WDM-upgraded PONs for FTTH and FTTBusiness

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equipment and the feeder section (with the fiber installation cost) are shared ... offers already today a large cost advantage compared to new fiber installation or.
WDM-upgraded PONs for FTTH and FTTBusiness C. Bouchat 1, C. Dessauvages 1, F. Fredricx 1, C. Hardalov 1, R. Schoop 2, P. Vetter 1 1

Alcatel Research & Innovation F. Wellesplein 1, B-2018 Antwerp, Belgium Phone: +32-3-240 8579, [email protected] 2 Eindhoven University of Technology, The Netherlands Introduction It is expected that the ITU-T G983.1 and G.983.3 recommendations for broadband optical access network (BPON) will accelerate the installation of fiber in the access network. These recommendations allow the simultaneous deployment of bi-directional digital data and of extra services by using wavelength division multiplexing (WDM) overlay techniques. Such extra services can be of a distributive nature, like broadcast community access television (CATV), or dedicated connections, like leased wavelengths for particular users. In this paper, we present an analysis at the physical layer level of the extra wavelength band of BPONs (the socalled enhancement band) for WDM upgrades. The first considered service is video distribution based on low-cost coarse WDM (CWDM) components for the residential market. This service is already offered in Alcatel’s 7340 fiberto-the-user (FTTU) product. The second service is a set of dedicated bi-directional links on separate wavelengths, offering leased wavelength applications for particular users. The emphasis of the latter will be placed on the business segment, in order to allow the provisioning of high bandwidth services with transparency to transport format and services (e.g. IP, STM-x, Gigabit Ethernet, optical packets, SCM channels), and to enhance the privacy. The use of a shared and generic infrastructure implies a reduction of the costs inherent in the deployment of isolated point-to-point links. In addition, novel WDM technologies are becoming available at a lower cost and enable novel optical access architectures. Economics The drive to add capacity in the core network has been replaced by a need to make this vast capacity available to end customers. Despite the resulting increase in long-haul capacity, residential and business customers continue to face a bandwidth bottleneck in the metro/access networks. Fiber deployment is projected to shift from long-haul networks to metro-access networks over the next several years. ATM PON (APON) and more in particular Broadband PON (BPON) has proved its economic viability for residential users, as it allows to offer the triple service bundle of data, voice and video over a single infrastructure. Its shared character and the lower operations and maintenance costs associated with a purely passive fiber plant are the determining advantages. When we compare the cost of a PON system with the cost of point-to-point links for business users, the same strong arguments in favor of the PON system can be pointed out. On one hand, the costs of the line termination (LT) equipment and the feeder section (with the fiber installation cost) are shared between the users. On the other hand, at the central office, one single LT board is used for several users (typically 16 or 32, potentially even up to 64) instead of as many boards for point-to-point links, leading to an important cost reduction in terms of room occupation (rent, temperature maintenance,…) and operation and maintenance (connecting the fibers, localization of the user,…). A study made for the provisioning of telecommunications services to medium business customers and large business customers in an industrial park area has shown that APON upgrades have a cost advantage over SDH-alternatives in high capacity areas, in terms of investment costs and operation, administration and maintenance (OAM) costs [1]. WDM has the capacity to overcome time division multiplexing limitations in case of unexpected traffic growth, which may appear quite rapidly when several bandwidth-hungry users are connected on the same network. In case of a bottleneck in the infrastructure, WDM offers already today a large cost advantage compared to new fiber installation or leasing of fibers for point to point links. For large business customers with a need of their own physical link, WDM offers an economic alternative to conventional SDH ring solution. When we compare the cost of a dense WDM (DWDM) upgrade of an existing PON system with the cost of point-topoint links, we could first have the impression to deal with a black and white situation. As a matter of fact, the only part

which is shared between the users in DWDM is the feeder section. At the central office, we will have, as for the pointto-point links, as many LT boards as upgraded users. In the access network, fiber installation cost is the most significant cost, far above the component costs. In a DWDM upgrade PON approach, the price of the feeder section from the installed PON has already been allowed for depreciation. While in a point-to-point connection, we should take into account the dark fiber price (investment cost at the installation). Also, for a PON user, the operator can upgrade the PON with the DWDM solution from the central office. While for a point-to-point connection, there is a need to go into the field reconnecting the feeder with the drop section. This part could be costly, depending on the location and the access facility of the connection point. CATV overlay A

Implementation

The distribution of CATV signals is an essential part of the service bundle for fiber-to-the-home (FTTH). There are two possible technical implementations for delivering this downstream signal next to the bi-directional data signals. The frequency division multiplexing (FDM) approach uses a single optical network termination (ONT) receiver for both and requires the CATV and data signals to be well separated in the electrical spectrum. This either limits the possibility of downstream bit rate increases, or requires an anticipative shifting of the carriers to high frequencies, which seriously impacts the design of the CATV receiver. Moreover, the FDM approach is incompatible with the high requirements for analog CATV reception. On the other hand, with the WDM approach, the CATV signal is carried over a separate wavelength and detected by an individual receiver. This requires the use of additional optical filters, but as both signals are now optically separated they are independent in electrical frequency. Moreover, the dedicated CATV receiver does not require extensive electrical filtering and so allows for analog CATV. For these reasons, we have developed and characterized a WDM solution for CATV. A cost-effective implementation is integrated in Alcatel’s FTTU product. The WDM approach raises several technical impacts on the physical layer which must be thoroughly addressed in order to ensure both a correct CATV reception quality and to preserve the PON traffic from any perturbation. In order to guarantee signal quality, appropriate WDM filters are key components. They must ensure sufficient isolation between both signals (wavelength bands) and at the same time be as transparent as possible for the receivers at their respective wavelength bands. The demanding requirements for correct CATV reception and the high CATV optical powers involved determine the required isolations and insertion losses.

Access Node

ONT

CATV Head-End CW LD

MZI

EDFA WF2

WDM

Local TV

Tx Rx

CATV Rx

NT Logic I’faces

Cable TV

CC

OLT 1 Tx

WDM

LT Logic

Rx

WF1

PSTN Internet

... OLT N

Figure 1. CATV by WDM overlay for BPON

The main physical layer blocks of the BPON network with CATV overlay are depicted on Figure 1. At the central office (CO), an additional optical head-end injects the CATV signal into the optical distribution network (ODN) via a WDM filter or a 2xN coupler. The head-end consists of a continuous wave laser diode at 1550 nm, externally modulated with the signal by a Mach-Zehnder Interferometer (MZI), and followed by an erbium doped-fiber amplifier (EDFA) for increasing the optical power to 17 dBm. At the network termination (NT), the CATV signal is optically demultiplexed and detected by a CATV receiver. Both analog and digitally modulated channels can be viewed (the latter with a set-top box). The up- and downstream data paths are carried in different wavelength bands (respectively in 1260-1360 nm and 1480 – 1500 nm), and are also separated by WDM filters.

B

Signal performance

In order to guarantee a correct signal quality, the CATV signal after reception at the ONT should respect several quality parameters. The first is the carrier to noise ratio (CNR), which expresses the ratio of the signal power (unmodulated channel carrier power) to the total noise power. In order to guarantee a good CNR value, the receiver should have low intrinsic noise and the detected signal power should be sufficiently high to dominate this power. The required power increases with the amount of channels (decreasing optical modulation index) and for analog channels the CNR requirement is higher than for digital channels. This is the reason for the high required CATV optical power at the ONT, and hence the need for the EDFA. Please note that if the signal is limited to a small amount of purely digital channels, the EDFA could be avoided. Other noise contributions are the PIN-receiver shot noise, the laser RIN noise and the signal-ASE noise generated by the EDFA. And last but not least, any leaking data signal has also to be considered as noise and will cause a degradation of the CNR. The other quality parameters, Composite Second Order (CSO) and Composite Triple Beat (CTB), concern the non-linear distortions. Similarly, the overlay of the extra wavelength band should not interfere with the data transmissions. The quality parameter is expressed here in Bit Error Rate (BER), with an acceptable limit of 10-10. Any leaking CATV signal will introduce noise contributions in the data receivers and will degrade the BER. C.

Measurements on CATV signal

A characterization was carried out in order to determine the required isolation values for the ONT WDM (WF2). Two values are determined by the CATV reception. Given the fact that analog channels are the most sensitive to noise, we performed measurements on an amount of analog carriers. A test set-up was mounted which allows to generate an arbitrary modulated or unmodulated video carrier and to change independently the power of the data signal and of the CATV signal. In order to analyze the worst-case situation, the ONT was considered at the maximal ODN loss for a Class B system (25 dB). The main parameter under investigation is the CNR. The considered optical modulation index (OMI) is 0.04, which corresponds to 80 channels. Starting at a received power of –9 dBm, a CNR of 46 dB is achieved. The CNR degrades with increasing data levels, and the limit of 43 dB is reached at –41 dB. Figure 2.a illustrates the agreement between the measurements and the simulation.

Figure 2.a: CNR degradation in function of downstream data signal power

D.

Figure 2.b : BER degradation in function of CATV power (OMI = 0.015)

Figure 2.c : BER degradation in function of CATV power (OMI = 0.04)

Measurements on data signal

The downstream data path was also monitored by measuring the BER at the worst-case ONT (ODN loss of 25 dB) in function of the CATV signal level. As can be seen from Figure 2.c, the initial BER of 10-11 degrades with increasing CATV signal level, and crosses the limit of 10-10 at –38 dBm (respectively –30 dBm) for Class B (respectively Class C). This result then determines the third minimal isolation value of the WDM WF2. E.

CATV in BPON

The experiments have shown that the equivalent of 80 analog channels can be transmitted by WDM overlay in a Class B BPON. This determines the quality of the ONT WDM filters. These channels can also be shared out amongst a number of analog and digital channels, by applying a back-off for the OMI of the digital channels. A Class C system will impose more limitations in terms of amount of channels or in optical reach. A triplexer implementing all optical blocks into a single component represents a cost-effective ONT implementation.

DWDM overlay A.

WDM upgrades

The most radical way to upgrade the capacity with DWDM consists in replacing the entire system (both LT and NTs), while keeping only the deployed fiber plant. A WDM router replaces the passive splitter in the field, and there is no APON signal anymore. It requires a single splitting stage in the network, or in case of multiple stages it implies the rearrangement of the drop fibers. This is not considered further in this article, as we are interested in evolutionary approaches. A more flexible way consists of a WDM overlay over an APON system. In those systems, the LT generates both the PON signal, in the basic band, and the DWDM channels that are multiplexed, in the enhancement band. All signals are sent in the feeder section through a CWDM multiplexer. Three different types of such overlays are described hereafter. -

Separate APON and WDM network. The user has to choose between a WDM and an APON connection. The splitter is replaced by a multiplexer separating all the WDM channels and the APON signal, with a bypass for the upstream APON signal (see Figure 3). WDM channels

To the LT

:

To W D M ONUs

: To the LT

:

To A PO N ONUs A PO N upstream

Figure 3. Separate APON and DWDM network.

:

To the ONUs

APON upstream and APON downstream

Figure 4. Advanced WDMPON upgrade.

-

Advanced WDMPON upgrade. Each user has both a WDM connection and an APON connection. The splitter is replaced by a multiplexer separating all the WDM channels (each multiplexed with the APON signal), and a bypass on each link for the upstream APON signal (see Figure 4).

-

WDMPON based upon a power splitter and DWDM filters. The passive splitter is kept and all the signals (APON and WDM channels) are broadcast to all NTs (see Figure 5). This configuration has the advantage not to replace the splitter (lower upgrade cost). There are two possible ONT configurations: either it accepts both the PON and the DWDM signals (as the second branch on Figure 5), or it accepts only one of the two (as the first and last branches on Figure 5).

We have studied in detail the DWDM upgrade of APON based upon a power splitter. We have chosen this evolution because it is the most promising in terms of flexibility. The new users and/or the users requiring a higher bandwidth can be added to the existing system one by one, and it does not require any work in the field. B.

DWDM overlay PON based upon a power splitter

Figure 5 shows the architecture of a DWDM upgrade PON based upon a power splitter. At the central office, the LT is the interface between the core network and the access network. For the DWDM branch, signals at different wavelengths belonging to the ITU-T grid, situated between 1539 nm and 1565 nm (ITU-T G.983.3), also called the Enhancement Band, are generated and multiplexed via a wavelength multiplexer. For the upstream part, the signals coming from the different DWDM NTs via the feeder section are demultiplexed and sent to receivers. The DWDM upstream signals have different wavelengths belonging to the Enhancement Band. The wavelengths of the upstream signals are different from the ones of the downstream signals. The PON LT board generates the PON downstream signal at a wavelength situated in the Basic Band (between 1480 nm and 1500 nm), as for the case with CATV overlay. The PON downstream signal is multiplexed into the feeder section with the other DWDM signals via a CWDM multiplexer noted WF1 in Figure 5.

At the NT side, a CWDM demultiplexer, noted WF2 in Figure 5, separates the Enhancement Band from the PON signals (called the Basic Band). For the DWDM branch, a DWDM filter select the downstream signal destined to the specific ONT. The PON ONT board generates the PON upstream signal (situated between 1260 nm and 1360 nm). PON Branch

WF2

1310 nm 1490 nm

1310 nm

APON Tx APON Tx

APON APONTx Tx APON APONRx Rx

CWDMAA CWDM

APON APONRx Rx

CWDMBB CWDM

Blocking filter for Enhancement Band

1310 nm

WF1 WF1

APON Tx APON Tx APON APONRx Rx

DWDM filter WDM Rx DWDM filter WDM Rx Enhancement Band

WDM WDMTx Tx

Enhancement Band DWDM Branch WF2

WDM WDMRx Rx

Multiplexer / demultiplexer Multiplexer / demultiplexer

WDM WDMTx Tx

WF2

1490 nm

CWDMBB CWDM

1490 nm

DWDM DWDMfilter filter WDM WDMRx Rx WDM WDMTx Tx

Figure 5. DWDM overlay PON based upon a power splitter.

C.

Analysis method System parameter

An important system parameter to be computed for the DWDM part is the dynamic range of the ODN. The dynamic range of the APON part is of course already defined in the standard ITU-T G.983.1. We have fixed the minimum ODN loss at 10 dB. This value has been standardized for an APON Class B. The minimum ODN loss having been fixed, the relevant parameter to compute is then the maximum loss of the ODN. The maximum ODN loss is limited on one hand by the power budget of the upstream and downstream paths, and on the other hand by the linear crosstalk in upstream, from the demultiplexer, being here an array waveguide grating (AWG). Due to the inability of the AWG to perfectly isolate a channel, all the other channels will contribute to the interference noise. Because the DWDM channels of the upstream of a PON travel through the drop section (different losses depending on the branch), the contribution of the channels to this noise could be higher than the signal. A signal to interference ratio (SIR) higher than 7 dB gives a power penalty due to the linear crosstalk smaller than 1 dB [2]. For safety, we require a SIR higher than 10 dB. The worst-case situation is considered. The value of insertion losses, like for the CWDM filter, can vary from zero to the maximal value announced by the component’s specifications. To compute the SIR in the worst-case scenario, the optical power of the effective signal at the transmitter (Tx) is taken minimal and the losses it undergoes are taken maximal. For the signals interfering with the effective signal, the optical power at Tx is taken maximal and the losses minimal. In worst case, there are two adjacent channels. The limitation of the ODN maximum loss due to the power budget is computed considering the minimal transmitted output power, the sensitivity of the receiver (Rx) and the insertion loss of the CWDM multiplexer/demultiplexer, the AWG and the DWDM filter, for both upstream and downstream paths. The limitation of the maximum ODN loss due to the linear crosstalk in upstream is computed by solving the equation of the SIR for the maximum ODN loss. The power of the total interference is the sum of the received power at the DWDM LT Rx from the two upstream adjacent channels, all the upstream nonadjacent channels, the downstream

WDM channels that are reflected by the AWG and those that are reflected by the ODN. The power of the effective signal involves the output optical power at the DWDM ONT Tx, the insertion loss of the CWDM multiplexer/demultiplexer, the DWDM filter and the AWG, and the maximum ODN loss. Component requirements Another goal of the computations is to determine the component requirements, like the ONT DWDM filter isolation and the CWDM demultiplexers (WF1 and WF2) isolation to ensure a good separation between the Enhancement Band and the Basic Band. For the APON part of the system, the CWDM demultiplexers WF1 and WF2 from Figure 5 are already included in the initial system. For the OLT PON side, we consider that the DWDM signals pass through the CWDM demultiplexer WF1 with an attenuation equal to the attenuation undergone by the downstream PON signal. As has been done for the DWDM OLT, we take all the interference contributions arriving at the OLT PON Rx and we solve the equation of the SIR for the isolation of WF1 for the Enhancement Band in order to have a SIR bigger than 10 dB. The same technique is applied for the ONT PON side for the isolation of WF2 for the enhancement band, where we consider that the DWDM signals pass through WF2 transparently (i.e. worst-case). We have computed the required isolation of WF1 for 1310 nm and 1490 nm respectively, in order that their respective noise contribution at the DWDM LT Rx are 10 dB smaller than the sum of the noise contributions of the DWDM channels. We assumed that in the worst case the PON signals pass transparently through the AWG. At the ONT side, a DWDM filter selects the specific wavelength in the Enhancement Band. The DWDM filter has certain isolation, so a part of the optical power of the other wavelengths will pass through the filter and create noise at the DWDM ONT Rx. The required isolation of the DWDM filter is computed by solving the equation of the SIR for this isolation. The power of the total interference is the sum of the received power at the DWDM ONT Rx from the DWDM downstream signals, the DWDM upstream signal from the same branch reflected by the ODN and also by the DWDM filter itself, the APON downstream signal and the APON upstream signal in the same branch reflected by the ODN and by WF2. The power of the effective signal involves the output optical power at the DWDM ONT Tx, the insertion loss of the CWDM multiplexer/demultiplexer, the DWDM filter and the AWG, and the maximum ODN loss. D.

Results

To illustrate the contributions to the maximum ODN loss due to the linear crosstalk, the following basic configuration has been assumed. Users have both a DWDM and a PON ONT, with a DWDM Rx sensitivity of –23 dB and an optical output power varying between –1 and 2 dBm, both at the OLT and ONT side. In Figure 6, the ODN loss maximum due to the linear crosstalk has been computed in function of different free parameters: the total insertion loss of the optical components, the number of channels (whereby each user needs two channels, one per direction), the adjacent and nonadjacent crosstalk of the AWG, the directivity of the AWG, and the maximal optical output power at the NT (the minimal output power remaining –1 dBm). When one parameter varies, the others are fixed at the following values: the maximum insertion loss of the AWG is 5 dB, its uniformity being 1.5 dB. The adjacent and nonadjacent crosstalks of the AWG are respectively 30 dB and 40 dB. The directivity of the AWG is 55 dB, the number of channels is 8.

number of DWDM users connected, a=3, b=16

maximum ODN loss (dB)

24

adjacent crosstalk of the AWG (dB), a=25, b=35

22

nonadjacent crosstalk of the AWG (dB), a=35, b=45

20

directivity of the AWG (dB), a=50, b=63

18

insertion loss of the AWG (dB), a=0, b=10

16

maximum average optical power of the DWDM NT Tx (dBm), a= -5, b=5

14

a

b

Figure 6. Maximum ODN loss due to the linear crosstalk versus of several parameters.

Figure 6 shows the parameter having the highest influence on the maximum ODN loss due to the linear crosstalk is the adjacent crosstalk. The higher the isolation, the higher the maximum ODN loss due to the linear crosstalk. The nonadjacent crosstalk and the directivity have a much lower influence, at least for this configuration assuming 8 channels. The insertion loss of the optical components has a small influence as well, but that influence becomes more important when the insertion loss is higher than about 5 dB. The maximum ODN loss due to the linear crosstalk decreases with about 3 dB when the number of channels increases from 6 to 32. The maximal output power of the NT has a big influence on the maximum ODN loss due to the linear crosstalk because the worst case is considered for the computation of the signal-to-interference ratio (interfering signals coming from the NT with the highest optical output power and useful signal coming from the NT with the lowest optical output power).

Maximum ODN loss (dB)

In Figure 7 we compare the maximum ODN loss in function of the receiver sensitivity for different AWG parameters: the number of channels, the minimum and maximum insertion loss, and the adjacent crosstalk. To be realistic, specifications of current commercial components have been used. But other components with improved characteristics could be used to optimize the performances of the system in a given configuration. The average output optical power of the PON LT varies between –1 and +2 dBm, as well as the average output optical power of the DWDM LT and NT. The nonadjacent crosstalk is 40 dB in all cases. The maximum number of DWDM users is connected (2 channels per user). The optical budget for both directions is the limiting factor for moderate Rx sensitivities (-20, -23 dBm) and the maximum ODN loss linearly increases with its improvement until a certain value for which the linear crosstalk at the LT Rx becomes the limiting factor. The Rx sensitivity has an influence on the maximum ODN loss until a value of –26 dBm and –30 dBm for an adjacent crosstalk equal to 25 dB and 30 dB respectively. The highest value achievable for the maximum ODN loss in the different cases studied is 20.8 dB. The maximum ODN loss can vary from 10 dB to 20.8 dB. A very important remark is that this maximum ODN loss only applies to branches with DWDM NTs, while the PON NTs follow the specifications of ITU-T G.982. 21 20 19 18 17 16 15 14 13 12 11 10

4 channels, insertion loss: 1 .. 2.5 dB, adjacent crosstalk: 25 dB 8 channels, insertion loss: 2 .. 3.5 dB, adjacent crosstalk: 25 dB 8 channels, insertion loss: 3.5 .. 5 dB, adjacent crosstalk: 30 dB 16 channels, insertion loss: 3.5 .. 6 dB, adjacent crosstalk: 30 dB 16 channels, insertion loss: 3 .. 5 dB, adjacent crosstalk: 25 dB 32 channels, insertion loss: 3.8 .. 5 dB, adjacent crosstalk: 30 dB

-20

-23

-26

-30

-33

Receiver sensitivity (dBm)

Figure 7. Comparison of the maximum ODN loss in function of the DWDM Rx sensitivity for different AWG parameters.

E.

Real case configuration

In the previous section, we have computed the maximum ODN loss for different configurations in the worst case, so for the maximum number of connected users. But this situation (maximum number of connected users equal to the number of channels of the AWG divided by two) could not be reached due to the limitation of the maximum ODN loss. A way of computing the real ODN loss, taking into account the splices (the amount of splices depends on the fiber length), the connectors, the optical branching devices, is given by the recommendation ITU-T G.982 using the Gaussian statistical approach. The upper limit of the optical path loss, which constitutes the worst case, is derived by adding to the mean value of the resulting distribution a figure equal to three times the standard deviation. The maximum fiber length for each user can be computed depending on the splitting configuration of that particular user (e.g. the first user could be situated after 2 splitters of 1:2, while another user could be situated after 4 splitters of 1:2), and depending on the maximum ODN loss available. The exercise has been done for some splitting configurations and the results are presented in Figure 8.

For example, let’s take the configuration with an AWG with 16 channels and an adjacent crosstalk of 30 dB. From Figure 7, a DWDM user with a receiver sensitivity of –26 dBm (respectively -30 dBm) is allowed to have a maximum ODN loss equal to 15.5 dB (respectively 18.8 dB). Let’s take a splitting configuration with a cascade of 3 splitters 1:2 for seven of the DWDM users, and continue a branch of this cascade with one splitter 1:2 for the eighth DWDM user and the APON users. From Figure 8, the seven DWDM users are allowed to have a total maximum fiber length of 5 km (respectively 13 km). The eighth DWDM user should use a receiver with a sensitivity of at least –28 dBm, because from Figure 8, for a cascade of 4 splitters 1:2, a maximum fiber length of 0 km corresponds to a maximum ODN loss of 17.1 dB. In Figure 7, a maximum ODN loss of 17.1 dB for the chosen configuration corresponds to a receiver sensitivity of approximately –28 dBm. If the eighth DWDM user has a receiver with a sensitivity of –30 dBm, its maximum fiber length is 4 km. The APON users have a maximum ODN loss of 25 dB (from ITU-T G.982). 40

1 splitter 1:2

Maximum fiber length (Km)

35

30

1 splitter 1:4 or 2 splitters 1:2

25

1 splitter 1:8 or 1 splitter 1:4 and 1:2 or 3 splitters 1:2

20

15

1 splitter 1:8 and one 1:2 or 1 splitter 1:4 and 2 1:2 or 4 splitters 1:2

10

1 splitter 1:6 and 1:2

5

0 10

11

12

13

14

15

16

17

18

19

20

21

Maximum ODN loss (dB)

Figure 8. Maximum fiber length in function of the maximum ODN loss for different splitting configurations.

Conclusions In this paper we have considered two applications for the BPON enhancement band. Firstly, the delivery of CATV services has been demonstrated for Class B and is included in Alcatel’s 7340 FTTU product. Secondly, we have studied the feasibility of a DWDM upgraded PON. The results of the simulations we have performed show that with the components available today, we can upgrade a PON with up to 16 DWDM users. This upgrade will not perturb the existing PON, but the maximum ODN loss for the DWDM users is reduced compared to the one of the PON users. A demonstrator is being built and BER measurements performed in order to evaluate the influence of the basic band on the enhancement band and the influence of the crosstalk on the quality of the signal. References [1] EURESCOM project P614, Implementation strategies for advanced access networks, deliverable 7: “Enabling technologies for broadband access infrastructures”. Volume 1of 2, February 1998. [2] R. D. Feldman, E. Harstead, S. Jiang, T. Wood and M. Zirngibl, “An evaluation of architectures incorporating wavelength division multiplexing for broadband fiber access,” Journal of Lightwave Technology, vol. 16, n. 9, pp. 1546-1559, September 1998.