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Physical Layer:Data Encoding &

Transmission

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Network Interface Card (NIC)

• LL in part, PL in total are implemented in NIC– Ethernet card, 802.11 card, …– NIC is semi-autonomous

• Listens to the link independent of the CPU• Talks to CPU after reception of a new frame• CPU talks to the card to send a frame

– Has connectivity to both the I/O bus and the network link

CPU

cache

Memory

I/O Bus Link

Businterface

Linkinterface

NICI/O Bus Interface

PCILink

Interface

LinkInterface

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Steps in Transmission of a Datagram to a Neighbor

Datagram

DatagramH

DatagramH EDC

Bit error prone link

Datagram

DatagramH

D’ EDC‘

LL

Allbits in D’

OK?N

Detected error

Y

Encode Bits to the Link

LL

Decode Bits from the Link

PL PL

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Encoding - Definitions• Data: Something which

carries meaning– Can be analog, e.g., radio, TV

signals or digital such as data files etc.

– For this course, we are ONLY interested in digital data encoding

• Signal: Encoded data, i.e., what is transferred on the link (wired or wireless)– Analog: Represents data with

continuously varying electromagnetic waves

– Digital: Represents data with a sequence of voltage pulses

LL Frame

Datagram

DatagramH

DatagramH EDC

Add Framing Information

Encode Bits to the Link PL

LL

Link

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Encoding Digital Data• Digital Data (0s and 1s) can be encoded as

an Analog Signal– Signal on the link is an electromagnetic wave of

some frequency– Signal can also be photonic pulses of a given

wavelength over optical fiber

• 3 general methods– Amplitude Shift Keying– Frequency Shift Keying– Phase Shift Keying

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Digital Data to Analog Signal:ASK

Digital Data = m(t) Modem Analog Signal = s(t)

s(t) = A cos(2*PI*fc*t) – binary 1s(t) = 0 – binary 0

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Digital Data to Analog Signal:FSK

Digital Data = m(t) Modem Analog Signal = s(t)

s(t) = A cos(2*PI*f1*t) – binary 1s(t) = A cos(2*PI*f2*t) – binary 0

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FDM + FSK on Voice Grade Line

• Full-duplex FSK transmission on a Voice-Grade Line:– The frequency band 300Hz-3400Hz is divided into to halves:

• 300Hz-1850Hz & 1850-3400Hz – Frequency Division Multiplexing– Then– Frequencies 2025 & 2225 are used to encode 1 and 0 in one direction– Frequencies 1070 & 1270 are used to encode 1 and 0 in the other direction

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Digital Data to Analog Signal: PSK

Digital Data Modem Analog Signal

s(t) = A cos(2*PI*fc*t + PI) – binary 1s(t) = A cos(2*PI*fc*t) – binary 0

QPSK usess(t) = A cos(2*PI*fc*t + PI/4) – binary 11s(t) = A cos(2*PI*fc*t + 3*PI/4) – binary 10s(t) = A cos(2*PI*fc*t + 5*PI/4) – binary 00s(t) = A cos(2*PI*fc*t + 7*PI/4) – binary 01

• Each signaling element carries 2 data elements

– Signaling element rate: Baud-rate– Data element rate: Bit-rate

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Encoding Digital Data• Digital Data (0s and 1s) can also be encoded as a

“Digital Signal”– Signal on the link are discrete, discontinuous voltage pulses– Each pulse is a signal element– Binary data encoded into signal elements– E.g., 2 voltage levels, one of them representing digital 0, the

other one representing digital 1

• Lots of different encoding methods– Non-Return to Zero (NRZ)– Manchester Encoding– 4B/5B– NRZ-I– NRZ-M– Bi-Phase-Mark, Bi-Phase-Space– Differential Bi-Phase-Space, Differential Bi-Phase-Mark– …

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Digital Data to Digital Signal: NRZ

• Non-Return to Zero (NRZ)– 1=high signal (+5V), 0=lower signal (-5V)– One-to-one correspondence between data &

signal0 1 0 0 1 1 0 0 0 1 1

Digital Data

Digital Signal

Digital Data Encoder

Digital Signal

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Manchester Encoding

0 1 0 0 1 1 0 0 0 1 1Digital Data

• Manchester Encoding – Used in Ethernet– 0 is encoded as low to high transition– 1 is encoded as high to low transition– Not very efficient

• Signaling rate (baud-rate) is twice the data rate (bit-rate)

Clock

Digital Signal

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4B/5B• Attempts to address the inefficiency of the Manchester

encoding without suffering from the problem of having extended durations of high or low signals as was the case in NRZ

• Idea is to insert extra bits into the bit stream so as to break up long sequences of 0s and 1s– Specifically, every 4-bit of actual data is encoded into a 5-bit

code that is then transmitted to the receiver using NRZI

0000 111100001 010010010 101000011 101010100 010100101 010110110 011100111 01111

1000 10010

1001 10011

1010 10110

1011 10111

1100 11010

1101 11011

1110 11100

1111 11101

4-bitData Symbol

5-bitCode

4-bitData Symbol

5-bitCode

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Encoding Digital Data

• Why use an analog signal to carry digital data?– Because some mediums only propagate analog

signals• Optical fiber, air (unguided media)

– Because the network was designed to receive, switch and transmit analog signals

• PSTN handles analog signals in the voice range of about 300-3400Hz.

• Why do digital data-to-digital signal encoding?– Equipment for encoding digital data into digital

signal is less complex and less expensive

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Transmission of Encoded DataSender Receiver

Digital SignalNRZ

• After encoding the digital data, the sender simply puts the signal on the wire as shown above

• How does the receiver decode this signal?– Receiver must sample on the middle of each signaling

element as shown above

• Problem:– How do you synchronize the sender & receiver clocks?

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Sender/Receiver Clocking• Timing problems require a mechanism to

synchronize the sender and the receiver• Two mechanisms

– Provide a separate line to send the clock signal from the sender to the receiver. The receiver then uses the same clock to decode the incoming signal

• OK for short distances, but not practical for long distances• Way too $$

– Let the sender and receiver use independent clocks• Independent clocks can easily get out of sync. Then,• Have the receiver synch its clock from the incoming signal• Called Self-Clocking

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Self-Clocking Transmission• Two ways to sync the sender and the receiver

during self-clocking transmission– Asynchronous Transmission

• Data transmitted one character at a time with start/stop bits

– Each char is 5 to 8 bits

• Timing only needs to be maintained within each character• Re-sync after each character

– Synchronous Transmission• Data transmitted as a stream of bits with no start or stop

bits• The receiver must synch itself using the incoming signal

– Signal must have enough transitions to let the receiver sync itself from the signal

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Asynchronous (diagram)

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Asynchronous - Behavior• In a steady stream, interval between characters is

uniform (length of stop element)• In idle state, receiver looks for transition 1 to 0

– Start bit

• Then samples next 7 or 8 intervals (char length)• Then looks for next 1 to 0 for next char

• Simple• Cheap• Overhead of 2 or 3 bits per char (~20%)• Good for data with large gaps (keyboard)• Used in RS-232 Serial Port Communication

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Synchronous Transmission• Block of data transmitted without start or

stop bits• How to synchronize clocks?

– Need to have enough transitions on the signal• Manchester encoding: a transition for every bit makes

clock recovery very easy

• 4B/5B: Make sure there will be enough transitions within the signal regardless of the bit-stream, while keeping the overhead at 20% -- Used by some Ethernets

• NRZ: Long stream of 0’s or 1’s make clock recovery almost impossible. So NRZ is almost never used in synchronous transmission

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Summary• Covered Digital Data Encoding Methods

– Digital Data-to-Analog Signal Encoding (Modulation)• ASK• FSK• PSK

– Digital Data-to-Digital Signal Encoding• NRZ• Manchester• 4B/5B• …

• Covered transmission of encoded data over the link– Asynchronous Transmission: Data transmitted one

character at a time with start/stop bits

– Synchronous transmission: Data transmitted as a stream of bits with no start or stop bits