Most digital data processing devices have limited data transmission capability. Typically, they generate a simple digital signal and the distance across which they can transmit data is limited. Consequently, it is rare for such a device (terminal, computer) to attach directly to a transmission or networking facility.

Data communication interfacing.
Data Terminal Equipments (DTE) (for example, a PC), do not connect to a network directly. Such a piece of equipment communicates through Data Circuit terminating Equipment (DCE) (for example, a modem). We call the connection between the DTE and DCE the DTE-DCE interface.

DTE-DCE interface has four important characteristics:
(1) Mechanical
(2) Electrical
(3) Functional
(4) Procedural.
The mechanical characteristics pertain to the actual physical connection of the DTE and DCE. DTE and DCE are connected by pin connectors of opposite genders (male/female). Both DTE and DCE must use the same voltage levels to mean the same things, and must use the same duration of signal elements as electrical characteristics. Functions can be classified into the broad categories of data, control, timing, and electrical ground.
Procedural characteristics specify the sequence of events for transmitting data, based on the functional characteristics of the interface.
One important interface standard RS-232 developed by the Electronic Industries Association (EIA), defines the mechanical, electrical and functional characteristics of the interface between a DTE and DCE. RS-232 standard has been revised several times. Probably, the most common version is the one developed in the late 1960s, and known as RS-232-C. DTE and DCE are connected by a 25-line cable (sometimes called a DB-25 cable). 25-pin connectors are used to connect each device. Each line has a specific function in establishing communication between the devices.
EIA defines the functions assigned to each of the 25-pins in the DB-25 connector. Figure 2.30 shows the ordering and functionality of each pin of a male connector (female connector is the mirror image of male connector). As we can see from Figure 2.30 not every pin is functional. Pins 9 and 10 are reserved for future use. Pin 11 is as yet unassigned.

The function assigned to each of the 25 pins is shown in Table 2.2.

Both data rate to 20 Kbps and cable length to 50 feet (15 meters) are restricted by EIA-232 (RS-232). To meet the needs of users who require more speed and/or distance, EIA and the ITU-T have introduced additional interface standards: RS-449 and X.21.
RS-449 was designed to replace RS-232 and increase both distance and bandwidth. RS-449 standard, unlike RS-232, differentiates between operational and electrical specification. That is, RS-449 defines pin functions, but relies on one of two other standards RS-422 or RS-423, for electrical standards. RS-449 defines a 37-pin connection.
The two electrical standards RS-422 and RS-423, correspond to balanced circuits and unbalanced circuits, respectively. An unbalanced circuit uses one line for single transmission while a balanced circuit uses two lines for single transmission.
RS-449 using a balanced signal can transmit up to 10 Mbps over short distances (about 40 feet). Lowering the data rate to 100 Kbps allows transmission upto about4000 feet. Using an unbalanced circuit, the data rate is limited to 100 Kbps at 40 feet. It also allows for communications upto 4000 feet, but only at a data rate 1200 bps. The X.21 interface standard is defined by ITU-T. It uses a 15-pin connector and like RS-449, allows balanced (electrical standard X.27) and unbalanced (X.26 standard) circuits.
The difference between X.21 and the RS standards is that X.21 was defined as a digital signaling interface. Experts agree that analog communications systems eventually will be replaced entirely by digital ones.
X.21 is useful both as an interface to connect digital computers to analog devices such as modems and as a connector between digital computers and digital interfaces such as Integrated Services Digital Network (ISDN). X.21 is designed to work with balanced circuits at 64 Kbps, a rate that is becoming the industry standard.

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