WO2011005223A1

WO2011005223A1 - Method and system for wavelength allocation in a wdm/tdm passive optical network

Info

Publication number: WO2011005223A1
Application number: PCT/SG2010/000260
Authority: WO
Inventors: Xiaofei Cheng; Yong Kee Yeo; Yixin Wang; Jian Chen; Zhaowen Xu
Original assignee: Agency for Science Technology and Research Singapore
Current assignee: Agency for Science Technology and Research Singapore
Priority date: 2009-07-10
Filing date: 2010-07-09
Publication date: 2011-01-13
Anticipated expiration: 2012-01-10
Also published as: TW201138353A

Abstract

A method and system for wavelength allocation in a WDM/TDM passive optical network. The method comprises the steps of providing a fixed wavelength channel to a plurality of optical network units (ONUs) in the passive optical network for TDM downlink/uplink data transmission; determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for one or more of the plurality of ONUs; and allocating the alternative wavelength channel to said one or more ONUs.

Description

METHOD AND SYSTEM FOR WAVELENGTH ALLOCATION IN A WDM/TDM PASSIVE OPTICAL NETWORK

FIELD OF INVENTION

The present invention relates broadly to a method and system for wavelength allocation in a WDM/TDM passive optical network, to a method for data transmission in a WDM/TDM passive optical network, and to a WDM/TDM passive optical network.

BACKGROUND Wavelength division multiplexed passive optical network (WDM-PON) has been demonstrated as a next-generation solution for broadband optical access network due to its attractive features such as large capacity, privacy, format transparency, network security, and flexible customer-based upgradability. However, WDM components are typically expensive, and the WDM-PON solution is traditionally not considered as commercially viable.

Some alternatives and/or improvements are being sought in the art. For example, recently, extensive attention has been paid to hybrid wavelength division multiplexed / time division multiplexed passive optical network (hybrid WDM/TDM-PON). Hybrid WDM/TDM-PON combines the high bandwidth capacity of WDM-PON and the bandwidth efficiency of TDM-PON, thus capable of accommodating a large number of optical network units (ONUs) while the usual high bandwidth per ONU is substantially maintained. Hybrid WDM/TDM-PON has been regarded as a migration from TDM-PON to WDM-PON for possible near-future deployment.

Meanwhile, in broadband access networks, network traffic studies have shown that the traffic has a burst characteristic, that is, the traffic fluctuates significantly with time, especially when multiple services are aggregated. With the steadily increasing number of users and emerging bandwidth intensive applications, ONUs in a hybrid WDM/TDM-PON may need better quality of service (QoS) and more service-level agreements (SLAs), and thus require higher bandwidth, which may exceed the channel bit rate.

A need therefore exists to provide a method and system for wavelength allocation in a WDIWTDM passive optical network that seeks to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention, there is provided a method for wavelength allocation in a WDM/TDM passive optical network, the method comprising the steps of:

providing a fixed wavelength channel to a plurality of optical network units (ONUs) in the passive optical network for TDM downlink/uplink data transmission; determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for one or more of the plurality of ONUs; and

allocating the alternative wavelength channel to said one or more ONUs.

The method may further comprise transmitting downlink/uplink data of said one or more ONUs through the allocated alternative wavelength channel. The step of determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for the one or more ONUs may comprise determining whether a queue size on the fixed wavelength channel for said one or more ONUs is above a first threshold. The method may further comprise redirecting downlink/uplink data transmission to the fixed wavelength channel if a queue size of said one or more ONUs on the alternative wavelength channel is below a second threshold. The alternative wavelength channel may be provided simultaneously with the fixed wavelength channel via one distribution fibre to the ONUs.

The step of allocating the alternative wavelength channel to said one or more ONUs may comprise:

providing two or more alternative wavelength channels via the one distribution fibre to the ONUs; and

if it is determined that the fixed wavelength channel should be switched to an alternative wavelength channel for said one or more ONUs, allocating a selected one of the alternative wavelength channels.

The one or more ONUs may utilise an optical switch to change between the fixed wavelength channel and the alternative wavelength channel. Input ports of the optical switch may be connected to different outputs from a wavelength selective coupler.

In accordance with a second aspect of the present invention, there is provided a system for wavelength allocation in a WDM/TDM passive optical network, comprising:

means for providing a fixed wavelength channel to a plurality of optical network units (ONUs) in the passive optical network for TDM downlink/uplink data transmission;

means for determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for one or more of the plurality of ONUs; and

means for allocating the alternative wavelength channel to said one or more ONUs. The system may further comprise means for transmitting downlink/uplink data of said one or more ONUs through the allocated alternative wavelength channel.

The means for determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for the one or more ONUs may determine whether a queue size on the fixed wavelength channel of said one or more ONUs is above a first threshold.

The system may further comprise means for redirecting downlink/uplink data transmission to the fixed wavelength channel if a queue size of said one or more ONUs on the alternative wavelength channel is below a second threshold.

The alternative wavelength channel may be provided simultaneously with the fixed wavelength channel via one distribution fibre to the ONUs.

The means for allocating the alternative wavelength channel to said one or more ONUs may provide two or more alternative wavelength channels via the one distribution fibre to the ONUs; and if it is determined that the fixed wavelength channel should be switched to an alternative wavelength channel for said one or more ONUs, may allocate a selected one of the alternative wavelength channels.

The one or more ONUs may comprise an optical switch to change between the fixed wavelength channel and the alternative wavelength channel. input ports of the optical switch may be connected to different outputs from a wavelength selective coupler.

The one distribution fibre may be coupled to an output port of a cyclic arrayed waveguide grating (AWG) router at one end, and to the ONUs at the other end.

The fixed wavelength channel may be provided to the cyclic AWG router via a first feeder fibre coupled to a first input port of the cyclic AWG router.

The alternative wavelength channel may be provided to the cyclic AWG router via a second feeder fibre coupled to a second input port of the cyclic AWG router. In accordance with a third aspect of the present invention, there is provided a method for data transmission in a WDM/TDM passive optical network, the method comprising the method for wavelength allocation as provided in the first aspect. In accordance with a fourth aspect of the present invention, there is provided a WDM/TDM passive optical network comprising the system for wavelength allocation as provided in the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1A shows a block diagram illustrating a hybrid wavelength division multiplexed / time division multiplexed passive optical network (hybrid WDM/TDM-PON) according to an example embodiment. Figure 1 B shows a block diagram of a variation of the hybrid WDM/TDM-PON of

Figure 1A.

Figure 2 shows a block diagram illustrating a hybrid WDM/TDM-PON according to another embodiment.

Figure 3 shows a block diagram illustrating a hybrid WDM/TDM-PON according to a further embodiment.

Figure 4 shows various optical network unit (ONU) node architectures suitable for use in the system of the example embodiments.

Figure 5 shows a flow chart illustrating a method for wavelength allocation in a WDM/TDM passive optical network according to an example embodiment. DETAILED DESCRIPTION

Figure 1A shows a block diagram illustrating a hybrid wavelength division multiplexed / time division multiplexed passive optical network (hybrid WDM/TDM-PON) 100 according to an example embodiment. Figure 1B shows a block diagram of a variation of the hybrid WDM/TDM-PON of Figure 1A.

As can be seen from Figure 1A, in the hybrid WDM/TDM-PON 100 of the example embodiment, optical network units (ONUs) 102, 112 are grouped in respective sub TDM-PONs. Typically, at the service provider's central office 140, a plurality of laser diodes LD₁, LD₂... are used to generate a plurality of respective continuous wave lights.

In the example embodiment, each continuous wave light is separated into two parts.

One part is used as a fixed (i.e. default) wavelength channel of a respective ONU group, e.g. sub TDM-PON 130, and is sent to a modulator DO for carrying original downlink data of that ONU group. The other part is used as an alternative wavelength channel of an ONU group (which is dynamically allocated/assigned in the example embodiment, as discussed in detail below) for ONUs which require higher quality of service (QoS) guarantee. Coarse wavelength division multiplexing (CWDM) couplers 104 are used to combine a pair of fixed and alternative wavelength channels, e.g. λ< and λ_N/2+i (where N is the number of channels).

In the example embodiment, the coupled fixed and the alternative wavelength channels are multiplexed via a cyclic 2^*N arrayed waveguide grating (AWG) multiplexer 106 and are routed to the two output ports 1 and N/2+1 of the 2*N AWG multiplexer 106 respectively. Table 1 shows an example input/output table of the 2*N AWG multiplexer

106 for N = 8.

Table 1

Alternatively, the CWDM couplers 104 and the 2*N AWG multiplexer 106 may be replaced by two separate 1^*N AWG multiplexers 156a, 156b. As shown in Figure 1B, the fixed wavelength channels are provided to the 1*N AWG multiplexer 156a, while the alternative wavelength channels are provided to the 1^*N AWG multiplexer 156b, respectively. This configuration may advantageously simplify the network architecture and reduce insertion loss. in addition, in both configurations, two 3-port circulators 108, 110 are used to separate downlink and uplink signals. The multiplexed fixed and alternative wavelength channels are then sent to a remote node 118 via two feeder fibres: an upper feeder fibre 116 and a lower feeder fibre 114, respectively. The remote node 118 comprises an N x N cyclic AWG 120 which connects to the two incoming feeder fibres 116, 114 from the central office 140 via e.g. input ports 1 and N/2+1 of the AWG 120, respectively. In the example embodiment, each output port of the N x N cyclic AWG 120 is connected to a respective sub TDM-PON, i.e. an ONU group. Downstream signals from the remote node 118 are sent to ONUs 102, 112 via respective distribution fibres 124 and couplers 122.

It will be appreciated that the N x N cyclic AWG 120 can be disposed at and operate from the central office 140 in alternate embodiments, for example, in already deployed PONs such as Ethernet PON (EPON) or Gigabit PON (GPON) for upgrading from TDM-PON to hybrid WDM/TDM-PON. In such embodiments, the remote node 118 is effectively eliminated and its functionality incorporated into the central office 140. At the ONU end, a wavelength selective coupler such as a CWDM coupler 126 is typically used to separate the fixed and the alternative wavelength channels. In the example embodiment, one of the two wavelength channels is selected as the downlink/uplink wavelength channel by a low-speed 2 x 1 optical switch 128. The selected downlink wavelength channel is further separated into two parts; one part is sent to a receiver 132 for receiving the downlink data, the other part is sent to a transmitter 134, e.g. a reflective semiconductor optical amplifier (RSOA), for upstream transmission (to be discussed in detail below). For example, in the embodiment shown in Figures 1A, 1B, a wavelength channel with wavelength λ, is provided for ONU group / as the fixed wavelength channel to transmit downlink/uplink data. The fixed wavelength channel is transmitted to the remote node 118 via the upper feeder fibre 116 and distributed to each ONU in that group, e.g. ONU 102. When one or more ONUs in ONU group / require a higher bandwidth for downlink/uplink transmission, the optical line terminal (OLT) (i.e. the central office 140) switches the downlink/uplink data from the original wavelength channel λ, to a selected alternative wavelength channel λ_N/₂₊i (e.g. λ-t+j when N = 8; i=i-8 for 4+i>8) in the example embodiment. As a result, these ONUs form a sub TDM-PON using the alternative wavelength channel λ_N/2+l in parallel to the previous TDM-PON.

The alternative wavelength channels in the example embodiment are sent to the remote node 118 via the lower feeder fibre 114, and distributed to ONU groups such that each ONU group is provided with one predetermined alternative wavelength channel. Furthermore, in the example embodiment, the alternative wavelength channels are dynamically allocated to the ONUs of the respective ONU groups which require higher QoS guarantee.

With reference to Figures 1A, 1B, the method of dynamic wavelength allocation to an ONU according to an example embodiment is now described. A Flow Scheduler

150 disposed at the OLT / central office 140 is used in the example embodiment to investigate the status of traffic in every ONU periodically. Typically, the Flow Scheduler

150 is coupled to the receivers 152 and the modulators 154. For example, if the current size of a queue assigned to a specific ONU exceeds a first preset threshold value, and a shared alternative wavelength channel is available, then flows to that ONU is dynamically redirected to and carried by this shared alternative wavelength channel. The flows are redirected back to the original dedicated queue once the queue size on the alternative wavelength channel is less than a second threshold value. Similarly, if the current size of the uplink data queue in an ONU exceeds a third preset threshold value, the ONU sends a message to request the alternative wavelength channel from the OLT. The OLT selects the relevant alternative wavelength channel to be assigned to the request such that data flows to the requesting ONU are dynamically redirected to the selected alternative wavelength channel, and thus the uplink data from the ONU. The flows are redirected back to the original dedicated queue once the uplink data queue size on the alternative wavelength channel in the ONU is less than a fourth threshold value. It will be appreciated that other methods may be used for monitoring the downlink/uplink traffic status, for determining whether the fixed wavelength channel should be switched to the alternative wavelength channel for one or more of the ONUs. Also, the first threshold value may be equal to the third threshold value, and the second threshold value equal to the fourth threshold value, depending on the service provider. Furthermore, the uplink data queue may be monitored either at/by the ONU or remotely from the OLT / central office 140.

Figure 2 shows a block diagram illustrating a hybrid WDM/TDM-PON 100 according to another embodiment. The hybrid WDM/TDM-PON 100 in this embodiment comprises components corresponding to those in the embodiment of Figure 1A. In addition, a 1x2 optical coupler 138 is disposed between the lower feeder fibre 114 and the N x N AWG 120 at the remote node 118 for enhancing the dynamic wavelength allocation capability for each ONU. In this embodiment, three wavelength channels, e.g. λ° , A^₊₁ and ^₊, (for N = 8; and if /+2 >8 or 4+/ >8, /=/-8), are sent to each ONU group /, e.g. sub TDM-PON 130. Similar to the embodiment of Figures 1A, 1B, the ONUs in the ONU group / in this embodiment can dynamically select an alternative wavelength if one or more ONUs require a higher bandwidth. This configuration can increase the flexibility of dynamic wavelength allocation/assignment. Thus, this embodiment can enhance the dynamic allocation capability and further increase the bandwidth of the ONUs.

Figure 3 shows a block diagram illustrating a hybrid WDM/TDM-PON 100 according to a further embodiment. The hybrid WDM/TDM-PON 100 in this embodiment comprises components corresponding to those in the embodiment of Figure 1A. In addition, a 1χ(N-1) optical coupler 148 is disposed at the remote node 118 for further enhancing the dynamic wavelength allocation capability. In this embodiment, all available wavelengths, i.e. A°,,Λ'— Λ-_PΛ'₊Γ- -K ' ^{are sent to tne ONU} group /, e.g. sub TDM-PON 130. Each ONU in this ONU group can dynamically select an alternative wavelength with the highest flexibility to carry its downlink and uplink data, in other words, the hybrid WDM/TDM-PON in this embodiment can advantageously achieve full wavelength allocation ability.

As described above, in the example embodiments as shown in Figures 2 and 3, at least two alternative wavelength channels are provided to each ONU group, and the alternative wavelength channels are shared among the ONU groups. That is, in these example embodiments, the OLT / central office 140 can seek and select a suitable alternative wavelength channel to be allocated to a requesting ONU. Advantageously, the hybrid WDM/TDM-PON of the example embodiments has partial protection capability. For example, as shown in Figures 1A, 1B, when the up feeder fibre 116 fails, which results in data loss in all ONUs, or laser diode of wavelength λ, or modulator DO is damaged, which results in data loss in ONUs in ONU group /, the downlink/uplink data of the wavelength channel λ, can be switched to the alternative wavelength channel λ_Nn+l . The data transmission channel is recovered and data is sent to/from ONU group / through the alternative wavelength channel via lower feeder fibre 114. In other words, uplink/downlink data transmission is advantageously maintained even if one feeder fibre fails. In the various embodiments as described above, a hybrid WDM/TDM passive optical network architecture is provided. The network architecture comprises a central office 140, a remote node 118, plurality of optical network units (ONUs) 102, 112, a cyclic N x N arrayed waveguide grating (AWG) router 120. A plurality of continuous wave lights are separated into a first part and a second part in the central office, the first part carrying fixed downlink/uplink wavelength channels, the second part carrying alternative dynamic wavelength channels. The fixed downlink/uplink wavelength channels and the alternative dynamic wavelength channels are sent to the remote node 118 via two feeder fibres 116, 114, and routed to the optical network units (ONUs) via the cyclic N x N AWG router 120. The fixed wavelength channels and alternative dynamic wavelength channels are selected, and downlink channels are separated for detection at the ONU. Figure 4 shows various optical network unit (ONU) node architectures suitable for use in the system of the example embodiments. These ONU nodes can adaptiveiy choose a wavelength channel as downlink/uplink data transmission. Typically, as can be seen in Figure 4(a), the ONU comprises a CWDM coupler 126, an optical switch 128, a receiver 132 and a transmitter 134. The transmitter 134 is typically in the form of a Fabry-Perot Laser Diode (FPLD) (Figures 4(c)-(e)), a vertical-cavity surface-emitting laser (VESEL), or a reflective semiconductor optical amplifier (RSOA) (Figures 4(a)-(b)). Figures 4(b)-4(d) show ONU architectures with two receivers 132. Figure 4(e) shows a coherent detection and the architecture is suitable for selecting a wavelength channel from a multiple downlink wavelength channels. As can be seen in Figure 4(e), a local coherent wavelength 146 is additionally provided to the ONU such that the signal wavelength and the local coherent wavelength 146 beat and recover data transmitted on the same wavelength. Figure 4(f) shows an architecture employing a wavelength tunable laser 142 and a wavelength tunable receiver 144.

Advantageously, the hybrid WDM/TDM-PON architecture in accordance with the above example embodiments can provide dynamic wavelength channels for both downlink and uplink traffic which result in quality of service (QoS) guarantee for all ONUs. In addition, dynamic wavelength resource may be shared by all ONUs in the network in the example embodiments, thereby improving the dynamic wavelength resource utilisation efficiency. Also, the dynamic wavelength allocation scheme in the example embodiments is preferably provided to all ONUs without the requirement of extra laser diodes at the central office, which in turn can reduce cost. Furthermore, the hybrid WDM/TDM-PON of the example embodiments advantageously has partial network protection capability.

The hybrid WDM/TDM passive optical network architecture in accordance with the above example embodiments is applicable in broadband optical access networks, particularly wavelength division multiplexed passive optical networks.

Figure 5 shows a flow chart 500 illustrating a method for wavelength allocation in a WDM/TDM passive optical network according to an example embodiment. At step 502, a fixed wavelength channel is provided to a plurality of optical network units (ONUs) in the passive optical network for TDM downlink/uplink data transmission. At step 504, whether the fixed wavelength channel should be switched to an alternative wavelength channel for one or more of the plurality of ONUs is determined. At step 506, the alternative wavelength channel is allocated to said one or more ONUs. It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A method for wavelength allocation in a WDM/TDM passive optical network, the method comprising the steps of:

providing a fixed wavelength channel to a plurality of optical network units

(ONUs) in the passive optical network for TDM downlink/uplink data transmission; determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for one or more of the plurality of ONUs; and

allocating the alternative wavelength channel to said one or more ONUs.

2. The method as claimed in claim 1 , further comprising transmitting downlink/uplink data of said one or more ONUs through the allocated alternative wavelength channel.

3 The method as claimed in claims 1 or 2, wherein the step of determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for the one or more ONUs comprises determining whether a queue size on the fixed wavelength channel for said one or more ONUs is above a first threshold.

4. The method as claimed in any one of the preceding claims, further comprising redirecting downlink/uplink data transmission to the fixed wavelength channel if a queue size of said one or more ONUs on the alternative wavelength channel is below a second threshold.

5. The method as claimed in any one of the preceding claims, wherein the alternative wavelength channel is provided simultaneously with the fixed wavelength channel via one distribution fibre to the ONUs.

6. The method as claimed in claim 5, wherein the step of allocating the alternative wavelength channel to said one or more ONUs comprises:

providing two or more alternative wavelength channels via the one distribution fibre to the ONUs; and if it is determined that the fixed wavelength channel should be switched to an alternative wavelength channel for said one or more ONUs, allocating a selected one of the alternative wavelength channels.

7. The method as claimed in claims 5 or 6, wherein said one or more

ONUs utilise an optical switch to change between the fixed wavelength channel and the alternative wavelength channel.

8. The method as claimed in claim 7, wherein input ports of the optical switch are connected to different outputs from a wavelength selective coupler.

9. A system for wavelength allocation in a WDM/TDM passive optical network, comprising:

means for allocating the alternative wavelength channel to said one or more

ONUs.

10. The system as claimed in claim 9, further comprising means for transmitting downlink/uplink data of said one or more ONUs through the allocated alternative wavelength channel.

11. The system as claimed in claims 9 or 10, wherein the means for determining whether the fixed wavelength channel should be switched to an alternative wavelength channel for the one or more ONUs determines whether a queue size on the fixed wavelength channel of said one or more ONUs is above a first threshold.

12. The system as claimed in any one of claims 9 to 11 , further comprising means for redirecting downlink/uplink data transmission to the fixed wavelength channel if a queue size of said one or more ONUs on the alternative wavelength channel is below a second threshold.

13. The system as claimed in any one of claims 9 to 12, wherein the alternative wavelength channel is provided simultaneously with the fixed wavelength channel via one distribution fibre to the ONUs.

14. The system as claimed in claim 13, wherein the means for allocating the alternative wavelength channel to said one or more ONUs provides two or more alternative wavelength channels via the one distribution fibre to the ONUs; and if it is determined that the fixed wavelength channel should be switched to an alternative wavelength channel for said one or more ONUs, allocates a selected one of the alternative wavelength channels.

15. The system as claimed in claims 13 or 14, wherein said one or more

ONUs comprise an optical switch to change between the fixed wavelength channel and the alternative wavelength channel.

16. The system as claimed in claim 15, wherein input ports of the optical switch are connected to different outputs from a wavelength selective coupler.

17. The system as claimed in any one of claims 13 to 16, wherein the one distribution fibre is coupled to an output port of a cyclic arrayed waveguide grating (AWG) router at one end, and to the ONUs at the other end.

18. The system as claimed in claim 17, wherein the fixed wavelength channel is provided to the cyclic AWG router via a first feeder fibre coupled to a first input port of the cyclic AWG router.

19. The system as claimed in claim 17, wherein the alternative wavelength channel is provided to the cyclic AWG router via a second feeder fibre coupled to a second input port of the cyclic AWG router.

20. A method for data transmission in a WDM/TDM passive optical network, the method comprising the method for wavelength allocation as claimed in any one of claims 1 to 8.

21. A WDM/TDM passive optical network comprising the system for wavelength allocation as claimed in any one of claims 9 to 19.