Communication Systems I
CPE 313: Micro Processors and Interfacing
How can information flow frompoint A to point B?
Morse Code
Can you spell your name?
See http://morsecode.scphillips.com/jtranslator.html for a live demo
Morse Code in Hollywood
1996 Film
Fountain of Bits
How to transmit voice?
Speaker = Sound reproducer Mic = Sound transducerMax Frequency Range:
20-20,000 Hz
Electrical signal in a transmission line
How Are Phone Connected?
How are phone conversations connected?
Traditional Switched Network
Each conversation needs a dedicated line. Very expensive to build the network.
Using Multiplexing Techniques
Frequency Division Multiplexing (FDM)
Many connections can be multiplexed through a single line
Freqeuncy Channels
Traditional Telephone Lines Limitthe Sound Frequencies Transmitted
400 – 3,400 Hz
The bandwidth of this line is 3kHz
Can you think of
why?
Frequency Channels
If the main transmission line has a maximum bandwidth of 1MHz,How many concurrent phone connections can it carry?
What if we want to send digital data over the phone line?
Can it be done?010111001010111001
Digital to Analog Encoding
See http://www.iesmarenostrum.com/informatica/zmaterias/ral/flash/digit_modulation.swf for an interactive demo.
Example: A Traditional Modem
Modem Typical Maximum Speed
56Kbps
How data is sent with a traditional modem
Traditional Modem
Telephone Network
Home
Internet Gateway
DestinationPhone
Example II: ADSL Modem
ADSL
Why is ADSL much faster?
256 Channels of 4 kHz each
But What About the 4K Bandwidth Limitation?
Splitter ADSL Modem
DSLAM
Telephone Network
Data Network
Home
Digital Subscriber Line Access Multiplexer
How fast is ADSL?
ADSL = 256 x 4 kHz x 8 bits= 8 Mbps
ADSL2 = 12 Mbps
ADSL2+ = 25 Mbps
*ADSL2+ expands channel frequency from 1.1 to 2.2 MHz
Serial vs Parallel Transmission
Image © Quatec
Example of Serial Comm?
Example of Parallel Comm?
Which Mode is More Common?
Cost. Less cables. Smaller size. Simplicity. Keeping bits aligned in a high-
speed parallel line requires more complex electronics.
Serial
Why?
The Need for SpeedS
peed
Mbp
s
Synchronous vs Asynchronous
A B
A B
Synchronous uses a clock line
Asynchronous relies on a common clock on each side
Playing Catch with a Blind Receiver
If you are the blind catcher, which method would you choose?Method 1: Pitcher throws and sends a signalMethod 2: Pitcher throws every 10 seconds. No signal.
PitcherCatcher
RS-232 (Serial Communication) Baud Rate
Data bits
Start bit / Stop bits
Parity
RS-232 Pulses
Letter “V” ASCII = 86 (0101 01102)9600 bps, 8-N-1
Layout: Start Bit, Data Bits, Parity, Stop Bits
* Note: Rs-232 logic pulses are inverted
RS-232 Pulses
Letter “Y” ASCII = 89 (0101 10012)9600 bps, 8-Odd-1
Layout: Start Bit, Data Bits, Parity, Stop Bits
* Note: Rs-232 logic pulses are inverted
How about ASCII 255?
Layout: Start Bit, Data Bits, Parity, Stop Bits
The data transmission is unbalanced. Can cause transmission errors.
9600 bps, 8-N-1
Manchester Encoding (IEEE 802.3)
Bit 1 = Low to High
Bit 0 = High to Low
Example
Image: Wikipedia
Data = 255
Manchester Code
Encode the following transmissions using the Manchester standard
Letter “V” ASCII = 86 (0101 01102)
Letter “Y” ASCII = 89 (0101 10012)
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