1. Circuit- Switched Networks2. Datagram Networks3. Virtual- Circuit NetworksR. R. Malekar Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. Quite difficult using Star, Bus, mesh. A better solution is switching.2 R. R. MalekarSwitchingSwitched network: Series of interlinked nodes, called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch.NoteSwitching Switching3 R. R. MalekarSwitchingEnd Device: a, b, c, so onSwitches4 R. R. MalekarSwitching5 R. R. MalekarSwitching A circuit-switched network is made of a set of switches connected by physical links. In whicheach link is divided into nchannels by using FDM or TDM.6 R. R. MalekarSwitching Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link (physical path).N lines share the bandwidth of one link 7 R. R. MalekarSwitching8 R. R. MalekarSwitchingIn FDMA, the available bandwidth of the common channel is divided into bands that are separated by guard bands. Frequency- division multiple (FDM) 9 R. R. MalekarSwitchingThe bandwidth is just one that is timeshared between different stations Time-division multiple access (TDMA) 10 R. R. MalekarSwitchingThree phases are need to communicate two parties or multiple parties in a conference call): Connection setup (setup phase) data transfer Connection teardown. A trivial circuit- switched network 11 R. R. MalekarSwitchingA. The setup phase: means creating dedicated channels between the switches. Example : when system A needs to connect to system M: 1. A sends a setup request that includes the address of system M, to switch I. Switch I finds a channel between itself and switch IV. Switch I then sends the request to switch IV,which finds a dedicated channel between itself and switch III. Switch III informs system M of system A's intention at this time. 2. An acknowledgment from system M needs to be sent in theopposite direction to system A. 3. After system A receives this acknowledgment the connection established. Three Phases12 R. R. MalekarSwitching13 R. R. MalekarSwitchingIn 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. Note14 R. R. MalekarSwitchingB. Data Transfer Phase: After the establishment of the dedicated circuit (channels), the two parties can transfer data. C. Teardown Phase: When one of the parties needs to disconnect, a signal is sent to each switch to release the resources. circuit-switched network 15 R. R. MalekarSwitchingAs a trivial example, 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. 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 1 Example 116 R. R. MalekarSwitchingTelephone 1 is connected to telephone 7; 3 --- 8; and 2 -- 5; 4 -- 6. The situation may change when new connections are made. The switch controls the connections. Each link use FDMABandwidth of each link is then 8 kHz17 R. R. MalekarSwitchingSwitching at the physical layer in the traditional telephone network uses the circuit-switching approach. . Note18 R. R. MalekarSwitching Switching take place at physical layer ResourcesReserved Such as bandwidth in FDM and time slot in TDM Switch buffer Switch processing time Switch I/O ports Data transferred are not packetized, continuous flow No addressing involved during data transfer19 R. R. MalekarSwitching1. Circuit switching takes place at the physical layer. 2. Before starting communication, the stations must make a 3. reservation for the resources to be used during the communication. (such as channels bandwidth in FDM and time slots in TDM) 4. input/output ports, must remain dedicated during the entire duration of data transfer until the teardown phase. 5. Data transferred between the two stations are not packetized(physical layer transfer of the signal). The data are a continuousflow sent by the source station and received by the destinationstation, although there may be periods of silence. 6. There is no addressing involved during data transfer. Of course,there is end-to-end addressing used during the setup phase. 20 R. R. MalekarSwitchingExample 2As another example, consider a circuit-switchednetwork that connectscomputers intworemoteofficesof a private company. The offices are connectedusing a T-l line leased froma communication serviceprovider. Therearetwo 4 X 8 (4 inputs and 8 outputs)switches inthis network. For eachswitch, four outputports are folded into the input ports to allowcommunication between computers in the sameoffice.Four other output ports allow communication betweenthetwooffices.21 R. R. MalekarSwitching22 R. R. MalekarSwitchingtwo 4 X 8 (4 inputs and 8 outputs) switches circuit-switched networks are not as efficient as the other two types of networks because resourcesare allocatedduring the entire duration of the connection. These resources are unavailable to other connections.In a telephone network, people normally terminatesthe communication when they have finished their conversation. However, in computer networks, a computer can be connected to another computer even if there is noactivity for a long time. In this case, allowing resources to be dedicated means that other connections are deprived. 23 R. R. MalekarSwitching Although a circuit-switched network normally has low efficiency, the delayin this type of network is minimal. During data transfer the data are not delayed at each switch; the resources are allocated for the duration of the connection. The total delay = time needed to create the connection, transfer data, and disconnect the circuit.R. R. MalekarSwitching 24 Three phases Setup phase, data transfer phase, teardown phase Delay in a circuit- switched network25 R. R. MalekarSwitching In data communications, 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. Thesize of the packet isdetermined by the network and the governing protocol.26 R. R. MalekarSwitchingIn a packet-switched network, there is no resource reservation; Resources are allocated on demand. Note The allocation is done first come, first served When a switch receives a packets , the packet must wait if there are other packets being processed, this lack of reservation may create delay. e.g.27 R. R. MalekarSwitching1. Each packet is treated independently of all others. Even if a packet is part of a the same message. 2. Packets in this approach are referred to as datagrams.3. All packets (or datagrams) belong to the same message may travel different paths to reach their destination. 4. This is so because the links may be involved in carrying packets from other sources.Datagram Networks Datagram Networks28 R. R. MalekarSwitching In a datagram network, each packet is treated independently of all others. Referred to as connectionless networks.8-29 R. R. MalekarSwitchingOut of order Different delays Switching --- Network layerlost or dropped because of a lack of resources.Upper-layer protocol to reorder the datagrams.No setup or teardown phases.30 R. R. MalekarSwitchingR. R. MalekarSwitching 31 Datagram Switching is done at the network layer. This approach can cause the datagramsof a transmission toarrive at their destination out of order with different delays between the packets. Packets may also be lost or dropped because of a lack ofresources. In most protocols, it is the responsibility of an upper-layerprotocol to reorder the datagramsor ask for lost datagramsbefore passing them on to the application. The datagram networks are referred to connectionlessnetworks. There are no setup or teardown phases. How are the packets routed to their destination?? Datagram Networks Datagram Networks32 R. R. MalekarSwitching Each packet switch has a routing table which is based on the destination address. The routing tables are dynamic and are updated periodically. The destination addresses and the corresponding forwarding output ports are recorded in the tables. This is different from the table of a circuit switched network in which each entry is created when the setup phase is completed and deleted when the teardownphase is over. R. R. MalekarSwitching 3334 R. R. MalekarSwitchingRemains the same during the entire journey of the packetA switch uses ---- routing table ---based on ---the destination addressDestination addresses ---and the corresponding forwarding ---Output ports Every packet in a datagram network carries a header that contains, among other information, the destination address of the packet. When the switch receives the packet, this destination address is examined; the routing table is consulted to find the corresponding port through which the packet should be forwarded. This address remains the same during the entire journey of the packet. R. R. MalekarSwitching 35 The efficiency of a datagram network is better than that of a circuit-switched network. Resources are allocated only when there are packets to be transferred. If a source sends a packet and there is a delay of a few minutes before another packet can be sent, the resources can be reallocated during these minutes for other packets from other sources. R. R. MalekarSwitching 36Delay in a Datagram Network Datagram network may have greater delay. Delay is not uniform.Note37 R. R. MalekarSwitching38R. R. MalekarSwitchingTotal delay =3T + 3 + WI + W2 T: transmission times : propagation delays(slopes) (WI + w2) : waiting times Ignore the processing time in each switch Switching in the Internet is done by using the datagram approach to packet switching at the network layer. Note39 R. R. MalekarSwitchingR. R. MalekarSwitching 40A virtual-circuit network is a cross between a circuit-switched network and a datagram network.. It has some characteristics of both.. R. R. MalekarSwitching8-41 A source and destination need to go through three phases: setup, data transfer, and teardown. In the setup phase, the source and destination use their global addresses to help switches make table entries for the connection. In the teardown phase, the source and destination inform the switches to delete the corresponding entry. Data transfer occurs between these two phases In a virtual-circuit network, twotypes of addressing are involved: 1. Global and 2. Local (virtual-circuit identifier). Global Addressing An address that can be unique in the scope of the network or internationally if the network is part of an international network. A global address in virtual-circuit networks is used only to create avirtual-circuit identifier .R. R. MalekarSwitching 4243 R. R. MalekarSwitchingLocal Address Virtual- circuit identifierThe identifier that is actually used for data transfer (VCI). A VCI, unlike a global address, is a small . VCI 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 VCl. A VCI does not need to be a large number since each switch can use its own unique set ofVCls. 44 R. R. MalekarSwitching45 R. R. MalekarSwitchingAll switches need to have a table entry for this virtual circuit. The table, has four columns for each virtual circuit that is already set up. R. R. MalekarSwitching 46Setup Phase: In the setup phase, a switch creates an entry for a virtualcircuit. Two steps are required: 1. the setup request 2. the acknowledgment Teardown Phase: In this phase, source A, after sending all frames to B, sends a special frame called a teardown request. Destination B responds with a teardown confirmation frame. All switches delete the corresponding entry from their tables. Virtual-circuit network 47 R. R. MalekarSwitchingNote: The switch, in the setup phase acts as a packet switch ; It has a routing table used to know the output portnumber.When Destination B receives the up frame , and it is ready to receive frames from A, it assign a VCI (in this case :77) . This VCI lets the destination know that the frames comes from A not other sources .Note48 R. R. MalekarSwitching49 R. R. MalekarSwitchingR. R. MalekarSwitching8-50 In this phase, source A, after sending all frames to B, sends a special frame called a teardown request. Destination B responds with a teardown confirmation frame. All switches delete the corresponding entry from their tables. Teardown Phase In virtual-circuit switching, all packets belonging to the same source and destination travel the same path; but the packets may arrive at the destination with different delays if resource allocation is on demand. Note51 R. R. MalekarSwitching52 R. R. MalekarSwitchingTotal delay = 3T+ 3 +Setup delay + teardown delay R. R. MalekarSwitching 53 As in a circuit-switched network, 1.setup 2.teardown phases 3.data transfer phase. Resources can be allocated during the setup phase, as in a or on demand, as in a datagram network. As in a datagram network, data are packetized and eachpacket carries an address in the header VCI.. All packets follow the same path established during theconnection as in circuit switch network. A virtual-circuit network is implemented in the data link layer, A circuit-switched network is implemented in the physical layer and a datagram network in the network layer. R. R. MalekarSwitching 54Efficiency : In datagram network, resources are allocated only when there are packets to be transferred. In virtual-circuit network, all packets belonging to the same source and destination travel the same path; Delay: In datagram network, each packet may experience a wait at a switch before it is forwarded. In addition, the delay is not uniform for the packets of a message. In a virtual-circuit network, there is a one-time delay for setup and a one-time delay for teardown. If resources are allocated during the setup phase, there is no wait time for individual packets. Datagram VS. Virtual circuit networks