NETWORK HARDWARE

 Broadly speaking, there are two types of transmission technology that are in widespread use: broadcast links and point-to-point links. 

Point-to-point : links connect individual pairs of machines. To go from the source to the destination on a network made up of point-to-point links, short messages, called packets. 

Point-to-point transmission with exactly one sender and exactly one receiver is sometimes called unicasting. 

In contrast, on a broadcast network, the communication channel is shared by all the machines on the network; packets sent by any machine are received by all the others. An address field within each packet specifies the intended recipient. Upon receiving a packet, a machine checks the address field. If the packet is intended for the receiving machine, that machine processes the packet; if the packet is intended for some other machine, it is just ignored.                                                                                      For Example, consider someone standing in a meeting room and shouting ‘‘Watson, come here. I want you.’’ Although the packet may actually be received (heard) by many people, only Watson will respond; the others just ignore it.                                                                                                                  Broadcast systems usually also allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every machine on the network.                                                                                                This mode of operation is called broadcasting. Some broadcast systems also support transmission to a subset of the machines, which known as multicasting. 

An alternative criterion for classifying networks is by scale. Distance is important as a classification metric because different technologies are used at different scales.                                                            At the top are the Personal Area Networks, networks that are meant for one person. Beyond these come longer-range networks. These can be divided into Local, Metropolitan, and Wide Area Networks, each with increasing scale.                                                                                                                                    Finally, the connection of two or more networks is called an internetwork. The worldwide Internet is certainly the best-known (but not the only) example of an internetwork. 

1. Local Area Networks (LAN)                                                                                                                    A LAN is a privately owned network that operates within and nearby a single building like a home, office or factory. LANs are widely used to connect personal computers and consumer electronics to let them share resources (e.g., printers) and exchange information. When LANs are used by companies, they are called enterprise networks. 

Wireless LANs are very popular these days, especially in homes, older office buildings, cafeterias, and other places where it is too much trouble to install cables. In these systems, every computer has a radio modem and an antenna that it uses to communicate with other computers. In most cases, each computer talks to a device in the ceiling as shown in Fig. This device, called an AP (Access Point), wireless router, or base station, relays packets between the wireless computers and also between them and the Internet. There is a standard for wireless LANs called IEEE 802.11, popularly known as WiFi, which has become very widespread. It runs at speeds anywhere from 11 to 100Mbps. 



Wired LANs use a range of different transmission technologies. Most of them use copper wires, but some use optical fiber. LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance. 

Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps, have low delay (microseconds or nanoseconds), and make very few errors. Newer LANs can operate at up to 10 Gbps. Compared to wireless networks, wired LANs exceed them in all dimensions of performance. It is just easier to send signals over a wire or through a fiber than through the air. The topology of many wired LANs is built from point-to-point links. IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN. Fig.  shows a sample topology of switched Ethernet. 

Each computer speaks the Ethernet protocol and connects to a box called a switch with a point to-point link. Hence the name. A switch has multiple ports, each of which can connect to one computer. The job of the switch is to relay packets between computers that are attached to it, using the address in each packet to determine which computer to send it to.                                                                                      To build larger LANs, switches can be plugged into each other using their ports. 

2. Metropolitan Area Networks                                                                                                                A MAN (Metropolitan Area Network) covers a city. The best-known examples of MANs are the cable television networks available in many cities.                                                                                              These systems grew from earlier community antenna systems used in areas with poor over-the air television reception. In those early systems, a large antenna was placed on top of a nearby hill and a signal was then piped to the subscribers’ houses.                                                                                When the Internet began attracting a mass audience, the cable TV network operators began to realize that with some changes to the system, they could provide two-way Internet service in unused parts of the spectrum. At that point, the cable TV system began to morph from simply a way to distribute television to a metropolitan area network. To a first approximation, a MAN might look something like the system shown in Fig. In this figure we see both television signals and Internet being fed into the centralized cable headend for subsequent distribution to people’s homes.                                                  Cable television is not the only MAN, though. Recent developments in high speed wireless Internet access have resulted in another MAN, which has been standardized as IEEE 802.16 and is popularly known as WiMAX.

 


3. Wide Area Networks                                                                                                                                A WAN (Wide Area Network) spans a large geographical area, often a country or continent. We will begin our discussion with wired WANs, using the example of a company with branch offices in different cities.                                                                                                                                          The WAN in Fig.  is a network that connects offices in different cities. Each of these offices contains computers intended for running user (i.e., application) programs. We will follow traditional usage and call these machines hosts. The rest of the network that connects these hosts is then called the communication subnet, or just subnet for short. 

In most WANs, 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. Most companies do not have transmission lines lying about, so instead they lease the lines from a telecommunications company.                                                                                                  Switching elements, or just switches, are specialized computers that connect two 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.                                                                                          Originally, subnet’s only meaning was the collection of routers and communication lines that moved packets from the source host to the destination host. Readers should be aware that it has acquired a second, more recent meaning in conjunction with network addressing. The WAN as we have described it looks similar to a large wired LAN, but there are some important differences that go beyond long wires. Usually in a WAN, the hosts and subnet are owned and operated by different people.



 A second difference is that the routers will usually connect different kinds of networking technology. The networks inside the offices may be switched Ethernet, other may have different technology. Some device needs to join them. This means that many WANs will in fact be internetworks, or composite networks that are made up of more than one network. 

A final difference is in what is connected to the subnet. This could be individual computers, as was the case for connecting to LANs, or it could be entire LANs. This is how larger networks are built from smaller ones. As far as the subnet is concerned, it does the same job. 

Two other varieties of WANs. First, rather than lease dedicated transmission lines, a company might connect its offices to the Internet This allows connections to be made between the offices as virtual links that use the underlying capacity of the Internet. This arrangement, shown in Fig, is called a VPN (Virtual Private Network). Compared to the dedicated arrangement, a VPN has the usual advantage of virtualization, which is that it provides flexible reuse of a resource (Internet connectivity). Consider how easy it is to add a fourth office to see this. A VPN also has the usual disadvantage of virtualization, which is a lack of control over the underlying resources. With a dedicated line, the capacity is clear. With a VPN your mileage may vary with your Internet service. 



The second variation is that the subnet may be run by a different company. The subnet operator is known as a network service provider and the offices are its customers. This structure is shown in Fig. The subnet operator will connect to other customers too, as long as they can pay and it can provide service. Since it would be a disappointing network service if the customers could only send packets to each other, the subnet operator will also connect to other networks that are part of the Internet. Such a subnet operator is called an ISP (Internet Service Provider) and the subnet is an ISP network. Its customers who connect to the ISP receive Internet service. 



How the network makes the decision as to which path to use is called the routing algorithm. How each router makes the decision as to where to send a packet next is called the forwarding algorithm.