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COMPUTER NETWORKS Introduction Computer network is a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. Why Networks? Distribute computation among nodes Coordination between processes running on different nodes Remote I/O Devices Remote Data/File Access Personal communications (e-mail, chat, A/V) World Wide Web Applications Business applications Home Applications Access to remote information. Person-to-person communication. Interactive entertainment. Electronic commerce. WHY DO WE NEED A STANDARD? Many types of connection media: telephone lines, optical fibers, cables, radios, etc. Many different types of machines and operating systems Many different network applications To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network. The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented. In a sense, each layer is a kind of virtual machine, offering certain services to the layer above it. BASED ON CONNECTION 1. BUS Network: Bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. Bus networks are the simplest way to connect multiple clients, but often have problems when two clients want to communicate at the same time on the same bus. Advantages Easy to implement and extend Well suited for temporary networks (quick setup) Typically the cheapest topology to implement Failure of one station does not affect others Disadvantages Difficult to administer/troubleshoot Limited cable length and number of stations A cable break can disable the entire network Maintenance costs may be higher in the long run Performance degrades as additional computers are added Low security (all computers on the bus can see all data transmissions on the bus) One virus in the network will affect all of them (but not as badly as a star or ring network) 2. STAR NETWORK: Star network is one of the most common computer network topologies. In its simplest form, star network consists of one central or hub computer which acts as a router to transmit messages. Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. Advantages Easy to implement and extend, even in large networks Well suited for temporary networks (quick setup) The failure of a non-central node will not have major effects on the functionality of the network. Disadvantages Limited cable length and number of stations Maintenance costs may be higher in the long run Failure of the central node can disable the entire network. One virus in the network will affect them all 3.RING NETWORK: Ring network is a topology of computer networks where each user is connected to two other users, so as to create a ring. The most popular example is a token ring network. Advantages All stations have equal access Each node on the ring acts as a repeater, allowing ring networks to span greater distances than other physical topologies. When using a coaxial cable to create a ring network the service becomes much faster. Disadvantages Often the most expensive topology 4.TREE OR EXPANDED STAR A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs. 4.Mesh Network: Mesh Network is a way to route data, voice and instructions between nodes. It allows for continuous connections and reconfiguration around blocked paths by "hopping" from node to node until a connection can be established. Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed. This is applicable to wireless networks, wired networks, and software interaction. A mesh network is a networking technique which allows inexpensive peer network nodes to supply back haul services to other nodes in the same network. A mesh network effectively extends a network by sharing access to higher cost network infrastructure Mesh is a network topology in which devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network. There are two types of mesh topologies: full mesh and partial mesh. Full mesh topology occurs when every node has a circuit connecting it to every other node in a network. Full mesh is very expensive to implement but yields the greatest amount of redundancy, so in the event that one of those nodes fails, network traffic can be directed to any of the other nodes. Full mesh is usually reserved for backbone networks. Partial mesh topology is less expensive to implement and yields less redundancy than full mesh topology. With partial mesh, some nodes are organized in a full mesh scheme but others are only connected to one or two in the network. Partial mesh topology is commonly found in peripheral networks connected to a full meshed backbone. Star-Bus Network: Star-Bus Network is a combination of a star network and a bus network. A hub (or concentrator) is used to connect the nodes to the network. It is a combination of the linear bus and star topologies and operates over one main communication line. TYPES OF NETWORK Networks can be divided into three types based on geographical areas covered: LANs, MANs, and WANs LANs: Local Area Networks: LANs may use a transmission technology consisting of a cable, to which all the machines are attached A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring. Developed in 1970s. Medium: optical fibers, coaxial cables, twisted pair, wireless. Low latency (except in high traffic periods). High speed networks (10 to 100 Mb/sec). Speeds adequate for most distributed systems MAN: Metropolitan Area Networks: The best-known example of a MAN is the cable television network available in many cities. Generally covers towns and cities (50 kms) Developed in 1980s. Medium: optical fibers, cables. Data rates adequate for distributed computing applications. A typical standard is DQDB (Distributed Queue Dual Bus). Typical latencies < 1 msec. Message routing is fast. Man Based On Cable TV DQDB: Distributed Queue Dual Bus Defined in IEEE 802.6 Data Over Cable Service Interface Distributed Queue Dual Bus (DQDB) is a Data-link layer communication protocol for Metropolitan Area Networks (MANs), specified in the IEEE 802.6 standard, designed for use in MANs. DQDB is designed for data as well as voice and video transmission based on cell switching technology (similar to ATM). DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards such as SMDS, which is based on the DQDB standards. For a MAN to be effective it requires a system that can function across long, city-wide distances of several miles, have a low susceptibility to error, adapt to the number of nodes attached and have variable bandwidth distribution. Using DQDB, networks can be thirty miles long and function in the range of 34 Mbps to 155 Mbps. The data rate fluctuates due to many hosts sharing a dual bus as well as the location of a single host in relation to the frame generator, but there are schemes to compensate for this problem making DQDB function reliably and fairly for all hosts. The DQDB is composed of a two bus lines with stations attached to both and a frame generator at the end of each bus. The buses run in parallel in such a fashion as to allow the frames generated to travel across the stations in opposite directions. Below is a picture of the basic DQDB architecture: Upstream(A) means upstream on bus A Downstream(A) means downstream on bus A Upstream(B) means upstream on bus B Downstream(B) means downstream on bus B Head(A) means the uppermost node on bus A Head(B) means the uppermost node on bus B WAN: Wide Area Networks: It contains a collection of machines intended for running user (i.e., application) programs. The machines are connected by a communication subnet. In most wide area networks, the subnet consists of two distinct components: transmission lines and switching elements. Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or even radio links. Switching elements are specialized computers that connect three or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line on which to forward them. These switching computers have been called by various names in the past; the name router is now most commonly used. Developed in 1960s. Generally covers large distances (states, countries, continents). Medium: communication circuits connected by routers. Routers forwards packets from one to another following a route from the sender to the receiver. Store-and-Forward Hosts are typically connected (or close to) the routers. Typical latencies: 100ms - 500ms. Problems with delays if using satellites. Typical speed: 20 - 2000 Kbits/s. Not (yet) suitable for distributed computing. New standards are changing the landscape. In this model, each host is frequently connected to a LAN on which a router is present, although in some cases a host can be connected directly to a router. The collection of communication lines and routers (but not the hosts) form the subnet. WAN 4. ROUTING Routing is the act of moving information across an internetwork from a source to a destination. Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. The routing algorithm is that part of the network layer software responsible for deciding which output line an incoming packet should be transmitted on. Two major classes: • Static Routing • Dynamic Routing A static routing table is created, maintained, and updated by a network administrator, manually. A static route to every network must be configured on every router for full connectivity. A dynamic routing table is created, maintained, and updated by a routing protocol running on the router. Examples of routing protocols include RIP (Routing Information Protocol), EIGRP (Enhanced Interior Gateway Routing Protocol), and OSPF (Open Shortest Path First). Non-adaptive algorithms do not base their routing decisions on measurements or estimates of the current traffic and topology. The choice of the route to use is computed in advance, off-line, and downloaded to the routers when the network is booted (Static Routing). Adaptive algorithms attempt to change their routing decisions to reflect changes in topology and the current traffic. (Dynamic Routing). 5. IP address Every machine on the Internet has a unique identifying number, called an IP Address. A typical IP address looks like this: 216.27.61.137 To make it easier for us humans to remember, IP addresses are normally expressed in decimal format as a "dotted decimal number" like the one above. But computers communicate in binary form. Look at the same IP address in binary: 11011000.00011011.00111101.10001001 The four numbers in an IP address are called octets, because they each have eight positions when viewed in binary form. If you add all the positions together, you get 32, which is why IP addresses are considered 32-bit numbers. Since each of the eight positions can have two different states (1 or 0) the total number of possible combinations per octet is 28 or 256. So each octet can contain any value between 0 and 255. Combine the four octets and you get 232 or a possible 4,294,967,296 unique values! Out of the almost 4.3 billion possible combinations, certain values are restricted from use as typical IP addresses. For example, the IP address 0.0.0.0 is reserved for the default network and the address 255.255.255.255 is used for broadcasts. The octets serve a purpose other than simply separating the numbers. They are used to create classes of IP addresses that can be assigned to a particular business; government or other entity based on size and need. The octets are split into two sections: Net and Host. The Net section always contains the first octet. It is used to identify the network that a computer belongs to. Host (sometimes referred to as Node) identifies the actual computer on the network. The Host section always contains the last octet. There are five IP classes plus certain special addresses: Default Network - The IP address of 0.0.0.0 is used for the default network. There are different classes of IP addresses such as o o o o o Class A Class B Class C Class D Class E Class A: This class is for very large networks, such as major international companies might have. The first octet is used as Net identifier. The other three octets are used to identify each Host. In Class A networks, the high order bit value in the first octet is always zero. Eg: Net 115. Host 24.53.107 Loopback: The IP address 127.0.0.1 is used as the loopback address. This means that it is used by the host computer to send a message back to itself. It is commonly used for troubleshooting and network testing. Class B: Class B is used for medium sized networks. Class B addresses include the second octet as part of the Net identifier. The other two octets are used to identify each host. Class B networks have a first bit value of 1 and a second bit value of 0 in the first octet. Eg: Net 145.24 Host 53.107 Class C: Class C is used for small to mid-size businesses. Class C addresses include the second and third octets as part of the Net identifier. The last octet is used to identify each host. Class C networks have a first bit value of 1, a second bit value of 1 and a third bit value of 0 in the first octet. Eg: Net 195.24.53 Host 107 Class D: Class D is used for multicast. Class D networks have first bit value of 1, second bit value of 1, third bit value of 1 and fourth bit value of 0 in the first octet. The other 28 bits are used to identify the group of computers the multicast message is intended for. Eg: Net 224. Host 24.53.107 Class E: Class E is used for experimental purposes only. Class E networks have first bit value of 1, second bit value of 1, third bit value of 1 and fourth bit value of 1 in the first octet. The other 28 bits are used to identify the group of computers the multicast message is intended for. Eg: Net 240. Host 24.53.107 Broadcast: Messages that are intended for all computers on the network are sent as broadcasts. These messages always use the IP address 255.255.255.255. 6. DOMAIN NAME SERVICE (DNS) The Domain Name System (DNS) is the method by which Internet addresses in mnemonic form such as sunc.scit.wlv.ac.uk. are converted into the equivalent numeric IP address such as 134.220.4.1. To the user and application process this translation is a service provided either by the local host or from a remote host via the Internet. The DNS server (or resolver) may communicate with other Internet DNS servers if it cannot translate the address itself. The system accesses the DNS through a resolver. The resolver gets the hostname and returns the IP address or gets an IP address and looks up a hostname. The resolver returns the IP address before asking the TCP to open a connection or sending a datagram using UDP DNS Name Structure DNS names are constructed hierarchically. The highest level of the hierarchy being the last component or label of the DNS address. Labels can be up to 63 characters long and are case insensitive. A maximum length of 255 characters is allowed. Labels must start with a letter and can only consist of letters, digits and hyphens. The root of the DNS tree is a special node with a null label. The seven 3-character domain names. meaning code com Commercial. Now international. edu Educational. gov Government. int International Organization. mil Military. net Network related. org Miscellaneous Organization. Fig: 1 DNS 7. PROTOCOL A protocol is a set of guidelines or rules. A communications protocol is a formal description of digital message formats and the rules for exchanging those messages in or between computing systems and in telecommunications. Protocols may include signaling, authentication and error detection and correction capabilities. To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network. The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented. The fundamental idea is that a particular piece of software (or hardware) provides a service to its users but keeps the details of its internal state and algorithms hidden from them. Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n protocol. Basically, a protocol is an agreement between the communicating parties on how communication is to proceed. A five-layer network is illustrated in Fig. The entities comprising the corresponding layers on different machines are called peers. The peers may be processes, hardware devices, or even human beings. In other words, it is the peers that communicate by using the protocol. 8. THE INTERNET The Internet is not a network at all, but a vast collection of different networks that use certain common protocols and provide certain common services. It is an unusual system in that it was not planned by anyone and is not controlled by anyone. The Internet was largely populated by academic, government, and industrial researchers. One new application, the WWW (World Wide Web) changed all that and brought millions of new, nonacademic users to the net. Architecture of the Internet The modem is a card within the PC that converts the digital signals the computer produces to analog signals that can pass unhindered over the telephone system. These signals are transferred to the ISP’s POP (Point of Presence), where they are removed from the telephone system and injected into the ISP’s regional network. From this point on, the system is fully digital and packet switched. The ISP’s regional network consists of interconnected routers in the various cities the ISP serves. If the packet is destined for a host served directly by the ISP, the packet is delivered to the host. Otherwise, it is handed over to the ISP’s backbone operator. International backbone networks, with thousands of routers connected by high-bandwidth fiber optics. Large corporations and hosting services that run server farms (machines that can serve thousands of Web pages per second) often connect directly to the backbone. If a packet given to the backbone is destined for an ISP or company served by the backbone, it is sent to the closest router and handed off there. Packets can be forwarded from any backbone to any other backbone.