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Lecture Note on Transmission Basic

Lecture Note on Transmission Basic. Direct Connections in Communication Networks. Building Blocks links: coax cable, optical fiber... nodes: general-purpose workstations... Connectivity Types point-to-point multiple access. Indirect Connections in Communication Networks.

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Lecture Note on Transmission Basic

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  1. Lecture Note on Transmission Basic

  2. Direct Connections in Communication Networks • Building Blocks • links: coax cable, optical fiber... • nodes: general-purpose workstations... • Connectivity Types • point-to-point • multiple access

  3. Indirect Connections in Communication Networks • Connectivity Types • switched networks => switches • inter-networks => routers

  4. Connectivity Examples • Internet: • Best-effort (no performance guarantees) • Packet-by-packet • Point-to-point link: • Always-connected • Fixed bandwidth • Fixed delay • Zero-jitter

  5. Delays in Networks • Propagation time or propagation delay, tprop • Time required for a signal or waveform to propagate (or move) from one point to another point. It is defined as d/v where d is the distance between the two points and v is the velocity of the signal. • Electromagnetic signal sent through space from the Sun to the Earth. • Velocity is the speed of light, 3 × 108 meters per second. • Say distance is 148.8 million kilometers. • Propagation time is (148.8 /3) × 109 / 108 = 496 seconds. • Transmission time, ttrans • Links or channels for digital transmission are defined in bits/second or bps. This is called the Data Rate, R, and is the rate at which the transmitter can push bits onto the transmission line. Transmission time is the time it takes to send some number, say B bits, or B/R. • What is the transmission time for a packet of 1500 bytes on a 10 Megabit link. • 1500 × 8 / 107 = 1.2 milliseconds. • Queuing time or queuing delay • Any time related to processing or waiting of packets in buffers.

  6. Representation of Information • Block Oriented Information: specific length blocks • Text files • Fax documents • Example: A4 Page • 200 pixels per inch (horizontal) by 100 pixels per inch (vertical) requires 256 kilobytes prior to compression (a pixel is black or white). Compression reduces by a factor of 8 to 16. • JPEG color images • 8 x 10 inch photo uncompressed is about 38.4 Mbytes before compression and about 1.2 – 8 Mbytes after compression. • Note that each of 3 colors (RGB) requires one byte to represent the value • Stream Oriented Information: continuous stream of data • Voice PCM: 4 kilohertz voice is 64 kbps, standard voice channel • Voice ADPCM, compression technique reduces rate to 16-32 kbps • Audio MPEG audio MP3 compression • Video MPEG 2 • 1920 x 1080 pixel frames at 30 frames /second requires about 1.5 Gbps (includes 20bits/pixel and vertical/horizontal blanking and embedded audio) uncompressed is reduced to 19-38 Mbps compressed

  7. A4 Fax Document Standard 1/16 m2, 5 grams / page typically 210 millimeters x 297 millimeters (slightly bigger than 8 1/2 by 11)

  8. Data Compression Techniques • Why data compression: reduce amount of storage space required to represent the document • Compression ratio: size of uncompressed / size of compressed • Lossless data compression • Example: run length encodings • Sequences of likely symbols encoded into runs • 25 0’s follows by a 1 is encoded as a fixed m-bit code word • Example: encode 000000000000100001001 using a 3 bit code • Lossy data compression • Compressed file is an approximation of the original data • Better compression ratio is achieved. • For example, MP3 can achieve a factor of 14 reduction. • Example: unencoded MP3 requires about 1.4 Mbits/sec (2 channels, 16 bits per sample, at a rate of 44100 samples per second). Thus, 1 hour is about 317 MB (about ½ of an audio CD and 1/14 of common DVDs) Through MP3 compression, we can compress to say 128 kbits /second.

  9. Analog Information • Based on a wave that varies continuously with time • Example • Sound • Electromagnetic radiation • Periodic signal repeats over time • Representation of a analog signals • Amplitude versus time • Frequency representation • Representation as sums of sines and cosines

  10. Frequency Spectrum • Frequency: cycles per second (Hertz) of a sinusoidal signal • Bandwidth: usable range of frequencies high – low: f2 – f1 • Typical telephone voice channel is 4k Hz Some approximate frequencies 300 Hz – 20,000 Hz human voice / sound 50 kHz navigation (ships, submarines, etc) 1 MHz AM radio (20 k Hz channels) 10 MHz CB, short wave 100 MHz FM radio, TV 1 GHz UHF TV, mobile telephony 10 GHz amateur satellite 100 GHz upper microwave – many uses (up to about 300 GHz) 10 T Hz = 1013 Hz Infrared, 1015 Hz Visible light, 1018 X-rays, . . . .

  11. Bandwidth Versus Channel Rate • Shannon's theorem: maximum bit rate for noisy channel C = Bandwidth * log2 (1 + S/N) • 4 kHz voice channel equates to 56 or 64 k bits / second. • This is achievable by modern modems • Some sample bit rates Radio LAN in the 2.4GHz band: 2 Mbps Fast Ethernet: 100 Mbps, Gigabit Ethernet, Optical fiber transmission: 2.4 – 9.6 Gbps over one wavelength

  12. Multiplexing • Multiplexing is fundamentally sharing some amount of bandwidth by many different users, channels, etc. • FDM: Frequency Division Multiplexing: dividing up the frequency spectrum into multiple channels, each channel dedicated to a particular use • TDM: Time Division Multiplexing: using the entire frequency bandwidth allotted but uses “logical slots” to transmit different channels at specific points of relative time within a logical frame that is transmitted. • A digital telephone speech signal (corresponding to a 4 kHz channel) is obtained as follows • Sample the signal at twice the maximum frequency or 8000 times a second • Quantize the sampled value ( the amplitude of the signal) to 7 bits or 128 different value. • This gives a bit rate of 56 kbits/second

  13. T-1 Carrier System • T-1 was developed to carry digitally multiplexed channels between central offices using TDM. • T-1 multiplexed 24 voice channels in a T-1 frame as follows Channel 1 Channel 2 . . . . . . Channel 24 7 bit data, 1 bit control 1 bit framing Sampling rate: 1 frame is sent every 125 µsec (8000 frames /sec) 8 bits / channel x 24 channels + 1 bit = 193 bits per frame. Therefore, 193 bits / 125 µsec = 1.544 Mbps This standard signaling rate became known as DS-1 (digital signal 1)

  14. DS Hierarchy • DS-1: basic block, 1.544 Mbits/sec • DS-2: 6.312 Mbits/sec (4 DS-1 or 96 voice, plus some control) • DS-3: 44.736 Mbits/sec • International standards (based on the ITU hierarchy developed in Europe) is somewhat different • E1: 2.048 Mbits/sec supporting 30 voice channels at 64 kbits/sec each with two channels for control.

  15. SONET Digital Hierarchy • OC-1 51.84 Mbps • OC-3 155.52 Mbps • OC-12 622.08 Mbps • OC-48 2.48832 Gbps • OC-192 9.95328 Gbps • OC-768  40 Gpbs • Compare to the evolution of standard rates for Ethernet • 10 Mbps • 100 Mbps • 1 Gbps • 10 Gbps • 40 Gbps

  16. Wavelength Division Multiplexing (WDM) • Combining multiple wavelengths over the same optical fiber • For example 16 wavelengths, each at 2.5 Gbps can provide a rate of about 40 Gbps. (OC-48 x 16) • Using optical add drop multiplexers • Optical switches • A lightpath is a logical connection from a start node to an end node going through intermediate nodes. • Same wavelength throughout • Reallocating of a wavelength at intermediate nodes (switching) • Protection paths • Wavelength switching capabilities

  17. Parallel transfer of information (bits) Parallel Wires 8 bits, 16 bits, etc. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

  18. Serial Transfer (Bit Serial) 0 0 0 0 1 1 0 0 1 0 0 0 0 SOURCE DEST Data line Control line / info Transfer, for example, an octet a single bit at a time using a fixed time interval for each bit. A clock signal helps determine when to sample the bit. Why use a serial line? As distances increase, multiple wire costs increase, complexity of line drivers and receivers increase, etc. Thus, to send a character, first serialize the bits, send over the line, and then receive the bits, and convert back to character.

  19. Communication Modes • Simplex: Data is transmitted in one direction only • Half duplex: Alternating exchange of data between two devices. Need to switch between sender and receiver mode • Full duplex: Data can be exchanged in both directions simultaneously

  20. Determining Received Bit Pattern Determine • Start of each bit (or center of bit). Bit or clock synchronization • Start and end of the unit, a character, a byte, etc. Byte synchronization • Start and end of message unit or frame. Frame synchronization

  21. Asynchronous Transmission • Receiver and transmitter clocks are not synchronized. They are independent. Receiver resynchronizes at the start of each unit, say a character. • To determine beginning of character, have a “start” bit • To be sure about end of character, have a stop bit. • Know how many bits are in a character, say 8. • Clock frequency for sampling is usually 16 times bit rate. Receiver attempts to sample in the center of the bit. • Start bit is usually 1 bit long and stop bits are often 1, 1.5, or 2 bits (idle) start 1 0 0 1 1 1 1 0 stop (idle)

  22. Synchronous Transmission • Sender and receiver clocks are synchronized (often through the sending of a clock signal on a control line) • Start and stop bits are not used. Characters can be sent one after the other without the overhead of start and stop bits. • Framing is still important.

  23. 1850 – Invention of telegraph by Morse 1874 - Invention of time-division multiplexing by Baudot 1876 - Invention of telephone by Bell 1899 - Invention of radio by Marconi 1928 – Development of sampling theory by Nyquist 1936 - Invention of pulse code modulation by Reeves 1948 – Development of channel capacity theory by Shannon Invention of transistor by Bell Laboratories 1962 – First 1.544 Mb/s T1 cable carrier system by Bell System Telestar, first communications satellite be Bell System 1965 – Early Bird, first geostationary communications satellite by INTELSAT 1966 – Low-loss optical fiber proposed by Kao 1980 – AT&T introduces fiber optic transmission at T3 data rate 1984 – Divestiture of the Bell System increases competition and accelerates introduction of digital transmission in the United States 1988 – First transatlantic fiber optic cable (TAT-8) installed 1989 – First transpacific fiber optic cable (HAW-4/TPC-3) installed 1992 – Conversion of major U.S. networks to digital transmission completed 1990s- Deployment of digital transmission worldwide in support of the Synchronous Digital Hierarchy and Integrated Services Digital Network.

  24. Performance of Transmission System Circuit length Type of service Reference circuit Analog vs.digital Multiplex hierarchy Performance objectives

  25. Performance objectives Quality Availability Bit error rate Bit count integrity Jitter Delay Propagation reliability Equipment reliability

  26. Digital output Analog input Low-pass filter Quantizer Coder \ ´ Predictor Voice Transmission For digital transmission applications, voice signal must first be converted to a digital representation by the A/D process • PCM (Pulse Coded Modulation) • Different PCM • Delta modulation Block Diagram of Analog-to-Digital Converter

  27. Standard for voice digitization • For single channel: PCM at a transmission rate of 64 kb/s (8 kHz sampling with 8 bit quantization) • For multi-channel: North American standard: 24 voice channels at a total transmission rate of 1.544 Mb/s European CEPT standard: 30 voice channels at a total transmission rate of 2.048 Mb/s • Transmission quality of digitized voice channels • A/D technique • Number of tandem A/D and D/A conversions (accumulation of quantization noise) • Characteristics of transmission impairment

  28. Data Transmission Data Transmission Network Interfaces data Interface timing control data Interface timing control DTE DCE DCE DTE Data terminal equipment Data circuit terminating equipment Communications Network Data circuit terminating equipment Data terminal equipment

  29. Timing Signals Asynchronous transmission Synchronous transmission (a) Codirectional timing (a) Contradirectional timing Timing Timing DTE DCE Communication Network DCE DTE Data Data Timing Timing DTE DCE Communication Network DCE DTE Data Data

  30. (c) Buffered timing Synchronous Data Transmission Configuration Timing Timing DTE Buffer DCE Data Data Communication Network ••• ••• Timing Timing DTE Buffer Data Data

  31. Interface standards End-to-end______________ Layer7_______________ End-to-end______________ Layer6_______________ End-to-end______________ Layer5_______________ End-to-end______________ Layer4_______________ ________________________ Layer3_______________ ________________________ Layer2_______________ ________________________ Layer1_______________ DTE DCE Architecture of Open Systems Interconnection Model Application Presentation Session Transport Network Network Data Link Communications Equipment Physical

  32. Video Transmission • Compression schemes are essential because of very large amounts of information to be transmitted. (e.g.) 8-bit PCM applied to 6-MHz TV signal results in 96 Mb/s • Compression schemes - Compression in space: transform coding, sub-band coding - Compression in time: motion estimation/compensation

  33. Failure Model Failure : departure from specified performance • Failure rate ( ): average rate at which failures can be expected to occur throughout useful life of equipment • Reliability (R(t)): probability that equipment will perform without failure for a period of time t given by • Note that the above equation is based on the Poisson assumption on the failure process.

  34. Performance Objectives • Availability is defined as the probability or fraction of time that circuit continuity is maintained and usable service is provided. • Quality is considered only when the circuit is available and the quality parameters commonly used include bit error rate, timing slips, jitters, and delay. • Design objectives (DO) vs. Operation and Maintenance (O&M) standards of performance. DO=O&M + Margin • Mean Time Between Failure (MTBF): ratio of total operating time divided by the number of failures in the same period.

  35. Mean Time To Repair (MTTR): average time to repair the failed equipment including fault isolation, equipment replacement or repair, and test time, but excluding administrative time for travel and locating spare modules of equipment. • Mean Time to Service Restoration (MSTR): average time to restore service including repair and administrative time. • Availability (A): probability that equipment will be operable at any given point in time or the fraction of time that the equipment performs the required function over a stated period of time, which can be defined as follows: • Unavailability (U): complement of availability : • For large MTBF and small MTSR (usually the case)

  36. Outage (O): condition whereby the user is deprived of service due to failure within the communication system. • Mean Time Between Outages (MTBO): ratio of total operating time divided by the number of outages in the same period. • Redundant equipment • MTBO1: If the equipment consists of n identical units, each with the same MTBF, for which at least r must be operable for the system to be operable, the first type of outage is given by where the sensing of a failure and resulting switchover is itself to occur within failure.

  37. System availability • Series combination: • Parallel combination: • Series parallel combination: • Parallel-series combination:

  38. Series Combination Parallel Combination N 1 2 1 2 N

  39. Series-Parallel Combination A1 B1 N1 B2 A2 N2 AM BM NM

  40. Parallel-Series Combination A1 AN A2 B2 B1 BN M1 M2 MN

  41. Error Objectives • Error parameters • Bit Error Rate (BER): ratio of erred bits to the total transmitted bits in a measurement interval • Error-Free Second (EFS) or Error Seconds (ES): percentage or probability of one-second measurement intervals that are error free (EFS) or in error (ES) • Error-Free Blocks (EFB): percentage or probability of data blocks that are error free.

  42. Relationships between error parameters • Basic assumption: statistically independent bit errors with an average probability of error p=BER, R = bit rate • EFS and BER: • P (block error)

  43. 1 Block length Error second probability Probability of block error Bit error probability 0 1 R(data rate)

  44. Bit Count Integrity • Definitions • Bit Count Integrity (BCI): preservation of precise number of bits (or characters or frames) that are originated in a message or unit of time. • Slip: loses of BCI caused by a shift of a digital signal. • Performance Impact of One Slip Service Potential Impact Encrypted Text Encrypted key must be resent Video Freeze frame for several seconds - Loud pop on audio Digital Data Deletion or repetition of data - Possible reframe Facsimile Deletion of 4-8 scan lines – Drop call Voice Band Data Transmission errors for 0.01 to 2 sec. – Drop call Voice Possible click

  45. CCTTT Performance Objectives for slip rate on 64-kb/s International Connection (Rec. G.822)(13) Performance Mean Slip Rate Measurement Percentage of Classification Thresholds Average Period Total Time Unacceptable >30 slips in 1 hr 1yr <0.1% Degraded >5 slips in 24 hrs and 1yr <30 slips in 1 hr Acceptable <5 slips in 24 hrs 1yr >98.9%

  46. Jitter • Definitions • Jitter: short-term variation of sampling instant from its intended position in time or phase • Wander (or drift): long-term variation of sampling instant fro its intended position in time or phrase • Transmission impairment • By jitter: * Degradation in system error rate performance * Slips * Distortion in the recovered analog signal • By wander: * Buffer overflow (fill) or underflow (empty)

  47. Sources of jitter • Regenerative repeaters: imperfect clock recovery process • Digital multiplexers: insertion/deletion of overhead of stuffing bits • Sources of wander • Oscillator instability • Propagation delay variation

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