Frequency Division Multiplexing:
Frequency division multiplexing (FDM) is the assignment of non-overlapping frequency ranges to each "user" of a medium. The technique used by FDM will assign each user a different channel when sharing the medium for multiple signals. A channel is an assigned set of frequencies that is used to transmit the user's signal. When assigning these channels, the signals are either discrete analog (digital) or analog. Radio stations, cable stations, and various telecommunications companies are all examples of businesses that use the frequency division multiplexing technique for transmitting multiple signals through one medium.
When assigning these channels dynamically to various users, it can be less wasteful to do so. However, when channels are static, much like radio stations, wastefulness can occur. Radio stations will assign guard bands which are frequencies that are inserted between two different signals to a form of insulation. This will prevent the interruption of other signals that are close in range.
Devices that accept incoming transmissions from one or more users, this device is normally called the multiplexor. The device that is attached to the receiving end of the transmission that splits the signal into different channels is called the demultiplexor.
Time Division Multiplexing:
Time division multiplexing allows only one user at a time to transmit signals. To share the medium for transmission, the medium will be divided by available transmission time. The users will have the entire spectrum of the bandwidth, but it will only be for a short amount of time. Time division multiplexing has been divided into two separate technologies: synchronous time division multiplexing and statistical time division multiplexing.
Synchronous time division multiplexing (sync TDM) gives each incoming source signal a turn to be transmitted. The sync TDM accepts one piece of data from device one and transmits it and then accepts another piece of data from a second device and transmits that data. The technique will then start over by accepting another piece of data from the first device and transmitting it and then moving to the second device.
If a device has nothing to transmit, then the sync TDM must still allocate a time slot for the device. The time slot will only be empty when the data its transmitted. If only one device is transmitting data out of five other devices, the sync TDM must still allocate time slots for each device.
To maintain synchronization between the multiplexor and demultiplexor, the data that is being transmitted will be packaged into simple frames that will include synchronization bits. There are two types of synchronous time division techniques which are T-1 multiplexing and SONET/SDH. The T-1 multiplexing technique provides 24 separate digitized voice/data channels of 64 kbps each. Users can use all 24 channels or some can request a fraction of the 24 separated channels. This technique works exactly the same way as we had explained in the beginning of this section. The only difference is that there are 24 digitized voice/data channels.
Synchronous optical network (SONET) and synchronous digital hierarchy (SDH) are two standards for multiplexing data over single mediums. Both techniques are synchronous multiplexing techniques, but they both have a single clock that controls the timing of all transmissions. Both are also capable of transmitting varying speed streams of data onto on fiber optic cable. The SONET technique uses data transmission rates called synchronous transport signals (STS). The STS are supported by physical specifications called optical carriers (OC). The reason for the data rate relationship is to keep SONET simple. Each SONET frame contains 810 bytes which is equivalent to 6480 bits. The frames will also consist of a section overhead, line overhead, path overhead, data, and the synchronous bit.
Statistical time division multiplexing (Stat TDM) transmits data that is from active devices only. This technology does not transmit empty time slots. If there are two devices that are active out of a total of four devices, the multiplexor will only send data from the two active devices. If that were to change, however, the multiplexor will have to provide an address within each byte of data to identify which device had sent the data. To transmit various forms of data from each data, the multiplexor will add a length field that will describe the length of data being transmitted. The frame layout produced by Stat TDM includes a flag bit, control bit, packets that include the length and address, the frame check sequence (FCS), and then another flag bit.
Wavelength Division Multiplexing:
Wavelength division multiplexing(WDM) multiplexes multiple data streams onto a single fiber-optic line. This technique assigns input sources to separate sets of frequencies. Wave division multiplexing uses different wavelengths (frequencies) lasers to transmit various signals at the same time over a medium. Lambda are wavelengths or differently colored frequencies that are transmitted.
The division multiplexing technique is also considered to be scalable.
- Coarse wavelength division multiplexing (CWDM) – a less expensive form of wavelength division multiplexing that involves the transfer of a small number of streams of data over a single optical fiber using multiple lasers of differing wavelengths
- Code division multiplexing (CDM) – a multiplexing technique used extensively by the military and cellular phone companies in which binary 1’s and 0’s are replaced with larger, unique, binary sequences to allow multiple users to share a common set of frequency.
- Compression – the process of manipulating data such that it fits into a more compact space
- Demultiplexor – a multiplexer that un multiplexes that data stream and delivers the individuals streams to the appropriate devices
- Dense wavelength division multiplexing (DWDM) – an expensive form of wavelength division multiplexing that involves the transfer of a large number of data stream over a single optical fiber using multiple lasers of differing wavelengths
- Discrete multitone (DMT) – a modulation technique used in digital subscriber line
- DS-1 signaling – the signaling technique used to transfer 1.544 MBPS over a T-1 system
- Fiber exhaust – what happens when a fiber optic cable is transmitting at its maximum capacity
- Frequency division multiplexing (FDM) – the oldest and one of the simplest multiplexing techniques, FDM involves a signing non-overlapping frequency ranges to different signals or to each user of a medium
- Guard band – a set of unused frequencies between two channels on a frequency division multiplex system
- JPEG – a technique commonly used to compress video images
- Lambda – in wavelength division multiplexing the wavelength of each differently colored laser
- Lossless compression – a compression technique in which data is compressed and then decompressed such that the original data is returned and no data is lost
- Lossy compression – a compression technique in which data is compressed and then decompressed but this process does not return the original data, some data is lost due to compression
- MP3 – a compression technique that allows a high-quality audio sample to be reduced to a much smaller size file
- MPEG – a technique used to compress motion picture images or moving videos
- Multiplexing – transmitting multiple signals on one medium at the same time
- Multiplexor – the device that combines multiple input signals for transmission over a single medium and then demultiplexes the signal back into multiple signals
- Perception encoding – a compression technique applied to audio and video files in which aspects of the data with characteristic that are usually not noticed by the average person are removed from the data or compressed
- Run-length encoding – a compression technique in which a commonly occurring symbol in a data set is replaced with a simpler character and a count of how many times that symbol occurs
- Statistical time division multiplexing (Stat TDM) – a form of time division multiplexing in which the multiplexor creates a data packet of only those devices that have something to transmits
- Synchronous Digital Hierarchy (SDH) – a high-speed synchronous time division multiplexing technology developed in Europe by ITU-T that uses fiber-optic cables for a high-bandwidth transmission in the megabit to gigabit range for a wide variety of data types. Almost identical to SONET
- Synchronous Optical Network (SONET) – A high-speed synchronous time division multiplexing technology developed in the United States by ANSI that uses fiber optic cables for high bandwidth transmission in the megabit to gigabit range for a wide variety of data types. Two common users of SONET are the telephone company and companies that provide an internet backbone service Almost identical to SDH
- Synchronous time division multiplexing (Sync TDM) – a multiplexing technique that gives each incoming source a turn to transmit, proceeding through the sources in round-robin fashion
- Synchronous transport signals (STS) – the signaling techniques that used to support SONET transmissions when the data is transmitted in electrical form and not in optic form
- T-1 Multiplexing – A type of synchronous time division multiplexing (involving T-1 multiplexors) where the data stream is divided into 24 separate digitized voice/data channels of 64 kbps each. Together, T-1 multiplexing and T-1 multiplexers provide a T-1 service.
- Time division multiplexing (TDM) – a multiplexing technique in which the sharing of a single is accomplished by dividing the available transmission time on a medium among the medium users
- Wavelength division multiplexing (WDm) – the multiplexing of multiple data streams onto a single fiber-optic cable through the use of lasers of varying wavelengths
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