Part II: Data Communications

Chapter 5: Overview of Data Communications
Although it includes concepts from physics and mathematics, data communications provides a foundation that is used to construct practical communication systems. 

• Three main topics define the scope of data communications:
• Sources of data can be arbitrary types
• Transmission uses a physical system
• Multiple sources of information can share the underlying medium
• Data communications has subtopics:
• Information sources. Analog or digital
• Source encoder and decoder. Data compression.
• Encryptor and decryptor. Cryptographic techniques and algorithms.
• Channel encoder and decoder. Detect and correct transmission errors.
• Multiplexor and demultiplexor. Multiple sources combine data for transmission. Simultaneous sharing and turn-taking techniques.
• Modulator and demodulator. Analog and digital modulation schemes, and modems.
• Physical channel and transmission. Transmission media and modes. Bandwidth, noise and interference, channel capacity, modes such as serial or parallel.

Chapter 6: Information Sources and Signals
Throughout the study of data communications, it is important to remember that the source of information can be arbitrary and includes devices other than computers.

Sine waves are fundamental to input processing because many natural phenomena produce a signal that corresponds to a sine wave as a function of time.

• Four important characteristics of signals relate to sine waves:
• Frequency. Oscillations per unit time.
• Amplitude. Difference between maximum and minimum signal heights.
• Phase. Shift of the sine wave start position from reference time
• Wavelength. Length of a cycle as a signal propagates across a medium.
• The importance of composite signals and sine functions:
• Modulation usually forms a composite signal.
• Can decompose a composite signal into its constituent parts: a set of sine functions each with frequency, amplitude and phase.
•  Discovered by Fourier
• The bandwidth of an analog signal is the difference between the highest and lowest frequency of its components. 
• If the signal is plotted in the frequency domain, the bandwidth is trivial to compute.
• Digit signals and signal levels
• A communication system that uses two signal levels can only send one bit at a given time.
• A system that supports 2^n signal levels can send n bits at a time.
• An alternative method of increasing the amount of data that can be transferred in a given time consists of decreasing the amount of time that the system leaves a signal at a given level.
• bits per second = baud x [log sub2(levels)]
• Converting from digital to analog
• Approximating an analog signal from a digital one using Fourier results in an infinite set of sine waves, so approximation is used in practice.
• As few as three are necessary.
• A digital signal has infinite bandwidth.
• Line coding
• A variety of coding techniques are available that differ in how they handle synchronization as well as other properties such as the bandwidth used.
• Manchester Encoding - detecting a transition in the signal level is easier than measuring the  signal level. 1 corresponds to the transition from 0V to a positive voltage level. Similarly, 0 corresponds to the transition from a positive voltage level to zero.
• Differential Manchester Encoding (Conditional DePhase Encoding) - uses relative transitions rather than absolute. Transition always occurs in the middle of the bit time. The logical value of the bit is represented by the presence or absence of a transition at the beginning of the bit time. 0 = transition, 1=no transition.
• Converting from analog to digital
• Two basic approaches:
• Pulse code modulation (PCM)
• PCM encoder consists of a sequence of sampling (recording), quantization (converting recording into small integer values) and encoding (onto a specific format).
• Used in data systems that expect data values to be lost or changed during transmission.
• Delta modulation
• Also takes samples, but instead of quantization for each sample, delta modulation sends one quantization value followed by a string of values that give the difference between the previous value and the current value.
• Transmitting differences requires fewer bits than full values, particularly if the signal does not vary rapidly.
• Drawback is in the propagation of errors. One lost or damaged item will call all successive values to be misinterpreted.
• Nyquist Theorem and Sampling Rate
• Too few samples is undersampling (results in crude oversimplification of the original signal), too many is oversampling (generates unnecessary data which uses more bandwidth).
• Ideal sampling rate = 2 x highest frequency in the composite signal
• Digitized voice call = 8K samples/second x 8 bits/second = 64K bits/second
• Encoding and data compression
• Lossy - some information is lost during compression
• JPG, MP3
• Lossless - all information is retained in the compressed version.
• Most implementations use the dictionary approach where compression finds repeated strings and compresses by building and referencing a dictionary of those strings.

Chapter 7: Transmission Media

• Guided and unguided transmission:
• By type of path
• Communication can follow an exact path such as a wire, or can have no specific path, such as a radio transmission.
• By form of energy
• Electrical energy is used on wires, radio transmission is used for wireless, and light is used for optical fiber.
• Background radiation and electrical noise
• The random electromagnetic radiation generated by devices such as electric motors can interfere with communication that uses radio transmission or electrical energy sent over wires.
• Random electromagnetic radiation (noise) permeates the environment
• When it hits metal, electromagnetic radiation induces a small signal
• Because it absorbs radiation, metal acts as a shield.
• Twisted pair copper wire
• Unshielded Twisted Pair (UTP)
• Twisting eliminates the potential difference that builds up along parallel wires.
• Coaxial cable
• Heavy shielding and symmetry make coaxial cable immune to noise, capable of carrying high frequencies, and prevent signals on the cable from emitting noise to surrounding cables.
• Shielded Twisted Pair (STP)
• Cat 7 = 600 Mbps
• Media using light energy and optical fibers
• Optical fibers
• Angle of incidence is:
• Less than the critical angle = refraction
• Equal to the critical angle = absorption
• Greater than the critical angle = reflection
• Types of fiber and light transmission
• Multimode, Step Index: least expensive because boundary between fiber and cladding is abrupt causing frequent reflection and signal dispersion.
• Mutimode, Graded Index: slightly more expensive than step index, fiber density increased near the edge, reducing reflection and lowering dispersion.
• Single Model: most expensive, least dispersion. Small diameter and other properties to reduce reflection. Used for long distances and higher bit rates.
• Transmission:
• Light Emitting Diode (LED)
• Injection Laser Diode (ILD)
• Reception
• Photo-sensitive cell
• Photodiode
• Compared to copper wire
• Immune to electrical noise
• Less signal attenuation
• Higher bandwidth
• Higher cost
• More expertise and equipment required
• More easily broken
• InfraRed transmission
• Best suited for indoors in situation where the path between sender and receiver is short and free from obstruction.
• Point-to-point lasers
• Transmitter and receive must be aligned precisely
• Typical installations affix the equipment to a permanent structure.
• Electromagnetic (Radio) Communication
• Signal propagation
• Low frequency, <2Mbps, waves follow Earth's curvature but can be blocked by unlevel terrain.
• Medium frequency, 2-30Mbps, wave can reflect from layers of the atmosphere
• High frequency, >30Mbps, wave travels in a direct line and will be blocked by obstructions.
• Wireless technologies are classified into two broad categories:
• Terrestrial. Communication uses equipment such as radio or microwave transmitters, is relatively close to the Earth's surface.
• Nonterrestrial. Some of the equipment used in communication is outside the Earth's atmosphere.
• Types of satellites:
• Low Earth Orbit (LEO): low delay, moves across the sky
• A cluster of LEO satellites work together to forward messages.
• Medium Earth Orbit (MEO): elliptical orbit, polar communications
• Geostationary Earth Orbit (GEO): fixed position but further away (so more delay)
• Distance required = 35,785 km (1/10th distance to the moon)
• Radio wave to the GEO satellite and back = 0.238 seconds
• Can cover the whole earth with three satellites at 120 degrees from one another.
• Tradeoffs Among Media Types
• Cost: materials, installation, operation and maintenance
• Data rate
• Channel capacity = the maximum data rate that the medium can support
• Claude Shannon's Theorem determines the maximum data rate that could be achieved over a transmission system that experiences noise. C= data rate in bps, B=hardware bandwidth, S/N=signal to noise ratio or average signal power divided by the average noise power.
• C = B log sub2(1 + S/N)
• S/N often expressed in dB
• dB=10 log sub10[P2/P1]
• Delay: time required for signal propagation or processing
• Propagation delay = the time required for a signal to traverse the medium
• Affect on signal: attenuation or distortion
• Environment: susceptibility to interference and electrical noise
• Security: susceptibility to eavesdropping
• Significance of Channel Capacity
• Nyquist's Theorem encourages engineers to explore ways to encode bits on a signal because a clever encoding allows more bits to be transmitted per unit time.
• Shannon's Theorem informs engineers that no amount of clever encoding can overcome the laws of physics that place a fundamental limit on the number of bits per second that can be transmitted in a real communications system.

Chapter 8: Reliability and Channel Coding

• There are three main sources of transmission errors:
• Interference - electromagnetic radiation emitted from devices or from background cosmic radiation
• Distortion - all physical systems distort signals. Wires have properties of capacitance and inductance that block signals at some frequencies while admitting signals at other frequencies.
• Attenuation - as a signal passes across a medium, the signal becomes weaker.
• Although transmission errors are inevitable, error detection mechanisms add overhead. A designer must choose which error detection and compensation mechanisms will be used.
• Effect of Transmission Errors on Data
• Single bit error - only a single bit in a block of bits is changed. Often results from very short duration interference.
• Burst error - multiple bits in a block of bits are changed. Results from longer duration interference.
• Erasure (ambiguity) - signal that arrives at the receiver is ambiguous, not clearly a logical 1 or 0. Can result from distortion or interference.
• Two strategies for handling channel errors:
• Forward Error Correction (FEC) mechanisms
• Block Error Codes: block code divides the data to be sent into sets of blocks and attaches extra information known as redundancy to each block. The encoding for a given block depends only on the bits themselves, not on bits that were sent earlier. Block error codes are memoryless in the sense that the encoding mechanism does not carry state information from one block of data to the next.
• Single parity checking (SPC) is a basic form of channel coding in which a sender adds an extra bit to each byte to make an even (or odd) number of 1 bits and a receiver verifies that the incoming data has the correct number of 1 bits.
• Convolutional Error Codes: convolutional code treats data as a series of bits, and computes a code over a continuous series. Thus, the code computed for a set of bits depends on the current input and some of the previous bits in the stream. Convolutional codes are said to be codes with memory.
• An ideal channel coding scheme is one where any changes to bits in a valid codeword produces an invalid combination. There is a tradeoff between error detection and overhead
  
• Hamming Distance measures a code's strength and can be used on strings in a codebook.
• To find the maximum number of bit changes that can transform a valid codeword into another valid codeword, compute the minimum Hamming distance between all pairs in a codebook.
• Row and Column (RAC) encoding
• Allows a receiver to correct any single bit error and to detect errors in which two or three bits are changed.
• 16-bit Checksum Used in the Internet
• Two forms of zero:
• All 0s, meaning unused
• All 1s, represents a checked all 0s
• Cyclic Redundancy Codes (CRCs)
• Three key properties:
• Arbitrary message length
• Excellent error detection
• Fast hardware implementation
• Many disciplines have studied CRC.
• Automatic Repeat reQuest (ARQ) mechanisms
• Requires the sender and receiver to communicate metainformation
• Receiving side sends a short acknowledgement message back
• If no acknowledgement received after T time units, the sender retransmits a copy assuming the original message has been lost.

Chapter 9: Transmission Modes

• Taxonomy of transmission modes:
• Serial - one bit sent at a time
• Advantages:
• Can be extended over long distances without timing problems
• Less expensive (fewer physical wires and intermediate electronic components are less expensive)
• Transmission order
• Most significant bit (MSB or big-endian)
• Least significant bit (LSB or little-endian)
• Bit order and byte order are independent of one another
• Ethernet uses byte big-endian bit little-endian
• Timing of Serial Transmission
• Asynchronous - can occur at any time with an arbitrary delay between the transmission of two data items.
• Sends extra information before each transmission that allows a receiver to synchronize with the signal
• EIA RS-232-C is an accepted standard for asynchronous, serial communication over short distances and precedes each character with a start bit, sends each bit of the character, and follows each character with an idle period at least one bit long (stop bit).
• Synchronous - occurs continuously with no gap between the transmission of two data items.
• When compared to synchronous transmission an asynchronous RS-232 mechanism has 25% overhead per character.
• Framing
• Although the underlying mechanism transmits bits continuously, the use of an idle sequence and framing permits a synchronous transmission mechanism to provide a byte-oriented interface and to allow idle gaps between blocks of data.
• Isochronous - at regular intervals with a fixed gap between the transmission of two data items.
• Designed to provide a steady bit flow for multimedia applications like video.
• Accepts data at a fixed rate
• For an isochronous connection operating at fixed rate R, there is an underlying synchronous mechanism that operates at slightly more than R bps
• Parallel - multiple bits sent at the same time
• Two chief advantages:
• High speed
• Matches the communication mode of the underlying hardware
• A communication channel is classified a one of three types depending on the direction of transfer:
• Simplex - unidirectional data transfer
• Full-Duplex - concurrent bidirectional data transfer
• Half-Duplex - shared transmission mechanism that allows bidirectional data transfer where only one side transmits at a given time
• Data Communications Equipment (DCE) is equipment owned by the phone company, while Data Terminal Equipment (DTE) is equipment owned by the subscriber.

Chapter 10: Modulation and Modems

• Analog modulation schemes
• Modulation refers to changes made in a carrier according to the information being sent
• Three primary techniques exist to modulate an electromagnetic carrier according to a signal:
• Amplitude modulation
• Varies the amplitude of a carrier in proportion to the information being sent (the signal). Frequency stays constant.
• In practice modulation only changes the amplitude of a carrier slightly depending on the modulation index constant.
• Prevents amplitude from reaching zero, where Shannon's theorem predicts the signal to noise ration would also approach zero. The larger the signal to noise ratio the more bits per second can be transferred.
• Frequency modulation
• Varies the frequency of a carrier in proportion to the information being sent. Amplitude of the carrier is unaltered (continues as a sine wave).
• Phase Shift modulation
• Phase is the offset from a reference time at which the sine wave begins. It is possible to represent a signal by using changes in phase.
• This technique seldom used with an analog signal. For analog signal phase shift modulation is essentially a special case of frequency modulation.
• Modulation, Digital Input and Shift Keying
• Shift keying is the digital equivalent of Analog modulation
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
• Changes the phase of the carrier abruptly to encode data. Each change is called a phase shift.
• The chief advantage of mechanisms like phase shift keying arises from the ability to represent more than one data bit at a given change. 
• A constellation diagram shows the assignment of data bits to phase changes.
• Although many variations of phase shift keying exist, noise and distortion limit the ability of practical systems to distinguish among arbitrarily small differences in phase changes.
• Quadrature Amplitude Modulation (QAM)
• Uses both change in phase and change in amplitude to represent values to increase the data rate.

Chapter 11: Multiplexing and Demultiplexing (Channelization)

• There are four basic approaches to multiplexing:
• Frequency Division Multiplexing
• Because carrier waves on separate frequencies do not interfere, FDM provides each sender and receiver pair with a private communication channel over which any modulation scheme can be used.
• Long-lived (since early experiments with radio)
• Widely used (radio, TV, cable, AMPS cellular telephone)
• Analog (accepts and delivers analog signals, even if the carrier is modulated to contain digital information FDM treats the carrier as an analog wave; also makes it susceptible to noise and distortion)
• Versatile (filters on ranges of frequencies without examining other aspects of the signals)
• Can use a range of frequencies in FDM to:
• Increase the data rate
• Increase the immunity to interference
• Hierarchical FDM
• It is possible to build a hierarchy of frequency division multiplexing in which each stage accepts as inputs the outputs from the previous stage
• Wavelength Division Multiplexing
• When frequency division multiplexing is applied to optical fiber, prisms are used to combine or separate individual wavelengths of light, and the result is known as wavelength division multiplexing.
• Time Division Multiplexing
• Means simply transmitting an item from one source, then transmitting an item from another source, etc.
• Broad concept that appears in many forms 
• Synchronous TDM
• The synchronous TDM mechanism used for digital telephone calls includes a framing bit at the beginning of each round. The framing sequence of alternating 1s and 0s insures that a demultiplexor either remains synchronized or detects the error.
• Inverse Multiplexing
• When the only connection between two points consists of multiple transmission media, but no single medium has a sufficient bit rate, inverse multiplexing allows one to spread high-speed digital input over multiple lower-speed circuits for transmission and combine the results on the receiving end.
• Code Division Multiplexing
• Relies on mathematical idea: values from orthogonal vector spaces can be combined and separated without interference.
• CDM incurs lower delay than TDM when a network is highly utilized.

Chapter 12: Access and Interconnection Techniques
A typical residential subscriber receives much more information than the subscriber sends, Internet access technologies are designed to transfer more data in one direction than the other.

• Internet access technologies can be divided into two broad categories based on the data rate they provide:
• Narrowband
• Generally <= 128 kbps
• Dialup telephone connections, leased circuit using modems, fractional T1 data circuits, ISDN and other telco data services
• Broadband
• Delivers >=128 kbps, though some professionals suggest broadband is really >1Mbps
• DSL, cable modem, wireless access technologies, data circuits at T1 or higher
• Because it uses FDM, ADSL and plain old telephone service (POTS) can use the same wires simultaneously.
• VDSL=52Mbps
• ADSL uses an adaptive technology in which a pair of modems probe many frequencies on the line between them, and select frequencies and modulation techniques that yield optimal results on the line.
• Cable modems use FDM, so a cable modem can be easily attached directly to existing cable wiring without a splitter.
• Access technologies the employ optical fiber
• FTTC - Fiber to the Curb. Uses twisted copper for feeder circuits that cover the final distance to building or home.
• FTTB - Fiber to the Building. Business subscribers.
• FTTH - Fiber to the Home. Residential subscribers.
• FTTP - Fiber to the Premises. Encompasses FTTB and FTTH
• Wireless Access Technologies
• 3G services - third generation cellular telephone services for data
• WIMAX - wireless access technology up to 155 Mbps using radio frequencies
• Satellite - data services over satellite
• High Capacity Connections at the Internet Core
• Digital circuits leased from common carriers form the fundamental building blocks for long-distance data communications. The cost depends on the circuit capacity and distance.
• A digital circuit needs a device known as a DSU/CSU at each end. The DSU/CSU translates between the digital representation used by phone companies and the digital representation used by the computer industry.
• Synchronous Optical NETwork (SONET)
• Although the SONET standard defines a technology that can be used to build a high-capacity ring network with multiple data circuits multiplexed across the fibers that constitute the ring, most data networks only use SONET to define framing and encoding on a leased circuit.