Chapter 6: Bandwidth Utilization: Multiplexing and Spreading

What is bandwidth utilization?

the wise use of available bandwidth to achieve specific goals

Multiplexing

increases efficiency

set of techniques allowing simultaneous transmission of multiple signals across a single data link.

Spreading

increases privacy and anti-jamming

multiplexing categories

1. frequency-division multiplexing

2. wavelength division multiplexing

3. time division multiplexing

frequency-division multiplexing

combines analog signals

wavelength division multiplexing

combines optical signals

time division multiplexing

combines several low rate channels into one high rate channel

data rate is n times faster, unit rate is n times shorter

Empty slots

occurs when source has no data to send

Strategies for Data Rate Management

1. Multi-level multiplexing

2. Multiple slot allocation

3. Pulse stuffing

Multi-level multiplexing

input lines multiplexed together to provide a data rate equal to the other input lines

Multiple slot allocation

an input line broken down using a serial to parallel converter to make it equal to the other input lines

Pulse stuffing

adds dummy bits into an input line with data rates lower than the others when the input lines are not multiples of each other

Why do we frame bits

to help the demultiplexer identify the start of a new frame to align its reception with transmitted data.

synchronization bits are added to the beginning of each frame.

Statistical TDM

1. slots are dynamically allocated to improve the bandwidth efficiency

2. no slot is left empty as long as there is data to be sent by any input line unlike synchronous TDM

Spread spectrum

combines signals from different sources to fit a larger bandwidth

Spread spectrum techniques

1. frequency hopping spread spectrum

2. direct sequence spread spectrum synchronous.

Why use SS techniques?

add redundancy to prevent eavesdropping and jamming

Principles to achieve SS

1. bandwidth allocated to each station needs to be larger than what is needed

2. Spreading process occurs after the signal is created by the source

FHSS (frequency hopping spread spectrum)

1. transmits radio signals by rapidly switching between a set of predetermined frequencies

2. generated using pseudo-random sequence known to both transmitter and receiver

frequency selection in FHSS

FHSS transmits data on one frequency hop then switches to another frequency rapidly

hopping is predetermined and synchronized between transmitter and receiver.

DSSS (direct sequence spread spectrum synchronous)

used in wireless communication systems

spreads data signal over wider bandwidth than original

the spreading sequence generated is known to both transmitter and receiver