WHAT IS A NETWORK | The definition of Network,

WHAT IS A NETWORK 

OVERVIEW OF NETWORKS

As we noted earlier, in the simplest case, a direct point-to-point link provides the connectivity between two nodes. As an example, we mentioned that a smart phone and a personal computer may be directly linked using USB ports on each device. Another example would be the Bluetooth link between your smart phone and a Bluetooth-enabled headset. It is also sometimes useful to interpret a link as a direct link when the receiving node is an intermediate node used to collect and consolidate packets from several different sources; this would be the case, for example, when you send a message from your cell phone to a cell tower using cellular technology. More commonly, nodes are connected together using network technology. A network offers more flexibility, since it is usually necessary or desirable to be able to select a receiving node from a number of possibilities. By connecting individual networks together, it then becomes possible to build multilink channels that can address and link billions of different nodes together. Before we consider the methods used to build such huge networks of networks, however, let us focus on the important characteristics and properties of various types of networks in use.

Network Topology

To begin our discussion of different types of networks, we offer a brief introduction to the concept of network topology. Network topology describes the fundamental configuration, or layout, of a network. Topology is an important characteristic of all networks, large and small. It defines the path, or paths, between any two points in the network, which, in turn, defines the links between nodes that we have previously discussed. The topology of a network affects the performance of the network, particularly in terms of availability, speed, and traffic congestion.

Network topologies can provide a useful template when designing a network or when analyzing a network’s behavior. If you picture the packets in a network as tiny automobiles (actually, this is often a useful way to think about networks), there is an obvious similarity to automobile traffic.

(a)  Mesh Network:- Mesh networks provide multiple paths between end nodes. The failure of an individual intermediate node will slow, but not stop network traffic as long as an alternative path is available. As you will see shortly, large networks are usually made up of a mixture of local area networks, links, and connecting nodes, with switches and routers connecting the different networks and links together. By default, the result is usually mesh network. It is also possible to create a mesh network intentionally by design to meet the physical constraints of a particular organization. The “best” configuration for connecting a number of end nodes would be to provide direct point-to-point channel connecting each pair of nodes. This scheme, known as a full mesh network, is not practical for most installations, however, because the number of lines required increases too rapidly as the number of nodes increases. Furthermore, each node requires an interface for each connecting line. A mesh network with five nodes. Even this simple case requires ten connections to provide full connectivity. Since each node is connected to four others, the network also requires four interfaces for each node, for a total of twenty interfaces. Simply increasing the number of nodes to twenty increases the number of connections to 190 and requires 380 interfaces. For 500 computer nodes, we would require nearly 125,000 interconnecting cables! In general, the number of connections for a fully connected mesh network with N nodes is the sum of all integer values from 1 to N−1. Fortunately, this reduces to a simple formula:

number of connections = (nodes) × (nodes − 1)∕2.

(b) Bus Topology:- To communicate, a sending node “broadcasts” a message which travels along the bus. Every other node receives the message. Each node compares its address to that of the message; therefore the message is ignored by every node except that of the desired recipient. Each end of the bus is equipped with a terminator to prevent signals from echoing. Branches can be added to a bus, expanding it into a tree. Messages are still broadcast through the tree. Terminators are placed at the ends of each branch in the tree. Bus topology is the easiest to wire. It is only necessary to run a single pair of wires from one end of the network space to the other. Bus topology also has the advantage of low cost, however, traffic congestion is a major issue with bus topology.

(c)  Star Topology:- This topology is used primarily for local area networks, although it is sometimes used in metropolitan and wide area networks to connect individual centers of activity to a central office. In this configuration, all nodes are connected point-to-point to a central device. Nodes communicate through the central device. Switching in the central device connects pairs of nodes together to allow them to communicate directly and steers data from one node to another as required. Most modern switches allow multiple pairs of nodes to communicate simultaneously. The switch itself is usually not considered to be a node; once set, it is transparent to the data flowing through it.

(d) ring topology:- A ring topology consists of a point-to-point connection from each node on the network to the next. The last node on the network is connected back to the first to form a closed ring. Each node retransmits the signal that it receives from the previous node to the next node in the ring. Packets are placed on the loop at a node, and travel from node to node until the desired node is reached. Although the ring is inherently unidirectional (data passes through it in one direction), it is possible to build a bidirectional ring network. Ring networks were popular in the past because they provided a controlled way in which to guarantee network performance. This was an important issue when increased network capacity incurred a large incremental cost. Today, that is no longer the case. It is often cheaper and easier to increase capacity than it is to try to wring the last bit of performance out of a network. Nonetheless, there are legacy token-ring local area networks and Fiber Distributed  Data Interface (FDDI) fiber-optic backbone and metropolitan area networks still in service, although ring networks are essentially obsolete for new network designs.

Types of Networks

With an understanding of network topology in hand, we are now ready to consider the design of different types of networks. There are numerous ways to categorize networks: by medium (coaxial cable, wireless, fiber, for example), by protocol group and/or type of network (TCP/IP, Frame Relay, Ethernet, USB), by standard specification number (802.3, 802.11, X.25), by usage (Web server, database server, peer-to-peer, storage area network), or by range of service  (Bluetooth, LAN, MAN, WAN) to name a few. The most familiar, and often most practical and useful, way to categorize networks is by  their geographical range of service. A common approach is to categorize them hierarchically. From the smallest range to the largest, the major categories are local area networks, backbone networks, metropolitan area networks, and wide area networks. We will also include Internet backbones and the Internet. These designations are somewhat arbitrary, and more a matter of style and architecture than of rigid rule, but they are helpful as a starting point for visualizing and designing networks. We will also mention briefly some special cases: intranets, extranets, and personal area networks (also known as piconets), that do not fit neatly into the standard categories. Recall that the path between end nodes in a large network usually consists of multiple links; each link connects a pair of nodes together. As we observed in the previous section, some of the links may be direct connections. In general, though, nearly all of the links will connect nodes within a network, most commonly, a local area network. Most of the nodes will actually be used to interconnect networks.

The components used to provide inter-network connections at nodes are gateways and routers, which we will describe at the end of this section, following the discussion of the different types of networks. For now, we shall just assume that each inter-network node provides a means for forwarding its data to the corresponding link in the next network in the chain.

LOCAL AREA NETWORKS A local area network (LAN) is a network that connects computers and other supporting devices over a relatively small localized area, typically a room, the floor of a building, a building, or multiple buildings within close range of each other. Usually, most of the computers in a LAN are personal computers or workstations, although there may be larger server computers present, and, sometimes, mobile devices as well. It may be wired or wireless. Most commonly, a LAN will conform to a star or bus topology. Supporting devices might include printers, external storage devices, and routers. Routers, and perhaps gateways, will be used to connect the LAN to other networks. Some LANs are further limited in geographical scope by the particular medium in use Wireless Ethernet, commonly identified by its trade name, Wi-Fi, for example, is limited to a maximum range of a few hundred feet under ideal conditions by the usable strength of the radio signal that is used to carry the data. Walls and other obstructions will limit the range of the signal even more.  Since all communication channels are limited in the amount of data that they can carry, it is sometimes useful to design a LAN to minimize extraneous traffic on the network where possible. One common way to do this in business is to create separate LANs for different business functions or departments. Traffic between the different LANs is enabled by connecting the LANs together with a backbone network, as described later in this section. For example, there would be a LAN for the accounting department, a LAN for the marketing department, and so on. The interconnection between networks allows the different departments to communicate with each other, as well as to access data stored on central company servers. 

(a)  Hub-based Ethernet is based on the bus topology shown in Although it looks physically like a star topology, a hub is a central connection device used to simplify wiring and maintenance. The simplest form of hub is passive. All of the connections at the hub are simply tied together inside the hub. The word “passive” means that the hub performs no operation or modification of the signals as they arrive at the hub. In contrast, an active hub recreates the arriving signals. In either case, signals arriving at the hub are simply broadcast in their original form to every other device connected to the hub. In other words, logically the hub is basically a “squeezed-down” version of a multipoint bus topology. Network interface units from various computers, computer peripherals, and other network support devices such as routers are connected to the hub and share the “bus”.  Hubs have been used primarily in local area networks, but are also sometimes seen in older backbone networks. However, the use of hubs is essentially outmoded, because better performance can be obtained from other devices that can isolate and operate on individual nodes, particularly switches, discussed next.

(b)  Switched Ethernet. Switched Ethernet is based logically on a star topology. Each node of the network is connected to a central switch that is capable of connecting any two nodes together. When a node on the network wishes to communicate with another node, the switch sets up a direct connection between the two. Standard Ethernet cables contain at least two pairs of wires, which are used to make the connections full duplex. Multiple pairs of nodes can communicate at full bandwidth through the switch simultaneously. For wired local area networks, switched Ethernet is the prevalent method in use today.

(c)  Wireless Ethernet, or “Wi-Fi” is a radio based, compatible extension to the Ethernet standard. The basic configuration of a Wi-Fi local area network. Each wireless unit is connected by radio to a base station central access point that is somewhat equivalent to a hub. However, the access point is an active node, since it must transmit and receive radio waves to communicate with the nodes.

 

BACKBONE NETWORKS Backbone networks are used to interconnect local area networks. A backbone can tie several local area networks together to provide for the passage of data between the individual networks and from the networks to the Internet or other external network resources. A primary motivation for a backbone network is to improve overall performance of a larger network by creating separate local area networks for groups of users who communicate primarily with each other. Network traffic can be isolated into small areas of usage, replacing one large heavily used local area network with a number of smaller, isolated LANs. The backbone enables communication between the individual LANs when it is required. For example, a college campus might have multiple LANs built around dormitory areas, plus wireless access points in classrooms, study areas, libraries, dining halls, and various other points around the campus where people congregate. A backbone network would provide the interconnections between all of these LANs. The backbone network also makes it possible to extend the overall range of the combined networks well beyond that of a single LAN. In this case, fiber-optic cables in the backbone combined with the use of switches makes coverage of a large geographical area, such as a large college campus, feasible. One simple way to view a backbone network is to picture it as a large local area network where each node is, itself, a local area network. Figure 12.18 shows an example of such an Ethernet-based backbone network implementation. Some network designers actually call this backbone network layout hierarchical LAN.

METROPOLITAN AREA NETWORKS A metropolitan area network (MAN) is usually defined as a network larger in geographical scope than a local area network, but generally within a range of less than 30 miles or 50 kilometers. A MAN would be used to connect several buildings in an area together or, perhaps, connect a company’s buildings in a city or region together. Some communities have built or plan to build MANs, both for their own use, and as  a service utility for their residents and businesses. When the area is relatively small, it may be possible to implement a MAN almost entirely with a combination of LANs and one or more backbone networks, plus some easy-to-manage form of Internet access. More commonly, there is a desire to create network links to connect properties over areas that would require right-of-way access, that is, permission to run wires through somebody else’s property. To obtain right-of-way access, a company generally requires services from a service provider (SP) or other public carrier, and the infrastructure of the MAN begins to resemble that of a wide area network. A service provider is a company that provides the equivalent of a link or links between nodes that are not directly accessible to simple forms of connection, like wire or fiber-optic cable. A connection to a provider occurs at an access point on the customer’s premises. The access point is usually connected to the company networks with a switch, a router, or a gateway, depending on the type of connection. The connection is often referred to as an edge connection, because it sits at the “edge” of the local network. Thus, a router at the access point would be called an edge router.

 

WIDE AREANETWORKS(WANs) Wide area networks are networks designed to facilitate communications between users and applications over large distances—between the various corporate offices of an international organization that are located in cities all over the world,  for example.

There are two primary compelling reasons for designing and building wide area network capabilities:

1. An organization requires data communication links between widely spread facilities and between an organization and its business partners, customers, and suppliers.

2. An organization requires fast access to the Internet, either as a consumer or as a provider of Internet services, or both.

These two requirements, may, of course, overlap substantially. For example, an extranet is a connection between a business and its business partners, used for the exchange of information and services, and for collaboration, coordination, and planning. The Internet is generally preferred as the medium for extranet activities. The main distinguishing feature that characterizes the wide area network concept is the extensive reliance on service providers to supply the required connectivity between the various locations of the network nodes. The distances are too large to connect directly with a network owner’s own resources and it is impractical to obtain rights of access to all of the intervening property, public or private. Plus, it just isn’t practical for a company to lay its own cable across the Pacific Ocean! Wide area networks require the use of resources that are within the sphere of public switched telephone networks (PSTNs), large cable companies, and other common carrier service providers. A company builds its network at each location out to an edge access point, usually a gateway or router, at which point it is connected to the carrier’s facilities with a leased line to the carrier’s nearest point of presence.

Despite the distances between nodes, it is still possible to view the networks as a whole in the same way as we have viewed other, much smaller networks. Local area networks and backbone networks, and, perhaps even metropolitan area networks, are linked to form a large wide area network. However, it is common to represent the services provided by the carrier as a “black box”. (Actually, they are usually represented as a cloud!) Our interest in the details of the carrier network are generally limited to the edge connections and to the performance of the network as a whole. For clarity, the carrier network is sometimes represented as a collection of private virtual circuits within the cloud, which reflect the logical connections of the wide area network as a whole.

 

INTERNET BACKBONES AND THE INTERNET In theory, it should be possible to link any two computers or computer-based devices in the world using nothing but the routing capabilities of interconnected networks, TCP/IP, routers and gateways, plus appropriate software and physical connections. And indeed, the Internet is just a gigantic network of interconnected networks, connecting a high percentage of all the computers in the world. In practice, though, the number of intermediate nodes, measured as hops between nodes, would make this scheme impractical. The connections would be too slow, the order of arrival of packets too erratic, and the traffic too heavy, to sustain the effort for long. Although the Internet concept postulates that such connections can occur, it is more practical to provide fast connections between distant points to reduce the time it takes to traverse long distances, to reduce the number of hops to just a few, and to reduce the traffic on the local connections. The Internet can be compared to a structure of roads and highways. We travel on long distance, high-speed, limited access superhighways for the longest legs of a journey and use the local roads for initial access to the highways and for the final access to our destination. There might even be a middle tier of medium-speed highways that provide a means to get from the nearest superhighway exit to the network of local roads. In the United States, for example, Interstate highways provide the long legs of the journey, through national and state highways, the connections to the local roads of cities and towns, and the local roads to start and finish our journeys.

Although there is no official central backbone for the Internet and no official guidance for its development, the Internet has developed similarly. All access to the Internet is provided by ISPs.

The arrangement is approximately hierarchical. A small number of large ISPs, known as national  or international service providers, have built high-speed fiber-optic Internet backbones that carry traffic between large cities throughout the world. The speeds of these backbones generally range from 45 to 625 GBps, with faster backbones on the way. Interchanges between these backbones occur at network access points (NAPs). Smaller ISPs, known as regional ISPs, receive their Internet access from one or more national service providers. In addition to their interconnection with the national service providers, most regional ISPs also interconnect among themselves. Local ISPs receive their service from the regional ISPs. Most of us are customers of local ISPs, although large businesses and others with stringent requirements may connect directly to the regional or, even, national service providers. We connect to the Internet at one or more service provider’s points of presence.

 

PICONETS Piconets, or personal area networks (PANs) are a different category than the other networks previously discussed. These are networks created for the personal use of an individual. They generally have ranges of thirty feet or less, sufficient for an individual to interconnect his personal computing devices. Connections between different cooperating users are possible, but rare. Bluetooth is the primary medium for personal area networks. Bluetooth is used for such purposes as the interconnection between a cell phone or GPS and a car radio or hands-free speaker/microphone device, or for transferring and synchronizing pictures and other data between a tablet or cell phone and a computer.

Network Interconnection

PACKET ROUTING In the previous section, you saw that the typical communication channel is made up of a series of intermediate nodes, connected together by links. Packets are passed along the links from node to node. This section presents a general overview of how the packet moves from link to link and how the path is selected. There are two basic techniques for selecting the path through a channel: circuit switching and packet switching. A third technique, virtual circuit switching, is an alternative to ordinary packet switching that also operates on packets. Traditional telephony uses circuit switching. Circuit switching dedicates a path for the exclusive use of the sender–receiver pair for the entire length of time of the connection. A virtual circuit is a multilink channel path that is established for communication between two end nodes. There are two types of virtual circuits: a permanent virtual circuit (PVC) is a virtual circuit that is created when a network is built; a switched virtual circuit (SVC) is set up temporarily when a connection is established and maintained until the connection is closed. For either type, data is sent through the channel in packets; each packet follows the same channel links. However, the links and intermediate nodes are shared with other connections, making the use of the channel more effective.

ROUTERS AND GATEWAYS A key element in the routing of packets is the presence of intermediate link that connect nodes belonging to various networks together. Except for the simplest case, where the interconnection can be made by direct link, a component at each intermediate node routes the packet to the next appropriate node of a different network. It also converts the data format of the packet to the format required for the next link, if necessary. The component may be a computer programmed to do routing, but it’s more likely to be a router or a gateway. Routers and gateways are specialized devices used to interconnect networks and pass packets from one network to another. Technically, the difference between routers and gateways is that routers connect similar networks together; gateways perform the additional format conversion that is necessary to interconnect dissimilar networks. However, many network designers don’t bother to distinguish between routers and gateways and simply use the term “router” in both cases.