In the past, networks used to be designed so that the physical cables transported data in the same form in which the data was created: analog voice data generated by telephones used to be carried by analog transmission cables and digital computer data used to be carried by digital transmission cables. Today, it is simple to separate the different types of data (analog voice or digital computer) from the actual physical cables used to carry the data. In most cases, the cheapest and highest-quality media are digital, which means that most data today are transmitted in digital form. Thus the convergence of voice and video and data at the physical layers is being driven primarily by business reasons: digital is better.
The change in physical layers also has implications for organizational structure. Voice data used to be managed separately from computer data because they use different types of networks. As the physical networks converge, so too do the organizational units responsible for managing the data. Today, more organizations are placing the management of voice telecommunications into their information systems organizations.
This also has implications for the telecommunications industry. Over the past five years, the historical separation between manufacturers of networking equipment used in organizations and manufacturers of networking equipment used by the telephone companies has crumbled. There have been some big winners and losers in the stock market from the consolidation of these markets.
Summary
Circuits Networks can be configured so that there is a separate circuit from each client to the host (called a point-to-point configuration) or so that several clients share the same circuit (a multipoint configuration). Data can flow through the circuit in one direction only (simplex), in both directions simultaneously (full duplex), or by taking turns so that data sometimes flow in one direction and then in the other (half duplex). A multiplexer is a device that combines several simultaneous low-speed circuits on one higher-speed circuit so that each low-speed circuit believes it has a separate circuit. In general, the transmission capacity of the high-speed circuit must equal or exceed the sum of the low-speed circuits.
Communication Media Media are either guided, in that they travel through a physical cable (e.g., twisted-pair wires, coaxial cable, or fiber-optic cable), or wireless, in that they are broadcast through the air (e.g., radio, infrared, microwave, or satellite). Among the guided media, fiber-optic cable can transmit data the fastest with the fewest errors and offers greater security but costs the most; twisted-pair wire is the cheapest and most commonly used. The choice of wireless media depends more on distance than on any other factor; infrared and radio are the cheapest for short distances, microwave is cheapest for moderate distances, and satellite is cheapest for long distances. Digital Transmission of Digital Data Digital transmission (also called baseband transmission) is done by sending a series of electrical (or light) pulse through the media. Digital transmission is preferred to analog transmission because it produces fewer errors; is more efficient; permits higher maximum transmission rates; is more secure; and simplifies the integration of voice, video, and data on the same circuit. With unipolar digital transmission, the voltage changes between 0 volts to represent a binary 0 and some positive value (e.g., +15volts) to represent a binary 1. With bipolar digital transmission, the voltage changes polarity (i.e., positive or negative) to represent a 1 or a 0. Bipolar is less susceptible to errors. Ethernet uses Manchester encoding, which is a version of unipolar transmission.
Analog Transmission of Digital Data Modems are used to translate the digital data produced by computers into the analog signals for transmission in today’s voice communication circuits. Both the sender and receiver need to have a modem. Data is transmitted by changing (or modulating) a carrier sound wave’s amplitude (height), frequency (length), or phase (shape) to indicate a binary 1 or 0. For example, in amplitude modulation, one amplitude is defined to be a 1 and another amplitude is defined to be a 0. It is possible to send more than 1 bit on every symbol (or wave). For example, with amplitude modulation, you could send 2 bits on each wave by defining four amplitude levels. The capacity or maximum data rate that a circuit can transmit is determined by multiplying the symbol rate (symbols per second) by the number of bits per symbol. Generally (but not always), the symbol rate is the same as the bandwidth, so bandwidth is often used as a measure of capacity. V.44 is a data compression standard that can be combined with any of the foregoing types of modems to reduce the amount of data in the transmitted signal by a factor of up to six. Thus, a V.92 modem using V.44 could provide an effective data rate of 56,000 x 6 = 336, 000 bps.
Digital Transmission of Analog Data Because digital transmission is better, analog voice data is sometimes converted to digital transmission. Pulse code modulation (PCM) is the most commonly used technique. PCM samples the amplitude of the incoming voice signal 8,000 times per second and uses 8 bits to represent the signal. PCM produces a reasonable approximation of the human voice, but more sophisticated techniques are needed to adequately reproduce more complex sounds such as music.
