Time-Division Multiplexing (TDM) is a type of digital or (rarely) analog
multiplexing in which two or more signals or bit streams are transferred apparently
simultaneously as sub-channels in one communication channel, but physically are
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. After the last sub-channel 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
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 SDH and Synchronous Optical Networking (SONET) network
transmission standards, that has superseded PDH.
The RIFF (WAV) audio standard interleaves left and right stereo signals on a per-
sample basis The left-right channel splitting in use for Stereoscopic Liquid Crystal
shutter glasses 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: The GSM telephone system
TDM versus packet mode communication
In its primary form, TDM is used for circuit mode communication with a fixed number
of channels and constant bandwidth per channel.
What distinguishes time-division multiplexing from statistical multiplexing such
as packet mode communication (also known as statistical time-domain multiplexing,
see below) is that the time-slots are recurrent in a fixed order and pre-allocated to
the channels, rather than scheduled on a packet-by-packet basis. Statistical time-
domain multiplexing resembles, but should not be considered as, time division
multiplexing.
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. Dynamic TDMA is used in HIPERLAN/2; IEEE 802.16a.
History
For the SIGSALY encryptor of 1943, see PCM.
In 1962, engineers from Bell Labs developed the first D1 Channel Banks, which
combined 24 digitised voice calls over a 4-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 a byte's fixed position (temporal alignment) in the frame
to determine which call it belonged to.
Transmission using Time Division Multiplexing (TDM)
In circuit switched networks such as the Public Switched Telephone Network (PSTN)
there exists the need to transmit multiple subscribers’ calls along the same
transmission medium. To accomplish this, network designers make use of TDM. TDM
allows switches to create channels, also known as tributaries, within a transmission
stream. A standard DS0 voice signal has a data bit rate of 64 kbit/s, determined
using Nyquist’s Sampling Criterion. TDM takes frames of the voice signals and
multiplexes them into a TDM frame which runs at a higher bandwidth. So if the TDM
frame consists of n voice frames, the bandwidth will be n*64 kbit/s.
Each voice frame in the TDM frame is called a channel or tributary. In European
systems, TDM frames contain 30 digital voice frames and in American systems, TDM
frames contain 24 digital voice frames. Both of the standards also contain extra
space for signalling (see Signaling System 7) and synchronisation data.
Multiplexing more than 24 or 30 digital voice frames 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 x 64 kbit/s, where n = 120, 480, 1920, etc.
Synchronous Digital Hierarchy (SDH)
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 as follows:
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, so as 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 such as Ethernet, PPP and ATM.
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. The most common SDH Networking
functions are as follows:
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.
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. 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.
Statistical Time-division Multiplexing (STDM)
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 1 line. Many college and corporate campuses use this type
of TDM to logically distribute bandwidth.
If there is one 10MBit line coming into the building, 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.
Attributes and Credits
The information and facts supplied on this subject
derive from http://en.wikipedia.org/wiki/Main_Page
multiplexing in which two or more signals or bit streams are transferred apparently
simultaneously as sub-channels in one communication channel, but physically are
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. After the last sub-channel 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
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 SDH and Synchronous Optical Networking (SONET) network
transmission standards, that has superseded PDH.
The RIFF (WAV) audio standard interleaves left and right stereo signals on a per-
sample basis The left-right channel splitting in use for Stereoscopic Liquid Crystal
shutter glasses 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: The GSM telephone system
TDM versus packet mode communication
In its primary form, TDM is used for circuit mode communication with a fixed number
of channels and constant bandwidth per channel.
What distinguishes time-division multiplexing from statistical multiplexing such
as packet mode communication (also known as statistical time-domain multiplexing,
see below) is that the time-slots are recurrent in a fixed order and pre-allocated to
the channels, rather than scheduled on a packet-by-packet basis. Statistical time-
domain multiplexing resembles, but should not be considered as, time division
multiplexing.
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. Dynamic TDMA is used in HIPERLAN/2; IEEE 802.16a.
History
For the SIGSALY encryptor of 1943, see PCM.
In 1962, engineers from Bell Labs developed the first D1 Channel Banks, which
combined 24 digitised voice calls over a 4-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 a byte's fixed position (temporal alignment) in the frame
to determine which call it belonged to.
Transmission using Time Division Multiplexing (TDM)
In circuit switched networks such as the Public Switched Telephone Network (PSTN)
there exists the need to transmit multiple subscribers’ calls along the same
transmission medium. To accomplish this, network designers make use of TDM. TDM
allows switches to create channels, also known as tributaries, within a transmission
stream. A standard DS0 voice signal has a data bit rate of 64 kbit/s, determined
using Nyquist’s Sampling Criterion. TDM takes frames of the voice signals and
multiplexes them into a TDM frame which runs at a higher bandwidth. So if the TDM
frame consists of n voice frames, the bandwidth will be n*64 kbit/s.
Each voice frame in the TDM frame is called a channel or tributary. In European
systems, TDM frames contain 30 digital voice frames and in American systems, TDM
frames contain 24 digital voice frames. Both of the standards also contain extra
space for signalling (see Signaling System 7) and synchronisation data.
Multiplexing more than 24 or 30 digital voice frames 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 x 64 kbit/s, where n = 120, 480, 1920, etc.
Synchronous Digital Hierarchy (SDH)
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 as follows:
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, so as 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 such as Ethernet, PPP and ATM.
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. The most common SDH Networking
functions are as follows:
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.
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. 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.
Statistical Time-division Multiplexing (STDM)
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 1 line. Many college and corporate campuses use this type
of TDM to logically distribute bandwidth.
If there is one 10MBit line coming into the building, 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.
Attributes and Credits
The information and facts supplied on this subject
derive from http://en.wikipedia.org/wiki/Main_Page
| Time-Division Multiplexing (TDM) |
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