1 Physical Layer: Data Encoding & Transmission

2 Network Interface Card (NIC) LL in part, PL in total are implemented in NIC –Ethernet card, 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 Bus interface Link interface NIC I/O Bus Interface PCI Link Interface Link Interface

3 Steps in Transmission of a Datagram to a Neighbor Datagram H HEDC Bit error prone link Datagram H D’EDC‘ LL All bits in D’ OK? N Detected error Y Encode Bits to the Link LL Decode Bits from the Link PL

4 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 H HEDC Add Framing Information Encode Bits to the Link PL LL Link

5 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

6 Digital Data to Analog Signal:ASK Digital Data = m(t) Modem Analog Signal = s(t) s(t) = A cos(2*PI*fc*t) – binary 1 s(t) = 0 – binary 0

7 Digital Data to Analog Signal:FSK Digital Data = m(t) Modem Analog Signal = s(t) s(t) = A cos(2*PI*f1*t) – binary 1 s(t) = A cos(2*PI*f2*t) – binary 0

8 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 & Hz – 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

9 Digital Data to Analog Signal: PSK Digital Data Modem Analog Signal s(t) = A cos(2*PI*fc*t + PI) – binary 1 s(t) = A cos(2*PI*fc*t) – binary 0 QPSK uses s(t) = A cos(2*PI*fc*t + PI/4) – binary 11 s(t) = A cos(2*PI*fc*t + 3*PI/4) – binary 10 s(t) = A cos(2*PI*fc*t + 5*PI/4) – binary 00 s(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

10 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 –…

11 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 & signal Digital Data Digital Signal Digital Data Encoder Digital Signal

12 Manchester Encoding Digital 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

13 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 bit Data Symbol 5-bit Code 4-bit Data Symbol 5-bit Code

14 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 Hz. Why do digital data-to-digital signal encoding? –Equipment for encoding digital data into digital signal is less complex and less expensive

15 Transmission of Encoded Data Sender Receiver Digital Signal NRZ 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?

16 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

17 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

18 Asynchronous (diagram)

19 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

20 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

21 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