Chapter 5: Making Connections Efficient: Multiplexing and Compression

Introduction

Under the simplest conditions, a medium can only carry one signal at a time. However, we want a medium that can carry multiple signals at the same time. The technique of transmitting multiple signals over a single medium is multiplexing. For multiple signals to share one medium, the medium must be divided to give each signal a portion of the total bandwidth. So a medium can be divided in three basic ways: a division of frequencies, a division of time, and a division of transmission code. Another way to make a connection between two devices more efficient is to compress the data that is transferred over the connection.

Frequency Division Multiplexing

• Frequency division multiplexing (FDM) is the assignment of non-overlapping frequency ranges to each user of a medium. 
• To allow multiple users to share a single medium, FDM assigns each user a separate channel
• Channel: is an assigned set of frequencies that is used to transmit the user's signal
• Multiplexor: the device that accepts input from one or more users
• Demultiplexor: the device attached to the receiving end of the medium that splits off each signal to deliver it to the appropriate receiver
• Guard band: a set of unused frequencies inserted between the two signals to provide a form of insulation to keep one signal from interfering with another signal

Time Division Multiplexing

• Time division multiplexing (TDM) allows only one user at a time to transmit and the sharing of the medium is accomplished by dividing available transmission time among users.
• A time division multiplexor calls on one input device after another, giving each device a turn at transmitting its data over  a high-speed line

1. Synchronous time division multiplexing

• Synchronous time division multiplexing (Sync TDM) gives each incoming source signal a turn to be transmitted, proceeding through the sources in round-robin fashion

• the demultiplexor on the receiving end of the high speed link must disassemble the incoming byte stream and deliver each byte to the appropriate destination
• when a device has nothing to transmit, the multiplexor must still allocate a slot for that device in the high speed output stream, but the slot still be empty.
• to maintain synchronization between sending multiplexor and receiving demultiplexor, the data from the input sources is often packed into a simple frame and synchronization bits are added somewhere within the frame

• T-I Multiplexing
  - T-1 multiplexed stream is a continuos repetition of frames
  - The frame of the T-1 multiplexor's output stream are divided into 24 separate digitized voice/data channels of 64 kbps each.
  - Each frame consists of 1 byte from each of the 24 channels (users) plus 1 synchronization bit
  - The input data from a maximum of 24 devices is assigned to fixed intervals. Each device can transmit only during that fixed interval. and if a device has no data to transmit, the time slot is still assigned to it and data such as blanks or zero is transmitted

• SONET/SDH Multiplexing
  - Synchronous Optical Networks (SONET) and Synchronous Digital Hierarchy (SDH) are powerful standards for multiplexing data streams over a single medium.
  - Both are synchronous multiplexing techniques
  - A single clock controls the timing of all transmission and equipment across an entire SONET network. Using only a single clock to time all data transmissions yields a higher level of synchronization because the system does not have to deal with two or more clocks having slightly different times
  - SONET defines a hierarchy of signaling levels, or data transmission rates, called Synchronous transport signal (STS)
  - Two common users for SONET are the telephone company and companies that provide an Internet backbone services

2. Statistical time division multiplexing

• Statistical time division multiplexing (Stat TDM) transmit data only from active users and does not transmit empty time slots
• To transmit data only from active users, the multiplexor creates a more complex frame that contains data only from those input sources that have something to send.
• If a workstation is not active, no space is wasted in the multiplexed stream

• A statistical multiplexor accepts the incoming data stream and create a frame containing the data to be transmitted
• To identify each piece of data, an address is included

Wavelength Division Multiplexing

• Wavelength division multiplexing (WDM) multiplexes multiple data streams onto a single fiber-optic line.
• It uses different wavelength (frequency) lasers to transmit multiple signals at the same time over single medium
• The wavelength of each different colored laser is called Lambda
• Each signal can be carried on the fiber-optic line at a different rate, this means that a single fiber-optic can support simultaneous transmission speeds
• Dense wavelength division multiplexing (DWDM): when WDM can support a large umber of lambdas
• Coarse wavelength division multiplexing (CWDM): is designed for short distance connections and has only a few lambdas, with greater space between lambdas

Code Division Multiplexing

• Code division multiplexing (CDM) allows multiple users to share a common set of frequencies by assigning a unique digital code to each user
• Uses direct sequence spread spectrum technology that spreads the transmission of a signal over a wide range of frequencies using mathematical value
• Each binary 1 and 0 is replaced with a larger, unique bit sequence
• To send a binary 1, a mobile device transmits the unique code
• To send a binary 0, a mobile device transmits the inverse of the code

Discrete Multitone

• Discrete multitone (DMT) is a multiplexing technique commonly used in digital subscriber line (DSL) systems 
• DMT essentially combines hundreds of different signals or subchannels into one stream and are destined for a single user
• Each of the subchannel is quadrature amplitude modulated-256 subchannels, each transmits 60 kbps, yields 15.36 Mbps
• Unfortunately, because of noise, not all 256 subchannels can  transmit at full speed of 60-kbps rate

Comparison of Multiplexing Techniques

Compression - Lossless vs. Lossy

• Compression: is the process of taking data and somehow packing more of it into the same space
• When data is compressed for transmission, it transfers more quickly, results in a more efficient connection
• It also allows more data to be stored in the same amount of disk space or memory

1. Lossless Compression

• Lossless compression: compresses data and then decompresses it back to original data, no data is lost due to compression
• Run-length encoding: replaces any repetition of the same bit or byte that occur in a sequence of data with a single occurrence of the bit/byte and run a count
• It compresses the 0s by counting the "runs" of 0s, that is it would start by counting the 0s until a binary 1s is encountered.
• The next step is to convert each of the decimal values into 4-bit binary values, or nibbles
• When encounter a decimal value of 15 or greater, we must convert it into multiple 4-bit nibbles

2. Lossy Compression

• Lossy compression: in the compression process, some of the data is lost
• Perceptual encoding or perceptual noise shaping: the compressed version of an audio stream sounds fairly close to the uncompressed version even though some of the original data as been removed
• MP3 is a common form of audio compression
• MP3 encoder produces a data stream that has a much slower rate than that of conventional CD music
• Video files can also be compressed by removing small details that the human eye will not notice
• JPEG: a technique that is very commonly used to compress video images that involves 3 phases:
  - Discrete Cosine transformation: the image is broken into multiple 8 by 8 blocks of pixels and then produce a new 8 by 8 blocks of values
  - Quantization phase: the object of this phase is to generate more zero entries in the 8  by 8 block
  Run-length coding: take the matrix of quantized values and perform run-length encoding on the zeros by going diagonal  to achieve longer runs of zeros
• MPEG (Motion Picture Expert Group) is a group of people that have created a set of standards that can use these small differences between frames to compress a moving video (and audio) to a fraction of its original size