3 Transmission Modes

Dataflow Transmission

1. simplex

2. half-duplex

3. full-duplex

simplex transmission,

data can only flow in one direction, which is usually from the source to the sink

half-duplex transmission (HDX),

data can flow in both directions, but never simultaneously. It first flows in one

direction, and then in the other direction

full-duplex transmission (FDX),

data can flow in both directions simultaneously. It can be viewed as a pair of simplex lines between the source and sink with one line going from the source to the sink and the other going from the sink to the source

Serial data transmission

when data is sent or received, the data bits are organized in a specific order since they can only be sent one after another

Asynchronous transmission

is used when data is transmitted as individual characters. In this method, each character is preceded by one start bit and one or two stop bits that are used by the receiver for synchronization purposes.

Synchronous transmission

is used to transmit large blocks of data at a time. In this scheme, data is usually organized in frames and each frame is preceded by a flag that consists of a few bits and terminated by another flag

Parallel data transmission

multiple data bits are transmitted over multiple channels at the same time.

Transmission media

is a communication channel that carries the information from the sender to the receiver in which data is transmitted through the electromagnetic signals via copper wire, fiber optics, atmosphere, water, and vacuum.

Guided media

enables the transfer of information between two or more points that are connected by an electrical conductor

Twisted Pair

- it consists of two insulated copper wires that are typically about 1mm thick and twisted together in a helical form

Coaxial Cable

- to facilitate two-way communication, the cable bandwidth is split into two sets of channels: upstream channels and downstream channels

Optical Fiber

- refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber the optical cable.

Unguided media

do not require physical links between two or more devices. Wireless communication is based on radio waves communications in the frequency spectrum

Radio waves

–These are easy to generate and can penetrate through buildings. The sending and receiving antennas need not be aligned. Example: AM and FM radios and cordless phones

Microwaves

– It is a line of sight (LOS) transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. These are majorly used for mobile phone communication and television distribution

Infrared

– Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems. It is used in TV remotes, wireless mouse, keyboard, printer, etc

Antennas

It is the metal rod or dish that catches radio waves and turns them into electrical signals feeding into something like a radio or television or a telephone system

Log Periodic Antennas

• It is a directional antenna that is made of a series of dipoles placed along its axis at different space intervals of time followed by a logarithmic function of frequency.

Wire Antennas

• These antennas use a length of wire for transmitting and receiving wavelengths of signals

Traveling Wave Antennas

• It is a directional aperiodic antenna that uses a travelling wave as its guiding structure. is a temporary wave that creates a disturbance and moves along the transmission line at a constant speed.

Microwave Antennas

• This type of antenna uses microwave transmission to broadcast signals between two or more locations.

• is a form of electromagnetic radiation that is used mostly in point-to- point communications and radar.

Frequency Spectrum

It is the range of frequencies contained by a signal

EXTREMELY LOW FREQUENCY

Frequency: 3 KHz to 30 KHz

Wavelength: 100 km to 10 km

LOW FREQUENCY

Frequency: 30 KHz to 300 KHz

Wavelength: 10 km to 1 km

MEDIUM FREQUENCY

Frequency: 300 KHz to 3000 KHz

Wavelength: 100 km to 10 km

HIGH FREQUENCY

Frequency: 3 MHz to 30 MHz

Wavelength: 100 m to 10 m

VERY HIGH FREQUENCY

Frequency: 30 MHz to 300 MHz

Wavelength: 10 m to 1 m

ULTRA HIGH FREQUENCY

Frequency: 300 MHz to 3000 MHz

Wavelength: 1 m to 100 mm

SUPER HIGH FREQUENCY

Frequency: 3 GHz to 30 GHz

Wavelength: 100 mm to 10 mm

EXTREMELY HIGH FREQUENCY

Frequency: 30 GHz to 300 GHz

Wavelength: 10 mm to 1 mm

Wave Propagation

These waves are basically defined as superimposed oscillations of an electric and a magnetic field in space with their direction of propagation perpendicular to both.

• Ground Wave

This is used for a low-frequency range transmission, mostly less than 1 MHz. This type of propagation employs the use of large antennas order of which is equivalent to the wavelength of the waves and uses the ground or Troposphere for its propagation

• Sky Wave

This is used for the propagation of EM waves with a frequency range of 3-30 Mhz.

• Space Wave

This is used for a line of Sight communication (LoS). Space satellite communication and very high- frequency waves use this propagation

method.

Multiplexing (MUX)

is a way of sending multiple signals or streams of information over a communications link at the same time in the form of a single, complex signal

demultiplexing

The receiver recovers the separate signals, a process called

Media Sharing Technique

The utilization of such media can be increased by allowing multiple users to transmit their data simultaneously, or close to simultaneously

• Frequency-division multiplexing (FDM)

where the frequency spectrum of the medium is partitioned into multiple frequency blocks called channels that are assigned to users who can use these channels simultaneously without interference from each other

• Time-division multiplexing (TDM)

where transmission time is divided into non-overlapping time slots that are assigned to users.

Spread spectrum (SS)

• where the output signal, which appears like noise, occupies more bandwidth than the original signal

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

1. Carrier Sense

2. Multiple Access

3. Collision Detection

Carrier Sense

o means that each station on the LAN continually listens to (tests) the cable for the presence of a signal before transmitting

Multiple Access

o means that many computers are attempting to transmit and compete for the opportunity to send data

Collision Detection

o means that when a collision is detected, the stations will stop transmitting and wait a random length of time before retransmitting the data

• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

o Instead of detecting and reacting to collisions, CSMA/CA tries to avoid them by having each computer signal its intention to transmit before transmitting.

• Token passing

o It is a media access method by which collisions are prevented. Collisions are eliminated under token passing because only a computer that possesses a free token (a small data frame) can transmit. This method also allows different priorities to be assigned to different stations on the ring.

• Demand priority

an active hub is an essential requirement that can control access to the network.