FlBEReWORLD BNR(I WDM Technologies in Telecommunications May 9, 1995

Contributors: CKelly, Greg May, Peter Roorda. D.Barriskil1 Issue: 1

Abstract: The principle of WDM has long been established within the academic arena. However i t i s only recently that advances in fabrication, material science and technology has made the application of WDM economically viable within the telecommunications in- dustry.

The drive towards WDM applications is motivated by a number of commercial and economic factors. These range from cost of installed fiber, issues regarding fiber right of way, to the increasing demand for bandwidth as the emerging broadband technol- ogies of ATM, Frame Relay and video on demand, consume the currently available bandwidth.

In this environment WDM is seen as a viable technique to increase the available band- width of the currently installed fiber base.

The advent of the optical amplifier technologies is closely coupled with the viability of WDM in the market place. With launch powers of exceeding lOdBm the initial drawback of insertion loss are largely overcome.

NT has been active in developing WDM technology. Products like the telecommuni- cations networkl6X, a 2.4GbitIs SDH ring system currently offer WDM. In WDM applications the capacity of a link may be doubled from 2.4Gbit/s to 4.8GbitIs. With high power transmitters a commercially viable 4.8Gbit/s link with spans exceeding lOOKm is possible. Another leading edge product offers lOGbit/s capacity at a single wavelength.

This paper discusses the main market drivers for WDM. I t also examines the advances of technology and materials, both of WDM and optical amplifiers that make the com- mercial application of this technology viable. Noise levels, ASE limitations, and laser material technologies are all considered. The paper deals with the practicalities of the technology, how technical problems are overcome and presents proven performance of WDM technology in Northern Tele- com products.

0 1995 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WCZR OBL. UK.

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FIBER~WORLD BNR(I 1 .O Introduction

Historically the telecommunications network supported principally telephone services (Plain Old Telephone Services, POTS), fax and limited corporate private networks. These were initially carried on analogue lines. With the advent of digital technology the telecommunications industry was quick to recognize the advantages of digital rransmission and Time Division Multiplexing(TDM). As the capacity required grew a standard Pleisyochronous Digital Hierarchy (PDH) evolved. The PDH provided bit rates for trunk transmission of up to 14OMbiVs (or 565 Mbit/s) and a standard multiplexing structure from 2MbiVs to 14OMbiVs. PDH also incorporated limited alarm reporting and management structures.

Today the telecommunications network looks very different. POTS stil l represents a significant proportion of the baseline load in any telecoms network however we are witnessing an unprecedented growth in the number of services available and bandwidth required over the telecommunications network.

Today, as in the past, the evolution of the telecommunications network is technology driven. The growth in new services such as broadband, Cable TV, distance learning, Interactive video etc. has arisen due to evolving technologies ability to deliver such services at economic costs.

These evolving services demand an increasingly flexible telecommunications network which is capable of providing high capacity, and reliable communication channels. The key elements in any telecommunications network are its capacity, survivability, dexibfity and expandabfity. These must be supported by a powerful control and monitoring system. The telecommunications network is evolving new topologies and technologies in order to meet these demands. To be successfully deployed within the telecommunications network WDM must be able to integrate effectively into these evolving networks.

2.0 Network Topology

The telecommunications network may be viewed as a layered network. The requirements and objectives of each layer differ. It is therefore important to examine the objectives of each layer as these shall govem the application of WDM within the particular layer.

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FIBER~WORLD BNRBI Figure 1 shows a layered network.

FIGURE 1. A Layered Network

service Layer

Transport Layer

(Tier1 &Tier2)

Access Layer

The layered network consists of a top service layer which is supported by a transmission layer. The transmission layer itself is subdivided into a number of layers or tiers.

Tier One

This is the backbone of the network. Its function is to provide high bandwidth interconnect.

The key requirements for this layer are

a) Capacity. Tier One provides the interconnect between the main centers of population. It also provides access to the main crossconnect sites, national switch centers and international switch centers. Tier One typically consists of the highest capacity links in the network. As the network grows so too must the capacity of the Tier One interconnect. The advent of broadband technologies has led to an almost exponential increase in expected bandwidth demand (Figure

It is expected that WDM shall provide the means to cost effectively expand the capacity of Tier One links beyond the limits of conventional electrical TDM technology without the need to

2 1.

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install new cable or duct routes. In the medium term optical routing technology is expected to provide significant cost benefits as the number of electro-optical conversions required falls. Interestingly the current trend towards deregulation has made the installation of additional Tier One capacity more expensive for established network operators. This is due to the increased competition for rights of way between operators. In addition many industries with established right of way are establishing their own competitive networks i.e. Electricity, Rail.

FIGURE 2. Expected Capacity

1 50 G bls

B W required 1 00 Gb/s

50 Gb/s

20 Gb/s

on largest routes

today TIME 2000

b) Survivability. A recent survey carried out by Northern Telecom of operators placed survivability as the most important characteristic of their network (see Figure 3 ). WDM and other optical technologies must provide an integrated solution within a network which will ensure high survivability within the network. The issues of survivability within networks employing WDM is dealt with in section 4.0 on page 13.

FIGURE 3. Operators Requirements Survey

End-customer Willingness to Pay' Percent ihcrease in I eiecom Tor siF vivability Customer Rank of Top Three

w Telecommu nicanon neq uiremeRts Reliability Operating Performance of use

cost

1 1 1 . 2 0 P.1S.m

Dnl Kmw

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FIBER~WORLD BNR4I Reach. The Tier One network is generally intercity links and the span of each link is generally expected to exceed the performance of current optical devices. Regenerators hence contribute a significant proportion of the expense of Tier One networks. The reach achievable in a single span is limited both by loss and dispersion effects (and non linear effects in very long haul or high launch power conditions). Optical postamplifiers and extemal modulators may extend the length of a single span on normal fiber, while optical line ampliiiers significantly reduce the need for electrical regenerators. Because of the increased spans available WDM becomes an increasingly attractive prospect as its coupling losses become less significant with respect to the total link budget. In addition optical amplifien are not wavelength dependant removing the need for independent electrical regeneration of each of the component wavelengths.

Control & Monitoring. SDH as well as providing standardized high bit rate transmission formats provides very extensive Operation And Maintenance (OAM) facilities for today’s telecommunications network.Telecom operators place great importance on the performance monitoring of the network. This will grow as the capacity of individual rates increases. The SDH format dedicates a large proportion of the SDH overhead to control and performance monitoring of regenerator sections. Optical devices such as WDM splitters and line amplifiers must be designed from day one to incorporate control and monitoring features. Northern Telecoms optical products are addressing these issues.

Expandability. Having learned from the expensive mistakes of the past telecommunications network provides shall expect network expansion to be catered for from day one. The expandability of the network must be catered for both in term of OAM and capacity. The operators current plans and potential future plans should be catered for. For example if an operator is installing a 2.4Gbit.k link and expects the capacity to grow in the future he should take into account the expected spans of a lOGbit/s link when deciding on site locations for regenerators or optical line amplifiers. If costs are an important issue and full crossconnect of lOGbit/s is not required he may choose to upgrade via a WDM route rather than by upgrading line terminating equipment to lOGbit/s. In the former case the loses of WDM couplers should be included in link budget calculations.

Flexibility. Flexibility is always an important element for a telecommunications network. It is closely coupled with the degree of OAM provided. As the tiers are descended towards the end user flexibility and the ability to provision Network Element (NE) services quickly and efficiently increases. Flexibility, particularly the ability to fully crossconnect higher bit rates (such as VC4 or 155MbiVs) across the entire bandwidth is important to Tier One. As the capacity of the Tier One links increases a trade off occurs between the ability to access the full bandwidth

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FI B E R m WORLD BNR(I Current networks are comprised of LWs which generally can access the full link capacity. As capacity increases the economics of network design employing such devices becomes less certain. Undoubtedly at some nodes full capacity access is required but at intermediate nodes only partial access may be required. In addition the management of such a large distributed crossconnect becomes extremely expensive. New design methodologies shall emerge to s i m p w network design and control (section 5.0 on page 16)

Tier l k o

This is the intermediate layer of the network. It is typified by bandwidths from 2Mbit/s to 622MBits/ s (STM-4). depending on network size. In this layer the emphasis shifts from capacity and reach towards flexibility and control. Electrical TDM techniques are well suited to this layer. They provide flexibility, powerful OAM and are cost effective for current bandwidth requirements. It is not expected that WDM will be deployed in Tier two in any significant proportion.1t may find application in areas suffering from fiber exhaust.

Tier Three (Access Layer)

WDM was expected to provide an economic means of providing high bandwidth in the local loop. This was to be achieved by removing the need to install new cables and ducts as these constitute a very significant percentage of the cost of the entire telecommunications network. Experimental work has established the feasibility of using WDM to provide high bandwidth in the local loop.

However the access layer requires powerful flexibility. The emphasis is on supporting a multitude of services and provisioning new services in a matter of minutes. It is a dynamic layer. WDM does not offer this flexibility. The initial experimental work was based on a static model of the access layer. Bandwidth on demand is the term that describes today's access layer. Other electrical technologies such as the Passive Optical Network (PON) and Asynchronous Transfer Mode (ATM) have evolved which can provide this flexibility. These employ TDM techniques to minimize the required fiber installation costs. WDM may still be expected to find some application for static loads such as CATV. In the future as local services require bandwidths exceeding the current electrical technologies WDM (or OTDM) is expected to again play an important part. This deployment however will require development of optical switches, and the issues of control and OAM of this maintenance layer to be addressed.

It is therefore expected that the deployment of WDM shall be best suited in the short to medium term to Tier One of the telecommunications network.

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BNRQs Northem Telecom has been active both in the initial research into these technologies and in the development of optical technologies into their product set. These products have been designed to specifically address the requirements of Tier One of the telecommunications network. In Europe Northem Telecoms current flagship transmission product is the TN16X.

TN16X is a 2.4Gbit.h Add Drop Multiplexor which conforms to the standards of the Synchronous Digital Hierarchy(SDH). The product offers high bandwidth transmission coupled with crossconnect ability and supports several low speed interface formats. The product addresses the survivability and control requirements of Tier One by supporting a number of Werent topologies and protection formats. These include both ring topologies (protection via SPRING and matched node protocols) and linear topologies (protection via 1+1 an 1:n). Element and network management is provided over embedded communication channel in the SDH frame structure.

3.0 TN16X WDM and Optical Performance

TN 16X has met the Tier One demand for greater reach and bandwidth through ongoing optical device development. Recently introduced into its product portfolio are a switchable optical post amplifierlline amplifier and a high performance transmitter(HPT) which incorporates a post amplifier and external modulator. By incorporating these optical devices into the Fiberworld product set many of the control and management problems of this new technology are addressed.

3.1 System Applications

For simplicity only 1550 nm optics shall be considered, however 13 lOnm lasers and crossband WDM are available as an option. With 1310nm optics EDFA amplification (Post amplification or line amplification) is not possible.

Figure 4 shows the reach over normal single mode fiber(Sh4F)

FIGURE 4. Normal TX -Standard Fiber

The 8OKm link budget is calculated as per Table 1 (Appendix A).

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BNRQW This reach may be sigmiicantly increased using HPT’s and optical line amplifiers. When replacing existing electrical regeneration with optical line amplifiers the existing 80km span may be used. However for new installations a line amplifier spacing of 92km may be achieved. The overall distance before electrical regeneration is required can be extended to 460 km. This is limited by ASE accumulation.

FIGURE 5. Extended Reach on N o d Fiber

460Km

The production of Northem Telecom’s 2.4Gbitls 80Km reach systems was achieved via the worlds first Strained Layer Multi-quantum-well DFB laser in volume production.

FIGURE 6. Structure of SLMQW DFB 2.4Gbitk Leser

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FI B E RI@ WORLD BNR(I WDM couplers typically have an insertion loss of 2dB per coupler. The additional loss of 4dJ3 per span would reduce the spacing by 32Km typical. Figure 7 shows such a bidirectional WDM application on normal fiber. Specific calculations are included in Table 2 (Appendix A).

FIGURE 7. Bidirectional-WDM Application with standard optics on normal fiber

64Km Spacing

In order to overcome the limited reach and as part of its ongoing optical evolution TN16X offers optical post amplifiers, optical line amplifiers and external modulators.

Figure 7 shows the improved performance using HIT’S. These HPT’s incorporate an optical post amplifier and an external modulator in one card. Together these give high launch power and low chlrp which can considerably improve the link budget even on standard fiber.

FIGURE 8. Bidirectional-WDM Application with HPT optics on normal fiber.

0 SMF

The quoted span is calculated on the basis of the currently available +9dBm H E .

High Per

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The reach of WDM systems may be further extended with the use of optical line amplifiers. The optical line amplifiers are inherently unidirectional in nature. However using four port WDM couplers bidirectional amplification is possible.

Unidirectional WDM does not require intermediate four port WDM splitters and the distance between electrical regeneration may be increased. However for the survivability considerations mentioned earlier Northem Telecom does not recommend such an application

FIGURE 9. Unidirectional WDM with Line AmpWers (+15dBm) and HPT

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3.2 Physical Characteristics

The HPT consists of an electrical to optical convertor in the form of a External Modulator assembly. This is then followed by an EDFA amplifier. Figure 10 details the assembly.

FIGURE 10. Block Diagram of HPT

U

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The optical post amplifier or optical line amplifier are the same card. Software provisions the particular application and controls the optical gains accordingly. Figure 11 shows a Line amplifier.

U

3.3 Operation and Maintenance ( O M )

The importance of operation and maintenance functions within the telecommunications network has been emphasized earlier. Electrical regenerators have a dedicated area of the traffic overhead with which they communicate and report alarm and status information to a network manager. They also accept control information from the network manager.

As optical devices are integrated into the network is important that all the aspects of survivability, control, performance monitoring etc. expected of Tier One equipment is preserved so that they may be seamlessly integrated into the telecommunications network. Currently the WDM devices are essentially dumb. This will change in the future as WDM with optical switching capabilities are developed.

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FIBER~WORLD BNR4I The optical amplilier and HPT however are intelligent devices and are provided with communication and control channels.

For example the following information is measured by the TN16X Optical LinePost Amplifier.

Wavelength for each signal component.

Total input power Input signal power to total input power ratio for each signal component

Total output power Output signal power to total output power ratio for each signal component

In addition the on board UP controls biasing of the Extemal Modulator and the EDFA (output power provisioning) and dithers the Continuous Wave (0 laser. It is also intended to use firmware capabilities to adjust al l DC parameters, hence eliminating the use of potentiometers on board. This allows automation of the factory calibration and test procedures.

The ability to measure the input signal power, noise levels and output power of each individual wavelength present in the system is important for performance monitoring and rapid fault isolation. All these parameters are continuously monitored throughout the optical path and are reported to a remote network manager. Hence the optical performance of the optical links can be accurately monitored from anywhere in the network.

The intelligence of these devices allows remote provisioning of power levels and full control and supervision. This facilitates their incorporation into the Tier One network and by providing localized processing power and firmware download facilities ensure the devices ability to cope with future evolution.

4.0 Protection Schemes and WDM

It is important the WDM is effectively integrated into the protection philosophy of the network.

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4.1 1+1 or 1:l Protection

1+1 and 1:l protection are commonly used protection schemes in the tn. The TN16X LTE supports 1+1 or 1: 1 on a single LTE. Figure 12 shows a typical configuration employing WDM in a protected network. Note that each fiber must carry bidirectional traffic such that no single point of failure exists. In 1+1 protection the traffic is automatically bridged at the head and. A tail switch at each LTE selects either traffic from the working or the protection fiber, depending on local signal conditions. Hence the same traffic is carried in both paths. The switch to the protected fiber may be either unidirectional or bidirectional as configured.

1: 1 protection behaves slightly differently. In this instance the traffic is not permanently bridged at the head end and hence the protection bandwidth is available for extra traffic. Under fault conditions a signal is sent to the head end and the traffic is then bridged to the protection, dropping any extra traffic. The tail end then follow s the switch to the protection. Unidirectional or bidirectional switches may be specified.

It may be seen that the presence of the WDM couplers in the network is transparent to the LTE. Unidirectional or bidirectional switches may still be selected. However it should be noted that as a single fiber carries both directions bidirectional switches will always occur.

FIGURE 12.1+1 or 1:l Protection with TN16X LTE

I I I I I I I I I I I I I I I I I I I I I I I

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4.2 l:N Protection

1:N protection with bidirectional two wavelength WDM is similarly transparent to the LTE's (Figure 12 ). However for multiple wavelength WDM the situation becomes more complex. In this instance a m :n (where men) may be used. The m:n is a relatively simple means of handling the four wavelength as it does not require any change to the l:n switch protocol.

FIGURE 13.l:N STM-16 Line Protection with WDM

.-------- I I

I I I

J I I

I 2

PLOOP PLOOP

L 1 5 3 3 b

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...

5.0 Future networks and Ongoing Development

It is expected that within the next year several lOGbit/s SDH terminals shall become commercially available. Northem telecoms TN64X shall be one such product. These products have pushed the limits of conventional TDM technology. They provide high bandwidth links without the disadvantages of WDM (i.e. they provide crossconnect & Add/Drop ability across the full lOGbit/s). Figun: 14 compares a lOGbits WDM system with a lOGbit/s TDM system.

FIGURE 14. Application drivers for lOGbitls LTE over WDM lOGbitls

- LTE-WDM fiber Reduction - 4:l NE Reduction - B/W Mgmt across larger pipe - Simplified Comms to NM - Many Protection Schemes

Beyond lOGbit/s electrical TDM techniques become uneconomic as current device limits are approached.

Undoubtedly WDM shall play an important part in advancing the bandwidth of transmission links beyond the lOGbit/s band.

Other optical techniques such as coherent detection and Optical Time Division Multiplexing (OTDM) are also under development.

Each of these technologies has the capability of extending bandwidth and shall undoubtedly find application in some markets. The technology, however, that shall become the congressmen of the leap to terrabit transmission shall be the technology which is best suited to the requirements of Tier One of the telecoms network.

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In the future terrabit transmission shall only be achieved by an optical path layer which shall reside above the current Tier One of the telecommunications network. In the short term this layer shall be relatively passive and shall depend upon electrical signal processing for protection switching, embedded communication and control. It would seam reasonable however to assume that as capacity increases that a whole new hierarchy the optical transmission hierarchy shall exist above the current SDH hierarchy.

FIGURE 15. Optical Path Layer

Optical Collector Multiple Plpe(signgle h) h Interconnect

wavelength management and gigabit service

carriage

rcuit traffic management and aggregation

future services

Optical path layers shall be defined depending on the transmission rate and supervisory and control information shall be associated with each layer. This shall enable autonomous protection and performance monitoring of each optical layer.

Unlike todays TDM based telecommunications network, the optical network of the future shall be a non-linear network. Performance of the network cannot be guaranteed by simple definition of accumulated jitter at network interfaces. Each optical path shall perform differently depending on span, number of wavelengths, number of line amplifiers etc.

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FIBER~WORLD BNR4B The telecommunication network must be analyzed as an analogue network. Protection of each layer shall require optical switches and adaptive amplifiers which can provision their optical power for the currently active span. Accumulation of dispersion and noise effects must be carefully controlled. The number of possible contigurations of an optical network under single and multiple fault conditions becomes vast as autonomous protection schemes seek to restore the network. In an attempt to restrict the permutations optical protection boundaries shad be defined and each potential route shall be verified or simulated. This opens a whole new area of analogue network design.

As complexity increases limitations on the flexibility of the Tier One interconnect shall be imposed by new traffic management philosophies. These shall be imposed in an attempt to keep the control and management of the network to a manageable level. Intermediate nodes shall no longer provide access to the full link capacity instead collector pipes shall be used to collect ‘local’ traffic. These pipes shall be relatively static and shall be comprised of one particular wavelength. The pipes shall return the collected traffic to a major node (VC4 crossconnect) for consolidation and grooming. Provisioning of these pipes shall be achieved via wavelength routing devices and shall be a relatively rare event.

6.0 Conclusion

The initial drive to WDM shall be motivated by the need for increased capacity extending beyond current electrical device limits. TN16X represents a product that is addressing that need today. Seamless integration of WDM and optical devices into the network is achieved by integrating these devices into the OAM and protection structure of the network. Northem Telecoms optical products are integrated into the product set and network topologies. Performance monitoring and fault isolation problems have been addressed through innovative design and intelligent devices. These monitor and remotely report the performance of any optical path on a per wavelength basis.

In future a new optical layer shall evolve. This shall initially be passive but shall migrate to an active layer with autonomous protection schemes and the ability to route optical path through the network. A whole new methodology of network design and verification shall be required to support such a layer.

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FIBER~WORLD BNRdI

Parameter

Minimum Transmit Power (P~, , )

Value Units -1.5 dBm

Minimum Received Power (Pw,,)

Maximum Path Penalty@ispersion<l5oops/ "Inax) System Gain

-27.2 dBm

2.1 dB

23 dB

~~ ~ ~~

Table 2 WDM 1533/1557 Direct Modulation on Normal Fiber

Recommended Link Margin

Good Quality Fiber Attenuation

Maximum Attenuation

Reach

3.0 dB 20.0 dB 0.25 dBm

80 Km

Parameter

Minimum Transmit Power (PT~, , )

Value Units -1.5 dBm

Minimum Received Power (Pw,,)

Maximum Path Penalty(Dispersion<l500ps/ W(PO-) WDM Losses l y 2

1.Link Optical return loss <-24dB 2. Includes O.ldB crosstalk penalty

-27.2 dBm 2.7 dB

4 dB

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System Gain

Recommended Link Margin

Maximum Attenuation

19 dB 3.0 dB 16.0 dB

Good Quality Fiber Attenuation

Reach

0.25 dBm

64 Km