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DIF7101 - Introduction to Computing -- UNIT V Computer networking, protocols, channels (ie. Copper, fiber, wireless etc), types (LAN, WAN etc.), networking components (NIC, Routers, Switch, Hub, etc), topologies (ring, bus, star, mesh, tree), the Internet (evolution, protocols, WWW) http://en.wikiversity.org/wiki/Introduction_to_Computers http://en.wikiversity.org/wiki/Computer_Networks

Computer Networks

A collection of computing devices – connected in various ways to communicate and share resources. Share : intangible resources – files and data ; tangible resources – printers, scanners, plotters, projectors and storage devices (?? monitor – well, display screens)

Networks are defined not only by physical connections but the ability to communicate. Networks make it easier to collect and dispense information ; users can collaborate Can contain devices other than computers. Networked printers can be used from all computers on the network. Also contain variety of devices for handling network traffic (routers, cables, modem, etc.) Node or host – generic terms for any device on the network.

Data transfer rate – speed at which data is moved from one place on the network to another. Bandwidth – another name for the data transfer rate. Protocol – a set of rules that defines how data is formatted and processed on a network. Client/server model – A distributed approach in which a client makes requests to a server and the server responds.

Network Protocols

A network protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications. A protocol may have a formal description. Protocols may include signaling, authentication and error detection and correction capabilities. A protocol definition defines the syntax, semantics, and synchronization of communication; the specified behavior is typically independent of how it is to be implemented. A protocol can therefore be implemented as hardware or software or both.

Data formats for data exchange. Address formats for data exchange. Address mapping. Routing. Detection of transmission errors Acknowledgments Loss of information - timeouts and retries. Direction of information flow Sequence control. Flow control

OSI Model

The Open Systems Interconnection model (OSI model) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers.

An instance of a layer provides services to its upper layer instances while receiving services from the layer below.

Layer 1: physical layer Layer 2: data link layer Layer 3: network layer Layer 4: transport layer Layer 5: session layer Layer 6: presentation layer Layer 7: application layer

Layer 1: physical layer The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.

Layer 2: data link layer The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer.

Layer 3: network layer The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the transport layer (in contrast to the data link layer which connects hosts within the same network). The network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors.

Layer 4: transport layer The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.

Layer 5: session layer The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

Layer 6: presentation layer The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.

Layer 7: application layer The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.

The Internet The Internet is vast collection of smaller networks that have decided to communicate with one another. No single person or company owns the Internet. A wide area network made of smaller networks which are owned and managed by persons or organizations. The Internet is defined by how connections are made between the networks.

The Internet Backbone High capacity data route that transfers traffic. Owned by large telecommunication companies. High redundancy with multiple carriers to prevent data loss.

The Internet Service Provider Company that provides individuals and other companies access to the Internet. Connect directly to the backbone or connect to other ISPs who connect to the backbone.

Many technologies to connect to the Internet. Using a phone modem to dial-up a computer that is permanently connected to the Internet – upto 64kbps which is the limit of analog voice communication. A digital subscriber line (DSL) to transfer data over copper phone lines to and from the phone company's premises – 10+ Mbps. Connect via the cable modem using cable TV wires used for TV -- ~10 Mbps. Fiber to the Home – Connection using Fiber Optics installed in each home – 100+ Mbps.

Open Systems and Protocols The Internet exists because of Open Systems and Protocols. Originally networks were proprietary and did not inter-operate with each other due to differences.

An Open System is one based on a common model of network architecture and a suite of protocols used in its implementation. The ISO Open System Interconnection (OSI) Reference Model.

The TCP/IP model

The TCP/IP model (Transmission Control Protocol/Internet Protocol) is a descriptive framework for the Internet Protocol Suite of computer network protocols created in the 1970s by DARPA, an agency of the United States Department of Defense.

It evolved from ARPANET, which was an early wide area network and a predecessor of the Internet. The TCP/IP Model is sometimes called the Internet Model. The TCP/IP model and related protocols are maintained by the Internet Engineering Task Force (IETF). The IETF conducts standard-setting work groups, open to any individual, about the various aspects of Internet architecture.

Resulting discussions and final standards are published in a series of publications, each called a Request for Comments (RFC), freely available on the IETF web site. The principal methods of networking that enable the Internet are contained in specially designated RFCs that constitute the Internet Standards.

The TCP/IP model describes a set of general design guidelines and implementations of specific networking protocols to enable computers to communicate over a network. TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination. Protocols exist for a variety of different types of communication services between computers.

Architectural principles more important than layering.

End-to-End Principle : a classic design principle of computer networking which states that application specific functions ought to reside in the end hosts of a network rather than in intermediary nodes, provided they can be implemented "completely and correctly" in the end hosts.

Robustness Principle : “Be liberal in what you accept, and conservative in what you send.” -- That is, it must be careful to send well-formed datagrams, but must accept any datagram that it can interpret.

The TCP/IP protocol loosely defines a four-layer model, with the layers having names, not numbers. Link Layer       Internet Layer        Transport Layer    Application Layer

The Internet Protocol Suite and the layered protocol stack design were in use before the OSI model was established. As per the IETF Internet protocol and architecture development is not intended to be OSI-compliant.

Link Layer: The Link Layer (or Network Access Layer) is the networking scope of the local network connection to which a host is attached. This is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP is able to be implemented on top of virtually any hardware networking technology. The Link Layer is used to move packets between the Internet Layer interfaces of two different hosts on the same link.

Internet Layer : The Internet Layer solves the problem of sending packets across one or more networks.

In the Internet Protocol Suite, the Internet Protocol performs two basic functions: Host addressing and identification: This is accomplished with a hierarchical addressing system Packet routing: This is the basic task of getting packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination.

Transport Layer : The Transport Layer's responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing.

End to end message transmission or connecting applications at the transport layer can be categorized as either connection-oriented, implemented in Transmission Control Protocol (TCP), or connectionless, implemented in User Datagram Protocol (UDP).

Application Layer The Application Layer refers to the higher-level protocols used by most applications for network communication. Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)), which in turn use lower layer protocols to effect actual data transfer.

Higher Level Protocols (Protocols built on top of TCP/IP) Simple Mail Transfer Protocol (SMTP) – Protocol used to specify the transfer of electronic mail.

File Transfer Protocol (FTP) – Protocol that allows a user to transfer files from one computer to another – secure version is SFTP.

Telnet – Protocol used to log into a computer from a remote computer – secure version is Secure Shell ie. SSH.

Hypertext Transfer Protocol (HTTP) – Protocol defining the exchange of World Wide Web documents.

All the protocols mentioned are built on TCP. Protocols using UDP are also available. Not popular as UDP lacks the reliability of TCP.

High-level protocols are assigned a port number. Port is a numeric designation which corresponds to a protocol. Servers and routers use the port number to help control and process network traffic.

MIME Types: MIME – Multipurpose Internet Mail Extension. Standard for attaching or including multimedia or otherwise specially formatted data with other documents like email. Applications can decide how to deal with data based on the MIME type. Exist for files created by many application programs as well as data from different content areas.

Channels (ie. Copper, fiber, wireless etc) 1.Channels           2 Phone Lines 3.Twisted Pair Cables  4. Coaxial Cables      5. Fiber Optic Communication

Channels A communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

Transmission media (channels) are classified as one of the following: Guided (or bounded)—waves are guided along a solid medium such as a transmission line. Wireless (or unguided)—transmission and reception are achieved by means of an antenna. Guided transmission media -- the waves are guided along a physical path phone lines, twisted pair cables, coaxial cables, and optical fibers Wireless transmission media -- transmission of data without the use of physical means to define the path it takes. microwave, radio or infrared

A transmission may be simplex, half-duplex, or full-duplex. In simplex transmission, signals are transmitted in only one direction; one station is a transmitter and the other is the receiver. In the half-duplex operation, both stations may transmit, but only one at a time. In full duplex operation, both stations may transmit simultaneously. In the latter case, the medium is carrying signals in both directions at same time.

Phone Lines A telephone line or telephone circuit is a single-user circuit on a telephone communication system. Refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network. Typically made of copper (sometimes aluminium) and were carried in balanced pairs on poles above the ground, and later as twisted pair cables. Twisted pair cables Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources. It was invented by Alexander Graham Bell. The twisted pair can be shielded on unshielded (UTP). Unshielded twisted pair (UTP) UTP cable is the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, utilizes UTP cables. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to optical fiber and coaxial cable.

Common cable categories Twisted pair cables A solid core cable uses one solid wire per conductor. Is supposed to be used for permanently installed runs. More prone to failure if repeatedly flexed. Stranded conductor uses multiple wires wrapped around each other in each conductor. For connections from wall-ports to end devices. Generally more expensive than solid core.

Advantages It is a thin, flexible cable that is easy to string between walls. More lines can be run through the same wiring ducts. UTP costs less per meter/foot than any other type of LAN cable. Electrical noise going into or coming from the cable can be prevented. Cross-talk is minimized.

Advantages It is a thin, flexible cable that is easy to string between walls. More lines can be run through the same wiring ducts. UTP costs less per meter/foot than any other type of LAN cable. Electrical noise going into or coming from the cable can be prevented. Cross-talk is minimized.

Disadvantages Twisted pair’s susceptibility to electromagnetic interference greatly depends on the pair twisting schemes staying intact during the installation – resulting in stringent requirements for maximum pulling tension as well as minimum bend radius – which makes the installation practices an important part of ensuring the cable’s performance. In video applications twisted pair cabling can introduce signaling delays known as skew causing ghosting and color defects. Can be prevented by changing cable lengths at the ends.

Coaxial cable Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880.

Coaxial cable is used as a transmission line for radio frequency signals. Applications include feedlines connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable television signals. In an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from external electromagnetic interference.

Fiber-optic communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fiber-optic communication systems played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, optical fibers have largely replaced copper wire communications in core networks in the developed world.

Optical Fiber Is a flexible, transparent fiber made of very pure glass (silica) not much wider than a human hair that acts as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. Typically consists of a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection. Fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those that only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,050 meters (3,440 ft).

Optical fiber cable An optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed. Optical cables send data at 182,000 km/s, resulting in 5.5 ms of latency for each 1000 km. Thus the round-trip delay time is around 11 ms.

Types (LAN, WAN etc.) 1.PAN           2.LAN            3.WAN 4.VPN           5.MAN            6.SAN

Personal Area Networks (PAN) Extremely small networks, often referred to as "piconets" that encompass an area around a single person. These networks, such as Bluetooth, have a range of only 1-5 meters, and tend to have very low power requirements, but also very low datarates.

Local Area Network (LAN) LAN networks can encompass a building such as a house or an office, or a single floor in a multi-level building  : group of computers in close proximity, A local area network (LAN) is created by a group of computers that are in close proximity to each other; hence local. The computers typically connect to each other by way of a router, which is a device that directs signals within the LAN. A wireless network is a LAN that is created using WiFi(a wireless technology) routers instead of wired ones. A LAN is typically created at schools that have a network their students can connect to. A WiFi hotspot, like the ones found at Starbucks, would also be a LAN. connect through a router – a device to direct signals within the LAN wireless network – LAN WiFi routers instead of wired ones

Wide Area Network (WAN) A wide area network connects multiple LANs across large physical distances. They're commonly created by leasing a wire from a cable company or internet provider, although WAN typically doesn't require the internet in order to work. If the LAN of one school was connected to a LAN at a school in another state, the two together would be a WAN. Often uses internet for connectivity across different component units (e.g.,located in different cities)

VPN (Virtual private networks) Virtual private networks provide a special way to create a WAN by using the internet to connect LANs together, without leasing a private cable. An example might be if you were at your house, away from school, you can create a VPN to connect to the schools network, without having to lease a communications cable (or access office network system from home)

Metropolitan Area Network (MAN) A large network which covers a campus or a city – more focused on an organization or area.

Sensor Area Networks These networks are low-data rate networks primarily used for embedded computer systems and wireless sensor systems.

Circuit Switching Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit. In circuit switching, the bit delay is constant during a connection. Each circuit cannot be used by others until the circuit is released and a new connection is set up. Even if no actual communication is taking place, the channel remains unavailable to other users. Channels that are available for new users are said to be idle. Circuit switching is commonly used for connecting voice circuits.

Packet Switching Packet switching is a digital networking communications method that groups all transmitted data into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams (sequences of packets) over a shared network. When traversing networks packets are buffered and queued, resulting in variable delay and throughput depending on the traffic load in the network. Connectionless packet switching, also known as datagram switching. In the first case each packet includes complete addressing or routing information. The packets are routed individually, sometimes resulting in different paths and out-of-order delivery. At the destination, the original message/data is reassembled in the correct order. Connection-oriented packet switching, also known as virtual circuit switching. A connection is defined and preallocated in each involved node during a connection phase before any packet is transferred. The packets include a connection identifier rather than address information, and are delivered in order.

Networking Components Network Hardware (Components) refers to equipment facilitating the use of a computer network. This includes routers, switches, hubs, gateways, access points, network interface cards, Networking cables, network bridges, modems, firewalls and other equipment. In the most common modern computer systems, Wireless networking has become increasingly popular, however, especially for portable and hand held devices.

Routers A device that forwards data packets between computer networks, creating an overlay internetwork. A router is connected to two or more data lines from different networks. When data comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.

Switches A network switch or switching hub is a computer networking device that connects network segments. Unmanaged switches — These switches have no configuration interface or options. Managed switches — These switches have one or more methods to modify the operation of the switch. Smart (or intelligent) switches Enterprise Managed (or fully managed) switches Mid-to-large sized LANs contain a number of linked managed switches.

Hub An Ethernet hub, active hub, network hub, repeater hub or hub is a device for connecting multiple twisted pair or fiber optic Ethernet devices together and making them act as a single network segment. A network hub is an unsophisticated device in comparison with, for example, a switch. A hub does not manage any of the traffic that comes through it: any packet entering any port is rebroadcast on all other ports.

Gateway A network node equipped for interfacing with another network that uses different protocols. The activities of a gateway are more complex than that of the router or switch as it communicates using more than one protocol. A gateway is an essential feature of most routers, although other devices (such as any PC or server) can function as a gateway.

Access Points A wireless access point (WAP) is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards. The WAP usually connects to a router (via a wired network), and can relay data between the wireless devices (such as computers or printers) and wired devices on the network. Home networks WAPs are mostly wireless routers, meaning converged devices that include the WAP, a router, and, often, an ethernet switch. Many also include a broadband modem.

Network Interface Cards A network interface controller (also known as a network interface card, network adapter, LAN adapter and by similar terms) is a computer hardware component that connects a computer to a computer network. Although other network technologies exist (e.g. token ring), Ethernet has achieved near-ubiquity since the mid-1990s. Every Ethernet network controller has a unique 48-bit serial number called a MAC address, which is stored in read-only memory.

Networking Cables Networking Cables are used to connect one network device to other or to connect two or more computers to share printer, scanner etc. Different types of network cables like Coaxial cable, Optical fiber cable, Twisted Pair cables are used depending on the network's topology, protocol and size. The devices can be separated by a few meters (e.g. via Ethernet) or nearly unlimited distances (e.g. via the interconnections of the Internet).

Network Bridges Bridging is a forwarding technique used in packet-switched computer networks. Bridging generally refers to Transparent bridging or Learning bridge operation which predominates in Ethernet. A bridge uses a forwarding database to send frames across network segments. The forwarding database is initially empty and entries in the database are built as the bridge receives frames.

Modems A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

Firewalls http://en.wikipedia.org/wiki/Firewall_(computing) In computing, a firewall is a network security system that controls the incoming and outgoing network traffic based on an applied rule set. A firewall establishes a barrier between a trusted, secure internal network and another network (e.g., the Internet) that is assumed not to be secure and trusted. Firewalls exist both as a software solution and as a hardware appliance. Many hardware-based firewalls also offer other functionality to the internal network they protect, such as acting as a DHCP server for that network. Many personal computer operating systems include software-based firewalls to protect against threats from the public Internet. Many routers that pass data between networks contain firewall components and, conversely, many firewalls can perform basic routing functions.

A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass. Many personal computer operating systems include software-based firewalls to protect against threats from the public Internet. Many routers that pass data between networks contain firewall components and, conversely, many firewalls can perform basic routing functions.

Network Topologies

Arrangement of various elements (links, nodes) of a network Physical topology: placement of the various components of a network, including device location and cabling ; concerns with capabilities of the network access devices and media, level of control or fault tolerance, associated cost factors

Logical topology: how data flows within a network, regardless of its physical design Physical and Logical topologies need not be the same the original twisted pair Ethernet using repeater hubs was a logical bus topology with a physical star topology layout

Token Ring is a logical ring topology, but is wired a physical star from the Media Access Unit. Two networks may have same topology, but Distances between nodes, physical interconnections, transmission rates, or signal types may vary

LAN Topologies

Point-to-Point       Bus            Star Ring               Mesh            Tree Hybrid           Daisy Chain        Peer-to-peer Client-server

Point-to-Point The simplest topology is a permanent link between two endpoints. Permanent (dedicated) link – unimpeded communications between the two endpoints. Switched link – set up dynamically using circuit-switching or packet-switching technologies, dropped when not needed (e.g., conventional telephone line)

Bus Each computer or server is connected to the single bus cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted.

Linear Bus : All the nodes of the network are connected to a common transmission medium which has exactly two endpoints (the 'bus', or backbone, or trunk).

Distributed Bus : All the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium (physically all nodes share a common transmission medium).

Advantages Easy to implement and extend. Well-suited for temporary or small networks not requiring high speeds (quick setup), resulting in faster networks. Less expensive than other topologies. Cost effective; only a single cable is used. Easy identification of cable faults.

Disadvantages Limited cable length and number of stations. If there is a problem with the cable, the entire network breaks down. Maintenance costs may be higher in the long run. Performance degrades as additional computers are added or on heavy traffic (shared bandwidth). Proper termination is required (loop must be in closed path). Significant Capacitive Load (each bus transaction must be able to stretch to most distant link). It works best with limited number of nodes. Commonly has a slower data transfer rate than other topologies. Only one packet can remain on the bus during one clock pulse

Star Each network host is connected to a central hub with a point-to-point connection. The network does not necessarily have to resemble a star to be classified as a star network, but all of the nodes on the network must be connected to one central device. All traffic that traverses the network passes through the central hub. The hub acts as a signal repeater.

Extended Star A network that is based upon the physical star topology has one or more repeaters between the central node and the peripheral nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes.

Distributed Star Individual networks that are based upon the physical star topology connected together in a linear fashion – i.e., 'daisy-chained' – with no central or top level connection point.

Advantages Better performance: prevents the passing of data packets through an excessive number of nodes. Isolation of devices: Each device is inherently isolated by the link that connects it to the hub.

Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Easy to expand, detect faults and to remove parts. No disruptions to the network when enabling or removing devices.

Disadvantages High dependence of the system on the functioning of the central hub Failure of the central hub renders the network inoperable

Ring A network topology that is set up in a circular fashion in which data travels around the ring in one direction and each device on the right acts as a repeater to keep the signal strong as it travels. Each device incorporates a receiver for the incoming signal and a transmitter to send the data on to the next device in the ring. The network is dependent on the ability of the signal to travel around the ring.

Advantages Very orderly network where every device has access to the token and the opportunity to transmit Performs better than a bus topology under heavy network load Does not require a central node to manage the connectivity between the computers

Disadvantages One malfunctioning workstation can create problems for the entire network Moves, adds and changes of devices can affect the network Communication delay is directly proportional to number of nodes in the network Bandwidth is shared on all links between devices

Mesh Is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network. There is often more than one path between a source and a destination in the network. Although mostly used in wireless scenarios, this concept is also applicable to wired networks and software interaction.

Fully Connected Mesh Each of the nodes is connected to each other. A fully connected network doesn't need to use switching nor broadcasting.

Major disadvantage – the number of connections grows quadratically with the number of nodes. C = n (n-1)/2 – impractical for larger meshes A two-node network is technically a fully connected network.

Partially Connected Mesh Some of the nodes of the network are connected to more than one other node in the network with a point-to-point link This makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network.

Tree Tree topology is a combination of Bus and Star topology. A top level central 'root' node is connected to one or more other nodes that are one level lower in the hierarchy with a point-to-point link Each of the second level nodes will also have one or more other nodes that are one level lower in the hierarchy connected to it with a point-to-point link. The top level central 'root' node is the only node that has no other node above it in the hierarchy.

Each node in the network having a specific fixed number of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the 'branching factor' of the hierarchical tree. This tree has individual peripheral nodes.

A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree. A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology. The branching factor is independent of the total number of nodes in the network.

The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network. If the nodes in a tree network are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy.

Hybrid Hybrid networks use a combination of any two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, a tree network connected to a tree network is still a tree network topology. A hybrid topology is always produced when two different basic network topologies are connected.

A Star ring network consists of two or more star topologies connected using a multistation access unit (MAU) as a centralized hub. A Star Bus network consists of two or more star topologies connected using a bus trunk (the bus trunk serves as the network's backbone). A Snowflake topology is really a "Star of Stars" network, with characteristics of a hybrid network topology.

Daisy Chain Connecting each computer in series to the next. If a message is intended for a computer partway down the line, each system bounces it along in sequence until it reaches the destination.

A daisy-chained network can take two basic forms: linear and ring. A linear topology puts a two-way link between one computer and the next was expensive in the early days of computing, since each computer (except for the ones at each end) required two receivers and two transmitters With advanced technologies, the cost is getting reduced

By connecting the computers at each end, a ring topology can be formed. – the number of transmitters and receivers can be cut in half, since a message will eventually loop all of the way around. When a node sends a message, the message is processed by each computer in the ring. In case of a single link  break, the transmission can be sent via the reverse path thereby ensuring that all nodes are always connected.

Peer-to-Peer Various computers on the network can act both as clients and servers – e.g., allow file and print sharing Advantages : no need for a network operating system, expensive server, specialist staff such as network administrators (users set their own file share permissions) ; easy to set up ; if one system fails, rest of the network not disrupted Disadvantages: performance slow down ; no central backup ; files not in specific 'shared area', difficult to locate if the owner does not have a logical filing system; virus problems ; no security besides permissions

Client -Server Clients typically host a small subset of the data in the application process space and delegate to the server system for the rest. model for vertical scaling, provides better data isolation, high fetch performance, and more scalability Better performance If you expect data distribution to put a very heavy load on the network the server system is itself a peer-to-peer system, with data distributed between servers. Client systems have a connection pool uses to communicate with servers and other members ; may also contain a local cache

Network Addresses

Hostname – unique identification that specifies a computer on the Internet. Generally readable words separated by dots. example.example.com           localhost.localdomain

IP address – An address made up of four numeric values separated by dots which uniquely identify a computer on the Internet. 192.168.1.4           10.0.0.2 Network Addresses : An IP address is 32 bits. Each number corresponds to a byte in the IP address. Each number in the IP address is in the range from 0 to 255. 10010100   01001110    11111010    00001100 148       .         78           .        250       .        12

No correspondence between the IP address and the hostname sections.

An IP address can be split into Network address – specifies the network Host number – specifies a particular machine on the network The split depends on the network “class”. Class A – First byte for network address, last 3 for hostname. Class B – First 2 bytes for network address, last 2 for hostname. Class C -- First 3 bytes for network address, last byte for hostname.

Very few Class A networks with many hosts. Many Class C networks with a maximum of 256 hosts each. Class C networks are assigned to most organisations. Class A and B are for very large organisations and Internet Service Providers. Running out of address. IPV6 – 128 bit, not widely used yet.

Domain Name System A hostname consists of a computer name followed by the domain name. server.example.com Here server is the computer name and example.com is the domain name.

A domain name is separated into two or more sections that specify the organization and possibly a subset of the organization, of which the computer is a part.

Domain names narrow in on a particular set of networks controlled by an organization. Two computers on can have the same name if they are on different domains since they can be identified by the full hostname. The last section of the domain name is called its top-level domain (TLD). Some TLDs have been around since the Internet was founded while many are new. Original TLDs : Some TLDs based on Country Codes Domain Name System (DNS) is chiefly used to translate hostnames into numeric IP addresses. DNS is a distributed database with no one organization responsible for maintaining the hostname/IP mappings. When a use specifies a hostname in a browser in a browser, a request is sent to a nearby domain name server.

If the server can resolve the hostname it does so. If not it asks another domain name server for help. If the second server can't resolve it the request propagates. Ultimately, either the request reaches a server which can resolve the name or the request expires taking too much time to resolve.

World Wide Web WWW – An infrastructure of information and the network software used to access it. WWW is distinct from the Internet – it runs on the Internet. The Internet allowed communication from the beginning. The WWW made communication easier. The Internet usage exploded after WWW arrived in the early 1990s.

Web Page – A document that contains or references various kinds of data. Link – A connection between one page and another. Website – A collection of related Web pages, usually controlled by a single person or company. Web Browser – A software tool that retrieves and displays Web pages. Web server – A computer set up to respond to requests to web pages. Uniform Resource Locator (URL) – A standard way of specifying the location of a Web page. Uniquely identifies the page from all the pages anywhere on the world. Part of the URL is the hostname of the computer on which the information is stored.

Hypertext Markup Language (HTML) Web pages are built using a language called Hypertext Markup Language (HTML). Hypertext – information is not stored linearly, links can allow user to jump from one place to another. Hypermedia – amalgamation of text, images, audio and video. Markup Language – A language that uses tags to annotate the information in the document. Tag – The syntactic element in a markup language that indicates how information should be displayed.

A HTML document consists of information that is annotated by tags that specify how a particular element should be treated and formatted. A web browser displays an HTML page without regard to extra spacing, blank lines or indentation. The tags alone guide the browser and a web page might look different in different browsers.

Extensible Markup Language (XML) A language that allows the user to describe the content of the document. Users can define their own tags unlike in HTML where the tags are fixed. XML is a metalanguage – it is used to define other languages. XML tags specify the nature of the data while HTML tags focus on the format of the displayed data.