| Regular phone lines, also known as
plain old telephone service (POTS), were designed to carry voice
traffic and are analog by definition. On the other hand, computers
are digital and communicate digitally. For example, in order for a
small office to connect to a central site, the digital equipment
needs to communicate over an analog connection. For this
communication to occur, the digital signal needs to be converted
into an analog tone. A modem performs this conversion.
A modem
(modulator/demodulator) converts digital signals to analog, and vice
versa. It converts digital to analog, only to be converted back to
digital, as seen in Figure
.
Modems can achieve data rates ranging from 300 bps up to 56 Kbps.
In order to achieve higher data rates, special conditions such as
the presence of only one analog to digital conversion in the
communication path must be adhered to. It is not uncommon for analog
paths to introduce noise and reduce the data rates modems can
achieve.
Universal Asynchronous Receiver/Transmitter
The transmission speed can also be limited by the Universal
Asynchronous Receiver/Transmitter (UART) type of a workstation. The UART is a
computer component that handles asynchronous serial communication.
Every computer contains a UART to manage serial ports, such as those
used for a mouse or an external modem. Every internal modem has its
own UART.
Controlled by a
clock usually running at 1.84 Mega-Hertz (MHz), UARTs have a maximum
throughput of 115 Kbps. UARTs have a memory buffer to temporarily hold
incoming data. This buffer varies by the model, but it is usually
quite small.
|
Type of UARTs: |
16C450 |
|
16450 |
|
16550 uses a 16-byte buffer |
|
16550af |
|
16750 uses a 64-byte transmit buffer and a 56-byte receive buffer |
Data terminal equipment (DTE) are end
devices such as PCs, workstations, and mainframe computers. End
devices communicate with each other through data communications
equipment (DCE) such as modems, channel service units (CSUs), and
data service units (DSUs).
Figure ,
displays the DTE/DCE interface. DCE can also be expanded to mean
data circuit-terminating equipment, which is the International
Telecommunication Union Telecommunication Standardization Sector (ITU-TSS,
or simply ITU-T), formerly known as CCITT. The Electronic Industries
Association (EIA) defines the DCE acronym as data communications
equipment. The EIA/TIA-232 standard defines the interface between
DTE and DCE. TIA stands for Telecommunications Industries
Association.
Note: The EIA/TIA-232-C
(formerly known as RS-232-C) standard is the most commonly used
asynchronous interface for data communications in North America. The
RS-232 standard was first issued in 1962, and its third revision,
RS-232-C, was issued in August 1969. Although the ubiquitous
D-shaped 25-pin connector (DB-25) has become the market norm for EIA/TIA-232-C
interfaces, it was not specified in the original RS-232-C standard.
Many EIA/TIA-232-C devices use other connectors, such as the DB-9 or
RJ-11/RJ-45 modular connectors. X.21 is a European standard that
defines the DCE/DTE interface.
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