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Communications software often is required and generally is embedded in the computer operating system. Alternatively, it can take the form of a systems task under the control of the computers operating system. The role of communications software is to assist the operating system in managing local and remote terminal access to host resources, to manage security and to perform certain checkpoint activities. The remote terminals interface to the operating system access methods, which contain the specific code required to transfer data across the network channels between the devices. Examples of access methods include IBMs BTAM (Basic Telecommunications Access Method) and VTAM (Virtual Telecommunications Access Method).
Alternatively, commercial communications management software can be used to control and manage access to the host. IBMs CICS (Customer Information Control System) is such a product. Such software resolves contention issues between diverse applications without impacting programs or terminals. It handles polling, selection, and program interrupts, thereby ensuring minimum response time. It also resolves error conditions at both the data and line levels. CICS and other TSRs (Terminate-and-Stay-Resident) software is maintained in RAM (Random Access Memory).
Also known as paths, circuits, or channels, networks provide the connections between computer resources in order to accommodate the flow of information. The network can be in the form of a Local Area Network (LAN), a Metropolitan Area Network (MAN), or a Wide Area Network (WAN), which provide communications over areas of increasing geographic scope. LANs, MANs, and WANs, which will be discussed in later chapters, also can be interconnected.
Developed in support of voice communications, circuit switches serve to provide for the flexible interconnection of circuits, in the process providing a communications path that supports a continuous stream of transmission. Traditional circuit switches still are widely used in support of data communications, although they have lost much ground to packet switches during the past 25 years. Frame and cell switches have appeared during the past few years, and represent the switching technique for data communications far into the future.
Packet, frame, and cell switches are highly advanced, computerized switching devices which have substantial capacity and which have the ability to share high-capacity transmission systems among large numbers of individual user transmissions. Such switches read the destination address of each segment (packet, frame, or cell) of data in a data stream. Each packet, frame or cell individually then is forwarded through the switch. Packet, frame, and cell switching will be discussed in detail in later chapters.
While we have discussed, and will discuss, data networks and switches in great detail in other chapters, this is a convenient and meaningful place to pause and explore the concept and detail of several types of data communications equipment. Specifically, we will examine modems, codecs, terminal adapters, CSUs and DSUs, and front end processors.
Modems MOdulate and DEModulate signals. In other words, they change the characteristics of the signal in some way. Modems are of several basic types, line drivers, short haul modems, and conventional modems.
Line drivers
Line drivers actually are interface converters, rather than modems in the classic sense. Line drivers are used to extend the distance of a digital connection, within finite limits, by converting the digital signal to a low-voltage, low-impedance signal that can be transmitted more effectively and over longer distances over dedicated, specially conditioned twisted pair circuits. As an example, the RS-232 specification generally limits the distance between devices to 50 feet at transmission rates of 56 Kbps. At lower speeds, line drivers can reshape the digital pulses to extend that distance. At speeds of up to 9.6 Kbps, line drivers can extend that limitation to 500-5,000 feet. The distance can be extended further through the cascading of line drivers, in a unidirectional network; bi-directional communications requires separate wire pairs and separate sets of line drivers.
Short-Haul (limited-distance) modems
Short-Haul (limited-distance) modems are used where line drivers fail in terms of either capacity or distance. Short-haul modems can work at distances between 5,000 and 100,000 feet, and usually are used for private line and hardwired links, but can operate over local loop facilities.
Conventional modems
Conventional modems provide for digital communication across an analog circuit, accomplishing the digital-to-analog conversion in order to resolve that dimension of incompatibility between the DTE and the network. The original de facto standards for modems were set by AT&T with the introduction of the DataPhone in 1961 [7-2]. Currently, standards are international in nature, designated as the V.xx family of standards from the ITU-T. The digital input is in the form of varying electrical voltages, which represent binary 1s and 0s. The output from the modem is a modulated analog carrier wave, that can be modulated in terms of its amplitude, frequency or phase, or a combination. Through this process, the 1s and 0s of the digital data world can be sent over the plain old voice telephone network.
Amplitude Modulation (AM)
Amplitude Modulation (AM) involves the modulation of the amplitude of the analog sine wave, as depicted in Figure 7.1. Using a single-bit AM technique, each 1 bit entering the transmitting modem is expressed as a relatively high amplitude sine wave or series of sine waves, and each 0 bit as low amplitude sine waves. It is possible to express multiple bits by defining four levels of amplitude. In a dibit (2-bit) coding scheme, for instance, the lowest level of amplitude represents a 00 bit pattern, the next highest a 01 bit pattern, the next a 10, and the highest a 11. In this fashion, the speed of data transmission is doubled at the same analog line speed; halving the connection time and reducing the cost of transmission. Amplitude modulation rarely is used individually as it is highly sensitive to the impacts of attenuation and line noise.
Figure 7.1 Amplitude Modulation.
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