Terminalto Modem Communication

This section deals with the protocols typically used on the asynchronous serial link between the host and the modem. First, you are introduced to the concept of data terminal equipment (DTE) and data circuit-terminating equipment (DCE). Then, the communications link between terminal and modem is divided into three layers. From the bottom up, they are as follows:

• RS-232 physical layer: This defines the mechanical, electrical, and hardware signaling used on the terminal-to-modem cable.

• Async framing layer: This specifies the format used to frame characters on an asynchronous serial link.

• AT command layer: This is a command language used by the host to configure and control the modem.

DTE and DCE

Various international standards bodies agreed on specifications that detail how to facilitate the connection of data communications equipment. These standards discuss the interface between DTE and DCE. The specifications describe the physical and electrical interface between a DTE and a particular type of DCE. As an example, the ITU-T V-series recommendations deal with the connection of a DTE to a modem (the DCE).

DTE is equipment that acts as a data source/sink from the point of view that it converts user information into signals to be transmitted by the DCE. The most frequently used example of a DTE is a computer. Correspondingly, DCE is the equipment that establishes and provides access to a communications link over a channel connecting the source and destination DTEs. Therefore, a DCE provides a data link service for DTEs to communicate over. In this chapter, the DCE will always be a modem. Figure 1-5 shows the logical location and function of both DTE and DCE. The practical significance in distinguishing between these two types of equipment is that they are pinned and cabled differently.

Figure 1-5 DTE and DCE Topology

Data Communications

Figure 1-5 DTE and DCE Topology

Data Communications

DTE Link

RS-232 Signaling

RS-232 is a serial transmission system designed to support communications for short distances between a DTE and a low-speed DCE. It has evolved through several generations of standards (EIA-232C, EIA-232D, EIA-232E, and a variant has separately been standardized by the ITU as V.24). RS-232 supports a variety of applications, including synchronous and asynchronous transmission. This discussion focuses on full-duplex async DTE links to contemporary modems and uses the term RS-232 in its generic sense; for more precise details, consult the standards.

A standard RS-232 link will use the DB-25 connector. Normally (but not necessarily), the DTE port is male, and the DCE port is female. PC DTE ports often use a DB-9 connector, whereas Cisco normally uses a nonstandard 8-pin modular (RJ-45) connector for its async ports. Table 1-5 summarizes the pinouts for all three of these interface types. (Pinouts are from the plug side. Jack side pinouts are rolled.)

NOTE Technically PC DTE ports use a DE-9 connector. The misnomer "DB-9" is not a connector that exists in practice, but it is mistakenly used so frequently that it has become the de facto term for a PC DTE interface. Consequently, this book will henceforth use the commonly used DB-9 nomenclature when referring to the connector of a PC DTE port.

Table 1-5 Pinouts for Different RS-232 Interfaces

DB-25

DB-9

RJ-45

Name

From

Description

1

GND

gnd

Protective (shield) Ground

7

5

4, 5

SG

gnd

Signal Ground

2

3

6

TxD

DTE

Transmitted Data

3

2

3

RxD

DCE

Received Data

4

7

8

RTS

DTE

Request to Send (hw flow control)

5

8

1

CTS

DCE

Clear to Send (hw flow control)

6

6

2

DSR

DCE

Data Set Ready (DCE ready)

20

4

7

DTR

DTE

Data Terminal Ready

22

9

RI

DCE

Ring Indicator

8

1

(2)

CD

DCE

Data Carrier Detect

21

RL

DCE

Remote Loop / sig quality

23

CH/CI

DTE/DCE

Signal Rate Selector

24

DA

DTE

DTE Tx Timing

15

DB

DCE

DCE Tx Timing

17

DD

DCE

Rx Timing

14

SBA

DTE

Secondary TxD

16

SBB

DCE

Secondary RxD

Table 1-5 Pinouts for Different RS-232 Interfaces (Continued)

DB-25

DB-9 RJ-45 Name

From

Description

19

SCA

DTE

Secondary RTS

13

SCB

DCE

Secondary CTS

12

SCF

DCE

Secondary DCD

Not all 25 conductors are used; for async applications, typically from 3 to 9 conductors will be used, depending on whether hardware (hw) flow control or modem control signaling is required.

RS-232 does not specify bit rates per se. However, for async transmission, the following rates have been typically seen: 50, 75, 110, 134.5, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, 115200, 230400 bps. As specified in the standards, RS-232 is officially considered to be suitable only for data rates of up to 20 Kbps and distances of up to 50 feet. In practice, RS-232 is often run at 115200 bps for distances of up to 20 feet, and at 9600 bps for distances of as much as 500 feet.

The RS-232 protocol defines nine electrical circuits to handle all the handshaking between a DTE and DCE. These electrical circuits (also referred to as leads, pins, or signals) are grouped into three categories: data interchange circuits, control interchange circuits, and the ground circuit.

Data leads are used to signal the exchange of data. Control leads govern the call signaling states between a DTE and a DCE and manage the flow control between them. As its name suggests, the ground lead is the reference ground for the DTE and the DCE. Table 1-6 details each of the nine RS-232 pins and their individual function.

NOTE Terminology, such as "raise" or "assert," with regard to the RS-232 pins implies putting a particular voltage on it. Likewise, the terms "lower" and "drop" imply changing the polarity of that voltage.

Table 1-6 RS-232 Circuits and Their Function

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