Chapter 2 Physical & Data-Link Layers

1. Chapter 2Physical & Data-Link Layers Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. Announcements • Textbooks: Computer Networks, by Peterson and Davie • CU book store’s order has arrived • Read Chapter 1, start Chapter 2, sections 2.1-2.8, skip Section 2.9 Network Adaptors • Larger classroom – maybe… • Lectures will be online a few days after each class • Follow the Lectures link on class Web site • Homework #1 will be handed out next class • Next, Chapter 2 Prof. Rick Han, University of Colorado at Boulder

3. Application Layer OSI Stack Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer The Layered Network Stack Internet Stack Application Layer Transport Layer (TCP/UDP) Network Layer (IP) Data Link Layer Prof. Rick Han, University of Colorado at Boulder Physical Layer

4. “1” or “0” over wire Host A Host B Layer 1: The Physical Layer • Host A encodes the bit into an analog signal. Host B decodes the analog signal into a received bit. Prof. Rick Han, University of Colorado at Boulder

5. Amplitude 0 1 +5 0 -5 -5 0 1 +5 1 +5 +5 time -5 -5 Encoding: Mapping Bits to Analog Waveforms • “1” -> +5 volts, “0” -> -5 volts on copper wire • light intensity on optical fiber • Pulse Amplitude Modulation (PAM) encodes bits as diff. levels. NRZ is special case. • Manchester encodes bits as diff. transitions • FSK (Frequency Shift Key) encodes bits as diff. frequencies. 1200 bps modems. Prof. Rick Han, University of Colorado at Boulder

6. Sending More Bits Per Second • Can reduce the size of time slot • Widely used • Limitation: smearing of the waveforms and noise limit the maximum bit rate (Shannon’s capacity theorem) • Square wave is sum of many different frequency components. [analogy: stereo signal] • High frequencies get attenuated in wire, thereby smearing the waveform • Reducing time slots causes more “smear” from previous slot to interfere with current slot: InterSymbol Interference (ISI) Prof. Rick Han, University of Colorado at Boulder

7. +7 V 111 110 +5 101 +3 100 +1 011 -1 010 -3 001 -5 000 -7 Sending More Bits Per Second (2) • Can send more bits per time slot using multi-level D/A converter • 8 discrete levels • “111” -> +7 V, …, “000” -> -7 V • …100110… -> +1, +5, … Symbols • 3 bits per symbol Prof. Rick Han, University of Colorado at Boulder

8. 111 +7 V 110 +5 101 +3 +1 100 011 -1 010 -3 001 -5 -7 000 Sending More Bits Per Second (3) • Transmit a sequence ofsymbols: S1=+1, S2=+5, S3, … • Baud rate = Symbols/sec • If baud rate is 1 symbol/sec, then bit rate = 3 bits/sec • Tripled the bit rate! • Limitations: noise • Datam(t)=Sm(t)*SqWv(t-mT) • Where Sm(t) is the level of m’th symbol, T= duration of each symbol, t=time, SqWv is square wave Prof. Rick Han, University of Colorado at Boulder

9. Sending More Bits Per Second (4) • Can send more bits over different frequencies, [analogy: AM/FM] • Modulate Sm(t) up to 3000 Hz, demodulate down • Datam(t) = Sm(t)*cos(2p3000t) + S’m(t)*cos (2p6000t) [simplified: sq wave not shown] • Can send more bits over the same frequency! • Datam(t) = Sm(t)*cos(2p3000t) + S’m(t)*sin (2p3000t) • This is called Quadrature Amplitude Modulation (QAM). • If Sm(t), and S’m(t) both have 4 levels, then this is called 16-QAM. 9600 bps modems use this. Baud? Prof. Rick Han, University of Colorado at Boulder

10. Physical Layer Effects • Goal: maximize the Signal-to-Noise ratio (SNR) to minimize the probability of bit error, then pass the bit up to Data-Link Layer • Unreliability: • due to“Smearing”, Interference, … (Wireless: shadowing, multi-path, doppler, …) • Apply advanced adaptive filtering and digital signal processing (DSP) to improve SNR • Propagation Delay • Speed of light c = 3x108 m/s • Over copper wires, propagation speed is 2/3 of c • Satellite links have long prop. delays (~120 ms one-way) • Interactivity requires <400 ms roundtrip Prof. Rick Han, University of Colorado at Boulder

11. 1011000…? Host A Host B Layer 2: The Data Link Layer • Next Problem: How do I send a message from Host A to Host B? • Data Link Layer, also called Layer 2, ensures that host B can decode a digital message from a stream of bits sent by host A • Examples: PPP (Point-to-Point Protocol), HDLC, LAPB, LAPD, Frame Relay, … Prof. Rick Han, University of Colorado at Boulder

12. The Data Link Layer (cont.) • A Data Link Layer Protocol implements: • Delimiting/framing of a message • Fragmenting of a long message • Retransmission of a lost message • … 1011000 Host A Host B Prof. Rick Han, University of Colorado at Boulder

13. 1011000 Defining a Protocol • A protocol is an agreement between two parties or endpoints as to how information is to be transmitted • A protocol implements this agreement via: • A Header • How each endpoint responds to control info in the header (& external input) Host A Host B Prof. Rick Han, University of Colorado at Boulder

14. …0110011011011000100110100001110001001… • Where is the start of the data? • Where is the end of the data? • Use a special flag to signal the start (and end) of packet data, e.g. flag = “1001101” • “Sentinel” approach – flags act as sentinels • What if this flag appears in the data? • Character stuffing for byte-oriented protocols • Bit stuffing for bit-oriented protocols Framing Receiver’s A/D: Prof. Rick Han, University of Colorado at Boulder

15. Data: AKLWXIKKEZLXKDKLEXYBE XAKLWEXIKKEEZLEXKDKLEEEXYBEEX Send: Character/Byte Stuffing • BiSync and PPP (common over modems) • Data is divided into 8-bit bytes • Byte boundaries synchronized btwn sender and receiver • Define a special start-of-packet N-byte flag, e.g. let flag be X (one byte flag) • Stuff X in data: replace X with escape character “E” (DLE in textbook) and X, i.e X -> (E, X) • Stuff E in data: replace E with (E, E) • At rcvr, first X is start of packet (variants exist) Prof. Rick Han, University of Colorado at Boulder

16. Bit Stuffing …01001000001111110000001110001001… • HDLC uses this • Similar to byte stuffing, except bit stuffing is not confined to byte boundaries • HDLC denotes beginning and end of a packet/frame with “01111110” flag • Since “01111110” may occur anywhere (across byte boundaries) in data, stuff it: • At sender, after 5 consecutive ones, insert a “0” • At receiver, “0111110” => stuffing, so destuff, “01111110” => end of frame, “01111111” => error Prof. Rick Han, University of Colorado at Boulder

17. Byte Counting • Deduce End-of-Packet with a length field in the header, rather than explicit sentinel flag • …010010000000000100000011100010010… • Start-of-Packet lengthdata • Length = 2 bytes • Limitation: the length field could be corrupted by a transmission bit error, due to noise, etc. • Bit corruption in header and start-of-packet usually catastrophic • Bit corruption can also occur in data • How do we handle errors caused by bit corruption? Prof. Rick Han, University of Colorado at Boulder

18. Error Detection • Simple technique: Send two copies of data in addition to the data, and then do majority logic decoding at the receiver Original data: 0100 Corrupt Bit Send: 0100 0100 0100 Receiver: 0100 0110 0100 Decode: 0100 • Inefficient • Two errors in the same bit => cannot detect error, • 0100 0110 0110 -> 0110 [Error] • so each technique has some probability of error Prof. Rick Han, University of Colorado at Boulder

19. Error Detection (2) • General technique: Add redundancy to the data and use this redundancy to detect bit errors • …01001000000000010011010100000011100010010… • Start-of-Packet lengtherr. det.data • The error detection field can be computed over the data only, the header only (IP) or both (TCP) • Can be located in header or at end of packet • The error detection field is also called a Cyclic Redundancy Check (CRC) • Most link-layer protocols use CRC’s for error detection, e.g. HDLC, PPP • A checksum is a special case of a CRC where the CRC is computed using binary addition Prof. Rick Han, University of Colorado at Boulder

20. Error Detection (3) • The Internet checksum: • Divide your data into 16-bit segments • Add each of the 16-bit segments, using one’s complement binary addition • One’s complement of the final sum is your 16-bit IP checksum • Send IP checksum with the IP packet • An alternative checksum: XOR instead of one’s complement • Checksum’s are relatively weak compared to CRC’s but are easy to compute • A bit error twice in the same offset position within the 16-bit segment will go undetected Prof. Rick Han, University of Colorado at Boulder

21. Probability of Packet Error • Suppose N bits are sent, and the link is independently corrupting each bit with some probability of bit error pb. What is the probability of packet error? • Prob[packet error] = Prob[at least 1 bit is corrupt] • = 1 – Prob[every bit is clean] • = 1-(1- pb)N • If pb =10-6, and N=10000 bits, then • Prob[packet error] = 9.95*10-3 ~= 1% • Optical links have much lower probabilities of bit error: 10–12 • Wireless links have much higher probabilities of bit error: 10-3 in fades [send small packets] Prof. Rick Han, University of Colorado at Boulder

22. 00 01 10 11 00000 10101 01010 11111 Error Correction • Forward Error Correction (FEC): • Rather than just detect a bit error, correct a bit error • Add K bits of redundancy to N bits, to form a (N+K)-bit long packet, or vector • N dimensions -> N+K dimensions • 2N patterns or vectors mapped into 2N+K possibilities • Spread out these vectors as far away from neighbors as possible in (N+K)-dimensional space • N=2, K=3, N+K=5 • Receive 01111 - closest • to 11111, so decode “11” • and correct one bit error Prof. Rick Han, University of Colorado at Boulder