Chapter 8 Multiplexing

Transcription

1 CEN 342 Introduction to Data Transmission Chapter 8 Multiplexing Dr. Mostafa Hassan Dahshan Computer Engineering Department College of Computer and Information Sciences King Saud University mdahshan@ccis.ksu.edu.sa Multiplexing Communication links are expensive Using one link/party is inefficient Many applications require modest data rates Sharing link is more cost efficient Link sharing requires multiplexing

2 Multiplexing Types Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) Statistical Time Division Multiplexing Frequency Division Multiplexing Possible with large bandwidth Multiple signals carried simultaneously Each signal modulated onto different carrier frequency Carrier frequencies must be sufficiently separated Bandwidths should not overlap

3 Frequency Division Multiplexing

4 Frequency Division Multiplexing Example: FDM of TV Signals

5 Example FDM of Voice Signals Bandwidth of voice signal ( Hz) Generally taken as 4 khz Using AM with carrier frequency 64 khz Spectrum of modulated signal is 8 khz 60 khz 68 khz To make efficient use of bandwidth transmit only lower sideband

6 FDM Problems Crosstalk spectrum overlap between adjacent component signals guard band should be added e.g. voice 4 khz instead of 3100 Hz Intermodulation noise amplifiers produce frequency components of other channels Wavelength Division Multiplexing FDM used in optical fiber Multiple signals use different frequency WDM is the commonly used term Each wavelength carry channel of data More channels, closely spaced dense wavelength division multiplexing DWDM 160 channels, each 10 Gbps now available

7 Time Division Multiplexing Multiple digital signals carried on single path Portions of each signal interleaved in time Interleaving: bit level, blocks of bytes, larger Data from each source is buffered Buffers scanned sequentially Data organized into frames Time Division Multiplexing

8 Rate of m c (t) must be Σ n i m i (t) Time Division Multiplexing

9 Synchronous TDM Time slots pre-assigned to sources Slot transmitted even if source has no data May waste capacity but simple to implement Different data rates are possible Fast source can be assigned multiple slots Slots dedicated to source called channel Framing Frame sync to identify frame boundaries Added-digit framing one control bit added to each TDM frame effectively another channel (control channel) bit pattern unlikely on data channel

10 Synchronizing Multiple Sources Most difficult problem in TDM design If each source has separate clock any variation among clock loss of sync Input data rates not related by simple rational number Pulse stuffing is used to solve this problem Pulse Stuffing Outgoing data rate of multiplexer > sum of max instantaneous incoming rates Extra capacity used to stuff extra bits Dummy bits/pulses added to each input until rate matches local clock Stuffed pulses added at fixed locations Removed by demultiplexer

11 Example Input Source 1: Analog, 2 khz Source 2: Analog, 4 khz Source 3: Analog, 2 khz Sources 4-11: Digital, 7200 bps

12 Example For analog sources Sources 1, 3 sampled at 4000 samples/s Source 2 at 8000 samples/s PCM, quantized using 4 bits/sample 2 sample / scan for source 2 (8 bits) 1 sample / scan for sources 1, 3 (8 bits) Total sources 1-3 = = 64 kbps Example For digital sources Pulse stuffing raise each source to 8 kbps For aggregate data rate = 64 kbps Frame bit allocation Suppose frame = 32 bits 16 bits for PCM sources 1-3 (1:4, 2:8, 3:4) 2 bits for each source from 4-11 = 16 bits

13 Digital Carrier Systems Synchronous TDM transmission structure TDM performed at multiple levels Hierarchy of TDM structures US, Canada, Japan use AT&T system Other countries use ITU-T system DS-1 Transmission Format

14 Voice Transmission Voice is PCM digitized 8000 samples/s Frame rate must be 8000 frames/s Frame length = = 193 bits Data rate = = Mbps Control bits used in every 6 th frame Data Transmission Same Mbps data rate used 23 channels for data, 1 for sync byte Within channel 7 bits used for data 1 bit indicate channel is user or sys control data = 56 kbps max rate / channel

15 SONET/SDH SONET (Synchronous Optical Network) optical transmission interface proposed by BellCore, standardize by ANSI SDH (Synchronous Digital Hierarchy) compatible version published by ITU-T few differences from SONET Signal Hierarchy SONET defines hierarchy of data rates Lowest STS-1 / OC-1: Mbps STS-1 carry 1 DS-3 or group of DS-1 Multiple STS-1 combined to form STS-N SDH lowest rate is STM-1: = STS-3

16 Signal Hierarchy Frame Format Frame consists of 810 octets Transmitted every 125 μs = 8000 frame/s bit/frame 8000 frame/s = 51.84Mbps Frame logically viewed as matrix 9 rows, 90 octets each transmitted one row at time first 3 cols (27 octets) are overhead payload includes a column for path overhead

17

18 Statistical TDM TDM does not efficiently utilize capacity Many times, slots are wasted Statistical TDM allocates slots on demand Number of lines n < number of time slots k Not slots are reserved for specific input line Multiplexer collects data until frame is filled Statistical TDM Output data rate < sum input rates Can take more sources than TDM at same output rate or less output rate for same sources as TDM More overhead than TDM slot positions must be identified address information must be included with data

19 Statistical TDM Frame Structure Control information is needed Two possible formats One data source per frame need to identify address of source work well under light load inefficient under heavy load Multiple sources per frame need to identify length of data of each source

20 Frame Structure Digital Subscriber Line (DSL) Subscriber line: customer to central office Carry voice grade signal: 0 4 khz Wire can support 1 MHz or more Provide high speed data over phone line Asymmetric DSL (ADSL) more downstream rate the upstream most home user traffic is downstream

21 ADSL Design Lowest 25 khz reserved for voice known as plain old telephone service (POTS) more than 4 khz to prevent crosstalk FDM or echo cancellation to allocate bands smaller upstream, larger downstream FDM used within each band bit stream split into multiple parallel bit streams each portion carried in separate frequency band

22 Echo Cancellation Allow simultaneous transmission in both directions on the same band To recover received signal, transmitter subtracts echo of its own transmission Frequency band of up/down stream overlap Echo Cancellation Advantages less attenuation in low frequency range more downstream band in good part of spectrum more flexible allocation of up/down stream bands Disadvantages logic for EC must be installed both sides more complexity

23 Discrete Multitone (DMT) Used in ADSL transmission Multiple carrier signals different frequencies Transmission band divided to 4kHz channels Send some bits on each sub-channel Substream converted to analog using QAM QAM can assign different bits / signal Total data rate = sum of sub-channel rates Discrete Multitone (DMT) Initially, DMT modem sends test signals Test signals sent on all channels to test SNR More bits assigned to better SNR channels Each channel carries between 0-60 kbps

24 ADSL/DMT Transmission ADSL/DMT Transmission Design uses 256 downstream sub-channels Each sub-channel is 4 khz Max possible rate 60 kbps 256 = Mbps In practice, limited by impairments Actual rates from 1.5 to 9 Mbps Rate depends on distance and quality

25 xdsl ADSL = Asymmetric DSL HDSL = High data rate DSL SDSL = Single line DSL VDSL = Very high data rate DSL Additional References DS0, DS1, DS3, T1, T3 Dedicated FAQ, dedicated-voicedata.alllongdistance.com/dedfaq.shtml An introduction to ADSL, people.seas.harvard.edu/~jones/cscie129/ nu_lectures/lecture13/dsl/dsl.html