CIS 321 – Data Communications & Networking Chapter 8 – Multiplexing

University of South Alabama Computer and Information Sciences 2 Multiplexing  Simultaneous transmission of multiple signals across a single data link  Can utilize higher capacity links without adding additional lines for each device – better utilization of bandwidth  Multiplexer (MUX) on sender’s end; demultiplexer (DEMUX) separates transmission stream and directs signals to intended receiving devices

University of South Alabama Computer and Information Sciences 3 Multiplexing vs. no multiplexing

University of South Alabama Computer and Information Sciences 4 Multiplexing Techniques  3 basic techniques Frequency-division multiplexing – analog Wave-division multiplexing – analog Time-division multiplexing – digital

University of South Alabama Computer and Information Sciences 5 Frequency-Division Multiplexing  Analog technique - when bandwidth of link is greater than combined bandwidth of signals to be transmitted  Signals from each sending device modulate different frequencies  Modulated signals are combined into a single composite signal  Bandwidth ranges are channels through which the signals travel, separated by guard bands

University of South Alabama Computer and Information Sciences 6 FDM

University of South Alabama Computer and Information Sciences 7 FDM MUX-DEMUX Process

University of South Alabama Computer and Information Sciences 8 Analog Hierarchy of Phone Network

University of South Alabama Computer and Information Sciences 9 Other Applications of FDM  AM and FM radio broadcasting Each station uses a different carrier frequency, shifting its signal and multiplexing Receiver filters (tunes) to the frequency desired  Same concept for TV broadcasting and first generation cell phones

University of South Alabama Computer and Information Sciences Wave-Division Multiplexing  Use light signals transmitted through fiber-optic channels  Very narrow bands of light are combined from several sources to make a wider band of light  A prism is used to bend the light beams based on the angle of incidence and frequency  Receiver’s DEMUX separates signals

University of South Alabama Computer and Information Sciences 11 WDM Picture from Cisco Systems, Inc. MPEG WDM example MPEG WDM example ©Sydney VisLab

University of South Alabama Computer and Information Sciences 12 WDM Applications  Application: SONET network Multiple optical fiber lines are muxed/demuxed  DWDM (dense WDM) allows muxing of large numbers of channels by spacing channels closer to one another to achieve greater efficiency

University of South Alabama Computer and Information Sciences Time-Division Multiplexing (TDM)  Process of combining digital signals from several sources whereby each connection occupies a portion of time in the link  Link is sectioned by time rather than frequency

University of South Alabama Computer and Information Sciences 14 TDM

University of South Alabama Computer and Information Sciences 15 Time Slots and Frames  Data flow of each connection is divided into units  Link combines one unit of each connection to make a frame  n input connections  n time slots  Data rate of link must be n times the duration of a time slot to guarantee flow of data  Time slots are grouped into frames; one complete cycle of time slots; each slot dedicated to one device

University of South Alabama Computer and Information Sciences 16 TDM

University of South Alabama Computer and Information Sciences 17 Interleaving  Process of taking a specific amount of data from each device in a regular order  May be done by bit, byte, or any other data unit

University of South Alabama Computer and Information Sciences 18 Synchronizing  Framing bits are used to alleviate timing inconsistencies that may be introduced  Usually one or two synchronization bits are added to beginning and end of each frame that allows the DEMUX to synchronize with the incoming stream so it can separate time slots accurately

University of South Alabama Computer and Information Sciences 19 Framing Bits example

University of South Alabama Computer and Information Sciences 20 Bit Padding  Used when differing speeds of devices are not integer multiples of each other (e.g. device A may be five and a half times as fast as device B)  MUX adds extra bits to a device’s source stream to force speed relationships between devices into integer multiples of each other  Extra bits are then discarded at the DEMUX

University of South Alabama Computer and Information Sciences 21 Digital Signal (DS) Service  Hierarchy of digital signals  DS-0 – single channel of 64 Kbps  DS-1 – single service or 24 DS-0 channels multiplexed to yield Mbps  DS-2 – single service or 4 DS-1 channels or 96 DS-0 channels to yield Mbps  DS-3 – single service, 7 DS-2 channels, 28 DS-1 channels, or 672 DS-0 channels to yield Mbps  DS-4 – 6 DS-3 channels, 42 DS-2 channels, 168 DS-1 channels, 4032 DS-0 channels to yield Mbps

University of South Alabama Computer and Information Sciences 22 DS Hierarchy

University of South Alabama Computer and Information Sciences 23 T Lines  Capacity lines matching data rates of DS-1 to DS-4 services ServiceLineRate (Mbps)Voice Channels DS-1T DS-2T DS-3T DS-4T

University of South Alabama Computer and Information Sciences 24 T Lines  Digital lines designed for digital data, voice, or audio  May be used for regular analog if sampled then multiplexed using TDM

University of South Alabama Computer and Information Sciences 25 T-1 line for multiplexing telephone lines

University of South Alabama Computer and Information Sciences 26 T-1 frame structure

University of South Alabama Computer and Information Sciences 27 Inverse TDM  Takes data stream from one high-speed line and breaks into portions and sends over several lower-speed lines simultaneously  Used in bandwidth-on-demand where channels can be used for several applications requiring different transmission rates (i.e. voice, data, video)

University of South Alabama Computer and Information Sciences 28 Inverse Multiplexing

University of South Alabama Computer and Information Sciences 29 More TDM Applications  Second-generation cell phone companies Divides into bands and uses FDM to combine Then uses TDM for sharing