Data Communications Principles

BASIC MODEL

Input Device ---> Transmitter ---> Channel ---> Receiver ---> Output Device
                    I(t)		x(t)	         R(t)      	       I(t)

Information: I(t) -> analog (continuous) or digital (discrete)

Analog Signal:  varies continuously between a maximum & minimum value
 - infinite number of values between 2 extremes; e.g. human voice, tv pictures

Digital Signal:  limited set of discrete values each representing a symbol such as alphabetical
   character or number;  e.g. telegraph signals, weather report, data from computer device

Transmitter:  couples message to be exchanged to channel so must be able to translate
  information from form created by human or machine to signal that can be transmitted over
  transmission medium
 - one of common ways to ŇtranslateÓ is through modulation of carrier wave of transmission 
   medium i.e. input signal used to modulate carrier wave
 - can also filter & amplify signal

Modulation:  systematic variation of some attribute of carrier signal - e.g. amplitude, phase
  or frequency

Digital Input - may encode input bits
  - often transmitter groups input bits into blocks of data => adds control information

I(t)  (information) -> can be analog or digital & can be transmitted as x(t) in either analog or
  digital form - i.e. can start in one form & be transmitted in another
  - if start as digital -> modem -> analog
  - if start as analog -> PCM -> digital


TERMINOLOGY

Transmission Media: guided or unguided as electromagnetic waves

Guided:  guided along physical path, e.g. twisted pair, coaxial cable, optical fiber (Fig. 25)

Unguided Media - transmit waves but do not guide them, e.g. propogation through air, 
  vacuum, sea water

Direct Link - transmission path between 2 devices such that signals propogate directly from 
  transmitter to receiver with no intermediate devices (other than amplifiers or repeaters to   
increase signal strength)  -  used for both guided & unguided media

Transmission can be:
 1) Simplex - signals only in one direction; one station transmitter, other receiver; e.g. radio
 2) Half-Duplex - both stations can transmit, but only one at a time
 3) Full-Duplex - both stations can transmit at same time i.e. medium is carrying signals in 
both directions at same time 

Any transmission signal can be expressed as function of time or function of frequency - the 
latter being more important.

Time Domain - signal as function of time;  signal s(t) is continuous if, for all  a  

		lim s(t)  =  s(a)
                          t-> a

i.e. no breaks or discontinuities in signal e.g. speech

Signal discrete if takes on finite number of values, e.g. binary 1Ős & 0Ős    Figure 8

Periodic: - signal s(t) is periodic iff, for all t,  s(t + T) =  s(t),  where constant T is smallest 
value that satisfied this equation, i.e. T is period; otherwise signal is aperiodic  -  Figure 9

Three important characteristics of periodic signal:  amplitude, frequency, phase

Amplitude:  instantaneous value of signal at any time;  as we refer to electric or 
electromagnetic waves => measure in volts

Frequency:  inverse of period (1/T)  i.e. number of repetitions of period/second and is 
expressed in cycles/second or hertz (Hz)

Phase - measure of relative position in time within single period of signal

Sine wave given in Figure 11


Frequency Domain - signal as function of frequency  - see figure 11 - components of signal 
are sine waves of frequencies f1 & 3f1

Note:  second frequency is integer multiple of first;  if all frequency components of signal 
integer multiples of one frequency => latter is fundamental frequency; period of total signal 
is period of Fundamental Frequency (Figure 11)

Result: using Fourier analysis, any signal is made up of components at various frequencies 
such that each component is sinusoid - very important result as effects of various 
transmission media on signal can be expressed in terms of frequencies

Spectrum of Signal - range of frequencies it contains; in Fig. 11, spectrum from f1 to 3f1 

Absolute Bandwidth of Signal - width of spectrum; in Fig. 11 -> BW = 2f1

DC Component of signal - compoent of zero frequency -> direct current component or 
constant component;  e.g. Figure 13 shows result of adding dc component to signal in Fig. 
11;  with no dc component, average amplitude is zero, but with dc there is a frequency term 
at f=0 and non-zero average amplitude

Relationship Between Data Rate & Bandwidth
 - although given waveform can have frequencies over very broad range, practically 
transmission medium only accomodates limited band of frequencies - > this limits data rate 
on medium

- by adding sine waves at frequency f1 & 3f1 => waveform begins to resemble square 
wave
- as add more odd multiples of f1 resulting waveform approaches square wave more 
closely
- actual square wave has infinite number of components but as add more & more 
components of k (f1) with amplitude of 1/k most of frequency of waveform is in first few 
components

Bandwidth & Data Rate
- consider digital transmission system that transmit signals with f1 = 1MHz
- try to transmit alternating 1Ős & 0Ős as square wave as in Figure 14(a) -> sufficiently close 
to distinguish 1Ős & 0Ős  =>  what is data rate?

HOMEWORK:  Double the fundamental frequency to 2 Mhz using the same square wave 
as in Figure 14(a).  What is the data rate?  What is the relationship to the above example?

In general, any digital waveform will have infinite bandwidth =>  if attempt to transmit 
waveform as signal over any medium => nature of medium will limit bandwidth
- for any given medium, greater the bandwidth transmitted => greater the cost
- but if limit bandwidth => distortions -. more limited the bandwidth, greater the distortion 
& hence greater potential for error by receiver

Figure 15 shows bandwidths for various values for digital bit streams with data rate of 
2000 bits/sec

Signal Strength:  as signal propogates along transmission medium there is loss (attenuation) 
of signal strength so add amplifiers at various points to gain signal strength

Analog Information over Digital Channels 
- becoming increasingly more common
Why?	- improved tolerance for noise - why?
	- possiblility of regeneration
	- convenience of electronic implementation
	- use of digital network for both analog & digital signal
- use PCM - Figure 16

Consider Telephone
- start with analog;  telephone channel -> bandwidth limited from 300 Hz to 3400 Hz 
(typically sample at twice highest frequency) so sampling frequency fs is 6800 Hz
- typically increase to 8kHz when encoding speech of telephone quality (internationally)
- so consider speech bandwidth approx. 4kHz and fs = 8kHz i.e. 8000 samples/sec
- each sample quantized into 1 of 256 levels, i.e. 8-bit word 
- so one voice channel has transmission rate of 8000 samples/sec x 8 bits/sample
	= 64 kbps (standard)

Data Transmission - Figure 17 - serial (modem) & parallel

Clock Drift - Figure 18 - dealt with by asynchronous transmission (Figure 19) with stop & 
start bit;  or synchronous (Figure 20) - can stay in synch for length of data

Data Signaling Formats - two formats:
  - digital signaling -> Figure 21(a) -> encoded - examples in figure 22
  - analog signaling -> Figure 21(b) -> modulated

Transmission over Analog Channel (Analog Signaling) - Figure 23,  Figure 24

Channel Capacity & Bandwidth - Shannon (1940s) showed that there are techniques 
(coding) such that possible to transmit information with arbitrarily low probability of error 
as long as information rate R is less than or equal to the capacity, C, of the channel, and 
there is no code if  R > C

Transmission Media - greater the bandwidth, the more information it can carry
	Figure 25,   Table 2

Open-wire:  e.g. telephone & telegraph - low attenuation (decrease in magnitude of current, 
voltage, or power of signal in transmission between points) of voice frequencies
 - sensitive to external interference from storms & high-voltage power lines

Paired Cable (twisted pair) - twisting minimizes susceptibility of cable to external 
interference & crosstalk
 - 6 to 3000 pairs in common sheath

Coaxial Cable - 4 to 24 coaxial tubes; very high frequencies so carry large number of voice 
channels, high-speed data, tv channels;  used in telephone network, transmission media for 
cable tv, LANs

Wave Guides - hollow metal tube - rectangular, elliptical, circular
  - very low attenuation at microwave frequencies
  - connect microwave transmitters to microwave antenna

Optical Fibers - Figure 26
  - low transmission losses and high bandwidth
  - immunity to electromagnetic interference
  - small & lightweight
  - becoming very popular - e.g. LAN, MAN, WAN, undersea
  - typicall digital communications with 1 -> pulse of photons of light & 0 -> absence of 
pulse
  - 3 concentric cylinders of di-electric materials: 
  - core & cladding -> transparent glass, guide light, reflects at their interface;
  -  jacket -> plastic, absorbs light, prevents crosstalk, protects surface of cladding
  - 2 main types
	1) Multimode - modal dispersion problem - modest rates (10s of Mbps to 100Mbps 
	   in ranges of 10 - 100 km, used in LANs
	2) Single Mode - medium to long range trunk lines, international, several giga-bps
	  - no spreading of signal since one one

Microwave - predominant long-haul media, effective way to avoid natural barriers, 
repeaters

Satellite - superhigh frequency range (SHF) - see Table 3

Multiplexing - combines & bundles together number of communication channels -> 
transmits over one physical common broadband channel
  - at receiver, demultiplexing separates & recovers original channels
  - efficient use of bandwidth
  - 2 types:  FDM, TDM
	1) Frequency Division Multiplexing (FMD)
	 - each transmitter allocated portion of frequency spectrum i.e. frequency band
	  e.g. telephone network, communication antenna tv, satellite systems
	 - in telephone network - standard 0-4 kHz voice signal modulated (i.e. frequency
	  shifted) to occupy specific frequency band   ->  Figure 27 - heirarchy of FDM

	2) Time Division Multiplexing (TDM) 
 	  - each station uses entire frequency bandwidth but for limited time
	  - 2 types:  synchronous, asynchronous
	    1) Synchronous - each source repeatedly assigned portion of transmission 
	     capacity - e.g. circuit switched telphone networks
	     2) Asynchronous - each source assigned portion of transmission capacity only
	     as it is needed

- e.g. T1 Digital System (Figure 28) time multiplexes 24 voice channels - recall 24 voice
 time slots x 8 bits/slot = 192 bits;  192 bits + 1 framing bit = 193 bits/frame; 8000 frames/
 second  x  193 bits/frame = 1.544 Mbps
 - uses Channel Associated Signaling (CAS) -> bits stolen - least significant bit in all 24 
time slots of frames 6 & 12 -> signaling bit