The physical layer is the basis of all networks. Nature imposes two fundamental
limits on all channels, and these determine their bandwidth. These limits
are the Nyquist limit, which deals with noiseless channels, and the Shannon limit,
which deals with noisy channels.
Transmission media can be guided or unguided. The principal guided media are twisted pair, coaxial cable, and fiber optics. Unguided media include terrestrial radio, microwaves, infrared, lasers through the air, and satellites.
Digital modulation methods send bits over guided and unguided media as analog signals. Line codes operate at baseband, and signals can be placed in a passband by modulating the amplitude, frequency, and phase of a carrier. Channels can be shared between users with time, frequency and code division multiplexing.
A key element in most wide area networks is the telephone system. Its main components are the local loops, trunks, and switches. ADSL offers speeds up to 40 Mbps over the local loop by dividing it into many subcarriers that run in parallel. This far exceeds the rates of telephone modems. PONs bring fiber to the home for even greater access rates than ADSL.
Trunks carry digital information. They are multiplexed with WDM to provision many high capacity links over individual fibers, as well as with TDM to share each high rate link between users. Both circuit switching and packet switching are important.
For mobile applications, the fixed telephone system is not suitable. Mobile phones are currently in widespread use for voice, and increasingly for data. They have gone through three generations. The first generation, 1G, was analog and dominated by AMPS. 2G was digital, with GSM presently the most widely deployed mobile phone system in the world. 3G is digital and based on broadband CDMA, with WCDMA and also CDMA2000 now being deployed.
An alternative system for network access is the cable television system. It has gradually evolved from coaxial cable to hybrid fiber coax, and from television to television and Internet. Potentially, it offers very high bandwidth, but the bandwidth in practice depends heavily on the other users because it is shared.
Transmission media can be guided or unguided. The principal guided media are twisted pair, coaxial cable, and fiber optics. Unguided media include terrestrial radio, microwaves, infrared, lasers through the air, and satellites.
Digital modulation methods send bits over guided and unguided media as analog signals. Line codes operate at baseband, and signals can be placed in a passband by modulating the amplitude, frequency, and phase of a carrier. Channels can be shared between users with time, frequency and code division multiplexing.
A key element in most wide area networks is the telephone system. Its main components are the local loops, trunks, and switches. ADSL offers speeds up to 40 Mbps over the local loop by dividing it into many subcarriers that run in parallel. This far exceeds the rates of telephone modems. PONs bring fiber to the home for even greater access rates than ADSL.
Trunks carry digital information. They are multiplexed with WDM to provision many high capacity links over individual fibers, as well as with TDM to share each high rate link between users. Both circuit switching and packet switching are important.
For mobile applications, the fixed telephone system is not suitable. Mobile phones are currently in widespread use for voice, and increasingly for data. They have gone through three generations. The first generation, 1G, was analog and dominated by AMPS. 2G was digital, with GSM presently the most widely deployed mobile phone system in the world. 3G is digital and based on broadband CDMA, with WCDMA and also CDMA2000 now being deployed.
An alternative system for network access is the cable television system. It has gradually evolved from coaxial cable to hybrid fiber coax, and from television to television and Internet. Potentially, it offers very high bandwidth, but the bandwidth in practice depends heavily on the other users because it is shared.
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