a very boring collection of notes that relate to data communications, any errors, email me, 'how dark the days were when memorising this information was the object':
LO1 Data Communications 'the process of accurate transmission of data from source to a receiver'
Analogue Can any value, accuracy is limited by the max & minimum values of the measuring instrument.
Digital Discrete number of fixed values, binary is the most common system.
Electromagnetic A periodic wave comprising of two mutually perpendicular waves, electric and magnetic.
EM Spectrum 10^-15 to 10^3m from shortest to longest in order:
Gamma, X-Ray, UV, IR,Visible, Microwave, S/M/L Radio
Visible Light 400-700nm
Speed Of Light 300,000,000 m/s (3x10^8) m/s
Time Period The inverse of wave frequency, 1s = 1Hz 0.001s =1000Hz 0.000,0003s =30,000,000s
V=Fλ Velocity = Frequency * Wavelength
Kilohertz 1,000 Hz = 1kHz (10^3)
Megahertz 1,000,000 Hz = 1MHz (10^6)
Gigahertz 1,000,000,000 Hz = 1GHz (10^9) then peta, exa, zetta, yotta increasing at 10^3
Millisecond 0.001s (ms) 10^-3s
Microsecond 0.000, 001s (μs) 10^-6s
Nanosecond 0.000, 000, 001 (ns) 10^-9s - then increasing in powers of (10^-3) pico, femto, atto, zepto, yocto
LO2 Factors Affecting Performance
Thermal Noise Random movement of electrons in a conductor, power is proportional to the temperature of the conductor. (Also called quantum noise)
Impulse Noise Spikes or disturbances (transient noise) more damaging to digital transmissions.
Cross Talk Noise Two conductors are placed two close together and the magnetic field from one couples into the other, causing a small ghost image to appear.
Near End NEXT pairs overly untwisted at the near end (low negative dB’s mean more noise), minimize with connectors properly fitted and tightly twisting wires.
Far End Less noise than near end FEXT – noise at the far end of the cable
PSNEXT Power Sum the cumulative effect of next from all wires in the cable mostly effects 1000BASE-T as its full duplex and so uses all the pairs.
ACR Attenuation -crosstalk ratio (dB) - difference between signal attenuation and near end cross talk.
SNR Signal:noise ratio +10dB means the signal has ten times, power of noise, +3dB means twice.
Attenuation -3dB means half of the power is lost, the opposite of amplifications, a form of distortion
LO3a Modulation 'modifying a carrier signal to carry information'
Audio Carrier Frequency of an audio carrier wave (F=1/T)
Amplitude Modulation Signal is contained in variations of the amplitude, simple but susceptible to noise and fading.
Modulation Index Ratio of signal to carrier – always less than 1
On off keying OOK – e.g. morse code is a form of AM-OOK
Frequency Modulation The frequency of the carrier is modulated (88-180MHz)
Phase Modulation The phase of the carrier is modulated e.g. 180degrees or 90degrees
Shift Keying A/P/F SK – Digital methods of modulation
QAM Quadrature amplitude modulation – ASK + PSK combined
Modulation Units Bits/Baud Baud Rate Bit Rate
ASK,FSK, 2-PSK Bit 1 N N
8-PSK, 8-QAM Tribit 3 N 3N
16-QAM Qaudbit 4 N 4N
64-QAM Hextabit 6 N 6N
LO3b Modems 'modulator demodulator'
V.21 Modem Uses FSK 1180HZ =0 980Hz = 1
V.34 Modem Baud 2400 – 12bits per signal change, 28Kbps
V.90 Modem PSK 24bits of data per baud 56Kbps
Baud Rate Signalling rate is number of signal units per second.
Data Rate Bit Rate, number of bits per second
Analogy: baud is analogous to a car while bit is the passengers (1 male, 0 female); the number of cars determines the traffic; the number of passengers does not.
LO4 Multiplexing 'the transmission of multiple pieces of data over the media
Synchronous TDM Every input gets a time slot (wasteful with empty slots)
Asynchronous TDM Any input can have a vacant time slot (less wasteful)
Orthogonal FDM High speed transmission (wireless and digital TV)
WDM (Dense) Used with fibre in light, different channels have one of 80 wavelengths. 2.5Gbps (Sonet, Broadband, T1, E1)
L05a Transmission Modes
Baseband Signalling Uses the entire media bandwidth, using a change in DC on the line (Ethernet & RS-232-C)
Broadband Signalling Multiple simultaneous channels (cable TV or ADSL)
PSTN Bandwidth 300-3300Hz
Serial Transmission One bit after another, slower but only needs 1 connection (modem, network)
Parallel Transmission Multiple bits at the same time, not practical over distance (printers) max length 15-3ft before parallel Data Skew occurs which is when data arrives at RX at different times.
Asynchronous Communication 8,N,1 – Data, Parity Even or Odd, Stop Bits 1 or 2,
Asynchronous Overhead Simple to Implement but 25% bandwidth loss due to start/stop/parity.
ASCII American standard code for information interchange (7 bits represent 128 characters)
Parity Check Even parity or Odd parity
Unicode Supports up to 65, 000 language sets
Synchronous Communication Data is sent in large frames, start and stop marked with flags.
Synchronous Clocking Embedded in the data stream or on a clock line, RX and TX are always in sync, more efficient and can carry more data.
Ethernet Frame 64-1500bytes Frame Relay 4096 bytes
HDLC High level data link control
SDLC Synchronous data link control
L05b Satellite Communication
'increase the line of site needed for transmission beyond the curvature of the earth'
Minimum Escape Velocity 17,000mph for orbit
Period of Orbit Using kelpers theorem R3/T2 (radius *3 / period of orbit *3)
Low Earth Orbit 90 miles (LEO)
Geosynchronous Orbit 22,300 miles (24h orbit)
KU Band Low frequency microwaves 10.7-18GHz (Kurz-unten or directly below the K band)
L06 Digital Line Encoding
'the representation of digital information using discrete voltage signals – binary 1s & 0s are translated into sequences of voltage pulses that are sent along the media'
Unipolar Encoding Single voltage level, 1 is positive 0 is nothing, this is rarely used due to - DC component, a DC signal prevents the signal passing through transformers - Synchronisation as there is no clock.
Polar Encoding: Two voltage levels +ve and -ve are used to reduce DC component:
NRZ: Voltage is always either positive or negative never zero, this provides poor sync as there are long strings of the same bit.
NRZi: 1 is a change 0 is no change, good for strings of ones but bad for zeros.
RZ: Uses +/-/0 changes for each bit good sync, disadvantage is that 2 changes are needed for a single bit.
Biphase Encoding: (instead of returning to zero it goes to the opposite polarity)
Manchester: Changes in the middle of the bit interval up is a 1 change down is a 0 (used in Ethernet)
Differential Manchester: The voltage changes up and down to provide sync, zero is marked by a change at the beginning of the transition so there are two changes for a zero one for the data and one for the interval. (used in Token Ring)
Bipolar Encoding: Uses (z/+/-) positive is an alternation 0 is zero.
AMI: Alternate Mark Inversion 1 is positive and negative alternately, poor sync on long strings of 0s.
B8ZS: Bipolar 8-zero substitution. Violation occurs when 8 zeros occur based on the polarity of the previous one, this is used for American ISDN.
Previous 1 positive (000+-0-+) Previous one Negative (000-+0+-)
HDB3: High Density Bipolar3. Violation occurs if 4 zeros occur four violation triggers:
1) Number of 1s since the last substitution is odd, violation on the last zero.(000v)
2) Number of 1s since last substitution is even, violation on first and last zero. (v00v)
3) When polarity of the previous bit was positive both violations are negative. (+ |-00-)
4) When polarity of the previous bit was negative both violations are positive. (- |+00+)
all you need to know is, the polarity of each bit alternates- and the number of ones since the last substitution
4BT3: There is a look up table and there are ternary bauds (bandwidth +25% = block code)
2B1Q: Encodes 2 binary into 4 quaternary voltage levels (doubles bandwidth)
4D-PAM5: 1000Base-T, 9 voltage levels, 17 during transmission. (susceptible to noise due to all pairs being used)