Unit IIIBandwidth Utilization: Multiplexing and Spectrum Spreading • In practical life the bandwidth available of links is limited.• The proper utilization of these bandwidths is a major challenge in data communication.• Many times we need to combine many low bandwidth channels to form a large bandwidth channel.• Some times we need to expand the BW of channel to achieve goals such as privacy and antijamming.

Unit IIIMultiplexing: • Whenever the bandwidth of the medium connecting two devices is larger than the bandwidth required by the devices we can share the medium among the devices.• Multiplexing is a set of techniques that allows transmission of multiple signals across a single link.• Today with the advancements in the technology we have mediums with higher bandwidth.• If the BW required by the devices is less than the capacity of the link the BW is wasted.• An efficient system utilizes this available bandwidth effectively.

Unit IIIMultiplexing: • In a multiplexed system n lines share the BW of one link.• Figure below shows a multiplexed system

• At the sender inputs from several devices are given to a device called as Multiplexer, which combines them into a single data stream (Many to One)

Unit IIIMultiplexing: • At the receiving end this multiplexed stream is separated by a device called as de-multiplexer (One to Many), which separates single data stream into respective separate data streams.• Here a link refers to a physical path while channel refers to a portion of a link which carries transmission between given pair of devices.• There are three basic multiplexing techniques• Frequency Division Multiplexing (FDM)• Wavelength Division Multiplexing (WDM)• Time division Multiplexing (TDM)

Unit IIIMultiplexing: • The first two methods are analog while the third one is a digital technique.• Frequency Division Multiplexing (FDM): FDM is an analog technique that is used when the BW of the link (In Hertz) is greater than the combined bandwidths of the signal to be transmitted.• In FDM signals generated by each sending device modulate different carrier frequency.• These modulated signals are then combined to form a composite signal that can be transported by the link.• Carrier frequencies are separated by sufficient bandwidths to accommodate the modulated signals.

Unit IIIMultiplexing: • These BW ranges are the channels through which various signals travels.• Channels are separated by unused BW called as guard bands, which avoids overlapping of channels.• Figure shows FDM process

Unit IIIMultiplexing: •

Unit IIISwitching: • A network is a set of devices.• A common problem in computer networks is how to connect multiple devices with each other efficiently.• One solution is to provide a point to point link between all devices. But this solution is costly and will waste the resources.• A better solution is switching. A switched network consist of series of interlinked nodes called as switches. • Switches are devices which are capable of creating temporary connections between two or more devices called as nodes.

Unit IIISwitching: • In a switched network, some of these nods are connected to end systems (Computers). Others are used only for routing.

Unit IIISwitching: • There are traditionally three methods of switching as• Circuit Switching• Packet Switching• Message Switching

• The first two methods are commonly used today.• The switching can happen at several layers of TCP/IP

protocol stack• At physical layer (Only Circuit Switching)• At DLL (Packet / Frame Switching)• At Network Layer ( Packet Switching)• At Application Layer ( Only Message Switching)

Unit IIICircuit Switching: • A circuit switched network consist of a set of switches connected by physical links.• The connection between two devices is using dedicated path of one or more links.• However each connection uses only one dedicated channel on each link.• Each link is normally divided into n channels using FDM or TDM.• Figure shows a circuit switched network

Unit IIICircuit Switching: •

Unit IIICircuit Switching: • Following are some of the characteristics of circuit switching• Circuit switching takes place at physical layer.• Before data communication systems needs to resource reservation such as reserving BW, switch buffers, Switch processing time, switch I/O ports.• These resources must remain dedicated during entire data transfer between two systems.• Data exchanged is not packetized, instead there is a continuous flow of data between two systems.• There is no addressing involved during data transfer. The switches routes the data based on their occupied band FDM or TDM.

Unit IIICircuit Switching: • Three phases of Circuit Switching: The actual communication of circuit switching involved three phases• Connection set up.• Data transfer• Connection termination.

• Set Up Phase: During this phase the end systems establishes a dedicated path / connection between them. It means that during this phase a dedicated channel is created between switches.• One of the two systems sends a connection request to

other system and other system acknowledges the same and then connection is established.

Unit IIICircuit Switching: • During the connection phase all the resources required for the connection are reserved.• Data Transfer Phase: Once the connection is established two devices can exchange the data using dedicated connection.• Connection Termination: Once the communication is over, either of the device terminates a connection and all the resources are relapsed.• The efficiency of circuit switched network is less than the other two methods because of the resource allocation.

Unit IIICircuit Switching: • Although a circuit switched network is les efficient the delay in this type of network is minimal.• During data transfer data are not delayed at each switch since resources are already allocated for the duration of the connection.• Figure shows delay in circuit switched network.• There is no delay at each switch.• The delay is at the time of connection set up, time required for data transfer and connection termination.

Unit IIICircuit Switching: •

Unit IIIPacket Switching: • In a packet switched network the data is divided into packets and are transported across the network.• The size of the packet is determined by network and the governing protocol.• In packet switching there is no resource allocation or reservation.• Resources are allocated on demand. This allocation is done on first come first serve basis.• At each router a packet must wait if other packets are there for processing.• This lack of reservation creates delay.

Unit IIIPacket Switching: • There are two types of packet switched networks• Datagram Networks• Virtual Circuit Networks.

• Datagram Networks: In a datagram network each packet is treated independently. • Packets in this network are called as datagram.• Datagram switching is normally done at physical

layer.• Figure shows the concept of datagram network.

Unit IIIPacket Switching: •

Unit IIIPacket Switching: • As shown four packets of the same message may reach to the destination using different path.• Because of this the packets of the same message may arrive at the receiver out of order with different delays between them.• Packet my be droppd or lost due to lack of resources.• In most of the protocols it is the duty of higher layer protocols to reorder the datagram and ask for lost datagram's.

Unit IIIPacket Switching: • As shown four packets of the same message may reach to the destination using different path.• Because of this the packets of the same message may arrive at the receiver out of order with different delays between them.• Packet my be dropped or lost due to lack of resources.• In most of the protocols it is the duty of higher layer protocols to reorder the datagram and ask for lost datagram's.• This network is sometimes called as connectionless network because the switch does not keep the information about the connection state.• Each packet is treated independently regardless of other.

Unit IIIPacket Switching: • Since there are no connection setup or termination phase, some mechanism is required to route the packet to the correct destination.• To do this in datagram network each router maintains a routing table which has a entry of destination address.• The routing tables are dynamic and are updated periodically. • The destination address and corresponding output port are recorded in table.

Unit IIIPacket Switching: • Figure shows a routing table

Unit IIIPacket Switching: • Each packet in a datagram network in its header carries the destination address of the receiver.• Using this address routing decisions are taken by routers.• This address remains same during the entire journey of a packet.• The efficiency of datagram networks is better than that of circuit switched networks because the resources are not allocated well in advance.• There may be greater delay in a datagram networks than VC networks.

Unit IIIPacket Switching: • Since there are no setup and termination phases each packet encounters a variable delay at switch before it can be forwarded.• Figure shows delay in datagram networks.

Unit IIIPacket Switching: • Virtual Circuit Networks: The virtual circuit networks is a combination of circuit switching and datagram networks.• Following are some of the characteristics of VC networks• As in circuit switching there is set up and termination phase along with data transfer phase.• Resources can be allocated during setup or on demand.• As in datagram network data are packetized and each packet carries address in the header of a packet. However the address mentioned in header is not end to end address. The address is of the next switch for a packet.

Unit IIIPacket Switching: • As in circuit switching all packets follow the same path.• A VC network is usually implemented in DLL.• Figure shows a VC network

Unit IIIPacket Switching: • In virtual circuit network two types of addressing is used i.e. Global and Local• Global Addressing: A global address is a address which remains same through out the scope of the network. In VC networks a global address is used to create a virtual circuit identifier.• The identifier which is actually used to transfer the data is called as Virtual Circuit Identifier (Local Address).• A VCI is a small number and which has scope between only two switches.• When a frame arrives at a switch it has one VCI and when it leaves the switch it has different VCI.

Unit IIIPacket Switching: • In the set up phase source and destination use their global address to set up a connection and helps the switches to make appropriate entries.• In the data transfer phase data is exchanged between two systems.• In the connection termination phase source and destination inform the switches to delete the corresponding entries. • Data Transfer Phase: To transfer the data from source to destination a switch need to have a entry in its table. Using this entry a switch forwards a packet to the appropriate port number.

Unit IIIPacket Switching: • Figure shows the entries in a switch for virtual circuits passing through it

Unit IIIPacket Switching: • As shown a frame arrives on port number 1 with a VCI 14. Switch looks for a entry in a table and understands that it should be forwarded at port number 3 with a VCI of 22.• Thus VCI of a frame changes at each switch.• Figure shows the total journey of a frame from source and destination and how its VCI changes during the trip.

Unit IIIPacket Switching: •

Unit IIIPacket Switching: • Set Up Phase: In the set up phase a switch creates an entry in its table. In the set up phase there are two actions i.e. Request and acknowledgement.• Figure shows how a what happens in set up request between the two systems A and B.• A set up request is received by switch 1 on port 1. Switch 1 knows that a frame received from A and whose destination is B must be forwarded on port no 3.• So it forwards frame to port no 3 towards switch no 2.• In its routing table it makes entry for port 1 and VCI of 14. It assigns port number 3 to this entry but it does not assigns a VCI to it at this moment.• The same procedure is repeated at each switch for this journey from system A to B.

Unit IIIPacket Switching: •

Unit IIIPacket Switching: • When the set up request frame reaches to system B and it accepts the connection request it sends back an acknowledgement frame back to the system A.• During the return journey of this acknowledgement frame the last entry in the routing table of each switch is filled.• For ex. If system B assigns a VCI of 77 to the connection accepted from system A. When this acknowledgement frame arrives at switch 3, switch 3 updates entry in its routing table and assigns a VCI of 77 to this connection.

Unit IIIPacket Switching: •

Unit IIIPacket Switching: • Connection Termination: In this phase either of the system sends a connection termination request and during the journey of this frame from source to destination switches deletes all the corresponding entries made for that connection.• The efficiency of VC network depends upon how resources are allocated i.e. during set up or on demand.• If resources are allocated during set up all packets follow same path and if allotted on demand each packet experiences a variable delay.• One big advantage of VC is that even though resources are allocated on demand, the source can check the availability of resources without actually reserving it.

Unit IIIPacket Switching: • The delay in a VC network is encountered during VC set up and termination. If resources are allocated during set up there is no delay at switches.