ISO-OSI Model

ISO= International Standards Organization

OSI = Open Systems Interconnection

OSI Model = a 7 layer model used to describe most networks today [1]

" 1. Physical Layer

This is a physical connection between machines. Electrical functional and mechanical specs are given here.

2. Data Link Layer

Provides reliable xfer of information across the physical links. Synchronization of frames, error control and flow control

3. Network Layer

isolates upper layers from net-specific item like establishing, maintaining and terminating connections in a switching network.

4. Transport Layer

error recovery and flow control

5. Session Layer

Control for communication, close and open connections

6. Presentation Layer

Data representation consistency; EBDIC -> ASCII, little endian to big endian

7. Application Layer

User Interface; i.e. kermit, ftp, appleshare file server, telnet, NFS mounts etc.

Transmission

Analog transmission -: a means of transmitting analog signals without regard to their content; e.g., binary data that passes through a modem

Digital transmission, a system which is concerned with the content; e.g. patterns of one's and zero's are recovered before retransmission.

noise is not cumulative in a digital transmission.

Digital transmission is the current trend, for voice and video.

Advantages are in digital technology, data integrity, security, capacity utilization and integration.

Transmission Impairments

* Attenuation and attenuation distortion

* delay distortion

* noise

Attenuation

For guided transmission Signal strength falls of logarithmically as a function of distance

Attenuation is an increasing function of frequency

Signal to noise ratio must be great enough

delay distortion :- velocity of a signal, in a guided media, is a function of its frequency. This causes intersymbol interference in digital transmissions and limits the maximum bit rate.

Noise

noise -: Undesired signals. Human interference (other communication systesm, ignition and commutator sparking, 60 cycle hum..) Natual noise-producin phenomena (atmospheric disturbances, extraterrestrial radiation and circuit noise..)

Mathematics of the fourier series

plot(f,x=0..2*Pi);

A sine wave:

sin(Pi x)

f := 2 ---------

Pi

the five term sawtooth wave:

f := 2 (sin(Pi x) - 1/2 sin(2 Pi x) + 1/3 sin(3 Pi x) - 1/4 sin(4 Pi x)

+ 1/5 sin(5 Pi x))/Pi

The ten term sawtooth wave:

f := 2 (sin(Pi x) - 1/2 sin(2 Pi x) + 1/3 sin(3 Pi x) - 1/4 sin(4 Pi x)

+ 1/5 sin(5 Pi x) - 1/6 sin(6 Pi x) + 1/7 sin(7 Pi x) - 1/8 sin(8 Pi x)

+ 1/9 sin(9 Pi x) - 1/10 sin(10 Pi x))/Pi

and

For the 100 term sawtooth wave:

For K terms in the sawtooth wave:

K

----- (n - 1)

(-1) sin(n Pi x)

) -----------------------

/ n

-----

n = 1

f := 2 -----------------------------

Pi

Fourier series

:= an expansion of a periodic function as a series of trigonometric functions. Thus,

f(x) = a0 + (a1 cosx + b1 sinx) + (a2 cos 2x + b2 sin2x) + ...

where a0, a1, b1, b2 etc are constants called Fourier coefficients.

The series was first formulated by Jean Baptist Joseph de Fourier (1768-1830) and is used in harmonic analysis (fourier analysis) to determine the harmonic components of a complex wave.

Sine-Cosine Representation

Power Spectral Density and Bandwidth

Time-limited signals have infinite bandwidth.

Effective bandwidth := the width of the spectra which contains the most power.

You will not be asked to compute PSD.