CWDM solution

Produse

Curs valutar

Optimizarea comunicaţiilor prin fibre optice, printr-o utilizare mai eficientă a devenit principala preocupare în ultimii ani, din cauza necesităţii de creştere a a traficului in reţea, atât în volum cât şi în viteză de transmisie a datelor.
Costul ridicat al instalarii şi operarii de noi reţele de comunicaţii, recomandă ca o soluţie utilizarea optimă a infrastructurii existente.
Dezvoltarea transportului în domeniul telecom caută noi soluţii pentru a extinde canalele de comunicare prin multiplexare.
CWDM şi DWDM două tehnologii care s-au dezvoltat foarte repede în ultimii 10 ani, au devenit accesibile şi pot fi implementate în reţelele de fibră optică de comunicaţii, ca o investiţie de eficienţă incontestabilă.

CWDM: cost mai redus pentru capacitate mai mare de transmisie

Tehnologia CWDM – Coarse Wavelength-Division Multiplexing s-a comercializat incepand cu anii 1980, pentru transportul canalelor multiple de comunicatii cu latimea de banda de 25 nm in fereastra 850nm, folosita in retele de tip LAN. La acel moment, tehnologia era numita simplu WDM (Wavelength Division Multiplexing), iar printre aplicatii se numarau transmisiuni distribuite video, bidirectionale si transmiterea de informatii de control si telemetrie in timp real pe o singura fibra optica.

La mijlocul anilor '90, tehnologia WDM devine din ce in ce mai folosita, deoarece ofera peste 80 de lungimi de unda cu spatii dense, in fereastra de 1550nm. Numita si banda conventionala sau banda C (1530-1565nm), aceasta fereastra a fost preferata deoarece asigura atenuarea scazuta a semnalului optic pe fibra si pentru ca puteau fi folosite amplificatoare cu fibra dopata cu Erbium (EDFA).

In urma dezvoltarii de noi produse pentru piata de comunicatii pe lunga distanta, a fost nevoie de folosirea unui alt termen, alternativ, pentru a diferentia latimile de banda si benzile de comunicatii.

Termenul Coarse WDM (CWDM) a fost adoptat si folosit intensiv incepand cu 1996 pentru a diferentia vechiile tehnologii WDM, de cele in continua dezvoltare CWDM. Incepand cu anii 2000, tehnologia CWDM s-a dezvoltat pentru aplicatii LAN, iar la scurt timp a patruns si in domeniul retelelor metropolitane.

Quante Corp. a comercializat un sitem cu fereastra de 800nm si patru canale a cate 140 Mb/sec fiecare. Aceste sisteme au fost folosite initial pentru transmisii ale semnalelor TV. Cu toate acestea, sistemele CWDM nu au generat un interes semnificativ in randul furnizorilor de servicii de comunicatii – pana acum.

Odata cu nevoile transportatorilor metro de a gasi solutii de transmitere mai ieftine, sitemul CWDM este din ce in ce mai acceptat ca si o importanta solutie arhitecturala de transport.

CWDM si DWDM

Spre deosebire de DWDM, sistemele CWDM s-au dezvoltat pe baza laserelor neracite DFB - Distributed Feedback Lasers. Aceasta tehnologie vine cu cateva avantaje deloc de neglijat cum ar fi consum minim de energie, dimensiuni mai mici si costuri mult mai reduse. Disponibilitatea comerciala a sistemelor CWDM ce ofera beneficiile mentionate mai sus fac din ea o alternativa viabila pentru solutiile DWDM pentru aplicatiile metro si de acces.

Latimea de banda a legaturilor de fibre optice poate fi marita prin transmiterea de date mult mai rapid sau transmiterea de multiple lungimi de unda printr-un singura fibra folosind metoda cunoscuta sub numele de WDM. Tehnologia Wavelength Division Multiplexing consta in folosirea de multiplexoare pentru a combina lungimi de unda care circula din mai multe fibre spre una singura. La capatul receptorului de legatura, un demultiplexor separa lungimile de unda si le fixeaza in diferite fibre, fibre ce au terminatia in receivere diferite (a se vedea in Figura 2.1). Spatiile dintre lungimile de unda individuale transmise prin aceeasi fibra ofera baza definirii solutiilor DWDM si CWDM.(a.s.v. Figura 2.2).

Figura 2.1: WDM se realizeaza prin folosirea unui multiplexor pentru combinarea lungimilor de unda ce se transmit de la mai multe firbre spre una singura. La capatul receptorului de legatura un demultiplexor separa lungimile de unda si la fixeaza pe diverse fibre.

Sitemele DWDM deobicei folosesc una din cele trei solutii de separare a lungimilor de unda: 200 GHz (1.6 nm) / 100 GHz (0.8 nm) / 50 GHz (0.4 nm)

Sistemele viitoare sunt proiectate pentro a avea un spatiu si mai ingust. Lungimile de unda operationale a sistemelor DWDM sunt definite conform standardelor actuale ale frecventelor/lungimilor de unda.

Fig. 2.2 Spatiul dintre lungimile de unda

individuale sunt transmise prin aceias fibra

ofera baza definirii solutiilor DWDM si

bineinteles WDM (CWDM). Spatiul standard

in sitemele CWDM este de 20 nm, in timp

ce majoritatea sitemelor DWDM astazi este

de 0.8-nm (100-GHz).

Ambele arhitecturi de spatiere (rara si densa) utilizeaza sitemul DFB. Insa sistemele DWDM necesita lasare reci DFB mai mari deoarece lungimea de unda a laserului semi-conductor fluctueaza cu aproape 0.08 nm / 1°C.

Lasere DFB reci stabilizeaza abaterea lungimilor de unde in afara canalelor filtrelor multiplexoarelor si demultiplexoarelor atunci cand temperatura fluctueaza in sistemele DWDM.

Pe de alta parte, sistemele CWDM , folosesc lasare DFB normale. Aceste sisteme functioneaza de la 0 la 70°C unde lungimea de unda fluctueaza cu aprox. 6 nm peste camp. Aceasta fluctuatie de lungime de unda cuplata cu variatia laserului pana la ±3 nm (datorita proceselor de fabricare a laserelor). Distanta canalului optic trebuie sa fie destul de lata sa poata gazdui variatia lungimii de unda a laserului neracit a sistemului CWDM.(a.s.v Figura 2.3). Spatiul canalului in acest sistem este de 20nm si cu o latime de banda de 13nm.

Fig. 2.3 Cursurile laserelor WDM

fluctueaza in lungimile de unda

neracite la o rata de aprox.0.08 nm/°C.

Hardware mult mai ieftin

Diferenta de cost intre sistemele CWDM si cele DWDM poate fi atribuita costurilor de hardware si de operare.

In timp ce laserele DWDM sunt mult mai scumpe decat cele CWDM, laserul racit DFB ofera solutii eficiente de pret pentru solutii de transport pe distanta scurta si inele metro ce necesita o mai mare capacitate. In ambele aplicatii, costul sitemelor DWDM este amortizat de numarul mare de clienti ce sunt deserviti de aceste sisteme.

Pe de alta parte, retelele metro-access necesita un cost mai scazut si o capacitate mai mica pentru a satisface cerintele pietei, bazandu-se in mare masura pe disponibilitatea clientului de a plati pentru serviciile broad-band.

Pretul modului DWDM de obicei este de patru-cinci ori mai mare decat cel al modulului CWDM, diferenta fiind atribuita unor factori ce tin de lasere. Durata de viata mult mai mare a laserului DWDM in comparatie cu cea a CWDM, este esentiala. Toleranta tipica a lungimilor de unda a laserelor DWDM este de ±0.1 nm, unde toleranta pentru laserele CWDM este de ±2-3 nm. De asemenea, diferenta de cost dintre multiplexoarele si demultiplexoarele DWDM si CWDM contribuie la un cost general mai mic, in favoarea CWDM. Filtrele CWDM sunt mult mai ieftine de fabricat decat cele de DWDM datorita straturilor mai putine. In mod standard sunt aproximativ 150 de straturi ce alcatuiesc filtrele de 100-GHz DWDM, pe cand cele CWDM au doar 50 de straturi la filtre. Acest lucru duce la fabricarea in masa a filtrelor CWDM. La fabricarea aparatelor CWDM cu trei porturi, toleranta de aliniere este relativ mai relaxata decat la aparatele DWDM, acesta fiind un alt motiv pentru care aparatele CWDM sunt cu mult mai ieftine.(a.s.v Figura 3.1).

Fig. 3.1 Designul filtrului WDM

(CWDM) foloseste mai putine straturi iar

toleranta la aliniere este mult mai relaxata

fata de cele DWDM ceea ce duce la un

pret mult mai scazut de fabricare a CWDM.

Costul filtrului CWDM este cu aprox. 50% mai mic decat cel al filtrului DWDM si se prognozeaza ca va mai scadea in urmatorii trei ani datorita automatizarii liniei de fabricatie. Implementarea noului filtru si a tehnologiei de multiplexare/demultiplexare va reduce pretul si mai mult.

Alimentare

Costurile operationale a sistemului de transport optic CWDM / DWDM depind de mentenanta si alimentare.

In timp ce pretul de mentenanta este acceptabil atat pentru CWDM cat si pentru DWDM, costul criteriilor de alimentare pentru DWDM este semnificativ mai mare. De exemplu, lasarele DWDM sunt stabilizate termic cu coolere Peltier integrate in pachetul lor de module. Coolerul impreuna cu monitorul si circuitul de control consuma 4W pe lungime de unda. In timp ce transmitatoarele laserului CWDM neracit folosesc doar 0.5W.

In timp ce numarul lungimilor de unda creste, apar fenomene de disipare a puterii de transmisie si de reglare a temperaturii pentru sistemele DWDM, ceea ce constuie o mare problema pentru designerii de placi de baza.

Cu cat cerintele de alimentare sunt mai mici folosinduse lasarele neracite ale sistemului CWDM cu atat implicatiile financiare devin pozitive pentru operatorii de sistem. De exemplu, trebuie luat in considerare costul redicat al bateriei de backup pentru transportul echipamentelor. Minimalizand puterea de alimentare si costurile asociate de backup si mentenanta, in cat mai mare masura, se reduc costurile operationale ale solutiei.

Dimensiuni

Lasarele CWDM sunt cu mult mai mici decat cele DWDM. Lasarele neracite sunt construite dintr-un monitor fotodioda montat intr-o carcasa de metal sigilata ermetic si cu o fereastra din sticla. Aceste carcase sunt aliniate cu o fibra de tip pigtail sau maneca de aliniere ce accepta un connector, formand un pachet cilindric denumit transmitter optical subassembly (TOSA).

Un laser TOSA tipic are aprox. 2cm in lugime si 0.5cm in diametru. Lasarele racite se gasesc in pachete de tip fluture sau dual inline si contin monitor fotodioda, thermister si cooler Peltier. Aceste lasare sunt de 4cm lungime, 2cm inaltime si 2cm latime. Aceste aparate sunt mai tot timpul conectate cu pigtail-uri, necesitand fibre management, un radiator cu monitor corespunzator si circuite de control. Dimesiunea transmitatorului laserului DWDM ocupa cam de 5 ori mai mult spatiu decat cel a transmitatorului CWDM, 100cm² in comparatie cu transmitatorul CWDM 20cm².

	DWDM	CWDM
Modul:	100 cm.2 (16 in.2)	20 cm.2 (3.1 in.2)
Laser:	Laser racit 4cm. Lungime, 2cm. Inaltime, 2cm. Latime .	Laser neracit (TOSA) 2cm. Lungime, 0.5cm. Diametru.
Caracteristici:	- Butterfly package (or) - Dual inline laser package - Laser die - Monitor photodiode - Thermister - Peltier cooler	- Laser die - monitor photodiode - Mounted in a hermetically sealed metal container with a glass window.

Fiabilitate

Fiabilitatea laserelor DFB folosite in sitemele de transport DWDM si CWDM s-a dovedit excelenta in ambele tipuri de lasere (racite sau neracite). Diferenta dintre cele doua tipuri de lasere este numarul in plus de componente electronice asociate in cazul laserelor DWDM. Totusi , nu exista nici o informatie importanta in ceea ce priveste diferenta de fiabilitate intre cele doua sisteme. Analize viitoare ar putea aduce alte argumente.

Lungimi de unda

Momentan sunt disponibile sistemele CWDM ce suporta 2 pana la 8 lungimi de unda. In viitor, este de asteptat ca acestea sa poata suporta pana la 16 lungimi de unda (1290-1610-nm). Majoriatea sitemelor CWDM sunt cu distantele intre canale de 1470 pana 1610 nm si ceva ameliorari in jurul ferestrei de 1300nm.

Lungimile de unda din regiunea undei de 1400nm sufera mari pierderi de semnal optic datorita apei reziduale prezenta in fibrele instalate. In timp ce aceasta pierdere scad performantele sistemului totusi nu prezinta un obstacol major in calea implementarii sistemelor CWDM.

Standarde

S-a constituit o organizatie ce incerca sa defineasca sandardele pentru sistemele CWDM - Commercial Interest Group (1400 CIG), ai carei membri sunt furnizorii de echipamente, distribuitori de sisteme si furnizori de servicii. Obiectivul grupului este in primul rand sa defineasca graficul lungimilor de unda CWDM si sa compare arhitectura sistemelor DWDM si CWDM din punct de vedere cost/performanta.

Figura 8.1 Graficul lungimilor

de unda standard WDM.

Graficul lungimilor de unda CWDM luat in considerare este impartit in 3 benzi:

O-band (1,290, 1,310, 1,330 si 1,350 nm.) – Original (cu fereastra standard 1,310 nm)

E-band (1,380, 1,400, 1,420 si 1,440 nm.) - Extended

S+C+L-band (8 lungimi de unda din 1,470 -1,610 nm. in 20 nm. ) – Scurte, Conventionale si Lungi

Aceste lungimi de unda au avantajul intregului spectru de fibra optica incluzand si sursele optice de 1,310, 1,510, and 1,550 nm, in timp ce isi maximalizeaza numarul de canale.

Impartirea la 20 nm a canalului aduce un cost mai mic pentru componente prin folosirea laserului ne racit si filtrelor wideband. De asemenea evita pierderile majore la 1270 nm si mentine un spatiu de 30 nm pentru o mai buna izolatie a benzilor de transmisie.(a.s.v Figura 8.1).

Standardele si dezvoltarea sistemului CWDM vine intr-un moment critic pentru providerii de metro-acces. Cum cerintele pentru latimea benzii este impinsa spre limitele retelei, nevoia pentru costuri mici de tranport a retelei este imperativa.

Tehnologia CWDM raspunde acestor cerinte, oferind solutii arhitecturale scalare pentru retelele de comunicatii.

The optimization of optical fiber communications through more efficient use is of major importance in recent years because of the need to increase in network traffic, both in volume and the speed of data transmission.
The high cost of installation and operation of new communications networks recommended as a solution, is the best possible use of existing infrastructure.
Transport development in the telecom field seeks new solutions to broaden the communication channels by multiplexing.
CWDM and DWDM two technologies have developed very sharp in the last 10 years, so they become accessible and can be implemented in fiber optic communications networks, as an indisputable efficiency investment.

Introduction 1.0

Coarse wavelength-division multiplexing (CWDM) technology was first commercially deployed in the early 1980s for transporting digital video signals over multimode fiber. Quante Corp. marketed a system in the 800 nm. window with four channels running 140 Mb/sec. each. These systems were primarily used in cable TV links. However, CWDM systems did not generate significant interest among service providers - until now.
As metro carriers seek cost-effective solutions to their transport needs, CWDM is becoming more widely accepted as an important transport architecture. Unlike DWDM, systems based on CWDM technology deploy uncooled distributed-feedback (DFB) lasers and wideband optical filters. These technologies provide several advantages to CWDM systems such as lower
power dissipation, smaller size, and less cost. The commercial availability of CWDM systems offering these benefits makes the technology a viable alternative to DWDM systems for many metro and access applications.

CWDM and DWDM 2.0

The bandwidth of a fiber-optic link can be increased by transmitting data faster, or transmitting multiple wavelengths in a single fiber, with a method known as WDM (Wavelength Division Multiplexing). Wavelength Division Multiplexing Technology involves using a multiplexer to combine wavelengths traveling on different fibers into a single fiber. At the receiver end of the link, a demultiplexer separates the wavelengths and routes them into different fibers, which all terminate at separate receivers (see Figure 2.1). The spacings between the individual wavelengths transmitted through the same fiber serve as the basis for defining DWDM and CWDM (see Figure 2.2).

DWDM systems typically use one of three possible wavelength separations:

200 GHz (1.6 nm)
100 GHz (0.8 nm)
50 GHz (0.4 nm)

Future systems are projected to have even narrower spacing. The operating wavelengths of DWDM systems are defined according to a standardized frequency/wavelength grid developed by the International Telecommunication Union (http://www.itu.int).

Figure 2.2

The spacings between the individual wavelengths transmitted through the same fiber serve as the basis for defining DWDM and coarse WDM (CWDM). Spacing in CWDM systems is typically 20 nm, while most DWDM systems today offer 0.8-nm (100- GHz) wavelength separation.

Both Course and Dense architectures utilize Distributed Feedback Lasers (DFB). However, DWDM systems require the larger cooled DFB Lasers. This is because a semiconductor laser wavelength drifts about 0.08 nm/°C with temperature. DFB lasers are cooled to stabilize the wavelength from drifting outside the passband of the multiplexer and demultiplexer filters as the temperature fluctuates in DWDM systems.
CWDM systems, on the other hand, use DFB lasers that are not cooled. These systems typically operate from 0 to 70°C with the laser wavelength drifting
about 6 nm over this range. This wavelength drift, coupled with the variation in
laser wavelength of up to ±3 nm (due to laser die manufacturing processes), yields a total wavelength variation of about ±12 nm. The optical filter passband and laser channel spacing must be wide enough to accommodate the wavelength variation of the uncooled lasers in CWDM systems (see Figure 2.3). Channel spacing in these systems is typically 20 nm with channel bandwidth of 13 nm.

Figure 2.3 Coarse WDM lasers drift in wavelengths uncooled at the rate of approximately 0.08 nm/°C.

Less Expensive Hardware 3.0

The cost difference between CWDM and DWDM systems can be attributed to hardware and operating costs. While DWDM lasers are more expensive than CWDM lasers, the cooled DFB lasers provide cost-effective solutions for long-haul transport and large metro rings requiring high capacity. In both of these applications, the cost of the DWDM systems is amortized over the large number of customers served by these systems. Metro-access networks, on the other hand, require lower-cost and lower-capacity systems to meet market requirements, which are based largely on what the customer is willing to pay for broadband services.
The price of DWDM transceivers is typically four or five times more expensive than that of their CWDM counterparts. The higher DWDM transceiver costs are attributed to a number of factors related to the lasers. The manufacturing wavelength tolerance of a DWDM laser die compared to a CWDM die is a key factor. Typical wavelength tolerances for DWDM lasers are on the order of ±0.1 nm; whereas tolerances for CWDM laser die are ±2-3 nm. Lower
die yields also drive up the costs of DWDM lasers relative to CWDM lasers. In addition, packaging DWDM laser die for temperature stabilization with a Peltier cooler and thermister in a butterfly package is more expensive than the uncooled CWDM coaxial laser packaging.
The cost difference between DWDM and the CWDM multiplexers and demultiplexers contributes to lower overall system costs in favor of CWDM as well. CWDM filters are inherently less expensive to make than DWDM filters due to the fewer number of layers in the filter designs. Typically, there are approximately 150 layers for 100-GHz filter designs used in DWDM systems, whereas there are approximately 50 layers in a 20-nm, CWDM filter. The result is higher manufacturing yields for CWDM filters. In the manufacture of CWDM three-port devices, the alignment tolerances are relaxed relative to those for DWDM devices so that the manufacturing costs for CWDM devices are significantly less (see Figure 3.1).

Figure 3.1

Coarse WDM (CWDM) filter designs use fewer layers, and the alignment tolerances are relaxed relative to those for DWDM devices, which significantly lowers the manufacturing costs of CWDM devices

The CWDM filter costs about 50% less than the DWDM filter and is projected to drop by a factor of three in the next two to three years as automated manufacturing takes hold. The adoption of new filter and multiplexing / demultiplexing technologies is expected to decrease costs even further.

Power Requirements 4.0

The operating costs of optical transport systems depend on maintenance and power. While maintenance costs are assumed to be acceptable for both CWDM and DWDM systems, the power requirements for DWDM are significantly higher. For example, DWDM lasers are temperature-stabilized with Peltier
coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4 W per wavelength. Meanwhile, an uncooled CWDM laser transmitter uses about 0.5 W of power. As the number of wavelengths in DWDM systems increases-along with transmission speeds- power dissipation and the thermal management associated with it become a critical issue for board designers.
The lower power requirement resulting from the use of uncooled lasers in CWDM systems has positive financial implications for system operators. For example, the cost of battery backup is a major consideration in the operation of transport equipment. Minimizing operating power and the costs associated with its backup, whether in a central office or a wiring closet, reduces operating costs.

Physical Size 5.0

CWDM lasers are significantly smaller than DWDM lasers. Uncooled lasers are typically constructed with a laser die and monitor photodiode mounted in a hermetically sealed metal container with a glass window. These containers are aligned with a fiber pigtail or an alignment sleeve that accepts a connector. The container plus sleeve forms a cylindrical package called a transmitter optical subassembly (TOSA). A typical TOSA is approximately 2 cm long and
0.5 cm in diameter. Cooled lasers are offered in either a butterfly or dual inline laser package and contain the laser die, monitor photodiode, thermister, and Peltier cooler. These lasers are about 4 cm long, 2 cm high, and 2 cm wide. These devices are almost always pigtailed, requiring fiber management, a heat- sink, and corresponding monitor and control circuitry. The size of a DWDM laser transmitter typically occupies about five times the board area of a CWDM transmitter, i.e., 100 cm2 (16 in.2) compared to a CWDM transmitter with an uncooled laser occupying 20 cm2 (3.5 in.2).

Physical Size Table 5.1
	DWDM	CWDM
Transmitter Board Area:	100 cm.2 (16 in.2)	20 cm.2 (3.1 in.2)
Laser Footprint:	Cooled laser 4 cm. long, 2 cm. high, 2 cm. wide.	Uncooled laser (TOSA) 2 cm. long, 0.5 cm. in diameter.
Package Features :	- Butterfly package (or) - Dual inline laser package - Laser die - monitor photodiode - Thermister - Peltier cooler	- Laser die - monitor photodiode - Mounted in a hermetically sealed metal container with a glass window.

Reliability 6.0

The reliability of DFB lasers used in DWDM and CWDM transport systems has been proved in both cooled and uncooled designs. The difference between the two laser designs is the number of additional components, including the Peltier cooler, thermister, and associated electronics in DWDM lasers. However, this author has no data to substantiate a significant reliability difference between the two types of systems in real world applications. Further analysis may or may not suggest otherwise.

Up to 16 Wavelengths 7.0

CWDM systems supporting two to eight wavelengths are commercially available today. These systems are anticipated to scale to 16 wavelengths in the
1,290-1,610-nm wavelength region in the future.
Today, most CWDM systems are based on 20-nm channel spacing from
1,470 to 1,610 nm, with some development occurring in the 1,300-nm window. Wavelengths in the 1,400-nm region suffer higher optical signal loss due to the attenuation peak caused by residual water present in most of the installed fiber. While this additional loss can limit system performance for longer links, it is not a major obstacle to CWDM deployment in most metro access spans.

Standards Underway 8.0

One organization working to define standards for CWDM systems is the
1,400-nm Commercial Interest Group (1400 CIG), whose participants include component suppliers, system vendors, and service providers. The group's focus to date has been primarily on defining the CWDM wavelength grid and investigating the cost/performance comparisons of DWDM versus CWDM architectures.

Figure 8.1

The 1,400- nm. Commercial Interest Group is working to define a standard coarse WDM wavelength grid

The CWDM wavelength grid under consideration for proposal is divided into three bands:

O-band (1,290, 1,310, 1,330 and 1,350 nm.)
E-band (1,380, 1,400, 1,420 and 1,440 nm.)
S+C+L-band (8 wavelengths from 1,470 -1,610 nm. in 20 nm. increments)

These wavelengths take advantage of the full optical-fiber spectrum, including the legacy optical sources at 1,310, 1,510, and 1,550 nm, while maximizing the number of channels. The 20-nm channel spacing supports lower component costs with the use of uncooled lasers and wideband filters. It also avoids the high-loss 1,270-nm wavelength and maintains a 30-nm gap for adjacent band isolation (see Figure 8.1).
CWDM systems development and standards efforts come at a critical time for metro-access service providers. As the demand for bandwidth is pushed to
the edge of the network, the need for low-cost transport systems is imperative. Today's CWDM technology fits these requirements, offering a scalable system architecture for metro and access networks.