Time-division multiplexing
Multiplexing |
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Analog modulation |
Statistical multiplexing (variable bandwidth) |
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Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. It can be used when the bit rate of the transmission medium exceeds that of the signal to be transmitted. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 19th century, but found its most common application in digital telephony in the second half of the 20th century.
History
Time-division multiplexing was first developed for applications in
In 1944, the
In 1953, a 24-channel TDM was placed in commercial operation by RCA Communications to send audio information between RCA's facility on Broad Street, New York, their transmitting station at Rocky Point and the receiving station at Riverhead, Long Island, New York. The communication was by a microwave system throughout Long Island. The experimental TDM system was developed by RCA Laboratories between 1950 and 1953.[2]
In 1962, engineers from Bell Labs developed the first D1 channel banks, which combined 24 digitized voice calls over a four-wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signal of 64 kbit/s. Channel banks used the fixed position (temporal alignment) of one byte in the frame to identify the call it belonged to.[3]
Technology
Time-division multiplexing is used primarily for
Application examples
- The plesiochronous digital hierarchy (PDH) system, also known as the PCM system, for digital transmission of several telephone calls over the same four-wire copper cable (T-carrier or E-carrier) or fiber cable in the circuit switched digital telephone network
- The synchronous digital hierarchy (SDH)/synchronous optical networking (SONET) network transmission standards that have replaced PDH.
- The Integrated Services Digital Network(ISDN).
- The RIFF(WAV) audio standard interleaves left and right stereo signals on a per-sample basis
TDM can be further extended into the time-division multiple access (TDMA) scheme, where several stations connected to the same physical medium, for example sharing the same frequency channel, can communicate. Application examples include:
Multiplexed digital transmission
In circuit-switched networks, such as the
Each voice time slot in the TDM frame is called a channel. In European systems, standard TDM frames contain 30 digital voice channels (E1), and in American systems (T1), they contain 24 channels. Both standards also contain extra bits (or bit time slots) for signaling and synchronization bits.[5]
Multiplexing more than 24 or 30 digital voice channels is called higher order multiplexing. Higher order multiplexing is accomplished by multiplexing the standard TDM frames. For example, a European 120 channel TDM frame is formed by multiplexing four standard 30 channel TDM frames. At each higher order multiplex, four TDM frames from the immediate lower order are combined, creating multiplexes with a bandwidth of n*64 kbit/s, where n = 120, 480, 1920, etc.[5]
Telecommunications systems
There are three types of synchronous TDM: T1, SONET/SDH, and ISDN.[7]
- Be synchronous – All clocks in the system must align with a reference clock.
- Be service-oriented – SDH must route traffic from End Exchange to End Exchange without worrying about exchanges in between, where the bandwidth can be reserved at a fixed level for a fixed period of time.
- Allow frames of any size to be removed or inserted into an SDH frame of any size.
- Easily manageable with the capability of transferring management data across links.
- Provide high levels of recovery from faults.
- Provide high data rates by multiplexing any size frame, limited only by technology.
- Give reduced bit rate errors.
SDH has become the primary transmission protocol in most PSTN networks. It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, in order to create larger SDH frames known as Synchronous Transport Modules (STM). The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame. SDH can also multiplex packet based frames e.g. Ethernet, PPP and ATM.[5][6]
While SDH is considered to be a transmission protocol (Layer 1 in the
- SDH Crossconnect – The SDH Crossconnect is the SDH version of a Time-Space-Time crosspoint switch. It connects any channel on any of its inputs to any channel on any of its outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and outputs are connected to other exchanges.[5]
- SDH Add-Drop Multiplexer – The SDH Add-Drop Multiplexer (ADM) can add or remove any multiplexed frame down to 1.544 Mb. Below this level, standard TDM can be performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used in End Exchanges where the channels from subscribers are connected to the core PSTN network.[5]
SDH network functions are connected using high-speed optic fibre. Optic fibre uses light pulses to transmit data and is therefore extremely fast. Modern optic fibre transmission makes use of wavelength-division multiplexing (WDM) where signals transmitted across the fibre are transmitted at different wavelengths, creating additional channels for transmission. This increases the speed and capacity of the link, which in turn reduces both unit and total costs.[5][6]
Statistical version
Statistical time-division multiplexing (STDM) is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over one line. Many college and corporate campuses use this type of TDM to distribute bandwidth.
On a 10-Mbit line entering a network, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96 Mb). A more common use however is to only grant the bandwidth when that much is needed. STDM does not reserve a time slot for each terminal, rather it assigns a slot when the terminal is requiring data to be sent or received.
In its primary form, TDM is used for
In
- HIPERLAN/2
- Dynamic synchronous transfer mode
- IEEE 802.16a
Asynchronous time-division multiplexing (ATDM),[7] is an alternative nomenclature in which STDM designates synchronous time-division multiplexing, the older method that uses fixed time slots.
See also
- Frequency-division multiplexing
- McASP
- Route reestablishment notification
- Time-division duplex
References
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on January 22, 2022. (in support of MIL-STD-188).
- ^ Wireless Set No. 10
- ^ US 2919308 "Time Division Multiplex System for Signals of Different Bandwidth"
- ^ María Isabel Gandía Carriedo (August 31, 1998). "ATM: Origins and State of the Art". Universidad Politécnica de Madrid. Archived from the original on June 23, 2006. Retrieved September 23, 2009.
- .
- ^ a b c d e f g h i j k Hanrahan, H.E. (2005). Integrated Digital Communications. Johannesburg, South Africa: School of Electrical and Information Engineering, University of the Witwatersrand.
- ^ a b c d "Understanding Telecommunications". Ericsson. Archived from the original on April 13, 2004.
- ^ ISBN 978-1-4188-3610-8.
- ISBN 978-1107143210.