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An Introduction to Computer Networks

An Introduction to Computer Networks. Lecture 5: Point-to-Point link. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Outline. Concepts: Data signal Links Link functions Encoding Framing Error Detection Error correction or reliable transmission.

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An Introduction to Computer Networks

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  1. An Introductionto Computer Networks Lecture 5: Point-to-Point link University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Introduction to Computer Network

  2. Outline • Concepts: • Data • signal • Links • Link functions • Encoding • Framing • Error Detection • Error correction or reliable transmission Introduction to Computer Network

  3. Data • Discrete data: an instance is binary. Computer works with discrete data. Discrete is encoded in 0s and 1s. • Continuous data: change with time or space. • It is converted to discrete data by sampling • Data is delivered by signals in the links Introduction to Computer Network

  4. Link Functions • Functions • Construct Frame with Error Detection Code • Encode bit sequence into analog signal • Transmit bit sequence on a physical medium (Modulation) • Receive analog signal • Convert Analog Signal to Bit Sequence • Recover errors through error correction and/or ARQ Signal Adaptor Adaptor Adaptor: convert bits into physical signal and physical signal back into bits Introduction to Computer Network

  5. Link Components NRZI Introduction to Computer Network

  6. Link Properties • Function • Duplex/Half Duplex • One stream, multiple streams • Characteristics • Bit Error Rate • Data Rate (this sometimes mistakenly called bandwidth!) • Degradation with distance Introduction to Computer Network

  7. Example: Optical Links Introduction to Computer Network

  8. Signals Electromagnetic waves propagating in the light speed. • Frequency • Wavelength • A (periodic) signal can be viewed as a sum of sine waves of different frequencies and strengths. • Every signal has an equivalent representation in the frequency domain. » What frequencies are present and what is their strength (energy) Introduction to Computer Network

  9. Signals (cont) Example- The following signal is the sum of sine waves. Introduction to Computer Network

  10. Modulation • Sender changes the nature of the signal in a way that the receiver can recognize. » Similar to radio: AM or FM • Digital transmission: encodes the values 0 or 1 in the signal. » It is also possible to encode multi-valued symbols • Amplitude modulation: change the strength of the signal, typically between on and off. » Sender and receiver agree on a “rate” » On means 1, Off means 0 • Similar: frequency or phase modulation. • Can also combine method modulation types. Introduction to Computer Network

  11. Modulation • The function of transmitting the encoded signal over a link, often by combining it with another (carrier signal) • E.g. Frequency Modulation (FM) • Combine the signal with a carrier signal in such a way that the instantaneous frequency of the received signal contains the information of the carrier • E.g. Frequency Hopping (OFDM) • Signal transmitted over multiple frequencies • Sequence of frequencies is pseudo random Introduction to Computer Network

  12. Link rate and Distance Links become slower with distance because of attenuation of the signal Amplifiers and repeaters can help Introduction to Computer Network

  13. Limits in sending signals • Noise: “random” energy is added to the signal. • Attenuation: some of the energy in the signal leaks away. We need repeaters. • Dispersion: attenuation and propagation speed are frequency dependent. » Changes the shape of the signal Introduction to Computer Network

  14. Noise • A signal s(t) sent over a link is generally • Distorted by the physical nature of the medium • This distortion may be known and reversible at the receiver • Affected by random physical effects • Shot noise • Fading • Multipath Effects • Also interference from other links • Wireless • Crosstalk • Dealing with noise is what communications engineers do Introduction to Computer Network

  15. Noise limits the link rate • Suppose there were no noise • E.g. Send s(t) always receive s(t+Δ) • Take a message of N bits say b1b2….bN, and send a pulse of amplitude of size 0.b1b2….bN • Can send at an arbitrarily high rate • This is true even if the link distorts the signal but in a known way • In practice the signal always gets distorted in an unpredictable (random) way • Receiver tries to estimate the effects but this lowers the effective rate • One way to mitigate noise is to jack up the power of the signal • Signal to Noise ratio (SNR) measures the extent of the distortion effects Introduction to Computer Network

  16. Channel capacity • Every transmission medium supports transmission in a certain frequency range. This is called channel capacity. » The channel bandwidth is determined by the transmission medium and the nature of the transmitter and receivers • A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H. » E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second Assumes binary amplitude encoding • Shannon extended this result by accounting for the effects of noise. • More aggressive encoding can increase the channel bandwidth. » Example: modems Introduction to Computer Network

  17. Bandwidth affects the data rate • There is usually a fixed range of frequencies at which the analog wave can traverse a link • The physical characteristics of the link might govern this • Example: • Voice Grade Telephone line 300Hz – 3300Hz • The bandwidth is 3000Hz • For the same SNR, a higher bandwidth gives a higher rate Introduction to Computer Network

  18. Signaling bits on a linkMulti-level Signaling 2-bits per symbol 3-bits per symbol 8 4 7 6 3 5 Levels Levels 4 2 3 2 1 1 00 11 10 01 00 000 010 101 001 110 Ultimately, what limits the number of bits I can send per symbol? Introduction to Computer Network

  19. Maximum Capacity/Data Rate Shannon Capacity: Bandwidth of link Signal-to-Noise ratio • For example: • Bandwidth of telephone link from telephone to a typical home is approx 3300Hz – 300Hz = 3kHz • Signal-to-noise ratio is approx 30dB = 10log10(S/N) • Therefore, C = 3000*log2(1001) ~= 30kb/s Optical fiber has a higher capacity because the bandwidth, B, of a fiber is much greater than for wire; and it is less susceptible to noise, N. Introduction to Computer Network

  20. Sampling Result (Nyquist) • Suppose a signal s(t) has a bandwidth B. • Sampling Result: Suppose we sample it (accurately) every T seconds. • If T≤ 1/2B then it is possible to reconstruct the s(t) correctly • Only one signal with bandwidth B has these sample points • There are multiple signals with these sample points for signals with bandwidth greater than B • Increasing the bandwidth results in a richer signal space • No noise allowed in the sampling result Introduction to Computer Network

  21. Copper Wire • Unshielded twisted pair » Two copper wires twisted - avoid antenna effect » Grouped into cables: multiple pairs with common sheath » Category 3 (voice grade) versus category 5 » 100 Mbps up to 100 m, 1 Mbps up to a few km » Cost: ~ 10cents/foot • Coax cables. » One connector is placed inside the other connector » Holds the signal in place and keeps out noise » Gigabit up to a km Introduction to Computer Network

  22. Copper Wire Introduction to Computer Network

  23. Microwaves • High frequency electromagnetic waves (>1GHz) q Line of sight terrestrial transmissions and for communications via satellites. q Some atmospheric interference occurs but reliable transmission can be obtained over distances up to 50 Km. q Microwaves is absorbed by rain and does not penetrate obstacles. Introduction to Computer Network

  24. Fiber Optic q Thin thread of glass or plastic q Lightweight. q Fibers act as wave-guides for light which is usually produced by lasers. q Visible light has frequency around 5*10 15 Hz, which ensures an extremely high bandwidth. q The raw materials are cheap. q Immune to electrical interference. q Difficult to join and tap. q Security advantages Introduction to Computer Network

  25. Leased Lines • Dedicated link from Telephone Companies • DS1 24 digital voice of 64Kbps • DS3 28 DS1 • STS stands for Synch. Transfer signal Introduction to Computer Network

  26. Last-Mile Links • Connect from home to network service providers • xDSl (Digital Subscriber line), runs on local loop on telephone line. ADSL, VDSL • CATV- Cabel TV, BW of 6 MHZ, is asymmetric. Introduction to Computer Network

  27. Encoding • Goal: send bits from one node to another node on the same physical media • Encode binary data onto signals • This service is provided by the physical layer • Bits flow is between network adaptors. • Problem: specify a robust and efficient encoding scheme to achieve this goal • How to send clock information? • Extract from changes in signals. Introduction to Computer Network

  28. Encoding (crieria) • Criteria for encoding: • Bit rate (in limited BW) • Recovering time information (in LAN) • Error detecting • Immunity to noise and interference • Complexity and cost of implementation. • Bit rates V.s Baud Rates • Baud rates is the number of pulses sent over the link or changing in signals. Introduction to Computer Network

  29. Assumptions • We use two discrete signals, high and low, to encode 0 and 1 • The transmission is synchronous, i.e., there is a clock used to sample the signal • In general, the duration of one bit is equal to one or two clock ticks • If the amplitude and duration of the signals is large enough, the receiver can do a reasonable job of looking at the distorted signal and estimating what was sent. Introduction to Computer Network

  30. NRZ (non-return to zero) Non-Return to Zero (NRZ) • 1  high signal; 0  low signal 0 0 1 0 1 0 1 1 0 Clock Introduction to Computer Network

  31. Problem with NRZ • Consecutive 0’s or 1’s may create problems. • Synchronization problem because of difference in the sender or receiver clocks. • The average of signals which is used to distinguish between low or high may move and make the decoding difficult. This is called baseline wander • Unable to recover clock Introduction to Computer Network

  32. NRZI (non-return to zero intverted) Non-Return to Zero Inverted (NRZI) • 1  make transition; 0  stay at the same level • Solve previous problems for long sequences of 1’s, but not for 0’s 0 0 1 0 1 0 1 1 0 Clock Introduction to Computer Network

  33. Manchester Manchester • 1  high-to-low transition; 0  low-to-high transition • Addresses clock recovery and baseline wander problems • Disadvantage: needs a clock that is twice as fast as the transmission rate • Efficiency of 50% 0 0 1 0 1 0 1 1 0 Clock Introduction to Computer Network

  34. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Clock Manchester NRZI Encodings (cont) Introduction to Computer Network

  35. 4-bit5-bit • 1000 10010 • 1001 10011 • 1010 10110 • 1011 10111 • 1100 11010 • 1101 11011 • 1110 11100 • 1111 11101 4-bit/5-bit (100Mb/s Ethernet) • Goal: address inefficiency of Manchester encoding, while avoiding long periods of low/high signals • Solution: • Use 5 bits to encode every sequence of four bits such that no 5 bit code has more than one leading 0 and two trailing 0’s • Use NRZI to encode the 5 bit codes • Efficiency is 80% 4-bit5-bit • Unused code are used for control. I.e. 11111 is for line idle or 00000 for the line is dead. • FDDI is using this scheme. • 0000 11110 • 0001 01001 • 0010 10100 • 0011 10101 • 0100 01010 • 0101 01011 • 0110 01110 • 0111 01111 Introduction to Computer Network

  36. Framing • How to distinguish between data and garbage. • Break sequence of bits into a frame • Typically implemented by network adaptor Bits Node B Node A Adaptor Adaptor Frames Introduction to Computer Network

  37. Byte-oriented Approaches • Sentinel-based • delineate frame with special pattern: 01111110 or STX, ETX, etc characters. • e.g., HDLC, SDLC, PPP • Problem: special pattern or characters appear in the payload. • Character escaping or stuffing: in BISYNC (from IBM) with DLE character. 8 16 16 8 Beginning Ending Body CRC Header sequence sequence Introduction to Computer Network

  38. Byte-oriented Approaches (cont) • Counter-based • include payload length in the header • e.g., DDCMP protocol from DEC. • problem: count field corrupted • solution: catch when CRC fails Introduction to Computer Network

  39. Bit-oriented Approaches • Frame is a collection of bits, SDLC (IMB) (synch. Data Link Control), later changed to HDLC (high) by OSI • Beginning and end with 01111110 • problem: How if the sequence appear in the body. • solution: Bit stuffing: add after 5 consecutive 1 a zero. 16 8 16 8 Introduction to Computer Network

  40. Payload Overhead 9 rows 90 columns Clock-Based Framing • e.g., SONET: Synchronous Optical Network • By Bellcore • Use NRZ , but scramble it for enough transition • Multiplex multiple low-speed links into a high sp • STS-n (STS-1 = 51.84 Mbps), each frame 125 us Introduction to Computer Network

  41. Error Detection • To send extra information to find error in the frame. • The simplest form is sending two copies, inefficient. • Sending the sum of values (?) in the frame, checksum. In Internet, consider 16 bits sequences and then use one-complement to find the result. • Sending parity, Odd or even parity. • Two dimensional parity. Introduction to Computer Network

  42. Error Detecting (goals) • Goals: • Reduce overhead, i.e., reduce the number of redundancy bits • Increase the number and the type of bit error patterns that can be detected Introduction to Computer Network

  43. Internet Checksum Algorithm • View message as a sequence of 16-bit integers; sum using 16-bit ones-complement arithmetic; take ones-complement of the result. u_short cksum(u_short *buf, int count) { register u_long sum = 0; while (count--) { sum += *buf++; if (sum & 0xFFFF0000) { /* carry occurred, so wrap around */ sum &= 0xFFFF; sum++; } } return ~(sum & 0xFFFF); } Introduction to Computer Network

  44. Even Parity Check • EDC field has 1 bit • Sender: If number of 1’s in payload is odd, the check bit is 1, else it is 0 • Receiver: Accept the packet if payload number of 1’s match the value of the check bit. • Can detect an odd number of bit errors Introduction to Computer Network

  45. Two-dimensional Parity • Add one extra bit to a 7-bit code such that the number of 1’s in the resulting 8 bits is even (for even parity, and odd for odd parity) • Add a parity byte for the packet • Example: five 7-bit character packet, even parity 0110100 1 1011010 0 0010110 1 1110101 1 1001011 0 1000110 1 Introduction to Computer Network

  46. How Many Errors Can you Detect? • All 1-bit errors • Example: 0110100 1 1011010 0 0000110 odd number of 1’s 1 error bit 1110101 1 1001011 0 Can detect AND correct 1-bit errors 1000110 1 Introduction to Computer Network

  47. Cyclic Redundancy Code (CRC) • Add r bits of redundant data to an n-bit message • Where r << n • e.g., r= 32 and n = 12,000 (1500 bytes) T R M r m MSB i.e. T = M.2r + R Modulo-2 addition (XOR) Introduction to Computer Network

  48. CRC(cont) • Represent n-bit message as n-1 degree polynomial • e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1 • Let k be the degree of some divisor polynomial • e.g., C(x) = x3 + x2 + 1 • Transmit polynomial T (x) that is evenly divisible by C(x) • shift left k bits, i.e., M(x)xk • subtract remainder of M(x)xk / C(x) from M(x)xk Introduction to Computer Network

  49. CRC (cont) • Receiver polynomial T(x) + E(x) • E(x) = 0 implies no errors • Divide (T(x) + E(x)) by C(x); remainder zero if: • E(x) was zero (no error), or • E(x) is exactly divisible by C(x) • All operation is done in modulo 2 in which there is no carry. Then, the operation can be done by XOR only. Introduction to Computer Network

  50. CRC(cont) Example: M(x)= 110101, C(x) = 1001 1 0 0 1 1 1 0 1 0 1 0 0 0 1 0 0 1 1 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 0 1 1 0 0 0 0 0 1 1 0 0 1 0 0 1 1 0 1 0 1 0 0 1 0 1 1 The final transmitted message is: T(x) = 1 1 0 1 0 10 1 1 R Introduction to Computer Network

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