COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara Chapter 8: Multiplexing

COE 341 (T061) – Dr. Marwan Abu-Amara 2 Lecture Contents 1.Introduction 2.Multiplexing Types 3.FDM 4.Synchronous TDM 5.Statistical TDM 6.ADSL

COE 341 (T061) – Dr. Marwan Abu-Amara 3 Introduction Multiplexing: A generic term used when more than one application or connection share the capacity of one link Objective is to achieve better utilization of resources

COE 341 (T061) – Dr. Marwan Abu-Amara 4 Multiplexing Types

COE 341 (T061) – Dr. Marwan Abu-Amara 5 Frequency Division Multiplexing (FDM)

COE 341 (T061) – Dr. Marwan Abu-Amara 6 FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands)  e.g. broadcast radio Channel allocated even if no data

COE 341 (T061) – Dr. Marwan Abu-Amara 7 FDM

COE 341 (T061) – Dr. Marwan Abu-Amara 8 FDM Multiplexing Process: Time-Domain View

COE 341 (T061) – Dr. Marwan Abu-Amara 9 FDM Multiplexing Process: Frequency-Domain View

COE 341 (T061) – Dr. Marwan Abu-Amara 10 FDM De-Multiplexing Process: Time-Domain View

COE 341 (T061) – Dr. Marwan Abu-Amara 11 FDM De-Multiplexing Process: Frequency-Domain View

COE 341 (T061) – Dr. Marwan Abu-Amara 12 FDM System – Transmitter

COE 341 (T061) – Dr. Marwan Abu-Amara 13 FDM System – Receiver

COE 341 (T061) – Dr. Marwan Abu-Amara 14 FDM of Three Voiceband Signals

COE 341 (T061) – Dr. Marwan Abu-Amara 15 Analog Carrier Systems Devised by AT&T (USA) Hierarchy of FDM schemes Group  12 voice channels (4kHz each) = 48kHz  Range 60kHz to 108kHz Supergroup  60 channels  FDM of 5 group signals on carriers between 420kHz and 612 kHz Master group  10 supergroups Jumbo group  6 master groups

COE 341 (T061) – Dr. Marwan Abu-Amara 16 Analog FDM Hierarchy

COE 341 (T061) – Dr. Marwan Abu-Amara 17 Time Division Multiplexing (TDM)

COE 341 (T061) – Dr. Marwan Abu-Amara 18 TDM Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level or blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources

COE 341 (T061) – Dr. Marwan Abu-Amara 19 Time Division Multiplexing

COE 341 (T061) – Dr. Marwan Abu-Amara 20 TDM Frames

COE 341 (T061) – Dr. Marwan Abu-Amara 21 TDM System – Transmitter

COE 341 (T061) – Dr. Marwan Abu-Amara 22 TDM System – Receiver

COE 341 (T061) – Dr. Marwan Abu-Amara 23 TDM Link Control No headers and trailers Data link control protocols not needed for MUXed line Flow control  Data rate of multiplexed line is fixed  If one channel receiver can not receive data, the others must carry on  The corresponding source must be halted  This leaves empty slots Error control  Errors are detected and handled by individual channel systems

COE 341 (T061) – Dr. Marwan Abu-Amara 24 Data Link Control on TDM

COE 341 (T061) – Dr. Marwan Abu-Amara 25 Framing So far, no flag or SYNC characters bracketing TDM frames on the MUXed line Must provide frame synchronizing mechanism Added digit framing  One control bit added to each TDM frame Looks like another channel - “control channel”  Identifiable bit pattern used on control channel e.g. alternating …unlikely on a data channel  Can compare incoming bit patterns on each channel with sync pattern

COE 341 (T061) – Dr. Marwan Abu-Amara 26 Framing in TDM C C C Four data channels Control Channel, C …. ….. A data Channel Unlikely to have …. over successive frames ….. MUXed frame RX knows the size of the MUXed frame It can check each frame bit frame-to-frame for the special pattern until it finds it Once the position of the control channel is established, RX knows where the channel sequence starts and sync is established with TX

COE 341 (T061) – Dr. Marwan Abu-Amara 27 Pulse Stuffing Problem - Synchronizing data sources  Clocks in different sources drifting Data rates from different sources not related by simple rational number Solution - Pulse Stuffing  Outgoing data rate (excluding framing bits) higher than sum of incoming rates  Stuff extra dummy bits or pulses into each incoming signal until it matches local clock  Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

COE 341 (T061) – Dr. Marwan Abu-Amara 28 TDM of Analog and Digital Sources

COE 341 (T061) – Dr. Marwan Abu-Amara 29 Digital Carrier Systems Hierarchy of TDM (similar to FDM) USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format  Multiplexes 24 PCM voice channels  Each frame has 8 bits per channel plus one framing bit  193 bits per frame (24 ch.  8 bits per ch. + 1 framing bit = 193 bits per frame)

COE 341 (T061) – Dr. Marwan Abu-Amara 30 DS Signals

COE 341 (T061) – Dr. Marwan Abu-Amara 31 DS & T Lines Rates ServiceLine Rate (Mbps) Voice Channels DS-1T DS-2T DS-3T DS-4T

COE 341 (T061) – Dr. Marwan Abu-Amara 32 Digital Carrier Systems For voice each channel contains one word of digitized data (PCM, 8000 samples per second)  Data rate 8000 x 193 = 1.544Mbps  Five out of six frames have 8 bit PCM samples  Sixth frame is 7 bit PCM word plus signaling bit  Signaling bits form stream for each channel containing control and routing info Same format for digital data  23 channels of data 7 bits per frame plus indicator bit for data or systems control  24th channel is sync DS-1 can carry mixed voice and data signals

COE 341 (T061) – Dr. Marwan Abu-Amara 33 DS-1 Transmission Format (8000 x 7 bits = 56 kbps)

COE 341 (T061) – Dr. Marwan Abu-Amara 34 T1 Due to 1 framing bit Per frame

COE 341 (T061) – Dr. Marwan Abu-Amara 35 T1 Frames

COE 341 (T061) – Dr. Marwan Abu-Amara 36 E1 E Line Rate (Mbps) Voice Channels E E E E

COE 341 (T061) – Dr. Marwan Abu-Amara 37 Mixed Data DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channels  DS-2 is four DS-1 giving 6.312Mbps

COE 341 (T061) – Dr. Marwan Abu-Amara 38 SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Utilize the large channel capacity of optical fibers Signal Hierarchy  Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) Frame of 810 octets every 125  s: 51.84Mbps  Carry DS-3 or group of lower rate signals (DS1, DS1C, DS2) plus ITU-T rates (e.g Mbps)  Multiple STS-1 combined into STS-N signal  ITU-T lowest rate is Mbps (STM-1)

COE 341 (T061) – Dr. Marwan Abu-Amara 39 SONET/SDH Frame Format

COE 341 (T061) – Dr. Marwan Abu-Amara 40 Statistical TDM In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame is full Data rate on line lower than aggregate rates of input lines

COE 341 (T061) – Dr. Marwan Abu-Amara 41 Statistical TDM

COE 341 (T061) – Dr. Marwan Abu-Amara 42 Statistical TDM Frame Formats

COE 341 (T061) – Dr. Marwan Abu-Amara 43 Performance Output data rate less than aggregate input rates May cause problems during peak periods  Buffer inputs  Keep buffer size to minimum to reduce delay

COE 341 (T061) – Dr. Marwan Abu-Amara 44 Performance I= number of input sources R= data rate of each source, bps M= effective capacity of multiplexed line, bps  = mean fraction of time each source is transmitting (0 <  <1 ) K= M / (IR) = ratio of multiplexed line capacity to total maximum input = measure of compression achieved by multiplexer (  < K < 1)  If K < , input will exceed multiplexer’s capacity

COE 341 (T061) – Dr. Marwan Abu-Amara 45

COE 341 (T061) – Dr. Marwan Abu-Amara 46 Performance Assume random (Poisson) arrivals and constant service time Average arrival rate Service time T s Utilization or fraction of total line capacity used

COE 341 (T061) – Dr. Marwan Abu-Amara 47 Buffer Size and Delay Increasing utilization increases  Buffer size  delay Utilization > 0.8 is undesirable

COE 341 (T061) – Dr. Marwan Abu-Amara 48 ADSL ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses The existing local loops can handle bandwidths up to 1.1 MHz ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line Use the following as a rule of thumb  XYZ km  XYZ Mbps

COE 341 (T061) – Dr. Marwan Abu-Amara 49 ADSL Hardware

COE 341 (T061) – Dr. Marwan Abu-Amara 50 ADSL Frequency Bands

COE 341 (T061) – Dr. Marwan Abu-Amara 51 DMT