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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