William Stallings Data and Computer Communications Chapter 8 Multiplexing
Multiplexing
Frequency Division Multiplexing 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
Frequency Division Multiplexing Diagram
FDM System
FDM of Three Voiceband Signals
Analog Carrier Systems AT&T (USA) Hierarchy of FDM schemes Group 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz and 612 khz Mastergroup 10 supergroups
Wavelength Division Multiplexing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs 100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps) Commercial systems of 160 channels of 10 Gbps now available Lab systems (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps Over 100km
WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Optical amplifiers amplify all wavelengths Typically tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHz
Dense Wavelength Division Multiplexing DWDM No official or standard definition Implies more channels more closely spaced that WDM 200GHz or less
Πολυπλεξία ιαίρεσης Κύµατος Wavelength division multiplexing.
Παραδείγµατα DWDM συστηµάτων TAT-14 (υπεραντλαντικό καλώδιο) 8 ζεύγη οπτικών ινών 16 διαφορετικά µήκη κύµατος ανά ζεύγος 2.4 Tbit/s συνολική χωρητικότητα FA-1: Flag Atlantic 1 (υπεραντλαντικό καλώδιο) 6 οπτικές ίνες 40 µήκη κύµατος ανά ίνα 10 Gbit/s ανά µήκος κύµατος 2.4 Tbit/s συνολική χωρητικότητα 2.4 Tbit/s µπορούν να µεταφέρουν 10.000.000 τηλεφωνικές κλήσεις
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 preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources
Time Division Multiplexing
TDM System
TDM Link Control No headers and tailers Data link control protocols not needed 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 quenched This leaves empty slots Error control Errors are detected and handled by individual channel systems
Data Link Control on TDM
Framing No flag or SYNC characters bracketing TDM frames Must provide 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 01010101 unlikely on a data channel Can compare incoming bit patterns on each channel with sync pattern
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
TDM of Analog and Digital Sources
Digital Carrier Systems Hierarchy of TDM USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format Multiplexes 24 channels Each frame has 8 bits per channel plus one framing bit 193 bits per frame
Digital Carrier Systems (2) For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 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
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
ISDN User Network Interface ISDN allows multiplexing of devices over single ISDN line Two interfaces Basic ISDN Interface Primary ISDN Interface
Basic ISDN Interface (1) Digital data exchanged between subscriber and NTE - Full Duplex Separate physical line for each direction Pseudoternary coding scheme 1=no voltage, 0=positive or negative 750mV +/- 10% Data rate 192kbps Basic access is two 64kbps B channels and one 16kbps D channel This gives 144kbps multiplexed over 192kbps Remaining capacity used for framing and sync
Basic ISDN Interface (2) B channel is basic iser channel Data PCM voice Separate logical 64kbps connections o different destinations D channel used for control or data LAPD frames Each frame 48 bits long One frame every 250µs
Frame Structure
Primary ISDN Point to point Typically supporting PBX 1.544Mbps Based on US DS-1 Used on T1 services 23 B plus one D channel 2.048Mbps Based on European standards 30 B plus one D channel Line coding is AMI usinghdb3
Primary ISDN Frame Formats
PDH (Plesiochronus Digital Hierarchy) Level Input Output G.740,G750 Mbps Channels T1/DS1 24X64 1.544 24 T1C/DS1C 2X1.544 Mbps 3.152 48 T2/DS2 4X1.544 Mbps 6.312 96 T3/DS3 7X6.312 Mbps 44.736 672 T4/DS4 6X44.736 Mbps 274.176 4.032
PDH (Plesiochronus Digital Hierarchy) Level Input Output G.740,G750 Mbps E1 30X64 KbpS 2.048 30 Channels E2 4X2.048 Mbps 8.448 120 E3 4X8.448 Mbps 34.368 480 E4 4X34.368 Mbps 139.264 1920 E5 4X139.264 Mbps 564.992 7680
PDH G.740,G750
SDH/SONET Definition Synchronous digital hierarchy (SDH) and synchronous optical network (SONET) refer to a group of fiber-optic transmission rates that can transport digital signals with different capacities
Sonet/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Signal Hierarchy Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) 51.84Mbps Carry DS-3 or group of lower rate signals (DS1 DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1)
SDH (G.707/708/709) Συµπτύσσει ευρωπαικά και αµερικάνικα δίκτυα Είναι οπτικό επιτρέπει όµως τη χρήση διαφορετικών µέσων Τα data για κάθε ρυθµό µετάδοσης µπορούν να τοποθετηθούν σε Containers (πλαίσιο σταθερού µεγέθους) και µετά σε ένα µεγαλύτερο πλαίσιο
Πλεονεκτήµατα SDH Απλοποιηµένες τεχνικές πολύπλεξης /αποπολύπλεξης Ενσωµάτωση καναλιών χαµήλοτερου bit rate χωρίς τη συνολική MUX/DEMUX του κυρίου σήµατος Βελτιωµένες τεχνικές διαχείρισης Εύκολα επεκτάσιµη δοµή σε υψηλότερες ταχύτητες
Πλεονεκτήµατα SDH Επιτρέπει τη χρήση των υπάρχοντων συστηµάτων ψηφιακών µεταδόσεων Προσφέρει ενοποίηση των δύο τεχνολογιών (Ευρωπαικής και Αµερικάνικης ) σε µία παγκόσµια τυποποίηση
οµές SDH STM-n Synchronous Transport Module βασική δοµή STM-1 155.52 Mbps STM-4 622.08 Mbps STM-16 2488.32 Mbps Container (C ) C-nx n 1-4 x=1,2 1 για 1.544 Mbps και 2 για 2.048 Mbps Virtual Container (VC) VC-nx Έ να C γίνεται VC µε τη προσθήκη ενός header POH (pathoverhead) πληροφορία δροµολόγησης
οµές SDH Tributary Unit (TU) TU-nx n=1..3 x=1,2 Ένα VC µε pointer.περιέχει πληροφορία για cross connection µεταξύ VC Tributary Unit Group (TUG) TUG-nx n=2,3 x=1,2 Administrative Unit (AU) AU-nx n=3,4 x=1,2
SDH path S-MUX D-MUX R MUX R Section Line Path
Σύνθεση σε STM-N
STM-1 frame
STM-1 frame
SONET/SDH Digital Hierarchy
SONET Frame Format
SONET STS-1 Overhead Octets
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 full Data rate on line lower than aggregate rates of input lines
Statistical TDM Frame Formats
Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network Local loop Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more
ADSL
ADSL Data Rate Wire Gauge Distance Wire Size Distance 1.5 or 2 Mbps 24 AWG 18,000 ft 0.5 mm 5.5 km 1.5 or 2 Mbps 26 AWG 15,000 ft 0.4 mm 4.6 km 6.1 Mbps 24 AWG 12,000 ft 0.5 mm 3.7 km 6.1 Mbps 26 AWG 9,000 ft 0.4 mm 2.7 km
ADSL
ADSL Design Asymmetric Greater capacity downstream than upstream Frequency division multiplexing Lowest 25kHz for voice Plain old telephone service (POTS) Use echo cancellation or FDM to give two bands Use FDM within bands Range 5.5km
ADSL Channel Configuration
Discrete Multitone DMT Multiple carrier signals at different frequencies Some bits on each channel 4kHz subchannels Send test signal and use subchannels with better signal to noise ratio 256 downstream subchannels at 4kHz (60kbps) 15.36MHz Impairments bring this down to 1.5Mbps to 9Mbps
DTM Bits Per Channel Allocation
DMT Transmitter
Technologies HDSL (High-bit-rate Digital Subscriber Line) E1/T1 dublex 4-wire UTP 12000 ft 80 KHz- 240KHz SDSL (Symmetric Digital Subscriber Line) 2-wire as HDSL RADSL (Rate Adaptive Asymmetric Digital Subscriber Line)
VDSL -- Very high data rate Digital Subscriber Line Downstream 12.96 Mbps (1/4 STS-1) 4,500 feet of wire 25.82 Mbps (1/2 STS-1) 3,000 feet of wire 51.84 Mbps (STS-1) 1,000 feet of wire Upstream 1.6-2.3 Mbps
xdsl High data rate DSL Single line DSL Very high data rate DSL
Required Reading Stallings chapter 8 Web sites on ADSL SONET