Networks are frequently classified according to the geographical boundaries the network spans. Two basic geographical designations for networks—local area network (LAN) and wide area network (WAN)—are the most common. A third designation, metropolitan area network (MAN), is also used, although its use has become clouded (because it might not be a clear-cut classification anymore) as networks continue connecting to the Internet. These three classifications, unlike the other methods used to describe networks, are based upon the specific levels of technology they use when going from one level to the other. The three geographical classifications are discussed because the geographical concepts and the increased emphasis they place on technology as you go from one level to the next still apply.
When we try to identify ourselves within the Network, we usually run a commands like ipconfig /all (on ms windows platforms). Apart from information related to the computer we work on, the command displays a list of IP related devices – in our example – a wireless adapter, a network card and an ADSL modem, devices which allow us to communicate to a network. Each of these devices exhibit a physical address – six bytes in hexadecimal representation and, for two of them, for which the connection is active, an IP address. These devices, which are, to some extent, part of our workstation, serves as a network interface. This clarifies a first issue, IP addresses are not for computers, as the general belief goes, but for interfaces, like network cards,wireless adapters, firewire ports. Also, interfaces serve as end points for communication links, which may be UTP cables, coaxial cables or radio waves. To make a distinction between a workstation (or another physical
network element) and its interfaces, a physical device consisting of a processing unit with one or more interfaces will be called a network device. Continue reading “Modeling the Network – Network Devices and their Interfaces”
The purpose of any telecommunications system is to transfer information from the sender to the receiver by a means of a communication channel.The information is carried by a signal, which is certain physical quantity that changes with time.
The signal can be a voltage proportional to the amplitude of the voice,
like in a simple telephone, a sequence of pulses of light in an optical fibre, or a radio-electric wave irradiated by an antenna.
For analog signals, these variations are directly proportional to some physical variable like sound, light, temperature, wind speed, etc. The information can also be transmitted by digital binary signals, that will have only two values, a digital one and a digital zero. Any analog signal can be converted into a digital signal by appropriately sampling and then coding it. The sampling frequency must be at least twice the maximum frequency
present in the signal in order to carry all the information contained therein. Random signals are the ones that are unpredictable and can be described only by statistical means.