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Chapter 6
The Data Communications
Interface
1
Data Flow: Simplex
Transmits in only one direction
rarely used in data communications
e.g., receiving signals from the radio or
TV station
the sending station has only one
transmitter, the receiving station has only
one receiver
2
Simplex Illustration
3
Data Flow: Half Duplex
data may travel in both directions, but
only in one direction at a time
provides non-simultaneous two-way
communication
computers use control signals to negotiate
when to send and when to receive
the time it takes to switch between
sending and receiving signals is called
turnaround time
4
Half Duplex Illustration
5
Data Flow: Full Duplex
complete two-way simultaneous
transmission
faster than half-duplex communication
because no turnaround time is needed
6
Full Duplex Illustration
7
While OSI model is increasingly out of
favor in application development, it is still
very useful in understanding networking
in a conceptual context
8
ISO’s Open Systems
Interconnection (OSI)
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
9
Physical Layer
Refers to transmission of unstructured bits
over physical medium
Deals with characteristics of and access to
the physical medium
10
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
Transmission
Timing problems require a mechanism to
synchronize the transmitter and receiver
Two solutions exist
Asynchronous
Synchronous
Both methods are concerned with timing issues
How 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 12
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
13
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
14
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
15
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
16
Generic Communications
Interface Illustration
17
DTE and DCE
18
RS-232 and RS-449
It is a physical protocol to interface computers with
modems
specify mechanical, electrical, functional, and
procedural interface
Protective Ground (1)
Transmit (2)
Receive (3)
Computer
or
Terminal
Request to Send (4)
Clear to Send (5)
Modem
Data Set Ready (6)
Common Return (7)
Carrier Detect (8)
Date Terminal Ready (20)
19
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
20
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
21
V.24/EIA-232-F
ITU-International
Telecommunication Union
ITU-T v.24
Only 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
ISO-International Standards
Functional v.24
Organization
Procedural v.24
22
RS-232 DB-25 Connectors
DB-25 Female
DB-25 Male
DB Connector-Data Bus Connector
23
RS-232 DB-25 Pinouts
24
RS-232 DB-9 Connectors
Limited RS-232
25
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
26
Electrical Specifications
Specifies signaling between DTE and DCE
Uses 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 fullduplex communication
Timing signals allow for synchronous
transmission (although asynchronous
transmission is more common)
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Procedural Specifications
Multiple procedures are specified
Simple 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)
31
Dial Up Operation (1)
32
Dial Up Operation (2)
33
Dial Up Operation (3)
34
Limited Distance Modem
Example (Point-to-Point)
Only a few circuits are Additional circuits
necessary:
necessary sometimes:
Signal Ground (7)
DTE Ready(20)
Transmitted Data (2)
Ring Indicator (22)
Received Data (3)
Request to Send (4)
Clear to Send (5)
DCE Ready (6)
Received Line Signal Detector
[Carrier Detect] (8)
35
Null Modem Cable
Allows DTE to DTE direct communication
SG
DTR
DSR
RTS
CTS
CD
TD
RD
SG
DTR
DSR
RTS
CTS
CD
TD
RD
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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
37
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
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