.

Circuit-switched network used in Example 8. 1 Figure 8. 4

Efficiency • Not as efficient as the other types • Resources are allocated during the entire duration of the connection • These resources are unavailable to other connections • Example: In telephone network, terminate they finished their communication when conservation • In computer n/w: computers are connected even if there is no activity for a long time

Delay in a circuit-switched network

Circuit-Switched Telephony Traditional PSTN Approach Signaling Class 5 Switch Typically analog “loop”, SS 7 Signaling Network Class 4 Switch Circuit-based Trunks voickb/s digital 64 Media e stream Data travels over a parallel (but separate) network Class 5 Switch

Data communication We need to send messages from one end system to another If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size The size of the packet is determined by the network and the governing protocol.

Packet switching • No resource allocation for a packet —It means no reserved BW & no scheduled processing time • Resources are allocated on demand —Allocation is done on a first come, first serve • When a switch receives a packet, no matter what is the source or destination —Packet must wait if there are other packets being processed

A datagram network. Figure with four switches (routers) 8. 7

DATAGRAM NETWORKS • Here, each packet is treated independently of all others. Even if a packet is part of multipacket transmission • It is normally done at the network layer • Packet travel different paths to reach their destination • Packets arrive at their destination out of order with different delays • Packetmay also be lost or dropped because of a lack of resources • It—It is a means connectionless network the switch does not keep info about the connection state —No setup or teardown phases

Routing table in a datagram network • Each switch has a routing table which is based on the destination address • Dynamic & updated periodically • Destination address and forwarding output ports are recorded in tables

Note A switch in a datagram network uses a routing table that is based on the destination address.

Note The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet. Efficiency is better than ckt switched network; resources are allocated only when there are packets to be transferred

Delay in a datagram network

VIRTUAL-CIRCUIT NETWORKS A virtual-circuit network is a cross between a circuit-switched network and a datagram network. It has some characteristics of both. There are setup, teardown, and data transfer phase like ckt Resources can be allocated during the setup phase, as in a ckt switched n/w, or on demand, as in a datagram In datagram, data are packetized and eack packet carries an address in the header In ckt switched n/w, all packets follow the same path established during the connection A virtual-ckt is implemented in data link layer, a ckt switch n/w in physical, datagram in network layer

Figure 8. 10 Virtual-circuit network

Addressing • Two types of address are involved: global and local Global Addressing • Source or destination needs to have global address —That can be Unique • Global address in virtual ckt is used only to create a virtual ckt identifier Virtual ckt identifier (VCI) • Identifier that is actually used for data transfer • A VCI, unlike a global, is a small number that has only switch scope • It is used by a frame between two switches • When a frame arrives at a switch, it has a VCI, when it leaves it has a different VCI

Virtual-circuit identifier Figure 8. 11

Data transfer phase • Frame from S to D , all switches need to have a table entry for this virtual ckt • It is active untill data transfer completes

Switch and tables in a virtual-circuit network

Source-to-destination data transfer in a virtual-circuit network

Setup phase • Source A needs to create a virtual ckt to B • 2 steps are required: —setup request —Acknowledgment

Figure 8. 14 Setup request in a virtual-circuit network

Figure 8. 15 Setup acknowledgment in a virtual-circuit network

Note In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packetsmay arrive at the destination with different delays if resource allocation is on demand.

Figure 8. 16 Delay in a virtual-circuit network

Note Switching at the data link layer in a switched WAN is normally implemented by using virtual-circuit techniques.