In TDM, slots are further divided into a) Seconds b) Frames c) Packets d) None of the mentioned. Which multiplexing technique transmits digital signals? A) FDM b) TDM c) WDM d) None of the mentioned. In TDM, slots are further divided into a) Seconds b) Frames c) Packets d) None of the mentioned. Distinguish between multilevel TDM, multiple-slot TDM, and pulse-stuffed TDM. Ans: Multilevel TDM: Multilevel TDM is used when the data rate of an input line is a multiple of others. Multiple Slot TDM: Multi slot TMD is used when many slot in a frame produce 1 single line. Pulse stuffed TDM: Pulse(stuffed TMD is used when bit rate sort are not multi-ple. Frequency division multiplexing FDM With FDM the frequency spectrum of a link from CS 101 at Jaypee Institute of Information Technology.
Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots.[1] The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2Gcellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. TDMA was first used in satellite communication systems by Western Union in its Westar 3 communications satellite in 1979. It is now used extensively in satellite communications,[2][3][4][5]combat-net radio systems, and passive optical network (PON) networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below.
TDMA is a type of time-division multiplexing (TDM), with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.
- 2TDMA in mobile phone systems
TDMA characteristics[edit]
- Shares single carrier frequency with multiple users
- Non-continuous transmission makes handoff simpler
- Slots can be assigned on demand in dynamic TDMA
- Less stringent power control than CDMA due to reduced intra cell interference
- Higher synchronization overhead than CDMA
- Advanced equalization may be necessary for high data rates if the channel is 'frequency selective' and creates Intersymbol interference
- Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA
- Frequency/slot allocation complexity
- Pulsating power envelope: interference with other devices
TDMA in mobile phone systems[edit]
2G systems[edit]
Most 2G cellular systems, with the notable exception of IS-95, are based on TDMA. GSM, D-AMPS, PDC, iDEN, and PHS are examples of TDMA cellular systems. GSM combines TDMA with Frequency Hopping and wideband transmission to minimize common types of interference.
In Tdm Slots Are Further Divided Into
In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instructs the mobile phone to transmit earlier and by how much. This compensates for the propagation delay resulting from the light speed velocity of radio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guard interval at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.
Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to actually know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; this is known as the random-access channel (RACH) in GSM. The mobile attempts to broadcast at the beginning of the time slot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the time slot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole time slot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome.[citation needed]
3G systems[edit]
In Tdm Slots Are Further Divided Into What
Although most major 3G systems are primarily based upon CDMA,[6] time-division duplexing (TDD), packet scheduling (dynamic TDMA) and packet oriented multiple access schemes are available in 3G form, combined with CDMA to take advantage of the benefits of both technologies.
While the most popular form of the UMTS 3G system uses CDMA and frequency division duplexing (FDD) instead of TDMA, TDMA is combined with CDMA and time-division duplexing in two standard UMTS UTRA.
TDMA in wired networks[edit]
The ITU-TG.hn standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables) is based on a TDMA scheme. In G.hn, a 'master' device allocates 'Contention-Free Transmission Opportunities' (CFTXOP) to other 'slave' devices in the network. Only one device can use a CFTXOP at a time, thus avoiding collisions.FlexRay protocol which is also a wired network used for safety-critical communication in modern cars, uses the TDMA method for data transmission control.
Comparison with other multiple-access schemes[edit]
In radio systems, TDMA is usually used alongside frequency-division multiple access (FDMA) and frequency division duplex (FDD); the combination is referred to as FDMA/TDMA/FDD. This is the case in both GSM and IS-136 for example. Exceptions to this include the DECT and Personal Handy-phone System (PHS) micro-cellular systems, UMTS-TDD UMTS variant, and China's TD-SCDMA, which use time-division duplexing, where different time slots are allocated for the base station and handsets on the same frequency.
A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for its own time slot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter frequency handovers, something which is difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in Universal Mobile Telecommunications System (UMTS). This in turn allows for co-existence of microcell layers with macrocell layers.
CDMA, by comparison, supports 'soft hand-off' which allows a mobile phone to be in communication with up to 6 base stations simultaneously, a type of 'same-frequency handover'. The incoming packets are compared for quality, and the best one is selected. CDMA's 'cell breathing' characteristic, where a terminal on the boundary of two congested cells will be unable to receive a clear signal, can often negate this advantage during peak periods.
A disadvantage of TDMA systems is that they create interference at a frequency which is directly connected to the time slot length. This is the buzz which can sometimes be heard if a TDMA phone is left next to a radio or speakers.[7] Another disadvantage is that the 'dead time' between time slots limits the potential bandwidth of a TDMA channel. These are implemented in part because of the difficulty in ensuring that different terminals transmit at exactly the times required. Handsets that are moving will need to constantly adjust their timings to ensure their transmission is received at precisely the right time, because as they move further from the base station, their signal will take longer to arrive. This also means that the major TDMA systems have hard limits on cell sizes in terms of range, though in practice the power levels required to receive and transmit over distances greater than the supported range would be mostly impractical anyway.
Dynamic TDMA[edit]
In dynamic time-division multiple access (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 broadband radio access network.
- IEEE 802.16a WiMax
- Military Radios / Tactical Data Link
See also[edit]
- Channel access methods (CAM)
- Duplex (telecommunications) (FDD, TDD)
References[edit]
- ^Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN1107143217.
- ^Maine, K.; Devieux, C.; Swan, P. (November 1995). Overview of IRIDIUM satellite network. WESCON'95. IEEE. p. 483.
- ^Mazzella, M.; Cohen, M.; Rouffet, D.; Louie, M.; Gilhousen, K. S. (April 1993). Multiple access techniques and spectrum utilisation of the GLOBALSTAR mobile satellite system. Fourth IEE Conference on Telecommunications 1993. IET. pp. 306–311.
- ^Sturza, M. A. (June 1995). Architecture of the TELEDESIC satellite system. International Mobile Satellite Conference. 95. p. 214.
- ^'ORBCOMM System Overview'(PDF).
- ^K. Jagannatham, Aditya (2016). Principles of Modern Wireless Communication Systems. McGraw-Hill Education.
- ^'Minimize GSM buzz noise in mobile phones'. EETimes. July 20, 2009. Retrieved November 22, 2010.
Multiplexing or (muxing) - To combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection. In other words, we can say that Muxing used for sharing of a medium and its link by two or more devices. It can provide both Efficiency and Privacy.
Multiplexing is done by using a device called multiplexer (MUX) that combines n input lines to generate one output line i.e. (many to one). Therefore multiplexer (MUX) has several inputs and one output.
At the receiving end, a device called demultiplexer (DEMUX) or (demuxing) is used that separates signal into its component signals. So DEMUX has one input and several outputs.
Concept of Multiplexing
As shown in figure multiplexer takes 4 input lines and diverts them to a single output line. The signal from 4 different devices is combined and carried by this single line. At the receiving side, a demultiplexer takes this signal from a single line and breaks it into the original signals and passes them to the 4 different receivers.
Advantages of Multiplexing
If no multiplexing is used between the users at two different sites that are distance apart, then separate communication lines would be required as shown in fig.
This is not only costly but also become difficult to manage. If multiplexing is used then, only one line is required. This leads to the reduction in the line cost and also it would be easier to keep track of one line than several lines. Multiplexing efficient for utilization of bandwidth.
The word path refers to the physical link. The word channel refers to a portion of the path that carries a transmission between a specific pair of devices. A road can have many (n) channels.
The signals are multiplexed using three basic techniques: frequency division multiplexing (FDM), time-division multiplexing (TDM) and wave division multiplexing (WDM). TDM further subdivided into synchronous RDM (usually referred to as TDM only) and asynchronous TDM, also called static TDM or concentrator.
The following are several examples of different multiplexing methods:
Frequency division multiplexing (FDM)
It is used in the analogue signal, a multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. FDM assigns a different carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. The carrier frequencies are separated by a sufficient bandwidth to accommodate the modulated signal. These ranges of bandwidth are the channels through which the different signals travel.
For example, television transmitters use FDM to broadcast several channels at once. The bandpass filter is used for separating channels and allows to pass a specific range of frequencies. During transmission of streams, it blocks lower and higher frequencies. In Frequency Division Multiplexing, channels are separated by unused strips of guard bands to prevent Overlapping. Guard bands increase the bandwidth for FDM.
The figure shows a conceptual view of FDM. In this illustration, the transmission path divided into three parts, each representing a channel that carries a transmission. As an analogy, imagine a point where three narrow streets meet to form a three-lane highway. Each of these streets corresponds to a lane of the highway. Each car entering the highway from one of the streets still has its lane and can travel without interfering with the cars on the other lanes.
Remember that although Figure shows the path as if it had a spatial division into separate channels, actual channel divisions are achieved by frequency, not by spatial separation.
The figure is a conceptual illustration in the time domain of the multiplexing process. The FDM is an analog process and is shown in the figure using phones as input devices.
Time Division Multiplexing (TDM)
A type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM is designed for digital signals, which combining several low-rate channels into high-rate one. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data stream. TDM, slots are further divided into Frames. In order to separate channels AND gates are used in a TDM receiver.
The Figure gives a conceptual view of the TDM. Note that the same link used as in FDM; However, here the link is shown sectioned by time instead of frequency.
In Fig. TDM the portions of signals 1,2,3 and 4 occupy a link sequentially. As an analogy, imagine a chairlift that has several streets. Each street has its line, and the skiers of each line take turn occupying the ski lift. As each chair reaches the top of the mountain, the skier who goes on, it gets off and skiing under the mountain where he waits again in the queue.
TDM can be implemented in two ways: synchronous TDM and asynchronous TDM. Synchronous means that the multiplexer always assigns the same time slot to each device, whether or not the device has something to transmit. For example, time slot A is assigned only to device A and cannot use for any other device. Each time the assigned time is up, the device has the opportunity to send a portion of its data. If the device is unable to transmit or has no data to send, its time slot remains empty.
Synchronous TDM does not guarantee that the full capacity of the link can use. It is more likely that only a portion of the time slots can be used at a given time. Because the time slots are pre-assigned and fixed, each time a connected device is not transmitting its corresponding time slot is empty, and that link capacity wasted. For example, imagine that the output of 20 identical computers on a line has been multiplexed. Using synchronous TDM, the line speed must be at least 20 times the speed of each input line. However, what happens if there are only 10 computers they use at the same time. Half the capacity of the line is wasted. Asynchronous multiplexing is designed to avoid this type of expense. As with the term synchronous, the term asynchronous means something different in multiplexing than it means in other areas of data communication. Here it means flexible or not fixed.
Like synchronous TDM, asynchronous TDM allows you to multiplex a certain number of low-speed input lines over a single high-speed line. However, unlike synchronous TDM, in asynchronous TDM, the total speed of the input lines may be greater than the capacity of the track. In a synchronous system, if we have n input lines, the frame contains a fixed m number of at least n time slots. In an asynchronous system, if there are n input lines, the frame does not contain more than n slots, with m less than n (see Figure). In this way, the asynchronous TDM supports the same number of input lines as the synchronous TDM with a smaller link capacity. Alternatively, given the same link, asynchronous TDM can support more devices than synchronous TDM.
The number of time slots in an asynchronous TDM frame (m) based on a statistical analysis of the number of input lines that are likely to transmit at a given time. Instead of being pre-assigned, each slot is available for any input device connected to the lines that have data to send. The multiplexer looks at the input lines, accepts portions of data until a frame is full and then sends the frame through the link. If there is not enough data to fill all the slots in a frame, the frame is transmitted partially filled; that is, the total link capacity may not use one hundred percent of the time. However, the ability to dynamically allocate time slots, associated with the lower ratio of time slots to input lines, dramatically reduces the probability and the degree of expense.
Wave Division Multiplexing (WDM)
It used in the analog signal, a type of multiplexing developed for use on optical fibre. The idea is the same: different signals on different frequencies are combined. However, the difference is that the frequencies are very high. WDM is the optical equivalent of FDM.
The figure gives a conceptual view of a WSM multiplexer and demultiplexer. Very narrow bands of light from different sources combine to achieve a wider band of light. In the receiver, the signals separated by the demultiplexer.
One can ask what the mechanism of the WDM is. Although technology is very sophisticated, the idea is straightforward. You want to combine multiple beams of light within a single light in the multiplexer and do the reverse operation in the demultiplexer. Combining and dividing beams of light are quickly resolved through a prism. Remember from fundamental physics that a prism curves a beam and light based on the angle of incidence and frequency. Using this technique, a demultiplexer can make that combines different beams of input light, each of which contains a narrow frequency band, into a single output beam with a broader frequency band. It can also be done in a demultiplexer to do the operation to reverse the process. The figure shows the concept.