Multiplexing and Compression Techniques

Multiplexing

is a technique used to divide a medium's bandwidth to allow multiple signals to share it.

Frequency Division Multiplexing (FDM)

This technique assigns non-overlapping frequency ranges to each user or signal, allowing them to be transmitted simultaneously. It is the oldest multiplexing technique and is commonly used in broadcast radio, television, and cable television.

Time Division Multiplexing (TDM)

divides the available transmission time on a medium among users and exclusively uses digital signaling.

Synchronous TDM

The multiplexor accepts input from devices in a round-robin fashion and transmits the data in a continuous pattern. If a device has nothing to send, the multiplexor still inserts something into the stream to maintain synchronization. Examples include T-1 and SONET/SDH telephone systems.

Statistical TDM

This method transmits data only from active workstations, making it more efficient as no space is wasted in the multiplexed stream for inactive devices. An address is included to identify each piece of data.

Wavelength Division Multiplexing (WDM)

Used for fiber-optic lines, ______multiplexes multiple data streams using different wavelength lasers, known as lambdas. Dense WDM (DWDM) combines many signals, while Coarse WDM (CWDM) combines only a few.

Dense WDM (DWDM)

Combines many signals.

Coarse WDM (CWDM)

Combines only a few.

Discrete Multitone (DMT)

Commonly found in DSL (Digital Subscriber Line) systems, DMT combines hundreds of subchannels into one stream for a single user.

Code Division Multiplexing (CDM)

Also known as Code Division Multiple Access (CDMA), this advanced technique allows multiple devices to transmit on the same frequencies at the same time by assigning a unique code to each device.

Compression

is used to reduce the size of a data file so it can be transferred in less time.

Lossless Compression

When the data is uncompressed, the original data is completely restored. This is essential for files where no data loss is acceptable, such as financial files. An example is run-length encoding, which replaces runs of identical data (like zeros) with a count.

Lossy Compression

When the data is uncompressed, you do not get the original data back. This is acceptable for audio and video files where some data loss is not perceptible to the human ear or eye.

Audio Compression

Techniques like MP3 work by taking advantage of psychoacoustic principles, such as a louder sound masking a softer one.

Video Compression

For moving video, techniques like MPEG (Motion Picture Experts Group) send only the differences between frames, since they are often very small.

run-length encoding

Replaces runs of identical data (like zeros) with a count.

MP3

Techniques like MP3 work by taking advantage of psychoacoustic principles, such as a louder sound masking a softer one.

MPEG (Motion Picture Experts Group)

Sends only the differences between frames, since they are often very small.

JPEG (Joint Photographic Experts Group)

_______ uses a three-phase process involving Discrete Cosine Transformations (DCT), Quantization (reducing the precision of the data), and run-length encoding.