UNIT V DIGITAL TRANSMISSION SYSTEMS Poin o poin link sysems consideraions Link Power budge Rise ime budge Noise effecs on sysem performance Operaional principles of WDM Solions EDFA s Basic conceps of SONET/SDH. Analog Transmission Sysem In phoonic analog ransmission sysem he performance of he sysem is mainly deermined by signal-o-noise raio a he oupu of he receiver. In case of ampliude modulaion he ransmied opical power P() is in he form of: P( ) = P [1 + ms( )] where m is modulaion index, and s() is analog modulaion signal. The phoocurren a receiver can be expressed as: i ( ) = R0MP [1 ms( )] s r +
By calculaing mean square of he signal and mean square of he oal noise, which consiss of quanum, dark and surface leakage noise currens plus resisance hermal noise, he S/N can be wrien as: S N = i i s N = q( R = q( I P 0 (1/ )( R0MmPr ) P + I ) M F( M ) B + (4k TB / R ) F r + I D D (1/ )( MmI P) ) M F( M ) B + (4k TB / R B B eq eq ) F I I P L : primary phoocurren = R0Pr ; I :Surface - leakage curren; F( M ) : excess phoodiode noise facor M B :effecive noise bandwidh; R F : noise figure eq of baseband amplifier; P D :primary bulk dark curren; : equivalen resisance of phoodeecor load and amplifier r : average received opical power x pin Phoodiode S/N For pin phoodiode, M=1: S N (1/ )( I Pm) (4k TB / R ) F B eq (1/ ) m R0 = (4k TB / R B eq Pr ) F Low inpu signal level S m R0Pr N 4qB Large signal level
SNR vs. opical power for phoodiodes
Phoonic Digial Link Analysis & Design Poin-o-Poin Link Requiremen: - Daa Rae - BER - Disance - Cos & Complexiy Analysis Mehods: - Link loss & S/N analysis (link power budge analysis and loss allocaion) for a prescribed BER - Dispersion (rise-ime) analysis (rise-ime budge allocaion) Sysem Design Choices: Phoodeecor, Opical Source, Fiber Phoodeecors: Compared o APD, PINs are less expensive and more sablewih emperaure. However PINs have lower sensiiviy. Opical Sources: 1- LEDs: 150 (Mb/s).km @ 800-900 nm and larger han 1.5 (Gb/s).km @ 1330 nm - InGaAsP lasers: 5 (Gb/s).km @ 1330 nm and ideally around 500 (Gb/s).km @ 1550 nm. 10-15 db more power. However more cosly and more complex circuiry. Fiber: 1- Single-mode fibers are ofen used wih lasers or edge-emiing LEDs. - Muli-mode fibers are normally used wih LEDs. NA and should be opimized for any paricular applicaion.
Link Power/Loss Analysis PT [ db] = Ps [ dbm] PR [ dbm] Toal Power Loss P = l [ db] + α [ db / km] L[ km] + Sysem Margin T c f
Receiver Sensiiviies vs. Bi Rae The Si PIN & APD and InGaAsP PIN plos for BER= 10 9 The InGaAs APD plo is for BER=. 10 11
Link Loss Budge
Link Power Budge Table Componen/loss Oupu/sensiiviy/loss Power margin (db) parameer Laser oupu 3 dbm 䦋カラットlog㧀좈 琰茞 ᓀ 㵂 Ü APD Sensiiviy @ -3 dbm.5 Gb/s Allowed loss 3-(-3) dbm 35 Source connecor loss 1 db 34 Jumper+Connecor 3+1 db 30 loss Cable aenuaion 18 db 1 Jumper+Connecor 3+1 db 8 loss Receiver Connecor loss 1 db 7(final margin) Example: [SONET OC-48 (.5 Gb/s) link] Transmier: 3dBm @ 1550 nm; Receiver: InGaAs APD wih -3 dbm sensiiviy @.5 Gb/s; Fiber: 60 km long wih o.3 db/km aenuaion; jumper cable loss 3 db each, connecor loss of 1 db each.
Dispersion Analysis (Rise-Time Budge) sys = [ x + mod + GVD + rx ] 1 / = x + 440 L B 0 q + D σ λ L + 350 B rx 1 / x [ ns] : ransmier rise ime rx [ ns] : receiver rise ime mod [ n] : modal dispersion B rx [ MHz ]:3dB Elecrical BW L[ km]:lengh of he fiber B [ MHz ]: BW 0 of he 1km of he fiber; q 0.7 GVD [ns]: rise-ime due o group velociy dispersion D[ ns /( km. nm)]:dispersion σ λ [nm]:specral widh of he source
Two-level Binary Channel Codes
Sysem rise-time & Informaion Rae In digial ransmission sysem, he sysem rise-ime limis he bi rae of he sysem according o he following crieria: sys sys < 70% of < 35% of NRZ bi period RZ bi period Example Laser Tx has a rise-ime of 5 ps a 1550 nm and specral widh of 0.1 nm. Lengh of fiber is 60 km wih dispersion ps/(nm.km). The InGaAs APD has a.5 GHz BW. The rise-ime budge (required) of he sysem for NRZ signaling is 0.8 ns whereas he oal rise-ime due o componens is 0.14 ns. (The sysem is designed for 0 Mb/s). Example: Transmission Disance for MM-Fiber NRZ signaling, source/deecor: 800-900 nm LED/pin or AlGaAs laser/apd combinaions. ; LED oupu=-13 dbm;fiber loss=3.5 db/km;fiber bandwidh 800 MHz.km; q=0.7; 1-dB connecor/coupling loss a each end; 6 db sysem margin, maerial dispersion ins 0.07 ns/(km.nm); specral widh for LED=50 nm. Laser ar 850 nm specral widh=1 nm; laser oupu=0 dbm, Laser sysem margin=8 db;
Example:Transmission Disance for a SM Fiber Communicaion a 1550 nm, no modal dispersion, Source:Laser; Receiver:InGaAs-APD (11.5 log B -71.0 dbm) and PIN (11.5log B- 60.5 dbm); Fiber loss =0.3 db/km; D=.5 ps/(km.nm): laser specral widh 1 and 3.5 nm; laser oupu 0 dbm,laser sysem margin=8 db;
WDM: WDM is he abbreviaion for Wavelengh Division Muliplexing. Wha i does is o spli he he ligh in an opic fiber ino a number of discree wavelenghs (colors). Each wavelengh (color) is a independen channel running a daa rae a.5gbi/s, 10Gbi/s, 40Gbi/s or even 100Gbi/s (sill under developmen). So if he ligh in he fiber is spli ino 16 wavelenghs (colors or channels), and each wavelengh is running a 40Gbi/s daa rae, we ge a oal of 40Gbi/s x 16 = 640Gbi/s rae. This is especially rue in long haul and ulra long haul fiber opic communicaion links. In addiion, fibers carrying 64 and more channels (wavelenghs) are already available on he marke now. Which means we can run,560gbi/s daa rae on a single fiber. How abou 48 fibers in a single fiber opic cable? Tha gives us an amazing,560gbi/s x 48 = 1,880Gbi/s link. Of course, his kind of high speed and high fiber coun links are usually only deployed for Inerne backbones. From aforemenioned samples, you can see he shocking ruh abou WDM. I dramaically increases capaciy of a fiber opic link while minimizes equipmen and fiber opic cable cos. Wha is DWDM? DWDM sands for Dense Wavelengh Division Muliplexing. Here "dense" means he wavelengh channels are very narrow and close o each oher. For 100 GHz dense WDM, he inerval beween adjacen channels are only 100 GHz, (or 0.8nm). For example, he adjacen channels could be 1530.33nm, 1531.1nm and 1531.90nm. DWDM are widely used for he 1550nm band so as o leverage he capabiliies of EDFA (Erbium Doped Fiber Amplifiers). EDFAs are commonly used for he 155nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L Band).
Why is DWM so imporan? The exploiaion of DWDM has fueled an explosion in ransmission capaciy. The amoun of informaion ha can be sen over he fiber cables ha span he world has increased so much ha here is now a glu of available capaciy. In pracice, more can be wrung ou of DWD sysems by exending he upper or lower bounds of he available ransmission window or by spacing wavelenghs more closely, ypically a 50GHz, or even 5 GHz. In doing his, suppliers can double or riple he number of channels. Each opical channel can currenly be rouinely used for ransmission of ligh pulses a 10Gbi/s, or even higher daa raes a 100 GHz spacing. Wih he help of WDM, a pair of fibers can provide daa capaciy of several hundred gigabis per second. WDM echnology does no require any upgrade or replacemen of he fiber infrasrucure ha has been pu in he ground. Hence, we can upgrade links from one capaciy level o he nex simply by reconfiguring or upgrading erminal equipmen and repeaers.
SONET: Synchronous Opical NETwork Designed for opical ranspor (high birae) Direc mapping of lower levels ino higher ones Carry all PDH ypes in one universal hierarchy ITU version = Synchronous Digial Hierarchy differen erminology bu ineroperable Overhead doesn increase wih rae OAM designed-in from beginning SONET was designed wih definie layering conceps Physical layer opical fiber (linear or ring) when exceed fiber reach regeneraors regeneraors are no mere amplifiers, regeneraors use heir own overhead
fiber beween regeneraors called secion (regeneraor secion) Line layer link beween SONET muxes (Add/Drop Muliplexers) inpu and oupu a his level are Virual Tribuaries (VCs) acually layers lower order VC (for low birae payloads) higher order VC (for high birae payloads) Pah layer end-o-end pah of clien daa (ribuaries) clien daa (payload) may be PDH ATM packe daa A SONET signal is called a Synchronous Transpor Signal The basic STS is STS-1, all ohers are muliples of i - STS-N The (opical) physical layer signal corresponding o an STS-N is an OC-N SONET Opical rae STS-1 OC-1 51.84M STS-3 OC-3 155.5M STS-1 OC-1 6.080M STS-48 OC-48 488.3M STS-19 OC-19 9953.8M