*2.4.2 Asynchronous time division multiplexing*

In synchronous TDM if a particular terminal has no data to transmit at a particular time period, the corresponding slot in a frame is wasted or an empty slot will be transmitted. Asynchronous TDM or statistical TDM is used to overcome this difficulty. It dynamically allocates the time slots on the demand to separate input channels, thus saving the channel capacity. Here the time slots are flexible, and the total capacity of input lines can be greater than the link capacity of the channel. In synchronous TDM if there are n input lines, there must be n time slots, but in asynchronous TDM if we have n input lines, then the frame may contain less than n slots. Here the number of slots in a frame is based on a statistical analysis of the number of input lines. The transmission of data with asynchronous TDM is shown in **Figure 10**.

**17**

**Figure 11.**

*Code division multiplexing (CDM).*

*Multiplexing*

*2.4.2.1 Disadvantages*

**Figure 10.**

• Frames have different sizes.

*Data transmission with asynchronous TDM.*

slots assigned for each user.

**2.5 Code division multiplexing**

• An output slot in synchronous TDM is totally occupied by data, in statistical TDM; a slot needs to carry data as well as the address of the destination.

• It requires buffer, and address information is needed as there is no separate

Code division multiplexing (CDM) [3] is a form of multiplexing in which the transmitter encodes the signal by using a unique chip code which is generated by a pseudorandom sequence generator. It uses spread-spectrum communication, and a narrowband signal is spread over a large band of frequency; it allows multiple signals from multiple users to share a common communication channel. CDM involves the modulation of data with this spreading code in the transmitter side.

*DOI: http://dx.doi.org/10.5772/intechopen.85866*

**Figure 9.** *Synchronous TDM transmission representation.*

*Multiplexing DOI: http://dx.doi.org/10.5772/intechopen.85866*

**Figure 10.**

*Multiplexing*

shown in **Figure 9**.

*2.4.1.1 Disadvantages*

send the data.

input lines.

shown in **Figure 10**.

• It is very complex to implement.

*2.4.2 Asynchronous time division multiplexing*

or statistical TDM or intelligent TDM.

*2.4.1 Synchronous time division multiplexing*

stream so that it can separate the time slots accurately. Normally, this synchroniza-

There are two types of TDM multiplexing, synchronous TDM and asynchronous

In synchronous TDM the slots are arranged in a round robin manner, i.e., if there are n sources, then a single frame consists of n time slots, and each time slot is dedicated to exactly one source for carrying data from the corresponding input. Each source places its data to the link only when the corresponding slot arrives. In synchronous TDM, if a device does not have data to send, then its time slots remain empty. The transmission of data with synchronous TDM is

• The channel capacity cannot be fully utilized when some source do not want to

• The capacity of the transmission link must be higher than the total capacity of

In synchronous TDM if a particular terminal has no data to transmit at a particular time period, the corresponding slot in a frame is wasted or an empty slot will be transmitted. Asynchronous TDM or statistical TDM is used to overcome this difficulty. It dynamically allocates the time slots on the demand to separate input channels, thus saving the channel capacity. Here the time slots are flexible, and the total capacity of input lines can be greater than the link capacity of the channel. In synchronous TDM if there are n input lines, there must be n time slots, but in asynchronous TDM if we have n input lines, then the frame may contain less than n slots. Here the number of slots in a frame is based on a statistical analysis of the number of input lines. The transmission of data with asynchronous TDM is

tion information consists of 1 bit per frame, alternating between 0 and 1.

**16**

**Figure 9.**

*Synchronous TDM transmission representation.*

*Data transmission with asynchronous TDM.*

## *2.4.2.1 Disadvantages*


## **2.5 Code division multiplexing**

Code division multiplexing (CDM) [3] is a form of multiplexing in which the transmitter encodes the signal by using a unique chip code which is generated by a pseudorandom sequence generator. It uses spread-spectrum communication, and a narrowband signal is spread over a large band of frequency; it allows multiple signals from multiple users to share a common communication channel. CDM involves the modulation of data with this spreading code in the transmitter side.

**Figure 11.** *Code division multiplexing (CDM).*

### *Multiplexing*

The receiver also wants to know the same code used at the transmitter side in order to decode the signal at the receiving side. Here different random sequences correspond to different communication channels from different stations. To separate other channels, CDM assigns each channel with its own code. The main advantage of CDM is protection from interference and tapping because only the sender and the receiver know the spreading code (**Figure 11**).

CDM is widely used in second-generation and third-generation wireless communication network.
