Multiplexing is a set of techniques that allows the simultaneous
transmission of multiple signals across a single data link. In a multiplexing
system,
In frequency division multiplexing (FDM), signals generated by each sending device modulate different carriers. This modulated signals are then combined into a single composite signal that can be sent by the link. Channels can be separated by strips of unused bandwidth-guard bands to prevent signal from overlapping.
Each source generates a signal of a similar frequency range. These similar
signals modulate different carriers (with different frequencies
The demultiplexer uses a series of filters to decompose the multiplexed signals into its constituent component signals. The individual signals are then passed to a demodulator that separates the input signals from their carriers and passes them to the output lines.
Telephone companies have traditionally multiplexed signals from lower-bandwidth lines onto higher-bandwidth lines.
| Analog multiplexing hierarchy | Classification |
|---|---|
| 12 voice channels | Group |
| 5 groups | Supergroup |
| 10 supergroups | Master group |
| 6 master groups | Jumbo group |
Time division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link. Instead of sharing a portion of the bandwidth as in FDM, time is shared. Each connection occupies a portion of time in the link.
In synchronous TDM, data units from each input connection are combined into
a frame. If we have in connections, a frame is divided into
The data rate of the output link must be in
TDM can be visualized as two fast-rotating switches, one on the multiplexing side and the other on the demultiplexing side. And the switch is unsynchronized and rotate at the same speed but in positive direction.
| Side | Description |
|---|---|
| Multiplexing | As the switch open in front of a connection, this connection can send a unit of data onto the link. This process is called interleaving. |
| Demultiplexing | As the switch open, the connection can receive a unit of data from the link. |
If a source does not have data to send, the corresponding slot in the output frame is empty.
Multilevel multiplexing is a technique used when the data rate of an input line is a multiple of others.
We can allocate more than 1 slot in a frame to a single input line.
If bit rate of sources are not multiple integers of each other, we can make the highest input data rate the dominant data rate and then add dummy bits to the input lines with lower rates. This technique is called pulse stuffing, bit padding, or bit stuffing.
If the multiplexer and the de-multiplexer are not synchronized, a bit belonging to one channel may be received by the wrong channel. For this reason, one or more synchronization bits are usually added to each frame.
Telephone companies implement TDM through a hierarchy of digital signals called digital signal (DS) service of digital hierarchy.
| Service | Description | Equation |
|---|---|---|
| DS-0 | Single digital channel of |
|
| DS-1 |
|
$$1,544 \textsf{ Mbps} = \underbrace{24 . 64 \textsf{ Kbps}}\textsf{phone channels} + \underbrace{8 \textsf{ Kbps}}\textsf{overhead}$$ |
| DS-2 |
|
|
| DS-3 |
|
|
| DS-4 |
|
The telephone companies use T-lines to implement the DS services.
E-lines are used in Europe.
In statistical time-division multiplexing, slots are dynamically allocated to improve bandwidth efficiency. Only when an input line has data to send, it is given a slot in an output frame. The number of slots in each frame is less than the number of input lines.
In statistical multiplexing, there is no fixed relationship between the inputs and outputs. We need to include the address of the receiver inside each slot.
Spread spectrum is designed to be used in wireless applications. Stations must be able to spare the medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder.
Spread spectrum techniques spread the original spectrum needed for each station. The expanded bandwidth allows the service to leap its message in a protective envelope for a more secure transmission.
The frequency hopping spread spectrum (FHSS) technique uses
A pseudorandom code generator creates a
If the number of hopping frequencies is