Multiplexing (kỹ THUẬT TRUYỀN số LIỆU SLIDE)

 E.g., broadcast radio Channel allocated even if no data An analog multiplexing technique to combine signals... FDM: Example 1Five voice channels, each with 3-kHz bandwidth, are to be

Multiplexing and Spreading

 E.g., broadcast radio

 Channel allocated even if no data

An analog multiplexing technique to combine signals

FDM: Multiplexing Process

FDM: Demultiplexing

Process

FDM: Example 1

Five voice channels, each with 3-kHz

bandwidth, are to be multiplexed

together

If there is a need for a guard band of 1.5

kHz, what is the minimum bandwidth of

the link?

Five voice channels, each with 3-kHz

bandwidth, are to be multiplexed

together

If there is a need for a guard band of 1.5

kHz, what is the minimum bandwidth of

the link?

f

3 kHz 3 kHz 3 kHz 3 kHz 3 kHz

1.5 kHz kHz1.5 kHz1.5 kHz1.5

21 kHz

FDM: Example 2

Four digital-data channels, 1 Mbps

each, use a satellite channel of 1 MHz

Design an appropriate configuration

using FDM

Four digital-data channels, 1 Mbps

each, use a satellite channel of 1 MHz

Design an appropriate configuration

using FDM

Jumbo group

The Advanced Mobile Phone System (AMPS) uses two bands The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving Each user has a bandwidth of 30 kHz in each direction How many people can use their cellular phones simultaneously?

Solution

Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we  get  833.33.  In  reality,  the  band  is  divided  into  832  channels.  Of  these,  42  channels  are  used  for  control,  which means only 790 channels are available for cellular  phone users. 

Example

 Medium Data Rate > Signal Data Rate

 Multiple digital signals interleaved in time

 Time slots

 are preassigned to sources and fixed

 are allocated even if no data

 do not have to be evenly distributed among sources

Time Division Multiplexing

(TDM)

T D M

A B C

A B C A B C A B C

Frame

Time slot

A digital multiplexing technique to combine data

one unit

TDM Frames

 A frame consists of one complete

cycle of time slots

TDM: Example

Four 1-Kbps connections are multiplexed

together

A unit is 1 bit Find:

(1) the duration of 1 bit before multiplexing,

(2) the transmission rate of the link,

(3) the duration of a time slot, and

(4) the duration of a frame?

Four 1-Kbps connections are multiplexed

together

A unit is 1 bit Find:

(1) the duration of 1 bit before multiplexing,

(2) the transmission rate of the link,

(3) the duration of a time slot, and

(4) the duration of a frame?

17

Empty Slot

 Multiplexer and demultiplexer must be

Synchronization: Example

 Identify all the framing bits from this

bit sequence output by a multiplexer

 2 channels

 1 unit = 2 bits

0 0 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 1 1

Multi-Level Multiplexing

TDM of Different Data Rates

 Data rate from one source may be faster than the

 Different data rates may not be

integer multiples of each other

 Solution – Bit padding

B B

B

4

DS Services and T Lines

 DS-0, DS-1, etc, are services

 T lines are used to implement these

services

T Lines and Analog Signals

T-1 Frame Structure

E Lines

 European's version of T lines

 Also used in Thailand

Inverse Multiplexing

low-speed lines

Inverse MUX

Inverse DEMUX

FHSS Cycles

Narrow Band Interference

Spread Spectrum Signal

DSSS Example