1© Jörg Liebeherr, 1998,1999CS457Communication Networks© Jörg Liebeherr, 1998,1999CS457• Problem: Given a set of devices that want toexchange information. (Device = telephone, computer, terminals, etc.)• Simple Solution: Connect each pair of devices by adedicated point-to-point link– The simple solution is sufficient if the number ofdevices is small.Communication Networks2© Jörg Liebeherr, 1998,1999CS457• With a large number of devices it is not practical to connecteach pair of devicesCommunication Networks© Jörg Liebeherr, 1998,1999CS457• A communication network provides a general solution to theproblem of connecting many devices:– Connect each device to a network node– Network nodes exchange information and carry theinformation from a source device to a destination device– Note: Network nodes do not generate informationCommunication Networks3© Jörg Liebeherr, 1998,1999CS457• A generic communication network:Communication NetworksOther names for “device”: station, host, terminalOther names for “node”: switch, router, gatewayCommunicationNetworkDeviceNode© Jörg Liebeherr, 1998,1999CS457Example: vBNS NetworkSource: National Science Foundation4© Jörg Liebeherr, 1998,1999CS457Example: vBNS more DetailedSource: National Science Foundation© Jörg Liebeherr, 1998,1999CS457Example: Campus Network of Univ. of VirginiaSource: University of Virginia5© Jörg Liebeherr, 1998,1999CS457• Communication networks can be classified based onthe way in which the nodes exchange information:Classification of CommunicationCommunicationNetworkSwitchedCommunicationNetworkBroadcastCommunicationNetworkCircuit-SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagram NetworkVirtual CircuitNetwork© Jörg Liebeherr, 1998,1999CS457Broadcast Communication Network do not have intermediateswitching nodes:– Each station has a transmitter/receiver that com-municates over a medium shared by other stations– Transmission from any station is received by all otherstationsBroadcast Communication NetworksPacket RadioNetwork:SatelliteNetwork:Bus LocalNetwork:6© Jörg Liebeherr, 1998,1999CS457•A switched communication network consists of aninterconnected collection of nodes. Data are transmitted fromsource to destination by being routed through the nodes•The switching method describes how data are processed androuted in the network•The basic switching methods are:– Circuit Switching– Packet Switching•Datagram Packet Switching•Virtual-Circuit Packet SwitchingSwitched Communication Networks© Jörg Liebeherr, 1998,1999CS457• In a circuit-switched network, a dedicatedcommunication path is established between twostations through the nodes of the network• The dedicated path is called a circuit-switchedconnection or circuit• A circuit occupies a fixed capacity of each link for theentire lifetime of the connection. Capacity unused bythe circuit cannot be used by other circuits• Data is not delayed at the switchesCircuit Switching7© Jörg Liebeherr, 1998,1999CS457• Circuit-switched communication involves three phases:1. Circuit Establishment2. Data Transfer3. Circuit Termination• “Busy Signal” if capacity for a circuit not available.• Most important circuit-switching networks:• Telephone networks• ISDN (Integrated Services Digital Networks)Circuit Switching© Jörg Liebeherr, 1998,1999CS457Circuit Switching• A node in a circuit-switching network:incoming linksNodeoutgoing linksswitching fabriccircuit8© Jörg Liebeherr, 1998,1999CS457Circuit Switching457CBA E1 2D63circuit 2circuit 1© Jörg Liebeherr, 1998,1999CS457Timing in Circuit SwitchingDATA1 2 3 4Circuit Establishment Data Transmission Circuit Termination Propagation delay between Host 1 and Node 2Propagation delay between Host 1 and Host 4Processing delayHost HostNode Node9© Jörg Liebeherr, 1998,1999CS457Packet Switching• Data are sent as formatted bit-sequences, so-called packets.• Packets have the following structure:• Header and Trailer carry control information• Each packet is passed through the network from node to node along somepath (Routing)• At each node the entire packet is received, stored briefly, and thenforwarded to the next node (Store-and-Forward Networks)• No capacity is allocated for packetsHeader Data Trailer© Jörg Liebeherr, 1998,1999CS457Packet Switchingincoming linksnodememoryoutgoing links10© Jörg Liebeherr, 1998,1999CS457Datagram Packet Switching• Packets are called datagrams• The network nodes process each packet independently:If Host A sends two packets back-to-back to Host B over a datagrampacket network, the network cannot tell that the packets belongtogether. In fact, the two packets can take different routes.• Implications of processing packets independently:• A sequence of packets can be received in a differentorder than it was sent• Each packet header must contain the full address of thedestination© Jörg Liebeherr, 1998,1999CS457• Exercise:: Most network applications (think of email and filetransfer) require that data is received in sequence. For suchapplications a datagram network appears to be inappropriate,since packets may need to get reordered. Question:: What are advantages of datagram networks?• The main example of a datagram packet-switching network isthe InternetExercise: Datagram Packet11© Jörg Liebeherr, 1998,1999CS457457CBA E1 2D63A.3A.2C.2A.1C.1A.3A.2A.1C.1C.2Datagram Packet SwitchingA.3A.2A.1C.1C.2A.3A.2C.2A.1C.1A.3A.2A.1C.1C.2A.2A.3A.1A.2A.2A.2C.2A.2A.3A.1A.2A.3A.2A.1© Jörg Liebeherr, 1998,1999CS457Packet 1Packet 2Packet 3Packet 1Packet 2Packet 3Timing of Datagram Packet SwitchingPropagation delay between Host 1 and Node2Packet 1Packet 2Packet 3 1 2 3 4ProcessingdelayTransmissiondelayHost HostNode Node12© Jörg Liebeherr, 1998,1999CS457Virtual-Circuit Packet Switching• As the name suggests:Virtual-circuit packet switching is a hybrid of circuit switchingand packet switching• All data is transmitted as packets• All packets from one packet stream are sent along a pre-established path (=virtual circuit)• Guarantees in-sequence delivery of packets• However: Packets from different virtual circuits may beinterleaved© Jörg Liebeherr, 1998,1999CS457Virtual-Circuit Packet Switching• Communication with virtual circuits (VC) takes place in threephases:1. VC Establishment2. Data Transfer3. VC Disconnect• Note: Packet headers