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Multiplexing

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Multiplexing

  1. 1. September 24, 2012 TDM in cellphones TDM-time division multiplexing Prateek Kumar CSE-D 1031010186
  2. 2. MULTIPLEXING In telecommunications and computer networks, multiplexing (also known as muxing) is a method by which multiple analog message signals or digital data streams are combined into one signal over ashared medium. The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy, and is now widely applied in communications. George Owen Squier is credited with the development of multiplexing in 1910. The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the high-level communication channel into several low-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX). 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.1
  3. 3. TELECOMMUNICATION MULTIPLEXING TDM Time Division Multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency. TDM was initially developed in 1870 for large system telegraphy implementation. Packet switching networks use TDM for telecommunication links, i.e., packets are divided into fixed lengths and assigned fixed time slots for transmission. Each divided signal and packet, which must be transmitted within assigned time slots, are reassembled into a complete signal at destination. TDM is comprised of two major categories: TDM and synchronous time division multiplexing (sync TDM). TDM is used for long distance communication links and bears heavy data traffic loads from end users. Sync TDM is used for high speed transmission. During each time slot a TDM frame (or data packet) is created as a sample of the signal of a given sub-channel; the frame also consists of a synchronization channel and sometimes an error correction channel. After the first sample of the given sub-channel (along with its associated and newly created error correction and synchronization channels) are taken, the process is repeated for a second sample when a second frame is created, then repeated for a third frame, etc;2
  4. 4. andthe frames are interleaved one after the other. When the time slot has expired, the process is repeated for the next sub-channel. Examples of utilizing TDM include digitally transmitting several telephone conversations over the same four-wire copper cable or fiber optical cable in a TDM telephone network; these systems may be PCM (pulse code modulation) or PDH (plesiochronous digital hierarchy) systems. Another example involves sampling left and right stereo signals using RIFF (Resource Interchange File Format), also referred to as WAV (Waveform Audio File Format), audio standard interleaves. Also SDH (synchronous Digital Hierarchy) and SONET (synchronous optical networking) network transmission standards have incorporated TDM; and these have surpassed PDH. TDM can also be used within time division multiple access (TDMA) where stations sharing the same frequency channel can communicate with one another. GSM utilizes both TDM and TDMA. TDMA TDMA technology, which stands for Time Division Multiple Access, is a cell phone standard that has been incorporated into the more advanced GSM standard, which is now the world’s most widely used cell phone technology. TDMA is used in second-generation (2G) cell phone systems such as GSM. Most major third-generation (3G) cell phone systems are primarily based upon GSM rival CDMA. 3G allows for faster data speeds over 2G. Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency3
  5. 5. channel by dividing the signal into different time slots. 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 2G cellular 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. It is also used extensively in satellite systems, combat-net radiosystems, and 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, with the special point that instead of having one transmitter connected to onereceiver, 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. TDMA frame structure showing a data stream divided into frames and those frames divided into time slots.4
  6. 6. TDMA CHARACTERISTICS -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 2G systems 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 withFrequency Hopping and wideband transmission to minimize common types of interference. 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 aguard 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, an5
  7. 7. entire time slot has to be dedicated to mobiles attempting to contact the network (known as the 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 mobiles 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. 3G systems Although most major 3G systems are primarily based upon CDMAtime 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 MF-TDMA MF-TDMA (Multi-Frequency, Time Division Multiple Access) is the leading technology for dynamically sharing bandwidth resources in an over-the-air, two- way communications network. Many variations of MF-TDMA technology (including simple TDMA) exist and are commonly used in multiple types of networks, including:  Most two-way communication satellite networks,  The most common cellular telephony networks (e.g., GSM), and  Some metro-wireless data access networks (e.g., WiMax) Some alternatives to MF-TDMA are Code Division Multiple Access (CDMA) and Carrier sense multiple access (CSMA). It is also possible to combine MF- TDMA technology with these other technologies. For satellite networks MF- TDMA is the dominant technology because it provides the most bandwidth and the greatest overall efficiency and service quality, while also allowing the dynamic sharing of that bandwidth among many (tens of thousands) of6
  8. 8. transmitters in a two-way communication mode. MF-TDMA networks can have either a star-topology, fully meshed or partially meshed topologies. TDMA FRAMES This image below shows a sequence of two successive TDMA frames passing through the satellite. The carrier bit rate is 250 kbit/s Explanation: Site 1 transmits a burst, starting at the beginning of each TDMA frame. The burst lasts 180 mS, so at a rate of 250kbit/s site 1 sends 45,000 bits per burst, or 45,000 bits per second. Site 2 transmits a burst, timed to arrive at the satellite just after the end of burst 1. The red, second, burst lasts 80 mS, so at a rate of 250kbit/s, site 2 sends 20,000 bits per burst, or 20,000 bits per second. The diagram shows a fixed time plan, where each VSAT has been allocated a predetermined portion of the total time. There is designed in 20mS guard period between each burst. This allows for slight mistiming in the transmission of the bursts. Severe mistiming would cause bursts to arrive overlapping or on top of each other, causing loss of service to both sites involved in the mutual interference. The long 20mS guard period is illustrative only, so you can see the white space in the figure above. In actual TDMA systems the guard band may be very much less and there may be very many more bursts per frame. The above is just an example. TDMA frame length may be as short as 2000 microseconds or as long as 1 second, as in the example above. The shortest TDMA frame periods are associated with the highest higher speed TDMA systems, operating at say 120.832 Mbit/s. On low speed 250kbit/s VSAT return links, with perhaps 2 to 5 sites sharing, and used for internet browsing and emails, the TDMA frame period is typically 500Ms.7
  9. 9. BIBLIOGRAPHY -GOOGLE -WIKIPEDIA -THEBRETANICA.COM -Computer Networks –by Tanenbaum -Computer Networks –by S.S. Shinde -asme.org8

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