Transcript Physical and Datalink Layer and LANS: Part I

Digital Interfaces
• An interface links two devices
• Interface Standards define:
– mechanical specifications - how many wires &
connector type
– electrical specifications - frequency, amplitude
and phase of signal
– functional - what does each wire do?
– Procedural – how & when to perform functions
Serial vs. Parallel
• In serial transmission one bit is sent with
each clock pulse.
• Two types of serial transmission:
– asynchronous
– synchronous
• In parallel transmission multiple bits are
sent with each clock pulse. Faster/more
expensive
upated 1/2002
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Serial Transmission
0110 0010
Parallel Transmission
0
1
1
0
0
0
1
0
0
1
1
0
0
0
1
0
Serial Transmission
• Asynchronous & Synchronous
• Concerned with timing issues
• How does the receiver know when the bit
period begins and ends?
• Small timing difference become more
significant over time if no synchronization
takes place between sender and receiver
Sender
clock
Synchronizations
Receiver
clock
time
1
0
0
1
0 0
1 0
0
1
1 1 0 0 1
1
1
1
1
1
1
1 1 1
0
0
Sender data
time
Received data
Asynchronous Transmission
• Serial communication
• Data transmitted 1
character at a time
• Character format is 1
start & 1 or more stop
bits, plus data of 5-8
bits
• Character may include
parity bit
• Timing needed only
within each character
• Resynchronization
each start bit
• Uses simple, cheap
technology
• Wastes 20-30% of
bandwidth
Asynchronous Transmission
START
BIT
0110 0010
STOP
BIT
0110 0010
Gaps of
indeterminate size
0110 0010
Synchronous Transmission
• Serial communication
• Large blocks of bits
transmitted without
start/stop codes
• Synchronized by clock
signal or clocking data
• Data framed by
preamble/post amble
bit patterns
• More efficient than
asynchronous
• Overhead typically
below 5%
• Used at higher speeds
than asynchronous
Synchronous Transmission
0110 0010 0110 0010 0110 0010 0110 0010 0010
Synchronization Choices
• Low-speed terminals and PCs commonly use
asynchronous transmission
– inexpensive
– “burst” tendency of communication reduces
impact of inefficiency
• Large systems and networks commonly use
synchronous transmission
– overhead too expensive; efficiency necessary
– error-checking more important
Generic Communications
Interface Illustration
DTE
Generates
Data
DCE
Converts to
transmission
media/ converts
to generated data
(Network)
DCE
Converts to
transmission
media/ converts
to generated data
DTE
Receives
Data
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
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
RS-232 DB-25
Connectors
DB-25 Female
DB-25 Male
RS-232 DB-9
Connectors
Limited RS-232
Electrical
Specifications
• Specifies signaling between DTE and DCE
• Uses NRZ-L encoding
– Voltage -15V to -3V = binary 1
– Voltage +15V to +3V = binary 0
• Rated for < 20Kbps and < 15Mts
– greater distances and rates are theoretically
possible, but not necessarily wise
Functional Specification
• 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)
Functional Specification DB-25 Pinouts
Procedural Specification
• Multiple procedures are specified
• Provides means of attachment between
computer and modem
– Specifies method of transmitting data between
devices
– Specifies method of cooperation for exchange of
data between devices
– Handshaking between the two equipment
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
Modems
• Modulator - converts digital signal to
analog signal
• Demodulator - converts analog signal to
digital signal
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Data Rate
• Encoding Technique
• Physical properties of medium
• Can increase speed by increasing frequency
but every line has upper and lower limits
• frequency range = bandwidth
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Bit Rate Summary
Encoding
ASK, FSK, 2-PSK
4-PSK, 4-QAM
8-PSK, 8-QAM
16-QAM
64-QAM
256-QAM
HDX
2400
4800
7200
9600
14,400
19,200
upated 1/2002
FDX
1200
2400
3600
4800
7200
9600
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