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SmartOptics AS

Phone: +47 213 79 180Mail: info@smartoptics.com

www.smartoptics.com Rev 1.2

IntroductionThis Whitepaper describes Wavelength DivisionMultiplexing (WDM) Technology. WDM multiplies fibercapacity by multiplexing optical light signals of differentwavelengths onto a single optical fiber. The technology iswell standardized in the ITU G.695 and G.694recommendation and widely used in datacom and telecomapplications.

Figure 2: Wavelength signal paths in a 8 ch. Mux/Demux unit.

Figure 1: Illustration of CWDM function with 8 channels.

Due to the physical nature of light, the signals on WDMwavelengths are completely independent from each otherand data stream with different line-rates and protocols canbe transported on a single fiber or fiber pair. This enables totransport traditional telecom signals (PDH, SDH/SONET)IP data (Ethernet, Gigabit Ethernet or 10G Ethernet) andstorage data (4G/2G/1G Fibre Channel) over a singleinfrastructure without complex protocol conversion tech-nologies.

Figure 3: Signal paths in a 1ch. OADM with wavelength reuse.

Figure 4: Point-to-Point link with two OADMs.

Passive Mux/Demux filters

to WDM Technology in far superior reliability.

OADM: An Optical Add/Drop Multiplexer (OADM) unitterminates a limited subset of wavelength in WDMsystems. OADMs typically add/drop 1, 2 or 4 wavelengthsand the remaining wavelengths are bypassed (expressed)through the fiber. It is possible to reuse the terminatedwavelengths on the remaining span.

To build an embedded WDM systems two components are

necessary:?Optical Multiplexer Units (OMU) which combine and

The ability to reuse wavelengths in OADMs enablessplit light signals of WDM wavelengthsnetwork designers to build redundant WDM networks. With?WDM transceivers, which generate light signal ofprotection switching usually taken care of in the clientspecific WDM wavelengthsequipment.This paper does not take active or amplified WDM

systems into consideration, as modern active WDMsystems are based on the same type of WDM transceiverand filters.

CWDM and DWDMWDM comes in two flavours; CWDM and DWDM.Coarse WDM is a robust technology able to create 16

channels at 2.5 Gbps and was first to adapt transceiversfootprints. CWDM is typically used for un-amplifiedtransmission up to 120 km.Dense WDM uses cooled lasers and can be amplified.

Figure 4. illustrates a point-to-point system with twoDWDM transceivers first became available to the market inOADMs. The first OADM creates one wavelength path2005. DWDM transmission allows the transmissionfrom (a) to (b). The second OADM generates twodistance to be extended further, because optical ampliferswavelength paths from (a) to (c) and (c) to (d). The 8 ch.and associated components can be used. 10 GbpsMux/Demux units have six remaining wavelength pathstransmission usually requires DWDM.from (a) to (d).

Designing WDM NetworksMux/Demux: A Mux/Demux unit terminates all wavelengthWhen comparing design complexity of a WDM and non-on the WDM system and is at the heart of point-to-pointWDM network, only the additional WDM filter losses haveconnections. The optical combining and splitting ofto be taken into account. All other design parameterswavelengths is a passive technology and a Mux/Demuxremain equal to a non-WDM approach.unit requires neither electrical power nor software resulting

WDM Technology Whitepaper

fiber pair

Mux/Demux

(d)

(c)(b)

(a)

MU Line Tx

47 Rx49 Rx51 Rx53 Rx55 Rx57 Rx59 Rx61 Rx

DMULine Rx

47 Tx49 Tx51 Tx53 Tx55 Tx57 Tx59 Tx61 Tx

fiber pair fiber pair OADM

OADM

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SmartOptics ASPhone: +47 213 79 180

Mail: info@smartoptics.comwww.smartoptics.com

Rev 1.2

WDM transceivers Case Study 1 - 10G WDM in Backbone NetworksOptical transceivers exist in a wide range of different form- WDM is an attractive technology for increasing capacity infactors, data-rates and functions. The laser transmitter of a area\s where fiber is exhausted. Fiber exhaust exists due

WDM transceivers must meet the ITU criteria for CWDM to a limited installed fiber base, swapping of fiber betweenand/or DWDM. The receivers are usually wavelength operators or expensive leasing/buying of fibers. Theindependent, but two different types of receivers exist. A Return of Investment for installing WDM instead of leasing,PIN photo diode based receiver is much lower in cost than buying or installing new fiber is usually calculated inthe high sensitivity APD (avalanche photo diode) based months.receiver.The modules are hot pluggable and digital diagnosticfunctions are available enabling optical power levelmonitoring and advanced alarm settings.

CWDM is typically used for data-rates up to 4.25 Gbps andwhilst DWDM can be used for 2.5 Gbps it is requiredtransport data-rates around 10Gbps. Below is a table

showing typical parameters for data-rate and distance for Here the fiber costs, which is often a significant part of thecommon WDM transceivers. operational costs, are reduced by 80%. Additionalwavelengths/capacity can easily be added by introducing

SFP_________________________________________ new wavelengths. This does not disturb or interrupt theCWDM PIN 100 Mbps - 2.5 Gbps 40 km - 50 km existing traffic.CWDM APD 100 Mbps - 2.5 Gbps 80 km - 100 kmDWDM APD 100 Mbps - 2.5 Gbps 80 km - 120 km Case Study 2 - CWDM in Broadband Access NetworkCWDM PIN Gigabit Ethernet 80 km One of the operators dilemmas in broadband accessCWDM APD Gigabit Ethernet 150 km networks is the capacity requirements in the second mileCWDM PIN 4x/2x/1x Fibre Channel 50 km from the access terminal (xDSL, CaTV, FTTH, ETTH) toCWDM APD 4x/2x/1x Fibre Channel 80 km the central office. Todays focus is on delivering TV

services over an IP/Ethernet network, but tomorrowsPlease note, that DWDM SFPs can only be used in equipment prepared focus will be driven by on-demand services. Capacityfor DWDM SFPs.

requirements from on-demand services predict muchhigher traffic volume from central office to accessXENPAK______________________________________terminals. 2 Gbps per access terminal enables video-on-XENPAK DWDM 10G Ethernet 80 kmdemand services and allows operators to generateadditional revenue streams.XFP ______________________________________CWDM offers the best and most cost efficient approach ofXFP DWDM PIN 10G Ethernet 40 kmdelivering 2 Gbps to the access network. The networkXFP DWDM APD 10G Ethernet 80 kmarchitecture is illustrated below:

Please note, that DWDM XFPs and XENPAKs can be used in alequipment supporting XFPs and XENPAKs.

Figure 5: Picture of a 4.25 Gbps Fibre Channel CWDM SFP.

Figure 7: A hubbed CWDM network for broadband access.RecommendationMany operators who currently do not have the capacity The central-office is populated with Mux/Demux unitsrequirement to deploy WDM can prepare their network for terminating all wavelengths. The access terminals arefuture upgrades by purchasing WDM transceivers (eg populated with OADMs for wavelength reuse. The logical1000Base-CWDM GBIC/SFPs instead of 1000Base-ZX connection in this architecture is protected point-to-pointGBIC/SFPs). WDM transceivers have a marginal higher wavelength connections between central office andcost than their non-WDM long distance counterparts. access terminals. In normal operation each accessLater on, the network operators can simply add passive terminal receives 2 Gbps capacity, in case of a fiber cut 1filters and additional wavelengths and therefore do not Gbps capacity survives.have to replace the existing transceivers.

Figure 6:Point-to-Point WDM Network.

BRAS WWW

IP-DSLAM

PSN

Fiber

FTTH/ETTH

CMTS

MUDMU

SDH 2.5GGigEGigEGigE

10GigEMUDMU

SDH 2.5GGigEGigEGigE10GigE