CS 422 - Networking Notes

August 31, 2004

What is a network?

    - A network is a group of inter-connected computers

Why is a network necessary?

    - Resource Sharing

            + Files

            + Printers

            + Databases

    - Communication Between applications

            + Email

            + Instant Messenger

            + VoIP phones

    - Remote Execution

            + telnet

            + ssh

Transmission Media

    - Signals (variations of energy over time) are used to send data

    - Types of Signals:

        + Electricity

        + Light (fiber optics)

        + Radio waves

        + Sound

    - Media used to transmit signals

• Copper Wire

            - Coax cable (High frequency/shielded)   OR

            - Twisted pair cable - outside "noise" cancels out

            - Twisted pair bandwidth  <  Coax cable bandwidth

• Optical Fiber

            - Not susceptible to "noise"

            - Able to transmit long distances

            - Difficult to handle

            - Expensive

• Radio Frequency

            - No need of wires

            - RF spectrum is limited

            - Most limited bandwidth of all mediums

            - Power must be regulated to prevent interference with other networks

                         Two networks in a specific area                            More networks in same area if

                                                                                                        power is reduced

Propagation Delay

 - Time required for a signal to travel across media

        Examples:

                            Light: c = 3x10⁸

                            Copper: 0.7c

Bandwidth

 - Maximum number of times per second that a signal can change

 - Bandwidth limits the throughput of a network

Types of Encoding

 - Analog: signal changes proportionately to the data sent

        Without Noise                                With Noise - Difficult to differentiate noise from data

 - Digital: data is encoded as zeroes and ones, easier to differentiate noise from data

        Without Noise                        With Noise - receiver reconstructs to ....            Receiver reconstruction

Standards

 - To ensure interoperability hardware vendors, standards are created by:

        + ITU - International Telecommunications Union

        + EIA - Electronic Industries Association

        + IEEE - Institute of Electronics and Electrical Engineers

        + FCC - Federal Communications Commission (Regulates content and frequency spectrum)

     RS-232 Stadard

        - Initially used to connect computers to dumb terminals (ttys)

        - PCs still have COM1 and COM2 ports

        - Many devices (routers, bridges, etc.) have serial ports for diagnostics

        - Specifies the transmission of characters

        - It is called serial because one bit is sent after another

        - Asynchronus communication

            + Asynchornus - receiver doesn't know when transmitter will start sending

            + Synchronus - receivers know when transmission will start

        - In asynchronus, the sender has to add a special sequence of bits to the receiver to tell that a byte will be transmitted

               * +15 Volts => 0

               * -15 Volts => 1

               * Uses start bit and stop bit

               * Limited to 50 feet

               * When idle, level is always -15V (can check this to see if cable is till connected

        - Sender and receiver must agree on:

              + Duration of each bit (baud rate)

              + Number of bits per character (7, 8)

        - Disagreement results in a framing error

September 2, 2004

A Brief History of Electricity...

Electrical Transmission

 - Problems:

        + Resistance of cable increases as signal travels

            P = RI²

            R = KD

            P = IV

            P = V²/R

            I = V/R

            * P - power, I - current, R - resistance, V- volts

     + For long distances, power is conducted using high voltages to reduce power loss in the conductor

     + The capacitance and inductance of the cable distorts the signal

     + Distortion results in loss of information

        Original Signal                        Conducted through a cable                    Distorted Signal

    + Also, noise is added due to electromagnetic waves

RS-232 deals with these problems by

- Tolerating lower voltages

- Takes multiple samples and chooses the bit with more votes

- Electrical current that is direct (two levels) cannot be transmitted over long distances

Carrier Waves

 - A sine wave is used to carry information over long distances

 - Properties of the sine wave are changed (modulated) to encode information (called a carrier wave)

 - The modulated carrier technique is used by radio and TV

Characteristics of a carrier wave:

P - Phase: Distance from origin to beginning of first cycle

F - Frequency: Number of cycles per second (Hz)

A - Amplitude: Height of wave

Types of Modulation

- Amplitude modulation

    + Very susceptible to noise

    + Telling the noise from the signal is very diffcult

 - Frequency modulation

    + Noise doesn't affect frequency much

 - Phase Shift modulation

    + Phase of signal changes based on the data

    + Tolerant to noise

    + Used to send data

Ex. Radio Signals use different parts of the frequency spectrum

Modems

 - Hardware used for long distance communication (modulator/demodulator)

 - Modulator modules a carrier wave with the data

 - Demodulator converts back or extracts the data from the carriers

Types of Modems                In                          Out

- Dialup                           RS-232               Phone Tones

- Cable                            Ethernet                  Ethernet

- DSL                              Ethernet              Multiple bands in the phone line

Full Duplex Modem:

    - 2 way communication

    - Allows simultaneous transmission both ways

    - Needs 4 wires

Half Duplex Modem:

    - 2 way communication

    - Only one way at a time

    - Uses 2 wires