WO2014016274A1

WO2014016274A1 - A broadband optical access system

Info

Publication number: WO2014016274A1
Application number: PCT/EP2013/065472
Authority: WO
Inventors: Felipe JIMÉNEZ ARRIBAS; Julio MONTALVO GARCÍA
Original assignee: Telefonica, S.A.
Priority date: 2012-07-25
Filing date: 2013-07-23
Publication date: 2014-01-30

Abstract

The system of the invention, comprising: - at least one wavelength division multiplexed passive optical network (WDM- PON) with a head-end optical line terminal (OLT) termed as WDM-PON OLT, arranged to a remote node (RN); and - a plurality of optical network units (ONUs), further comprising a: - at least one primary passive assembly installed in a manhole/handhole, adapted to select a WDM downstream optical band and/or a seed signal, to receive a WDM upstream optical band with data to and from a group of said plurality of ONUs, and to let a plurality of other group of signals to be bidirectionally transmitted through a primary single fibre bus, and -at least one secondary passive assembly remotely installed in a manhole/handhole, adapted to demultiplex and multiplex a downstream and/or a seed signal and an upstream band, and provided by a remote primary assembly to which it is connected through a single fibre, to deliver a single pair of wavelengths to each one of said plurality of ONUs of the group.

Description

A broadband optical access system

Field of the art

The present invention generally relates to a broadband optical access system, and more particularly to a system for network traffic aggregation for locations with low service demands.

Background of the invention

The ever increasing relevance of information technologies in everyday life makes it imperative to extend broadband Internet coverage to as many as possible citizens, regardless of their geographical location.

Traditional operator networks comprise several segments between the Internet Service Provider (ISP) connection or service platform locations and the end-users. Access technologies covering the last segment (physical connection to the end-user) can be implemented in multiple ways, by means of technologies going from wireless to "fixed" copper/fiber or any other alternative. Aggregation technologies cover the intermediate segment between access head ends (at local exchanges) and Core network, having typically a metropolitan or regional scope. Finally core network provides overall connectivity within the network segments, as well as to the service platforms and ISPs.

Plenty of high capacity transmission resources are available for aggregation purposes at urban/suburban environments, where main exchanges host aggregation nodes belonging to different network hierarchies. Less relevant exchanges are not aggregation locations but have adequate connectivity to main exchanges through dedicated fiber or an intermediate "pre-aggregation" stage.

Low populated areas may be completely lacking of aggregation network connectivity or this could have insufficient capacity in case high performance services are to be deployed. Provisioning high speed pipes to the correspondent local exchanges (or cabinets) or upgrading existing ones will be a difficult and costly task, and low expected incomes make it difficult to justify the necessary expenditures.

Common approaches for "pre-aggregation" in urban/suburban areas:

Telco operators usually deploy Metro/regional-wide networks based on one or several distinct switching technologies, such as Ethernet, Internet Protocol/Multi Protocol Label Switching (IP/MPLS), Optical Transport Network (OTN), etc. as an aggregation step towards their high capacity cores. These networks usually show a limited capillarity, due to both scalability and cost related issues.

Several technologies are in use to achieve the necessary connectivity from Telco premises (housing DSLAMs, OLTs or any other element providing "services" to the end users) to the aggregation network nodes.

In the simplest case, a suitable aggregation node will be already located in the exchange, and dark fiber in-building connections will be used. Dark fiber can also be used in some cases where aggregation node resides in some other exchange in the vicinity or when there are plenty of spare fiber resources.

In urban or suburban-like environments different transport systems can be available and appropriate for pre-aggregation purposes. These systems could range from traditional SDH (mostly ring topologies) to "pure" (optical) DWDM. Pre- aggregation systems based on CWDM are also commonly used on these environments, though their inherent limitations regarding reach and capacity have to be considered.

Problems with existing solutions for low populated areas:

In low populated environments resource scarcity will be commonplace and, thus, the following restrictions can be found:

· There is no co-located aggregation node in the exchange.

• There are no other exchanges in the vicinity housing a (suitable) aggregation node or able to provide connectivity with the network that are reachable through dark fiber links.

• It is not possible to use a DWDM optical route to reach the aggregation network.

• There is no possibility to setup an SDH connection to the aggregation network (or any intermediate exchange) with enough capacity.

Reduced service demand and, as a consequence, limited revenue expectations, restrict feasible pre-aggregation solutions to cost-effective setups that are easy to maintain and operate. CWDM technologies could offer enough capacity for pre- aggregation but only a small number of independent channels can be supported and do not optimally exploit deployed fiber resources.

Figure 1 highlight all above mentioned giving some examples of possible situations that can be found in urban/suburban and low populated areas. A linear bus Dense WDM-PON has been reported in [1] with a bus-star structure using standard Optical Add-Drop Multiplexers (OADM), which can be used as a cost-effective aggregation passive optical network in low-density population rural areas. Nevertheless, the physical reach is limited due to the re-modulation of the downstream signal for upstream transmission at the same optical wavelength. On the opposite, the most common approach for commercial WDM-PON systems is using different optical bands for upstream and downstream.

Patent US 2005/0117903 A1 proposes a WDM-PON with bus remote nodes (RN). Nevertheless, this patent has some relevant limitations such as the lack of an upstream tunability procedure for ONU/s (colored WDM-PON, not colorless), and the use of two different ports in the AWG in the RN for each ONU, which increases the number of required ports and components in the RN.

In patent US 2011/0188859 A1 , an externally seeded WDM-PON is proposed but however the RN configuration only allows a single star configuration, thus increasing the length of the fibres required for pre-aggregation through the system in low-populated areas and reducing the physical reach to access nodes.

Description of the Invention

The object of this invention is to provide a suitable network setup for traffic pre- aggregation in rural or any other environment with low service demands.

To that end, the present invention provides, a broadband optical access system for low demand locations, comprising:

- at least one wavelength division multiplexed passive optical network, WDM- PON, with a head-end optical line terminal, OLT, termed as WDM-PON OLT, arranged to a remote node, RN; and

- a plurality of optical network units, ONUs.

On contrary to the known proposals in the field, the system of the invention, in a characteristic manner further comprises:

- at least one primary passive assembly installed in a manhole/handhole, adapted to select a WDM downstream optical band and/or a seed signal and to receive data in a WDM upstream optical band, to and from a group of said plurality of ONUs (30), respectively, and to let a plurality of other signals to be bidirectionally transmitted through a primary single fibre bus, and

-at least one secondary passive assembly remotely installed in a manhole/handhole, adapted to demultiplex and multiplex a downstream and/or a seed signal and an upstream, respectively, and provided by a remote primary assembly to which it is connected through a single fibre, to deliver a single pair of wavelength to each one of said plurality of ONUs (30). The broadband optical system is a broadband WDM-PON access system with bus-tree fibre topology. The primary passive assemblies allow to install other primary passive assemblies in cascade forming a primary bus, and to simultaneously connect each primary passive assembly to a remote secondary passive assembly in a tree topology, thus allowing a WDM-PON system to serve a low populated area with a convenient bus-tree topology optimizing fibre use and increasing outside plant construction flexibility.

In an embodiment, said at least one secondary passive assembly has a cyclic response at each port thus supporting operation at several wavebands and allows a colorless deployment in the field of externally seeded WDM-PON technology.

In other embodiment, the secondary passive assembly is formed by a first WDM which separates the upstream and downstream bands, a colored demultiplexer and a colored multiplexer for downstream and upstream wavelengths, respectively, using an optical circulator at each port for combining signals on a single fibre port of the assembly. Figure 6. shows this embodiment, where tunable transmission technology of the ONUs for upstream can be used in order to allow colorless ONU deployment.

The broadband optical access system, in a preferred embodiment comprises a bus-tree topology deployment to provide connectivity from low to higher hierarchy exchanges using different optical bands for upstream and downstream transmission.

Other embodiments of the broadband optical access system are described according to appended claims 2- 16, and in a subsequent section related to the detailed description of several embodiments.

Brief Description of the Drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached drawings which must be considered in an illustrative and non-limiting manner, in which:

Figure 1 shows an example of an aggregation solution.

Figure 2 shows the solution outline proposed in the present invention.

Figure 3 illustrates the detailed network setup, according to an embodiment of the present invention. Figure 4 shows the preferred implementation of the primary assembly and logical connections for a generic "i" add/drop location, according to an embodiment of the present invention.

Figure 5 shows the preferred implementation of the colorless secondary assembly (a) and operation optical bands (b) for a generic "2i" WDM location, which is suitable for externally seeded WDM-PON systems, according to an embodiment of the present invention,

Figure 6 shows the preferred implementation of the colored secondary assembly and port connections for a generic "2i" WDM location, according to an embodiment of the present invention.

Detailed Description of Several Embodiments

The proposed invention provides a suitable network setup for traffic pre- aggregation in rural or any other environment with low service demands.

Central offices serving all those areas could probably lack of an efficient feasible connection to the aggregation network and new transmission deployments will have to be carried out. This invention proposes a novel implementation aimed at providing moderate transmission capacities in a scalable manner, optimizing inter- exchange link usage while avoiding any active outside plant element.

The approach that is detailed herein takes leverage of already existent access technologies to build the "pre-aggregation segment". This means that a cost-optimized technology, being designed for the access segment, will be used, for this specific application, as an alternative to classical transmission implementations.

Amongst currently available access technologies, WDM-PON meets the needs of provisioned bandwidth per-channel (≥ 1 Gbps) and maximum reach for pre- aggregation applications for sparsely populated environments.

Unlike traditional star-like structures deployed for the access segment, a bus- tree topology is proposed herein to provide Ethernet connectivity from small exchanges/cabinets to higher hierarchy offices with aggregation network connectivity.

Optical head-ends (WDM-PON OLTs) will reside in specific central exchanges where aggregation nodes are located or some sort of appropriate (in terms of achievable bandwidth) connectivity with aggregate/Core network exists. Optical network units will reside in low hierarchy exchanges (serving small villages or any other low populated areas) or could even be located in outside cabinets. Access multiplexers like xDSL DSLAMs or FTTH concentrators (G/E PON, 10G/E-PON) will be connected to these WDM-PON optical network units and will provide broadband access services to the end users.

A number of individual (pairs of) wavelengths linking the WDM-PON head-end with every remote ONU will be transported on the single fibre and single waveband add/drops at intermediate exchanges/cabinets and wavelength multiplexing and demultiplexing will be performed by primary and secondary passive assemblies, respectively.

Figure 2 outlines the proposed invention implementation, with primary passive assemblies at a manhole/handhole, forming a bus topology, and secondary passive assemblies pending from each primary passive assemble, the former being installed in the close vicinity of a local exchange with access multiplexers, inside a cabinet containing both ONUs and access equipment or at the fibre entry point of the exchange.

Several advantages with respect to current CWDM solutions or other alternative approaches are clearly identified:

• Single-fibre laying for both transmission directions, instead of (dual-fibre) CWDM counterparts.

• Single ONU type for all add-drop connections. Self-tuneable nature of WDM- PON ONUs (Optical Network Units) simplifies provisioning tasks and optimizes logistics as there is no need for different sets of spares.

• Extended capacity, with at least 32 concurrent channels over a single fibre being already available in commercial systems.

• Although the proposed solution is purely passive, there is a chance for amplification, as optical channels will lie in EDFA operation bands.

The invention is applicable to externally seeded WDM-PON solutions where both WDM-PON head-end (OLT or Optical Light Terminal) and ONU transmitters are "seeded" from the head-end location by means of a BLS (Broadband Light Source) or a laser array, thus achieving a colorless solution and avoiding the reach limitation due to remodulation of downstream signal for upstream transmission (201 1/0188859). Individual channels from OLT and ONU elements will belong to different bands (typically C for upstream and L for downstream) following main commercial approaches. Nevertheless, the invention is also applicable to systems with colored ONUs or ONUS with tunable transmitter for upstream.

As already indicated, this present invention proposes a network setup aimed at providing pre-aggregation capacities (≥ 1 Gbps) to several low populated areas within a coverage range (up to≥ 50 km) of a higher hierarchy exchange where aggregation network can be directly (co-located) or indirectly available.

A WDM-PON OLT located at mentioned higher hierarchy exchange will have several 10 GbE and/or GbE interfaces for upstream traffic to the aggregation/core network. Several (up to≥ 32) individual optical channels in groups occupying a certain waveband, these wavebands will be multiplexed downwards from the OLT and added/dropped at smaller offices/cabinets (primary passive assemblies) disposed along a bus-like topology.

The wavebands dropped in the primary passive assemblies are then delivered to manhole/handhole secondary exchanges where specific wavelengths will be multiplexed and demultiplexed in order to provide a dedicated pair of wavelengths to each ONU for data transmission.

Figure 3 illustrates the proposed network setup where the flow of the groups of wavelengths for downstream (ADi), ONU seeding (ASi) and upstream (AUi) are described in the primary passive assemblies (i=1...N), arrows indicate logical connections (optical "channels"), and all physical connections are supported over a single fibre.

Passive assembly elements will present very low insertion loss value and, thus, their cumulative loss from the OLT to the last ONU of the bus will have a limited impact in the overall optical budget. As a consequence, connections to local exchanges or cabinets located more than 50 km away from the head end can be made possible while performance levels are maintained.

Preferred Embodiments:

Figure 4 shows the preferred implementation of the primary passive assembly and logical connections (channels) for a generic "i" add/drop location. All the physical connections are supported over a single fibre.

Passive assembly "i" could consist of two optical filters (OF d,i for downstream and OF u,i for seed and upstream, i=1 ...N) and a combiner block for multiplexing and demultiplexing upstream and downstream signals. This block can consist of a WDM filter in case of externally seeded WDM-PON or an optical circulator in case of colored approaches.

Following the figure in the downstream direction: ^• OF d,i will receive the seed and all the downstream wavebands from the OLT (i=1 ) or from passive assembly (i>1 ), and will filter the "i" downstream band towards the "2i" secondary passive assembly.

^• In case of externally seeded WDMPON systems, OF u,i will receive the seed and all the downstream channels except and will demultiplex waveband

ASi from the seed.

^• Demultiplexed signals from OF d,i and OFu,i are sent to a combiner block and delivered to secondary passive assembly "2i".

• All the downstream channels reaching OF d,i, except "ADi", will go through the filter pair towards OF d,i+1 at the next add/drop location of the primary bus. If i=N no downstream signal will be present at OF u,i, as all the channels have already been filtered.

For the upstream direction, AUi waveband will be injected to the combiner block and directed through OF u,i towards the WDM-PON OLT, traversing OF d,i.

All the other upstream signals reaching OF u,i (from primary passive assemblies i+1 ... N) will go through OF u,i and OF d,i towards the WDM-PON OLT without being filtered.

Figure 5 shows the preferred implementation of a colorless secondary passive assembly "2i" using a cyclic AWG (Fig. 5a), where all the secondary passive assemblies have "m" ports to "m" ONUs. Each port of the AWG can operate at each corresponding wavelength of a given cycle (see Fig. 5b), thus Pj will operate at XD^*!}, XD2 ... XDN and XS \, XS2 ... XSN in downstream as well as U 1j, X\J2 ... X\JN in upstream (j=1 ...M). This implementation is suitable for externally seeded WDM-PON systems.

Figure 6 shows the preferred implementation for colored WDM-PON ONUs or WDMPON systems with tunable ONU transmitters with m ports at each secondary passive assembly. In this implementation, the WDMi multiplexer/demultiplexer connected to primary passive assembly "i" will deliver the downstream waveband ADi to a downstream demultiplexer WDMid and will receive AUi from the upstream multiplexer WDMiu. Both downstream and upstream WDM components, WDMid and WDMiu, will demultiplex and multiplex the corresponding wavelengths to each port, and a single fibre interface can be offered to ONUs using an optical circulator.

Advantages of the Invention:

Several advantages with respect to previous inventions are clearly identified: • It allows using a reduced number of fibres and total fibre length used for low populated areas access nodes in case of externally seeded WDM-PONs with colorless ONTs (201 1/0188859) and WDM-PON systems with colored or tunable ONU transceivers, by allowing a topology deployment in bus-tree topology, which offers more deployment flexibility than other approaches in the state of the art.

• Increased physical reach than in WDM-PONs with star topology. This is due to the smaller insertion loss along the path as a consequence of the reduced number of ports of the primary passive assemblies of the primary bus.

« lt allows simultaneous colorless ONU deployment and bus-tree topology in the same system due to the novel configuration of the primary passive assemblies. In summary, proposed implementation enables cost-limited deployment of broadband services in low demand geographical locations in a simple and optimized manner (less fibre, less fibre length and higher reach), simultaneously offering compatibility with ONU self-tuning capabilities in externally seeded WDM-PONs.

Dense WDM-PON technology evolution towards higher capacity and extended reach makes it a future-proof solution for further bandwidth-intensive service offerings.

ACRONYMS

AWG Array Waveguide Grating

BLS Broadband Light Source

CWDM Coarse Wavelength Division Multiplexing

DSLAM DSL Access Multiplexer

DWDM Dense Wavelength Division Multiplexing

EDFA Erbium Doped Fibre Amplifier

FTTx Fibre to the "x"

ISP Internet Service Provider

MPLS Multi-Protocol Label Switching

OTN Optical Transport Network

SDH Synchronous Digital Hierarchy

OADM Optical Add Drop Multiplexer

OLT Optical Line Terminal

OTDR Optical Time Domain Reflectometry

ONU Optical Network Unit

OSP Out Side Plant

WDM-PON Wavelength Division Multiplexing Passive Optical Network

REFERENCES

[1] A linear bus wavelength-reuse WDM-PON with simple add/drop nodes. Han-Hyum Lee, et al. Hong Kong: s.n., 2009. Optoelectronics and Communications Conference.

Claims

1. - A broadband optical access system for low demand locations, comprising:

- at least one wavelength division multiplexed passive optical network (WDM- PON) with a head-end optical line terminal (OLT) termed as WDM-PON OLT (1 ), arranged to a remote node (RN); and

- a plurality of optical network units (ONUs) (30),

wherein the broadband deployment system is characterized in that it further comprises:

- at least one primary passive assembly (2) installed in a manhole/handhole, adapted to select a WDM downstream optical band and/or a seed signal, to receive a

WDM upstream optical band with data to and from a group of said plurality of ONUs (30), and to let a plurality of other group of signals to be bidirectionally transmitted through a primary single fibre bus, and

-at least one secondary passive assembly (21 ) remotely installed in a manhole/handhole, adapted to demultiplex and multiplex a downstream and/or a seed signal and an upstream band, and provided by a remote primary assembly to which it is connected through a single fibre, to deliver a single pair of wavelengths to each one of said plurality of ONUs (30) of said group.

2. - The broadband optical access system of claim 1 , wherein said at least one secondary passive assembly (21 ) comprises a plurality of ports, each one of said plurality of ports adapted to have a cyclic response thus supporting operation at several wavebands and allowing a colorless deployment.

3. - The broadband optical access system of claim 1 or 2, wherein said at least one secondary passive aassembly (21 ) is formed by a first WDM which separates the upstream and downstream bands, a colored demultiplexer and a colored multiplexer for downstream and upstream wavelengths, respectively, using an optical circulator at each port of said plurality of ports for combining signals on a single fibre port of the assembly.

4. - The broadband optical access system of any of previous claims, wherein said at least one primary passive assembly (N) with its corresponding secondary passive assembly (2N) is cascaded with respective to a previous primary passive (N-1 ) assembly with its corresponding secondary assembly (2N-1 ) forming a bus-tree topology.

5. - The broadband optical access system of claim 1 , wherein said plurality of ONUs (30) are located in lower hierarchy exchanges that such of said at least one WDM-PON OLT (1 ).

6. - The broadband optical access system of claim 1 , wherein said plurality of ONUs (30) are located outside said exchanges.

7. - The broadband optical access system of claim 1 , characterized in that said at least one WDM-PON OLT (1 ) comprises several interfaces for upstream traffic network.

8. - The broadband optical access system of claim 1 , comprising a plurality of access multiplexers and/or a plurality of concentrators, arranged to each of said plurality of ONUs (30), in order to provide broadband access services to end users.

9. - The broadband optical access system of claim 1 , wherein all physical connections between said at least one WDM-PON OLT (1 ) and each one of said plurality of ONUs (30) are supported over a single optical fibre.

10. - The broadband optical access system of claim 9, further comprising a plurality of pair of wavelengths linking said at least one WDM-PON OLT (1 ) with each one of said plurality of ONUs (30).

1 1. - The broadband optical access system of claim 10, further comprising passive assemblies elements at a manhole/handhole located next to said lower hierarchy exchanges or outside said exchanges.

12.- The broadband optical access system of claim 1 , wherein each one of said plurality of ONUs (30) is a colorless ONU.

13. - The broadband optical access system of claim 12, wherein the technology for colorless ONU is based on externally seeded technology.

14. - The broadband optical access system of claim 12, wherein the technology for colorless ONU is based on tunable transceivers in the ONU.

15. - The broadband optical access system of claim 12, wherein each one of said plurality of ONUs has a colored optical transceiver.

16. - The broadband optical access system of claim 1 , wherein individual channels from said at least one WDM-PON OLT (1 ) and each one of said plurality of ONUs (30) belong to different communication bands.