Numero de Brotes

Digital information sources, such as personal computers, communicate with each other us- ing the POTS (plain old telephone system) telephone network in a manner very similar to the way analog information sources, such as human conversations, communicate with each other using the POTS telephone network. With both digital and analog information sources, special devices are necessary to interface the sources to the telephone network.

Figure 8 shows a comparison between human speech (analog) communications and computer data (digital) communications using the POTS telephone network. Figure 8a shows how two humans communicate over the telephone network using standard analog telephone sets. The telephone sets interface human speech signals to the telephone network and vice versa. At the transmit end, the telephone set converts acoustical energy (information) to electrical energy and, at the receive end, the telephone set converts electrical energy back to acoustical energy. Figure 8b shows how digital data are transported over the telephone net- work. At the transmitting end, a telco interface converts digital data from the transceiver to analog electrical energy, which is transported through the telephone network. At the re- ceiving end, a telco interface converts the analog electrical energy received from the tele- phone network back to digital data.

In simplified terms, a data communications system is comprised of three basic ele- ments: a transmitter (source), a transmission path (data channel), and a receiver (destina- tion). For two-way communications, the transmission path would be bidirectional and the source and destination interchangeable. Therefore, it is usually more appropriate to de- scribe a data communications system as connecting two endpoints (sometimes called

nodes) through a common communications channel. The two endpoints may not possess

the same computing capabilities; however, they must be configured with the same basic components. Both endpoints must be equipped with special devices that perform unique functions, make the physical connection to the data channel, and process the data before they are transmitted and after they have been received. Although the special devices are

Fundamental Concepts of Data Communications

Telephone network Acoustical energy Acoustical energy Human Telephone set Human Telephone set Electrical energy Electrical energy

(a) Transceiver 1 Digital data Transceiver Digital data Telco interface Analog electrical energy Analog electrical energy Telco interface Telephone network (b)

FIGURE 8 Telephone communications network: (a) human communications; (b) digital data communications

sometimes implemented as a single unit, it is generally easier to describe them as separate entities. In essence, all endpoints must have three fundamental components: data terminal

equipment, data communications equipment, and a serial interface. 8-1 Data Terminal Equipment

Data terminal equipment (DTE) can be virtually any binary digital device that generates,

transmits, receives, or interprets data messages. In essence, a DTE is where information originates or terminates. DTEs are the data communications equivalent to the person in a telephone conversation. DTEs contain the hardware and software necessary to establish and control communications between endpoints in a data communications system; however, DTEs seldom communicate directly with other DTEs. Examples of DTEs include video display terminals, printers, and personal computers.

Over the past 50 years, data terminal equipment has evolved from simple on-line printers to sophisticated high-level computers. Data terminal equipment includes the con- cept of terminals, clients, hosts, and servers. Terminals are devices used to input, output, and display information, such as keyboards, printers, and monitors. A client is basically a modern-day terminal with enhanced computing capabilities. Hosts are high-powered, high- capacity mainframe computers that support terminals. Servers function as modern-day hosts except with lower storage capacity and less computing capability. Servers and hosts maintain local databases and programs and distribute information to clients and terminals.

8-2 Data Communications Equipment

Data communications equipment (DCE) is a general term used to describe equipment that in-

terfaces data terminal equipment to a transmission channel, such as a digital T1 carrier or an analog telephone circuit. The output of a DTE can be digital or analog, depending on the ap- plication. In essence, a DCE is a signal conversion device, as it converts signals from a DTE to a form more suitable to be transported over a transmission channel. A DCE also converts those signals back to their original form at the receive end of a circuit. DCEs are transparent devices responsible for transporting bits (1s and 0s) between DTEs through a data communi- cations channel. The DCEs neither know nor do they care about the content of the data.

There are several types of DCEs, depending on the type of transmission channel used. Common DCEs are channel service units (CSUs), digital service units (DSUs), and data

modems. CSUs and DSUs are used to interface DTEs to digital transmission channels. Data

modems are used to interface DTEs to analog telephone networks. Because data commu- nications channels are terminated at each end in a DCE, DCEs are sometimes called data

circuit-terminating equipment (DCTE). Data modems are described in subsequent sections

of this chapter.

9 DATA COMMUNICATIONS CIRCUITS

A data modem is a DCE used to interface a DTE to an analog telephone circuit commonly called a POTS. Figure 9a shows a simplified diagram for a two-point data communications circuit using a POTS link to interconnect the two endpoints (endpoint A and endpoint B). As shown in the figure, a two-point data communications circuit is comprised of the seven basic components:

1. DTE at endpoint A

2. DCE at endpoint A

3. DTE/DCE interface at endpoint A

4. Transmission path between endpoint A and endpoint B

5. DCE at endpoint B

6. DTE at endpoint B

7. DTE/DCE interface at endpoint B

The DTEs can be terminal devices, personal computers, mainframe computers, front- end processors, printers, or virtually any other piece of digital equipment. If a digital com- munications channel were used, the DCE would be a CSU or a DSU. However, because the communications channel is a POTS link, the DCE is a data modem.

Figure 9b shows the same equivalent circuit as is shown in Figure 9a, except the DTE and DCE have been replaced with the actual devices they represent—the DTE is a personal computer, and the DCE is a modem. In most modern-day personal computers for home use, the modem is simply a card installed inside the computer.

Figure 10 shows the block diagram for a centralized multipoint data communications circuit using several POTS data communications links to interconnect three endpoints. The circuit is arranged in a bus topology with central control provided by a mainframe computer (host) at endpoint A. The host station is sometimes called the primary station. Endpoints B and C are called secondary stations. The primary station is responsible for establishing and maintaining the data link and for ensuring an orderly flow of data between itself and each of the secondary stations. Data flow is controlled by an applications program stored in the mainframe computer at the primary station.

At the primary station, there is a mainframe computer, a front-end processor (DTE), and a data modem (DCE). At each secondary station, there is a modem (DCE), a line control unit (DTE), and a cluster of terminal devices (personal computers, printers, and so on). The line control unit at the secondary stations is referred to as a cluster controller, as it controls data flow between several terminal devices and the data communications channel. Line con- trol units at secondary stations are sometimes called station controllers (STACOs), as they control data flow to and from all the data communications equipment located at that station. For simplicity, Figure 10 only shows one data circuit served by the mainframe com- puter at the primary station. However, there can be dozens of different circuits served by one mainframe computer. Therefore, the primary station line control unit (i.e., the front-end processor) must have enhanced capabilities for storing, processing, and retransmitting data it receives from all secondary stations on all the circuits it serves. The primary station stores software for database management of all the circuits it serves. Obviously, the duties

Fundamental Concepts of Data Communications

POTS Telephone network Transmission path DTE/DCE Interface DTE Endpoint A Endpoint B DTE DCE DCE DTE/DCE Interface (a) POTS Telephone network Transmission path DTE/DCE Interface Modem Modem _PC Endpoint B PC Endpoint A DTE/DCE Interface (b)

FIGURE 9 Two point data communications circuit: (a) DTE/DCE representation; (b) device representation

Modem (DCE) Front-end processor (DTE) Endpoint A Primary Station Endpoint B Secondary Mainframe computer (host) Parallel interface Modem (DCE)

Transmission medium (POTS links)

Line control unit (DTE) Printer Serial interface Serial interface PC1 Terminal devices PC2 Endpoint C Secondary Modem (DCE) Line control unit (DTE) Printer Serial interface PC1 Terminal devices PC2

FIGURE 10 Multipoint data communications circuit using POTS links

performed by the front-end processor at the primary station are much more involved than the duties performed by the line control units at the secondary stations. The FEP directs data traffic to and from many different circuits, which could all have different parameters (i.e., different bit rates, character codes, data formats, protocols, and so on). The LCU at the sec- ondary stations directs data traffic between one data communications link and a relative few terminal devices, which all transmit and receive data at the same speed and use the same data-link protocol, character code, data format, and so on.