Time-division multiplexing

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 is 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 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. 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.[4] 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.[4][5] 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.[4]

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.[4]

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.[4]

Telecommunications systems

There are three types of synchronous TDM: T1, SONET/SDH, and ISDN.[6]

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:[4][5]

  • 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.[4][5]

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.[4] 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.[4]
  • 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.[4]

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.[4][5]

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.[7] Dynamic TDMA is used in:

Asynchronous time-division multiplexing (ATDM),[6] is an alternative nomenclature in which STDM designates synchronous time-division multiplexing, the older method that uses fixed time slots.

See also

References

  1. ^ Wireless Set No. 10
  2. ^ ‹See Tfd›US 2919308 "Time Division Multiplex System for Signals of Different Bandwidth"
  3. ^ María Isabel Gandía Carriedo (31 August 1998). "ATM: Origins and State of the Art". Universidad Politécnica de Madrid. Archived from the original on 23 June 2006. Retrieved 2009-09-23.
  4. ^ 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.
  5. ^ a b c d "Understanding Telecommunications". Ericsson. Archived from the original on April 13, 2004.
  6. ^ a b White, Curt (2007). Data Communications and Computer Networks. Boston, MA: Thomson Course Technology. pp. 143–152. ISBN 1-4188-3610-9.
  7. ^ Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN 1107143217.
Carrier system

A carrier system is a telecommunications system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the principal voice frequency or data rate.Carrier systems typically transmit multiple channels of communication simultaneously over the shared medium using various forms of multiplexing. Prominent multiplexing methods of the carrier signal are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). A cable television system is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same coaxial cable by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal. For example, in the public switched telephone network, many telephone calls are sent over shared trunk lines by time-division multiplexing. For long distance calls several of these channels may be sent over a communications satellite link by frequency-division multiplexing. At a given receiving node, specific channels may be demultiplexed individually.

Channel access method

In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus networks, ring networks and point-to-point links operating in half-duplex mode.

A channel access method is based on multiplexing, that allows several data streams or signals to share the same communication channel or transmission medium. In this context, multiplexing is provided by the physical layer.

A channel access method is also based on a multiple access protocol and control mechanism, also known as medium access control (MAC). Medium access control deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions. Media access control is a sub-layer in the data link layer of the OSI model and a component of the link layer of the TCP/IP model.

Contention (telecommunications)

In statistical time division multiplexing, contention is a media access method that is used to share a broadcast medium. In contention, any computer in the network can transmit data at any time (first come-first served).

This system breaks down when two computers attempt to transmit at the same time. This is known as a collision. To avoid collisions, a carrier sensing mechanism is used. Here each computer listens to the network before attempting to transmit. If the network is busy, it waits until network quiets down. In carrier detection, computers continue to listen to the network as they transmit. If computer detects another signal that interferes with the signal it is sending, it stops transmitting. Both computers then wait for random amount of time and attempt to transmit. Contention methods are most popular media access control method on LANs.

DVB-SH

DVB-SH ("Digital Video Broadcasting - Satellite services to Handhelds") is a physical layer standard for delivering IP based media content and data to handheld terminals such as mobile phones or PDAs, based on a hybrid satellite/terrestrial downlink and for example a GPRS uplink. The DVB Project published the DVB-SH standard in February 2007.The DVB-SH system was designed for frequencies below 3 GHz, supporting UHF band, L Band or S-band. It complements and improves the existing DVB-H physical layer standard. Like its sister specification (DVB-H), it is based on DVB IP Datacast (IPDC) delivery, electronic service guides and service purchase and protection standards.

DVB-SH specifies two operational modes:

SH-A: specifies the use of COFDM modulation on both satellite and terrestrial links with the possibility of running both links in SFN mode.

SH-B: uses Time-Division Multiplexing (TDM) on the satellite link and COFDM on the terrestrial link.

Dynamic synchronous transfer mode

Dynamic synchronous transfer mode (DTM) is an optical networking technology standardized by the European Telecommunications Standards Institute (ETSI) in 2001 beginning with specification ETSI ES 201 803-1. DTM is a time division multiplexing and a circuit-switching network technology that combines switching and transport. It is designed to provide a guaranteed quality of service (QoS) for streaming video services, but can be used for packet-based services as well. It is marketed for professional media networks, mobile TV networks, digital terrestrial television (DTT) networks, in content delivery networks and in consumer oriented networks, such as "triple play" networks.

Electronic switching system

In telecommunications, an electronic switching system (ESS) is a telephone switch that uses digital electronics and computerized control to interconnect telephone circuits for the purpose of establishing telephone calls.

The generations of telephone switches before the advent of electronic switching in the 1950s used purely electro-mechanical relay systems and analog voice paths. These early machines typically utilized the step-by-step technique. The first generation of electronic switching systems in the 1960s were not entirely digital in nature, but used reed relay-operated metallic paths or crossbar switches operated by stored program control (SPC) systems.

First announced in 1955, the first customer trial installation of an all-electronic central office commenced in Morris, Illinois in November 1960 by Bell Laboratories. The first prominent large-scale electronic switching system was the Number One Electronic Switching System (1ESS) of the Bell System in the United States in New Jersey during May 1965.

Later electronic switching systems implemented the digital representation of the electrical audio signals on subscriber loops by digitizing the analog signals and processing the resulting data for transmission between central offices. Time-division multiplexing (TDM) technology permitted the simultaneous transmission of multiple telephone calls on a single wire connection between central offices or other electronic switches, resulting in dramatic capacity improvements of the telephone network.

With the advances of digital electronics starting in the 1960s telephone switches employed semiconductor device components in increasing measure.

In the late 20th century most telephone exchanges without TDM processing were eliminated and the term electronic switching system became largely a historical distinction for the older SPC systems.

Frame (networking)

A frame is a digital data transmission unit in computer networking and telecommunication. In packet switched systems, a frame is a simple container for a single network packet. In other telecommunications systems, a frame is a repeating structure supporting time-division multiplexing.

A frame typically includes frame synchronization features consisting of a sequence of bits or symbols that indicate to the receiver the beginning and end of the payload data within the stream of symbols or bits it receives. If a receiver is connected to the system during frame transmission, it ignores the data until it detects a new frame synchronization sequence.

G.9972

G.9972 (also known as G.cx) is a Recommendation developed by ITU-T that specifies a coexistence mechanism for networking transceivers capable of operating over electrical power line wiring. It allows G.hn devices to coexist with other devices implementing G.9972 and operating on the same power line wiring.G.9972 received consent during the meeting of ITU-T Study Group 15, on October 9, 2009, and final approval on June 11, 2010.G.9972 specifies two mechanisms for coexistence between G.hn home networks and broadband over power lines (BPL) Internet access networks:

Frequency-division multiplexing (FDM), in which the available spectrum is divided in two parts: frequencies below 10 or 14 MHz (specific value can be selected by the access network) are reserved for the access network, while frequencies above them are reserved for the in-home network.

Time-division multiplexing (TDM), in which the available channel time is split equally between both networks. 50% of time slots are allocated for the access network, and 50% are allocated to the in-home network.

Generalized Multi-Protocol Label Switching

Generalized Multi-Protocol Label Switching (GMPLS) is a protocol suite extending MPLS to manage further classes of interfaces and switching technologies

other than packet interfaces and switching, such as time division multiplexing, layer-2 switching, wavelength switching and fiber-switching.

H.100 (computer telephony)

H.100 and H.110 are legacy telephony equipment standard published by the ECTF that allow the transport of up to 4096 simplex channels of voice or data on one connector or ribbon cable. H.100 is implemented using Multi-Channeled Buffered Serial Ports (McBSP), typically included as a DSP peripheral. McBSP, also known as TDM Serial ports are special serial ports that support multiple channels by using Time-division multiplexing (TDM).The McBSP / TDM Serial Port Interface is as follow:

CK: Clock

FS: Frame Sync

DR: Data Receive

DX: Data Transmit

Media gateway

A media gateway is a translation device or service that converts media streams between disparate telecommunications technologies such as POTS, SS7, Next Generation Networks (2G, 2.5G and 3G radio access networks) or private branch exchange (PBX) systems. Media gateways enable multimedia communications across packet networks using transport protocols such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP).

Because the media gateway connects different types of networks, one of its main functions is to convert between different transmission and coding techniques. Media streaming functions such as echo cancellation, DTMF, and tone sender are also located in the media gateway.

Media gateways are often controlled by a separate Media Gateway Controller which provides the call control and signaling functionality. Communication between media gateways and Call Agents is achieved by means of protocols such as MGCP or Megaco (H.248) or Session Initiation Protocol (SIP). Modern media gateways used with SIP are often stand-alone units with their own call and signaling control integrated and can function as independent, intelligent SIP end-points.

Voice over Internet Protocol (VoIP) media gateways perform the conversion between Time-division multiplexing (TDM) voice to a media streaming protocol, such as the Real-time Transport Protocol, (RTP), as well as a signaling protocol used in the VoIP system.

Mobile access media gateways connect the radio access networks of a public land mobile network PLMN to a next-generation core network. 3GPP standards define the functionality of CS-MGW and IMS-MGW for UTRAN and GERAN based PLMNs.

Multiplexing

In telecommunications and computer networks, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing in 1910.

The multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, extracts the original channels on the receiver end.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX or DMX).

Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

Point-to-multipoint communication

In telecommunications, point-to-multipoint communication (P2MP, PTMP or PMP) is communication which is accomplished via a distinct type of one-to-many connection, providing multiple paths from a single location to multiple locations.Point-to-multipoint telecommunications is typically used in wireless Internet and IP telephony via gigahertz radio frequencies. P2MP systems have been designed with and without a return channel from the multiple receivers. A central antenna or antenna array broadcasts to several receiving antennas and the system uses a form of time-division multiplexing to allow for the return channel traffic.

Pseudo-wire

In computer networking and telecommunications, a pseudowire (or pseudo-wire) is an emulation of a point-to-point connection over a packet-switching network (PSN).

The pseudowire emulates the operation of a "transparent wire" carrying the service, but it is realized that this emulation will rarely be perfect. The service being carried over the "wire" may be Asynchronous Transfer Mode (ATM), Frame Relay, Ethernet or time-division multiplexing (TDM) while the packet network may be Multi-protocol Label Switching (MPLS), Internet Protocol (IPv4 or IPv6), or Layer 2 Tunneling Protocol Version 3 (L2TPv3).

The first pseudowire specifications were the Martini draft for ATM pseudowires, and the TDMoIP draft for transport of E1/T1 over IP.

In 2001, the Internet Engineering Task Force (IETF) set up the PWE3 working group, which was chartered to develop an architecture for service provider edge-to-edge pseudowires, and service-specific documents detailing the encapsulation techniques. Other standardization forums, including the International Telecommunication Union (ITU) and the MFA Forum, are also active in producing standards and implementation agreements for pseudowires.

Self-organized time-division multiple access

Self-organized time-division multiple access (STDMA) is a channel access method designed by Håkan Lans, based on time-division multiplexing.

The term "self-organized" describes the manner in which time slots are assigned to users. Time-division multiple access (TDMA) divides a channel into frames, which furthermore are subdivided into a vast number of time slots. Users transmit in rapid succession, one after the other, each using their own time slot. One of the drawbacks of TDMA is that it requires a central station for slot assignment and time synchronisation. STDMA proposes a method for assigning slots without the involvement of a central station. Time synchronisation is usually taken care of using Coordinated Universal Time (UTC).

STDMA is in use by the Automatic Identification System (AIS), a standard marine short-range coastal tracking system, and is the base of the International Civil Aviation Organization VHF Data Link Mode 4.

While the method was patented, a US patent ex-parte reexamination certificate was issued in 2010 canceling all claims.

Shared medium

In telecommunication, a shared medium is a medium or channel of information transfer that serves more than one user at the same time.Most channels only function correctly when one user is transmitting, so a channel access method is always in effect.

In circuit switching, each user typically gets a fixed share of the channel capacity. A multiplexing scheme divides up the capacity of the medium. Common multiplexing schemes include time-division multiplexing and frequency-division multiplexing. Channel access methods for circuit switching include time-division multiple access, frequency-division multiple access, etc.

In packet switching, the sharing is more dynamic — each user takes up little or none of the capacity when idle, and can utilize the entire capacity if transmitting while all other users are idle. Channel access methods for packet switching include carrier sense multiple access, token passing, etc.

Statistical time-division multiplexing

Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bitrate digital channels or data streams. The link sharing is adapted to the instantaneous traffic demands of the data streams that are transferred over each channel. This is an alternative to creating a fixed sharing of a link, such as in general time division multiplexing (TDM) and frequency division multiplexing (FDM). When performed correctly, statistical multiplexing can provide a link utilization improvement, called the statistical multiplexing gain.

Statistical multiplexing is facilitated through packet mode or packet-oriented communication, which among others is utilized in packet switched computer networks. Each stream is divided into packets that normally are delivered asynchronously in a first-come first-served fashion. In alternative fashion, the packets may be delivered according to some scheduling discipline for fair queuing or differentiated and/or guaranteed quality of service.

Statistical multiplexing of an analog channel, for example a wireless channel, is also facilitated through the following schemes:

Random frequency-hopping orthogonal frequency division multiple access (RFH-OFDMA)

Code-division multiple access (CDMA), where different amount of spreading codes or spreading factors can be assigned to different users.Statistical multiplexing normally implies "on-demand" service rather than one that preallocates resources for each data stream. Statistical multiplexing schemes do not control user data transmissions.

TSMP

TSMP, an acronym for Time Synchronized Mesh Protocol, was developed by Dust Networks as a communications protocol for self-organizing networks of wireless devices called motes. TSMP devices stay synchronized to each other and communicate in time-slots, similar to other TDM (time-division multiplexing) systems. Such deterministic communication allows the devices to stay extremely low power, as the radios only turn on for the periods of scheduled communication. The protocol is designed to operate very reliably in a noisy environment. It uses channel hopping to avoid interference -- the packets between TSMP devices get sent on different radio channels depending on time of transmission. TSMP distinguishes itself from other time-slotted mesh-based protocols, in that time-slot timing is maintained continuously and enables a network to duty-cycle on a transmitter-receiver pair-wise basis, as opposed to putting the entire network to sleep for extended periods of time (as is done in a beacon-based protocol, such as DigiMesh).

Dust Networks' underlying time synchronized mesh networking technology has been standardized by the HART Communications Foundation with the WirelessHART protocol, the International Society of Automation ISA100 standard and in internet protocol standards, such as IEEE802.15.4E MAC layer, and IETF 6TiSCh.

Time synchronized mesh networking is marketed for applications that require reliability and ultra long battery life, typically measured in years.

It is intended for the industrial market for manufacturing-process monitoring and control.

Yamaha YM2413

The YM2413, a.k.a. OPLL, is a cost-reduced FM synthesis sound chip manufactured by Yamaha Corporation and based on their YM3812 (OPL2). To make the chip cheaper to manufacture, many of the internal registers were removed. The result of this is that the YM2413 can only play one user-defined instrument at a time; the other 15 instrument settings are hard-coded and cannot be altered by the user. There were other cost-cutting modifications: the number of waveforms was reduced to two, and the channels are not mixed using an adder; instead, the chip's DAC uses time-division multiplexing to play short segments of each channel in sequence, as also done in the YM2612.

History
Pioneers
Transmission
media
Network topology
and switching
Multiplexing
Networks

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