DATA COMMUNICATION INTERFACE

DATA COMMUNICATION INTERFACE

For two devices linked by a transmission medium to exchange data, a higf

degree of cooperation is required. Typically, data are transmitted one bit at a timc

over the medium. The timing (rate, duration, spacing) of these bits must be thc

same for transmitter and receiver. Two common techniques for controlling this tim

ing-asynchronous and synchronous-are explored in Section 5.1. Next, we look at

the physical interface between data transmitting devices and the transmission line

Typically, digital data devices do not attach to and signal across the mediulr

directly. Instead, this process is mediated through a standardized interface that pro.

vides considerable control over the interaction between the transmitting/receiving

devices and the transmission line.

ASYNCHRONOUS AND SYNCHRONOUS TRANSMISSION

In this lesson, we are concerned with serial transmission of data; that is, data rate

transferred over a single signal path rather than a parallel set of lines, as is common

with 110 devices and internal computer signal paths. With serial transmission

signaling elements are sent down the line one at a time. Each signaling elemenl

may be

0 Less than one bit. This is the case, for example, with Manchester coding.

6 One bit. NRZ-L and FSK are digital and analog examples, respectively.

6 More than one bit. QPSK is an example.

For simplicity in the following discussion, we assume one bit per signaling ele.

ment unless otherwise stated. The discussion is not materially affected by this sim

plification.

Recall from Figure 2.15 that the reception of digital data involves sampling the

incoming signal once per bit time to determine the binary value. One of the diffi

culties encountered in such a process is that various transmission impairments will

corrupt the signal so that occasional errors will occur. This problem is compounded

by a timing difficulty: In order for the receiver to sample the incoming bits properly,

it must know the arrival time and duration of each bit that it receives.

Suppose that the sender simply transmits a stream of data bits. The sender has

a clock that governs the timing of the transmitted bits. For example, if data are tc

be transmitted at one million bits per second (1 Mbps), then one bit will be trans.

mitted every 1 microsecond (ms), as measured by the sender's clock. Typi

cally, the receiver will attempt to sample the medium at the center of each bit-time.

The receiver will time its samples at intervals of one bit-time. In our example, the

sampling would occur once every 1 ms. If the receiver times its samples based on its

own clock, then there will be a problem if the transmitter's and receiver's clocks are

not precisely aligned. If there is a drift of 1 percent (the receiver's clock is 1 percent

faster or slower than the transmitter's clock), then the first sampling will be 0.01 of

a bit-time (0.01 ps) away from the center of the bit (center of bit is .5 ps from beginning

and end of bit). After 50 or more samples, the receiver may be in error because

it is sampling in the wrong bit-time (50 X .01 = .5 ps). For smaller timing differences,

the error would occur later, but, eventually, the receiver will be out of step

with the transmitter if the transmitter sends a sufficiently long stream of bits and if

no steps are taken to synchronize the transmitter and receiver.

Asynchronous Transmission

Two approaches are common for achieving the desired synchronization. The first is

called, oddly enough, asynchronous transmission. The strategy with this scheme is

to avoid the timing problem by not sending long, uninterrupted streams of bits.

Instead, data are transmitted one character at a time, where each character is five

to eight bits in length.' Timing or synchronization must only be maintained within

each character; the receiver has the opportunity to resynchronize at the beginning

of each new character.

The technique is easily explained with reference to Figure 5.1. When no character

is being transmitted, the line between transmitter and receiver is in an idle

state. The definition of idle is equivalent to the signaling element for binary 1. Thus,

for NRZ-L signaling (see Figure 4.2), which is common for asynchronous transmission,

idle would be the presence of a negative voltage on the line. The beginning of

a character is signaled by a start-bit with a value of binary 0. This is followed by the

five to eight bits that actually make up the character. The bits of the character are

transmitted beginning with the least significant bit. For example, for ASCII characters,

the first bit transmitted is the bit labeled bl in Table 2.1. Usually, this is followed

by a parity bit, which therefore is in the most significant bit position. The parity

bit is set by the transmitter such that the total number of ones in the character,

including the parity bit, is even (even parity) or odd (odd parity), depending on the

convention being used. This bit is used by the receiver for error detection

. The final element is a stop, which is a binary 1. A minimum

length for the stop is specified, and this is usually 1, 1.5, or 2 times the duration of

an ordinary bit. No maximum value is specified. Because the stop is the same as the

idle state, the transmitter will continue to transmit the stop signal until it is ready to

send the next character.

If a steady stream of characters is sent, the interval between two characters is

uniform and equal to the stop element. For example, if the stop is one bit-time and

the ASCII characters ABC are sent (with even parity bit), the pattern is

01000001010010000101011000011111 . . . 111.^2T he start bit (0) starts the timing

sequence for the next nine elements, which are the &bit ASCII code and the stop

bit. In the idle state, the receiver looks for a transition from 1 to 0 to signal the

beginning of the next character and then samples the input signal at one-bit intervals

for seven intervals. It then looks for the next 1-to-0 transition, which will occur

no sooner than one more bit-time.

The timing requirements for this scheme are modest. For example, ASCII

characters are typically sent as &bit units, including the parity bit. If the receiver is

5 percent slower or faster than the transmitter, the sampling of the eighth information

bit will be displaced by 45 percent and still be correctly sampled. Figure 5.lc

shows the effects of a timing error of sufficient magnitude to cause an error in

reception. In this example we assume a data rate of 10,000 bits per second (10 kbps);

therefore, each bit is of 0.1 millisecond (ms), or 100 ms, duration. Assume that the

receiver is off by 7 percent, or 7 ms per bit-time. Thus, the receiver samples the

incoming character every 93 ms (based on the transmitter's clock). As can be seen,

the last sample is erroneous.

An error such as this actually results in two errors. First, the last sampled bit

is incorrectly received. Second, the bit count may now be out of alignment. If bit 7

is a 1 and bit 8 is a 0, bit 8 could be mistaken for a start bit. This condition is termed

a framing error, as the character plus start and stop bits are sometimes referred to

as a frame. A framing error can also occur if some noise condition causes the false

appearance of a start bit during the idle state.

Asynchronous transmission is simple and cheap but requires an overhead of

two to three bits per character. For example, for an 8-bit code, using a 1-bit-long

stop bit, two out of every ten bits convey no information but are there merely for

synchronization; thus the overhead is 20%. Of course, the percentage overhead

could be reduced by sending larger blocks of bits between the start and stop bits.

However, as Figure 5.lc indicates, the larger the block of bits, the greater the cumulative

timing error. To achieve greater efficiency, a different form of synchronization,

known as synchronous transmission, is used.

Synchronous Transmission

With synchronous transmission, a block of bits is transmitted in a steady stream

without start and stop codes. The block may be many bits in length. To prevent timing

drift between transmitter and receiver, their clocks must somehow be synchronized.

One possibility is to provide a separate clock line between transmitter and

receiver. One side (transmitter or receiver) pulses the line regularly with one short

pulse per bit-time. The other side uses these regular pulses as a clock. This technique

works well over short distances, but over longer distances the clock pulses are

subject to the same impairments as the data signal, and timing errors can occur. The

other alternative is to embed the clocking information in the data signal; for digital

signals, this can be accomplished with Manchester or Differential Manchester

encoding. For analog signals, a number of techniques can be used; for example, the

carrier frequency itself can be used to synchronize the receiver based on the phase

of the carrier.

With synchronous transmission, there is another level of synchronization

required, so as to allow the receiver to determine the beginning and end of a block

of data; to achieve this, each block begins with apreamble bit pattern and generally

ends with a postamble bit pattern. In addition, other bits are added to the block that

convey control information used in the data link control procedures discussed in

Lesson 6. The data plus preamble, postamble, and control information are called

a frame. The exact format of the frame depends on which data link control procedure

is being used.

Figure 5.2 shows, in general terms. a typical frame format for synchronous

transmission. Typically, the frame starts with a preamble called a flag, which is eight

bit-long. The same flag is used as a postamble. The receiver looks for the occurrence

of the flag pattern to signal the start of a frame. This is followed by some number of

control fields, then a data field (variable length for most protocols), more control

fields, and finally the flag is repeated.

FIGURE 5.2

For sizable blocks of data, synchronous transmission is far more efficient than

asynchronous. Asynchronous transmission requires 20 percent or more overhead.

The control information, preamble, and postamble in synchronous transmission are

typically less than 100 bits. For example, one of the more common schemes, HDLC,

contains 48 bits of control, preamble, and postamble. Thus, for a 1000-character

block of data, each frame consists of 48 bits of overhead and 1000 X 8 = 8,000 bits

of data, for a percentage overhead of only 4818048 X 100% = 0.6%.

LINE CONFIGURATION

Two characteristics that distinguish various data link configurations are topology

and whether the link is half duplex or full duplex.

Topology

The topology of a data link refers to the physical arrangement of stations on a transmission

medium. If there are only two stations, (e.g., a terminal and a computer or

two computers), the link is point-to-point. If there are more than two stations, then

it is a multipoint topology. Traditionally, a multipoint link has been used in the case

of a computer (primary station) and a set of terminals (secondary stations). In

today's environments, the multipoint topology is found in local area networks.

Traditional multipoint topologies are made possible when the terminals are

only transmitting a fraction of the time. Figure 5.3 illustrates the advantages of the

multipoint configuration. If each terminal has a point-to-point link to its computer,

then the computer must have one I10 port for each terminal. Also, there is a sepa-

rate transmission line from the computer to each terminal. In a multipoint configuration,

the computer needs only a single 110 port, thereby saving hardware costs.

Only a single transmission line is needed, which also saves costs.

Full Duplex and Half Duplex

Data exchanges over a transmission line can be classified as full duplex or half

duplex. With half-duplex transmission, only one of two stations on a point-to-point

link may transmit at a time. This mode is also referred to as two-way alternate,

suggestive of the fact that two stations must alternate in transmitting; this can be

compared to a one-lane, two-way bridge. This form of transmission is often used for

terminal-to-computer interaction. While a user is entering and transmitting data,

the computer is prevented from sending data, which would appear on the terminal

screen and cause confusion.

For full-duplex transmission, two stations can simultaneously send and receive

data from each other. Thus, this mode is known as two-way simultaneous and may

be compared to a two-lane, two-way bridge. For computer-to-computer data

exchange, this form of transmission is more efficient than half-duplex transmission.

With digital signaling, which requires guided transmission, full-duplex operation

usually requires two separate transmission paths (e.g., two twisted pairs), while

half duplex requires only one. For analog signaling, it depends on frequency; if a

station transmits and receives on the same frequency, it must operate in half-duplex

mode for wireless transmission, although it may operate in full-duplex mode for

guided transmission using two separate transmission lines. If a station transmits on

one frequency and receives on another, it may operate in full-duplex mode for wireless

transmission and in full-duplex mode with a single line for guided transmission.

Interfacing

Most digital data-processing devices have limited data-transmission capability. Typically,

they generate a simple digital signal, such as NRZ-L, 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. The

more common situation is depicted in Figure 5.4. The devices we are discussing,

which include terminals and computers, are generically referred to as data terminal

equipment (DTE). A DTE makes use of the transmission system through the mediation

of data circuit-terminating equipment (DCE). An example of the latter is a

modem.

On one side, the DCE is responsible for transmitting and receiving bits, one

at a time, over a transmission medium or network. On the other side, the DCE must

interact with the DTE. In general, this requires both data and control information

to be exchanged. This is done over a set of wires referred to as interchange circuits.

For this scheme to work, a high degree of cooperation is required. The two DCEs

that exchange signals over the transmission line or network must understand each

other. That is, the receiver of each must use the same encoding scheme (e.g., Manchester,

PSK) and data rate as the transmitter of the other. In addition, each DTEDCE

pair must be designed to interact cooperatively. To ease the burden on dataprocessing

equipment manufacturers and users, standards have been developed

that specify the exact nature of the interface between the DTE and the DCE. Such

an interface has four important characteristics:

Mechanical

Electrical

Functional

Procedural

The mechanical characteristics pertain to the actual physical connection of the

DTE to the DCE. Typically, the signal and control interchange circuits are bundled

into a cable with a terminator plug, male or female, at each end. The DTE and DCE

must present plugs of opposite genders at one end of the cable, effecting the physical

connection; this is analogous to the way residential electrical power is produced.

Power is provided via a socket or wall outlet, and the device to be attached must

have the appropriate male plug (two-pronged, two-pronged polarized, or threepronged)

to match the socket.

The electrical characteristics have to do with the voltage levels and timing of

voltage changes. Both DTE and DCE must use the same code (e.g., NRZ-L), must

use the same voltage levels to mean the same things, and must use the same dura

tion of signal elements. These characteristics determine the data rates and distances

that can be achieved.

Functional characteristics specify the functions that are performed by assigning

meanings to each of the interchange circuits. 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. The examples that follow

should clarify this point.

A variety of standards for interfacing exists; this section presents two of the

most important: V.241EIA-232-E, and the ISDN Physical Interface.

V.24/EIA-232-E

The most widely used interface is one that is specified in the ITU-T standard, V.24.

In fact, this standard specifies only the functional and procedural aspects of the

interface; V.24 references other standards for the electrical and mechanical aspects.

In the United States, there is a corresponding specification, virtually identical, that

covers all four aspects: EIA-232. The correspondence is as follows:

Mechanical: IS0 2110

Electrical: V.28

c Functional: V.24

Procedural: V.24

EIA-232 was first issued by the Electronic Industries Association in 1962, as

RS-232. It is currently in its fifth revision EIA-232-E, issued in 1991. The current

V.24 and V.28 specifications were issued in 1993. This interface is used to connect

DTE devices to voice-grade modems for use on public analog telecommunications

systems. It is also widely used for many other interconnection applications.

Mechanical Specification

The mechanical specification for EIA-232-E is illustrated in Figure 5.5. It calls for a

25-pin connector, defined in IS0 2110, with a specific arrangement of leads. This

connector is the terminating plug or socket on a cable running from a DTE (e.g.,

terminal) or DCE (e.g., modem). Thus, in theory, a 25-wire cable could be used to

connect the DTE to the DCE. In practice, far fewer interchange circuits are used in

most applications.

Electrical Specification

The electrical specification defines the signaling between DTE and DCE. Digital

signaling is used on all interchange circuits. Depending on the function of the interchange

circuit, the electrical values are interpreted either as binary or as control signals.

The convention specifies that, with respect to a common ground, a voltage

more negative than -3 volts is interpreted as binary 1 and a voltage more positive

than f 3 volts is interpreted as binary 0; this is the NRZ-L code illustrated in Figure

4.2. The interface is rated at a signal rate of <20 kbps and a distance of <15 meters.

Greater distances and data rates are possible with good design, but it is prudent to

assume that these limits apply in practice as well as in theory.

The same voltage levels apply to control signals; a voltage more negative than

-3 volts is interpreted as an OFF condition and a voltage more positive than +3

volts is interpreted as an ON condition.

Functional Specification

'Table 5.1 summarizes the functional specification of the interchange circuits, and

Figure 5.5 illustrates the placement of these circuits on the plug. The circuits can be

grouped into the categories of data, control, timing, and ground. There is one data

circuit in each direction, so full-duplex operation is possible. In addition, there are

two secondary data circuits that are useful when the device operates in a half-duplex

fashion. In the case of half-duplex operation, data exchange between two DTEs (via

their DCEs and the intervening communications link) is only conducted in one

direction at a time. However, there may be a need to send a halt or flow-control

message to a transmitting device; to accommodate this, the communication link is

equipped with a reverse channel, usually at a much lower data rate than the primary

channel. At the DTE-DCE interface, the reverse channel is carried on a separate

pair of data circuits.

There are fifteen control circuits. The first ten of these listed in Table 5.1

relate to the transmission of data over the primary channel. For asynchronous transmission,

six of these circuits are used (105, 106, 107, 108.2, 125, 109). The use of

these circuits is explained in the subsection on procedural specifications. In addition

to these six circuits, three other control circuits are used in synchronous transmis-

sion. The Signal Quality Detector circuit is turned ON by the DCE to indicate that

the quality of the incoming signal over the telephone line has deteriorated beyond

some defined threshold. Most high-speed modems support more than one transmission

rate so that they can fall back to a lower speed if the telephone line becomes

noisy. The Data Signal Rate Selector circuits are used to change speeds; either the

DTE or DCE may initiate the change. The next three control circuits (120,121,122)

are used to control the use of the secondary channel, which may be used as a reverse

channel or for some other auxiliary purpose.

The last group of control signals relate to loopback testing. These circuits

allow the DTE to cause the DCE to perform a loopback test. These circuits are only

valid if the modem or other DCE supports loopback control; this is now a common

modem feature. In the local loopback function, the transmitter output of the

modem is connected to the receiver input, disconnecting the modem from the transmission

line. A stream of data generated by the user device is sent to the modem

and looped back to the user device. For remote loopback, the local modem is connected

to the transmission facility in the usual fashion, and the receiver output of

the remote modem is connected to the modem's transmitter input. During either

form of test, the DCE turns ON the Test Mode circuit. Table 5.2 shows the settings

for all of the circuits related to loopback testing, and Figure 5.6 illustrates the use.

Loopback control is a useful fault-isolation tool. For example, suppose that a

user at a personal computer is communicating with a server by means of a modem

connection and communication suddenly ceases. The problem could be with the

local modem, the communications facility, the remote modem, or the remote server.

A network manager can use loopback tests to isolate the fault. Local loopback

checks the functioning of the local interface and the local DCE. Remote loopback

tests the operation of the transmission channel and the remote DCE.

The timing signals provide clock pulses for synchronous transmission. When

the DCE is sending synchronous data over the Received Data circuit (104), it also

sends 1-0 and 0-1 transitions on Receiver Element Signal Timing (115), with transitions

timed to the middle of each BB signal element. When the DTE is sending synchronous

data, either the DTE or DCE can provide timing pulses, depending on the

circumstances.

Finally, the signal ground/common return (102) serves as the return circuit

for all data leads. Hence, transmission is unbalanced, with only one active wire.

Balanced and unbalanced transmission are discussed in the section on the ISDN

interface.

Procedural Specification

The procedural specification defines the sequence in which the various circuits are

used for a particular application. We give a few examples.

The first example is a very common one for connecting two devices over a

short distance within a building. It is known as an asynchronous private line modem,

or a limited-distance modem. As the name suggests, the limited-distance modem

accepts digital signals from a DTE, such as a terminal or computer, converts these

to analog signals, and then transmits these over a short length of medium, such as

twisted pair. On the other end of the line is another limited-distance modem, which

accepts the incoming analog signals, converts them to digital, and passes them on to

another terminal or computer. Of course, the exchange of data is two-way. For this

simple application, only the following interchange circuits are actually required:

Signal ground (102)

Transmitted data (103)

Received data (104)

Request to send (105)

Clear to send (106)

DCE ready (107)

Received-Line Signal Detector (109)

When the modem (DCE) is turned on and is ready to operate, it asserts

(applies a constant negative voltage to) the DCE Ready line. When the DTE is

ready to send data (e.g., the terminal user has entered a character), it asserts

Request to Send. The modem responds, when ready, by asserting Clear to Send,

indicating that data may be transmitted over the Transmitted Data line. If the

arrangement is half-duplex, then Request to Send also inhibits the receive mode.

The DTE may now transmit data over the Transmitted Data line. When data arrive

from the remote modem, the local modem asserts Received-Line Signal Detector to

indicate that the remote modem is transmitting and delivers the data on the

Received Data line. Note that it is not necessary to use timing circuits, as this is

asynchronous transmission.

The circuits just listed are sufficient for private line point-to-point modems,

but additional circuits are required to use a modem to transmit data over the telephone

network. In this case, the initiator of a connection must call the destination

device over the network. Two additional leads are required:

Q DTE ready (108.2)

@ Ring indicator (125)

With the addition of these two lines, the DTE-modem system can effectively

use the telephone network in a way analogous to voice telephone usage. Figure 5.7

depicts the steps involved in dial-up half-duplex operation. When a call is made,

either manually or automatically, the telephone system sends a ringing signal. A

telephone set would respond by ringing its bell; a modem responds by asserting

Ring Indicator. A person answers a call by lifting the handset; a DTE answers by

asserting Data Terminal Ready. A person who answers a call will listen for

another's voice, and, if nothing is heard, hang up. A DTE will listen for Carrier

Detect, which will be asserted by the modem when a signal is present; if this circuit

is not asserted, the DTE will drop Data Terminal Ready. You might wonder how

this last contingency might arise; one common way is if a person accidentally dials

the number of a modem. This activates the modem's DTE, but when no carrier tone

comes through, the problem is resolved.

It is instructive to consider situations in which the distances between devices

are so close as to allow two DTEs to directly signal each other. In this case, the

V.24lEIA-232 interchange circuits can still be used, but no DCE equipment is provided.

For this scheme to work, a null modem is needed, which interconnects leads

in such a way as to fool both DTEs into thinking that they are connected to

modems. Figure 5.8 is an example of a null modem configuration; the reasons for

the particular connections should be apparent to the reader who has grasped the

preceding discussion.

LSDN Physical Interface

The wide variety of functions available with V.24lEIA-232 is provided by the use of

a large number of interchange circuits. This is a rather expensive way to achieve

results. An alternative would be to provide fewer circuits but to add more logic at

the DTE and DCE interfaces. With the dropping costs of logic circuitry, this is an

attractive approach. This approach was taken in the X.21 standard for interfacing to

public circuit-switched networks, specifying a 15-pin connector. More recently, the

trend has been carried further with the specification of an 8-pin physical connector

to an Integrated Services Digital Network (ISDN). ISDN, which is an all-digital

replacement for existing public telephone and analog telecommunications networks,

is discussed further in Lesson A. In this section, we look at the physical

interface defined for ISDN.

Physical Connection

In ISDN terminology, a physical connection is made between terminal equipment

(TE) and network-terminating equipment (NT). For purposes of our discussion,

these terms correspond, rather closely, to DTE and DCE, respectively. The physical

connection, defined in IS0 8877, specifies that the NT and TE cables shall terminate

in matching plugs that provide for 8 contacts.

Figure 5.9 illustrates the contact assignments for each of the 8 lines on both

the NT and TE sides. Two pins are used to provide data transmission in each direction.

These contact points are used to connect twisted-pair leads coming from the

NT and TE devices. Because there are no specific functional circuits, the transmit1

receive circuits are used to carry both data and control signals. The control information

is transmitted in the form of messages.

The specification provides for the capability to transfer power across the interface.

The direction of power transfer depends on the application. In a typical application,

it may be desirable to provide for power transfer from the network side

toward the terminal in order, for example, to maintain a basic telephony service in

the event of failure of the locally provided power. This power transfer can be

accomplished using the same leads used for digital signal transmission (c, d, e, f), or

on additional wires, using access leads g-h. The remaining two leads are not used in

the ISDN configuration but may be useful in other configurations.

Electrical Specification

The ISDN electrical specification dictates the use of balanced transmission. With

balanced transmission, signals are carried on a line, such as twisted pair, consisting

of two conductors. Signals are transmitted as a current that travels down one conductor

and returns on the other, the two conductors forming a complete circuit. For

digital signals, this technique is known as differential ~i g n a l i n ga,s~ t he binary value

depends on the direction of the voltage difference between the two conductors.

Unbalanced transmission, which is used on older interfaces such as EIA-232, uses a

single conductor to carry the signal, with ground providing the return path.

The balance mode tolerates more, and produces less, noise than unbalanced

mode. Ideally, interference on a balanced line will act equally on both conductors

and not affect the voltage difference. Because unbalanced transmission does not

possess these advantages, it is generally limited to use on coaxial cable; when it is

used on interchange circuits, such as ETA-232, it is limited to very short distances.

The data encoding format used on the ISDN interface depends on the data

rate. For the basic rate of 192 kbps, the standard specifies the use of pseudoternary

coding (Figure 4.2). Binary one is represented by the absence of voltage, and binary

zero is represented by a positive or negative pulse of 750 mV ?10%. For the primary

rate, there are two options: 1.544 Mbps using alternate mark inversion (AMI)

with B8ZS (Figure 4.6) and 2.049 Mbps using AM1 with HDB3. The reason for the

different schemes for the two different primary rates is simply historical; neither has

a particular advantage.