Multiplexing

Multiplexing

• Multiplexing is the set of techniques that allows simultaneous transmission of multiple signals across a single link.

FDM: Frequency Division Multiplexing

FDM: Frequency Division Multiplexing

The division of a transmission facility into two or more channels by splitting the frequency band into narrow bands ,each of which is used to constitute a distinct channel.

Multiplexer

• A device that can combine and transmit several signals over a single line

Multiplexer

• In the multiplexer similar signals are modulated onto different carrier frequencies (f1,f2,f3 ) .The resulting modulated signals are then combined into a single composite single that is sent over media link that has enough bandwidth to accommodate it

Multiplexer

DeMultiplexer

• A device that decode the single line signal into

multiple signals.

DeMultiplexer

• The DeMultiplexer uses a series of filters to decompose the multiplexed signal into its component signals .the individual signals are then passed to a demodulator that separate them from their carriers and pass them to the waiting receivers.

DeMultiplexer

Note

• Link refers to the physical path .• Channel refers to the portion of a link that carries a

transmission between a given pair of lines .

• One link may have many channels .

Note

Example

• Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency

domain without the use of guard bands.

Example

Example

• Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?

Ans :• For five channels, we need at least four guard bands.

This means that the required bandwidth is at least 5 x 100 + 4 x 10 = 540 KHz,

as shown in Figure 6.7.

Example

WDM

• WDM is designed to use the high data rate capability of fiber optic cable.

• WDM is conceptually the same as FDM,except the multiplexing and demultiplexing involves optic signals transmitted through fiber optic channels.

Figure 6.10 WDM

Figure 6.11 Prisms in WDM multiplexing and demultiplexing

6.19

Figure 6.12 TDM

Time division multiplexing

• Time division multiplexing: is a digital process that allows several connections to share the high bandwidth of a link .Each connection occupies a portion of time in the link

• TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.

Time division multiplexing

Time slots and frames

• The data flow of each connection is divided into units , and the link combines one unit of each connection to make a frame.

• The size of the unit can be 1 bit or several bits.

• For n input connections , a frame is organized into a minimum of n time slots

Time division multiplexing

• Note • In TDM , the data rate of the link that carries data

from n connections must be n times the data rate of a connection to guarantee the flow of data.

6.24

Figure 6.13 Synchronous time-division multiplexing

Time division multiplexing

Exercise

• 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?

Answer: • We can answer the questions as follows:

1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).• 2. The rate of the link is 4 Kbps.• 3. The duration of each time slot 1/4 ms or 250 ms. • 4. The duration of a frame 1 ms.

TDM

1) Synchronous time division multiplexing

2) Statistical time division multiplexing

6.28

Figure 6.26 TDM slot comparison

Synchronous time division

• time slot is fixed to each device .• Data rate of medium exceeds data rate of digital

signal to be transmitted• May be at bit level of blocks• Time slots pre-assigned to sources and fixed• Time slots allocated even if no data

Statistical time division Multiplexing

• Multiplexer dynamically allocates time slots on demand )

• A statistical multiplexer transmits only the data from active workstations

• If a workstation is not active, no space is wasted on

the multiplexed stream.

Stat syn

Addressing yes

synchronization bit

No

Slot size less

Bandwidth of link Less than the sum total bandwidth of each channel