Computer Networks II- Lecture notes Unit 1 Packet Switching Networks UNIT I: - Packet Switching Networks What is a Computer Network? • • • Communication Networks: “Sets of nodes that are interconnected to allow the exchange of information such as voice, sound, graphics, pictures, video, text, data, etc…” Telephone Networks: “ The first well established and most widely used communication networks which are used for voice transmission” – Telephone networks originally used analog transmission as a transmission technology for the information. However, digital transmission started to evolve replacing a lot of the analog transmission techniques used in telephone networks. Computer Networks: “Collection of autonomous computers interconnected by a technology to allow exchange of information” A network is a series of connected devices. Whenever we have many devices, the interconnection between them becomes more difficult as the number of devices increases . Some of the conventional ways of interconnecting devices are a. Point to point connection between devices as in mesh topology. b. Connection between central device and every other device – as in star topology c. Bus topology-not practical if the devices are at greater distances. The solution to this interconnectivity problem is switching. A switched network consists of a series of interlinked nodes called switches. A switch is a device that creates temporary connections between two or more systems. Some of the switches are connected to end systems (computers and telephones) and others are used only for routing. Taxonomy of switched networks Circuit switching Traditional telephone networks operate on the basis of circuit switching In conventional telephone networks, a circuit between two users must be established for a communication to occur Circuit switched networks requires resources to be reserved for each pair of end users The resources allocated to a call cannot be used by others for the duration of the call
Computer Networks II- Lecture notes Unit 1 Packet Switching Networks The reservation of the network resources for each user results in an inefficient use of bandwidth for applications in which information transfer is bursty or if the information is small Packet Switching Packet switched networks are the building blocks of computer communication systems in which data units known as packets flow across the networks. It provides flexible communication in handling all kinds of connections for a wide range of applications e.g. telephone calls, video conferencing, distributed data processing etc... Packet switched networks with a unified, integrated data infrastructure known as the Internet can provide a variety of communication services requiring different bandwidths. To make efficient use of available resources, packet switched networks dynamically allocate resources only when required. The form of information in packet switched networks is always digital bits. Differences between Circuit Switching and Packet Switching Circuit switching Packet switching 1. Call set up is required. 2. Dedicated connection between two Hosts. 3. Connection/Communication is lost, if any link in the path between the Hosts is broken. 1. Call setup is not required. 2. No dedicated connection between two Hosts. 3. Connection/Communication could continue between the Hosts since data have many routes between the Hosts. 4. Information could take different routes to reach the destination Host. 5. Information could arrive out of order to the destination 6. Bandwidth available is variable. 7. Congestion is packet based. 8. Bandwidth utilization is full. 9. It uses store-and forward transmission. 10. Not transparent. 11. Charging is packet based. 4. Information take the same route between the connected Hosts 5. Information always arrives in order. 6. Bandwidth available is fixed. 7. Congestion is call based. 8. Bandwidth utilization is partial. 9. It does not uses store-andforward transmission. 10. It is Transparent. 11. Charging is time based. Packet networks can be viewed from two perspectives: External view of network :- It is Concerned with the services that the network provides to the transport layer Internal operation of the network. Network services and internal network operation Essential function of network: The essential function of network is to transfer information among the users that are attached to the network. Transfer of information may be single block of information or sequence of blocks as shown in below figure.
Computer Networks II- Lecture notes Unit 1 Packet Switching Networks In case of single block of information, we are interested in having the block delivered correctly to destination and also interested in delay experienced in traversing the network. In case of sequence of blocks, we are interested not only in receiving the blocks correctly and in right sequence. Network service can be Connection-oriented service or connectionless service Connectionless service: Connectionless service is simple with two basic interactions (1) a request to network layer that it send a packet (2) an indication from the network layer that a packet has arrived It puts total responsibility of error control, sequencing and flow control on the end system transport layer Connection-oriented service The Transport layer can not request transmission of information until a connection is established between end systems Network layer must be informed about the new flow Network layer maintains state information about the flows it is handling During connection set up, parameters related to usage and quality of services may be negotiated and network resources may be allocated Connection release procedure may be required to terminate the connection It is also possible for a network layer to provide a choice of services to the user of network like: best-effort connectionless services Low delay connectionless services Connection oriented reliable stream services Connection oriented transfer of packets with guaranteed delay and bandwidth End To End argument for system design The end to end argument in system design state that an end-to-end function is best implemented at higher level than at a lower level. The reason is that the correct end-to-end implementation requires all intermediate low-level components to operate correctly. The higher-level components at the ends are in better position to determine that a function has been carried out correctly and in better position to take corrective action if they have not. Keeping the core of the network simple and adding the necessary complexity at the edge enhances the scalability of the network to larger size and scope Internal network operation
Computer Networks II- Lecture notes Unit 1 Packet Switching Networks The fig above shows the relation between the service offered by the network and the internal network operation The internal operation of the network is connectionless if packets are transferred within the network as datagrams Each packets are routed independently Packets follow different paths from end to end and arrive out of order The internal operation of the network is connection-oriented if packets follow a virtual circuit along a path that has been established from source to destination. Virtual circuit setup is done once, then packets are simply forwarded If resources are reserved then bandwidth, delay and loss guarantees are provided. Network layer essentials The Functions that need to be carried out at every node in the Network Layer are: Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements Forwarding: transfer of packets from NE inputs to outputs Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security Packet Network Topology How users access packet networks?