08 Multiplexing

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Information about 08 Multiplexing
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Published on November 29, 2007

Author: Coralie

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William Stallings Data and Computer Communications 7th Edition:  William Stallings Data and Computer Communications 7th Edition Chapter 8 Multiplexing Multiplexing:  Multiplexing Multiplexes the data from n-input lines De-multiplexes the data from with respect to the channel A communication link is logically divided into n-number of channels Multiplexing Techniques:  Multiplexing Techniques Frequency-division multiplexing (FDM) Analog Signals Transmission Time-division multiplexing (TDM) Digital Signals Transmission Wavelength-division multiplexing (WDM) Fiber Optic Communication Statistical or Asynchronous TDM Frequency Division Multiplexing:  Frequency Division Multiplexing Analog Technique Link-Bandwidth > Combined-Bandwidth Each signal is modulated to a different carrier frequency Modulated Signals combine into a Composite Signal Carrier frequencies separated so signals do not overlap (guard bands) Channel allocated even if no data Frequency Division Multiplexing Diagram:  Frequency Division Multiplexing Diagram FDM System:  FDM System FDM:  FDM Advantages Simple logic is required Disadvantages Band-pass filters are required Waste of bandwidth if source has no/less data to transmit Wavelength Division Multiplexing (WDM):  Wavelength Division Multiplexing (WDM) Same general architecture as other FDM (A form of FDM) Multiple optical signals travel at different frequency Wavelength identifies the unique frequency range for each transmitted optical signal Each colour of light (wavelength) carries separate data channel Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Optical amplifiers amplify all wavelengths simultaneously Synchronous Time Division Multiplexing:  Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level of blocks Time slots pre-assigned to sources and fixed Time slots allocated even if no data We call it synchronous because of pre-determine/pre-assign time slots S-TDM Diagram & Frame:  Number of input signals 1-bit or 1-character in length The buffer is sequentially scanned to formed the composite signal mc(t) Data rate of mc(t) ≥ ∑{data rate mi(t)} S-TDM Diagram & Frame Time is divided into Time-slots W.R.T the # of I/P signals A Frame consists of 1 complete cycle of Time-Slot In a system with n input lines, each frame has at least n slots User-1 transmits at TS-1, user-2 transmits at TS-2, and User-N transmits at TS-N & then finally the frame will transmit TDM Link Control:  TDM Link Control No headers and trailers used by TDM Frames because the Data link control protocols not needed Problem: In the absence header/trailer, what happens with following: Flow Control When one of the O/P lines stops accepting data? Error Control When one of the O/P lines detects error? Solutions Flow control Data rate of multiplexed line is fixed If one O/P line can not receive data, the others must carry on The bottom line is: the flow control should be implemented on per O/P line basis rather than per system bass Can be used HDLC for that purpose Error control Errors are detected and handled by individual channel systems The bottom line is: the error control should be implemented on per O/P line basis rather than per system Can be implemented HDLC for that purpose Framing:  Framing Synchronization bits are usually added to the beginning of each frame These bits, called Framing Bits, follow the pattern from frame to frame that allows Demux to synchronize. Synchronization-Information consists of 1 bit per frame, alternating between 0 & 1 (010101….) 0 1 0 1 Digital Carrier Systems:  Digital Carrier Systems Digital Signal 1 (DS1) is a Signaling scheme used in North America & Japan DS1 also known as T1 (sometime) Technically, DS1 is the transmission protocol used over a physical T1 line DS1 or T1 provides a data rate of 1.544 Mbps (i.e., 193 bits * 8000 samples/sec) DS1 or T1 Carrier structure:  DS1 or T1 Carrier structure DS1 or T1 has 24 channels where each channel has 8-bits Each channel can be considered as a Time-Slot Each channel is multiplexed at 8000 samples/sec 7 bits/channel is used for data where 1 bit/channel is used for control information 1 extra framing bit is used per transmitted frame for control information Control bit Data bits 8000 samples/sec Statistical or Asynchronous TDM:  Statistical or Asynchronous TDM In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand In Statistical TDM, each slot in a frame is not dedicated to the fix device Primary Differences Total speed of input lines can be grater than the capacity of the path A frame can have K slots for N users where K<N Advantage Save Bandwidth Support relatively large number of users Statistical TDM Multiplexing Process (1):  Statistical TDM Multiplexing Process (1) 4 computers are sharing the data link 3 slots per frame that are not assigned to any specific user All 4 computers have data to send (statistically a rare case) In Statistical TDM, 10 slots of data is transmitted using 4 frames-12 slots (only 2 slots are wasted in) In STDM, 10 slots of data is transmitted using 4 frames-16 slots (6 slots will be wasted) Statistical TDM Multiplexing Process (2):  Statistical TDM Multiplexing Process (2) DEMUX de-multiplexes the data & forwarded to the appropriate destination by looking at the index of the device. Asymmetrical Digital Subscriber Line (ADSL):  Asymmetrical Digital Subscriber Line (ADSL) ADSL (family member of DSL) The links between the user and network is analog (local loop) The goal is to make these digital (i.e., provide a Fast-Link between subscriber and network The solution is to utilize the available bandwidth efficiently (i.e., gives max bandwidth to user) Asymmetric means more bandwidth for downstream than the upstream direction Downstream (from central office to end user) download Upstream (from end user to central office) send email ADSL Bandwidth division:  ADSL Bandwidth division ADSL divides the bandwidth (1 MHz) of twisted Pair in 3 bands The 1st band is used for regular telephone service The 2nd band is used for the Upstream communication such as sending email The 3rd band is used for the Downstream communication such as watching movie online xDSL:  xDSL High data rate DSL Single line DSL Very high data rate DSL Required Reading:  Required Reading Stallings chapter 8 Web sites on ADSL

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