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Chapter 6The Data Communications Interface

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Data Flow: Simplex

Transmits in only one directionrarely used in data communicationse.g., receiving signals from the radio

or TV stationthe sending station has only one

transmitter, the receiving station has only one receiver

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Simplex Illustration

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Data Flow: Half Duplexdata may travel in both directions, but only

in one direction at a timeprovides non-simultaneous two-way

communicationcomputers use control signals to negotiate

when to send and when to receivethe time it takes to switch between

sending and receiving signals is called turnaround time

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Half Duplex Illustration

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Data Flow: Full Duplex

complete two-way simultaneous transmission

faster than half-duplex communication because no turnaround time is needed

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Full Duplex Illustration

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While OSI model is increasingly out of favor in application development, it is still very useful in understanding networking in a conceptual context

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ISO’s Open Systems Interconnection (OSI)

Application LayerPresentation LayerSession LayerTransport LayerNetwork LayerData Link LayerPhysical Layer

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Physical Layer

Refers to transmission of unstructured bits over physical medium

Deals with characteristics of and access to the physical medium

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Data Link Layer

Provides for reliable transfer of information across physical link

Includes: transmission of blocks of data

(“frames”) synchronization error control flow control

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Asynchronous & Synchronous TransmissionTiming problems require a mechanism to synchronize the

transmitter and receiverTwo solutions exist

Asynchronous Synchronous

Both methods are concerned with timing issuesHow does the receiver know when the bit period begins

and ends?Small timing difference becomes more significant over

time if no synchronization takes place between sender and receiver

Synchronization occurs on the data link layer

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Asynchronous Transmission

Used in serial communication

Data transmitted 1 character at a time

Character format is usually 1 start & 1+ stop bits, plus data of 5-8 bits

Character may include parity bit

Timing needed only within each character

Resynchronization is accomplished with each start bit

Uses simple, cheap technology

Wastes 20-30% of bandwidth

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Synchronous Transmission

Used in parallel communication

Large blocks of bits transmitted without start/stop codes

Synchronized by a clock signal or clocking data

Data framed by preamble (opening)/ postamble (closing) bit patterns

More efficient than asynchronous

Overhead typically below 5%

Used at higher speeds than asynchronous

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Synchronization Choices

Low-speed terminals and PCs commonly use asynchronous transmission inexpensive

Large systems and networks commonly use synchronous transmission overhead too expensive; efficiency necessary error-checking more important

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Digital Interfaces

The point at which one device connects to another

Standards define what signals are sent, and how

Some standards also define the physical connector to be used

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Generic Communications Interface Illustration

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DTE and DCE

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RS-232 and RS-449 It is a physical protocol to interface computers

with modems specify mechanical, electrical, functional,

and procedural interface

Computeror

TerminalModem

Protective Ground (1)

Transmit (2)

Receive (3)

Request to Send (4)

Clear to Send (5)

Data Set Ready (6)

Common Return (7)

Carrier Detect (8)

Date Terminal Ready (20)

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RS-232C (EIA 232C)

EIA’s “Recommended Standard” (RS)Specifies mechanical, electrical,

functional, and procedural aspects of the interface

Used for connections between DTEs and voice-grade modems, and many other applications

EIA-Electronics Industries Association

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Mechanical Specifications

25-pin connector with a specific arrangement of leads

DTE devices usually have male DB25 connectors while DCE devices have female

In practice, fewer than 25 wires are generally used in applications

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V.24/EIA-232-FITU-T v.24Only specifies functional and procedural

References other standards for electrical and mechanical

EIA-232-F (USA) Based on RS-232 Mechanical aspects are defined by ISO 2110 Electrical v.28 Functional v.24 Procedural v.24

ITU-International Telecommunication Union

ISO-International Standards Organization

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DB-25 Female

DB-25 Male

RS-232 DB-25 Connectors

DB Connector-Data Bus Connector

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RS-232 DB-25 Pinouts

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RS-232 DB-9 Connectors

Limited RS-232

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RS-422 DIN-8 Found on Macs, some IBM compatible computers,

network processor panels

DIN-8 Male DIN-8 Female

DIN Connector-Deutsche Industrie Norm Connector

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Electrical Specifications

Specifies signaling between DTE and DCEUses NRZ-L encoding

Voltage < -3V = binary 1 Voltage > +3V = binary 0 Voltage could be as high as 25 volts

Rated for >20Kbps and <15M greater distances and rates are theoretically

possible, but not necessarily wise

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RS-232 Signals (Asynch)

Even Parity

Odd Parity

No Parity

See ASCII Table 3.1, Page 83

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Functional Specifications

Specifies the role of the individual circuits

Data circuits in both directions allow full-duplex communication

Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

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Procedural Specifications

Multiple procedures are specifiedSimple example: exchange of

asynchronous data on private line Provides means of attachment between

computer and modem Specifies method of transmitting

asynchronous data between devices Specifies method of cooperation for

exchange of data between devices

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See Table 6.1, Page 184

For the older RS-232-C standard, some of the pin definitions are:

Pin Number Name (function)

2 TD (Transmitted Data)

3 RD (Received Data)

4 RS (Request to Send)

5 CS (Clear to Send)

6 DSR (Data Set Ready)

20 DTR (Data Terminal Ready)

8 CD (Carrier Detect)

21 SQ (Signal Quality detector)

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Dial Up Operation (1)

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Dial Up Operation (2)

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Dial Up Operation (3)

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Limited Distance Modem Example (Point-to-Point)

Only a few circuits are necessary: Signal Ground (7) Transmitted Data (2) Received Data (3) Request to Send (4) Clear to Send (5) DCE Ready (6) Received Line Signal

Detector [Carrier Detect] (8)

Additional circuits necessary sometimes: DTE Ready(20) Ring Indicator (22)

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Null Modem Cable

SGDTR

DSR

RTS

CTSCDTDRD

SGDTR

DSR

RTS

CTSCDTDRD

Allows DTE to DTE direct communication

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EIA-232-D

newer version of RS-232-C adopted in 1987

improvements in grounding shield, test and loop-back signals

the popularity of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace

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RS-449

an EIA standard that improves on the capabilities of RS-232-C

provides for a 37-pin connection, cable lengths up to 200 feet, and data transmission rates up to 2 million bps

equates with the functional and procedural portions of R-232-C the electrical and mechanical specifications are

covered by RS-422 and RS-423