Transcript Document 7566197

Chapter 6
The Data Communications
Interface
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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
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Simplex Illustration
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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
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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 Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical 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
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
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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
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)
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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-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
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RS-232 DB-25 Connectors
DB-25 Female
DB-25 Male
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 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)
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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 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)
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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
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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
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