CS6551 COMPUTER NETWORKS UNIT I FUNDAMENTALS & LINK LAYER Building a network — Requirements - Layering and protocols - Internet Architecture — Network Software — Performance ; Link layer Services - Framing - Error Detection - Flow control. Computer network, Computer network is a collection of computers and devices connected by communications channel that facilitates communications among users and allows users to share resources with other users. INTRODUCTION Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The effectiveness of a data communication depends on three characteristics 1. Delivery 2. Accuracy 3. Timeliness Delivery: The system must deliver data to correct destination. Accuracy: The system must deliver data accurately. Timeliness: The system must deliver data in a timely manner. Components of data communication ule , | Rule 2: fie cus | Rule n: Receiver Medium * Source: It is the transmitter of data. Some examples are Terminal, Computer, and Mainframe. Medium: The communications stream through which the data is being transmitted. Some examples are: Cabling, Microwave, Fiber optics, Radio Frequencies (RF), Infrared Wireless Receiver: The receiver of the data transmitted. Some examples are Printer, Terminal, Mainframe, and ComputerMessage: It is the data that is being transmitted from the Source/Sender to the Destination/Receiver. Protocol: It is steps that are to be followed for sending the data from the data and as well as interpreting the data received. « DCE: The interface between the Source & the Medium, and the Medium & the Receiver is called the DCE (Data Communication Equipment) and is a physical piece of equipment. DTE: Data Terminal Equipment is the Telecommunication name given to the Source and Receiver's equipment. 1 1 t Figure 1.2: DTE and DCE, Direction of Data Flow Data flow is the flow of data between 2 points. The direction of the data flow can be described as 1. Simplex 2. Half-Duplex 3. Full-duplex. Simplex: In this type of data communication the data flows in only one direction on the data communication line (medium). Examples are Keyboard, Monitor, Radio and Television broadcasts SaeeREenne eRe (One dicestio Tatovision Half-Duple: In this type of data communication the data flows in both directions but at a time in only one direction on the data communication line. Example Conversation on walkie-talkies is a half-duplex data flow. oe fon (CX, Sip Figure ; Haif-Duplex Data FlowFull-Duplex: In this type of data communication the data flows in both directions simultaneously. Example Telephones and Modems = = Ce = eS r= Figure: Full-Duplex Data Flow Different Communication Modes 1) Unicast - one to one Unicast packets are sent from host to host. The communication is from a single host to another single host. 2) Broadcast — one to all Broadcast is when a single device is transmitting a message to all other devices in a given address range. 3) Multicast - one to many Multicast is a special protocol for use with IP. Multicast enables a single device to communicate with a specific set of hosts, not defined by any standard IP address and mask combination. Categories of networks The three primary categories are of network are Local Area Network (LAN), Metropolitan Area Network (MAN), and Wide Area Network (WAN). The category into which a network fall is determined by its size, ownership, the distance it covers and its physical architecture. LAN « A LAN is usually privately owned and links the devices in a single office, building or campus. « A LAN can be as simple as two PCs or it can extend throughout a company. LAN size is limited to a few kilometers. e The most widely used LAN system is the Ethernet system developed by the Xerox Corporation. : Marketing department Finance Departmen: _ Bridge Figure 1.8: Local Area NetworkMAN A MAN is designed to extend over an entire city. It could be a single network such as cable TV network or connect a number of LANs into a larger network A MAN can be owned by a private company or it may be a service provided by a public company, such as local telephone company. Telephone companies provide a popular MAN service called (SMDS) Switched Multi-megabit Data Services. Figure : Metropolitan Area Network * A WAN provides long distance transmission of data, voice, image and video information over large geographic areas. * Transmission rates are typically 2 Mbps, 34 Mbps, 45 Mbps, 155 Mbps and 625 Mbps. WAN utilize public, leased, or private communication equipment usually in combinations and therefore span an unlimited number of miles. A WAN that is wholly owned and used by a single company is referred to as an Enterprise Network. Figure 1.10: Wide Area Network: es of Connections / Two types are 1. point to pint 2. multipoint Point to Point ° It provides a dedicated link between two devices. ¢ The entire capacity of the link is reserved for transmission between these two devices.* we can connect two devices by means of a pair of wires or using a microwave or satellite link © eg: computers connected by telephone line PPPcobbection between remote and TV Figure : Point-to-point link Multipoint «It is a connection in which more than two specific devices share a single link. «In this environment a single channel is shared, either spatially or temporally. If several devices can use the link at the same time it said to be spatially shared. «If the devices take turn to use the link then it is referred to as timesharing. Wertotation ——-Worfetation Gm Works Figure; Multipoint link ‘opology, Physical Topology refers to the way in which network is laid out physically. Two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and the linking devices tone another. The basic topologies are: *° Mesh Star Bus Ring Hybrid (combination of other types ) aMesh « In a mesh topology each device has a dedicated point to point link to every other device. « The term dedicated means that the link carries traffic only between the two devices it connects. * A fully connected mesh network therefore has n (n-1)/2 physical channels to link n devices. To accommodate that many links every device on the network should have (a-1) 1/0 ports. Merits © Eliminates the traffic problems that occur when the links are shared by multiple devices. If one link becomes unusable, it does not incapacitate the entire system. * Since every message travels along a dedicated line only the intended recipient will receive the message and hence the data is secure. Demerits * The amount of cabling and the I/O ports required increases with the number of devices connected in the network + Installation and reconnection are difficult + The sheer bulk of the wire accommodates more space than available. * The hardware required to connect each link can be prohibitively expensive. Star * Each device has a dedicated point to point link only to a central controller usually called a hub. If one device has to send data to another it sends the data to the controller, which then relays the data to the other connected device. Merits Less expensive than a mesh topology. Each device needs only one link and I/O port. Installation and reconfigure is easy. Robustness. If one link fails only that link is affected. Requires less cable than a mesh. Demerits ¢ Require more cable compared to bus and ring topologies. e Failure of the central controller incapacitates the entire network.Bus * One long cable acts as a backbone to link all the devices in a network. * Nodes are connected to the bus cable by drop lines and taps. *A drop line is a connection running between the device and the main cable. « A tap is a connector that either splices into the main cable to create a contact with a metallic core. Figure 116: Ring Topology « As the signal travels farther and farther, it becomes weaker. So there is limitation in the number of taps a bus can support and on the distance between those taps. Merits + Ease of installation. e Bus uses less cabling than mesh or star topologies. Demerits Difficult reconnection and isolation. Signal reflection at the taps can cause degradation in quality. A fault or break in the bus cable stops all transmission. It also reflects signals back in the direction of origin creating noise in both directions. ‘ach device has a dedicated point to point connection only with the two devices on either side of it. A signal is passed along the ring in one direction from device to device until it reaches the destination. eEach device in the ring incorporates a repeater EJ Merits: « Easy to install and reconfigure. *To add or delete a device requires Oo a changing only two connections. Deinerita Figure 1.18: Bus Topology eA break in the ring disables the entire network. It can be solved by using a dual ring or a switch capable of closing off the break.