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Time-division multiplexing

March 10, 2023

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. This method transmits two or more digital signals or analog signals over a common channel. 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

 
Telegraphic multiplexer, from 1922 Britannica

Time-division multiplexing was first developed for applications in telegraphy to route multiple transmissions simultaneously over a single transmission line. In the 1870s, Émile Baudot developed a time-multiplexing system of multiple Hughes telegraph machines.

In 1944, the British Army used the Wireless Set No. 10 to multiplex 10 telephone conversations over a microwave relay as far as 50 miles. This allowed commanders in the field to keep in contact with the staff in England across the English Channel.[1]

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 digital signals, but may be applied in analog multiplexing in which two or more signals or bit streams are transferred appearing simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel.[4] The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

Application examples

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 public switched telephone network (PSTN), it is desirable to transmit multiple subscriber calls over the same transmission medium to effectively utilize the bandwidth of the medium.[5] TDM allows transmitting and receiving telephone switches to create channels (tributaries) within a transmission stream. A standard DS0 voice signal has a data bit rate of 64 kbit/s.[5][6] A TDM circuit runs at a much higher signal bandwidth, permitting the bandwidth to be divided into time frames (time slots) for each voice signal which is multiplexed onto the line by the transmitter. If the TDM frame consists of n voice frames, the line bandwidth is n*64 kbit/s.[5]

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]

Plesiochronous digital hierarchy (PDH) was developed as a standard for multiplexing higher order frames. PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames. This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy (SDH). The requirements which drove the development of SDH were these:[5][6]

  • 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 OSI Reference Model), it also performs some switching functions, as stated in the third bullet point requirement listed above.[5] The most common SDH Networking functions are these:

  • 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 1.544Mb. 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 time-division multiplexing

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 = 9.96Mb). 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 circuit mode communication with a fixed number of channels and constant bandwidth per channel. Bandwidth reservation distinguishes time-division multiplexing from statistical multiplexing such as statistical time-division multiplexing. In pure TDM, the time slots are recurrent in a fixed order and pre-allocated to the channels, rather than scheduled on a packet-by-packet basis.

In dynamic TDMA, a scheduling algorithm dynamically reserves a variable number of time slots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream.[8] Dynamic TDMA is used in:

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

References

  •   This article incorporates public domain material from Federal Standard 1037C. General Services Administration. (in support of MIL-STD-188).
  1. ^ Wireless Set No. 10
  2. ^ US 2919308  "Time Division Multiplex System for Signals of Different Bandwidth"
  3. ^ María Isabel Gandía Carriedo (August 31, 1998). . Universidad Politécnica de Madrid. Archived from the original on June 23, 2006. Retrieved September 23, 2009.
  4. ^ Kourtis, A.; Dangkis, K.; Zacharapoulos, V.; Mantakas, C. (1993). "Analogue time division multiplexing". International Journal of Electronics. Taylor & Francis. 74 (6): 901–907. doi:10.1080/00207219308925891.
  5. ^ 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.
  6. ^ a b c d . Ericsson. Archived from the original on April 13, 2004.
  7. ^ a b White, Curt (2007). Data Communications and Computer Networks. Boston, MA: Thomson Course Technology. pp. 143–152. ISBN 978-1-4188-3610-8.
  8. ^ Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN 978-1107143210.

March 10, 2023
time, division, multiplexing, method, transmitting, receiving, independent, signals, over, common, signal, path, means, synchronized, switches, each, transmission, line, that, each, signal, appears, line, only, fraction, time, alternating, pattern, this, metho. 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 This method transmits two or more digital signals or analog signals over a common channel 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 Contents 1 History 2 Technology 3 Application examples 4 Multiplexed digital transmission 5 Telecommunications systems 6 Statistical time division multiplexing 7 See also 8 ReferencesHistory Edit Telegraphic multiplexer from 1922 Britannica Time division multiplexing was first developed for applications in telegraphy to route multiple transmissions simultaneously over a single transmission line In the 1870s Emile Baudot developed a time multiplexing system of multiple Hughes telegraph machines In 1944 the British Army used the Wireless Set No 10 to multiplex 10 telephone conversations over a microwave relay as far as 50 miles This allowed commanders in the field to keep in contact with the staff in England across the English Channel 1 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 EditTime division multiplexing is used primarily for digital signals but may be applied in analog multiplexing in which two or more signals or bit streams are transferred appearing simultaneously as sub channels in one communication channel but are physically taking turns on the channel 4 The time domain is divided into several recurrent time slots of fixed length one for each sub channel A sample byte or data block of sub channel 1 is transmitted during time slot 1 sub channel 2 during time slot 2 etc One TDM frame consists of one time slot per sub channel plus a synchronization channel and sometimes error correction channel before the synchronization After the last sub channel error correction and synchronization the cycle starts all over again with a new frame starting with the second sample byte or data block from sub channel 1 etc Application examples EditThe 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 Basic Rate Interface and Primary Rate Interface for the Integrated Services Digital Network ISDN The RIFF WAV audio standard interleaves left and right stereo signals on a per sample basisTDM 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 The GSM telephone system The Tactical Data Links Link 16 and Link 22Multiplexed digital transmission EditIn circuit switched networks such as the public switched telephone network PSTN it is desirable to transmit multiple subscriber calls over the same transmission medium to effectively utilize the bandwidth of the medium 5 TDM allows transmitting and receiving telephone switches to create channels tributaries within a transmission stream A standard DS0 voice signal has a data bit rate of 64 kbit s 5 6 A TDM circuit runs at a much higher signal bandwidth permitting the bandwidth to be divided into time frames time slots for each voice signal which is multiplexed onto the line by the transmitter If the TDM frame consists of n voice frames the line bandwidth is n 64 kbit s 5 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 EditThere are three types of synchronous TDM T1 SONET SDH and ISDN 7 Plesiochronous digital hierarchy PDH was developed as a standard for multiplexing higher order frames PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames This solution worked for a while however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy SDH The requirements which drove the development of SDH were these 5 6 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 OSI Reference Model it also performs some switching functions as stated in the third bullet point requirement listed above 5 The most common SDH Networking functions are these 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 544Mb 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 time division multiplexing EditStatistical 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 96Mb 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 circuit mode communication with a fixed number of channels and constant bandwidth per channel Bandwidth reservation distinguishes time division multiplexing from statistical multiplexing such as statistical time division multiplexing In pure TDM the time slots are recurrent in a fixed order and pre allocated to the channels rather than scheduled on a packet by packet basis In dynamic TDMA a scheduling algorithm dynamically reserves a variable number of time slots in each frame to variable bit rate data streams based on the traffic demand of each data stream 8 Dynamic TDMA is used in HIPERLAN 2 Dynamic synchronous transfer mode IEEE 802 16aAsynchronous 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 EditFrequency division multiplexing McASP Route reestablishment notification Time division duplexReferences Edit This article incorporates public domain material from Federal Standard 1037C General Services Administration in support of MIL STD 188 Wireless Set No 10 US 2919308 Time Division Multiplex System for Signals of Different Bandwidth Maria Isabel Gandia Carriedo August 31 1998 ATM Origins and State of the Art Universidad Politecnica de Madrid Archived from the original on June 23 2006 Retrieved September 23 2009 Kourtis A Dangkis K Zacharapoulos V Mantakas C 1993 Analogue time division multiplexing International Journal of Electronics Taylor amp Francis 74 6 901 907 doi 10 1080 00207219308925891 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 a b White Curt 2007 Data Communications and Computer Networks Boston MA Thomson Course Technology pp 143 152 ISBN 978 1 4188 3610 8 Guowang Miao Jens Zander Ki Won Sung Ben Slimane 2016 Fundamentals of Mobile Data Networks Cambridge University Press ISBN 978 1107143210 Retrieved from https en wikipedia org w index php title Time division multiplexing amp oldid 1136822914, wikipedia, wiki, book, books, library,

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