There are multiple technologies that can be used in the access network but as it is known, connection paths comprising more optical fiber (relative to copper) will typically yield greater data rates, so a network comprising fiber all the way to customer premises (FTTP) which may take the form of a passive optical network (PON) is the most preferred solution in terms of transmission capacities and speeds.
As demand for capacity of the fiber access networks increases, eventually multiple wavelengths are used between the access network head-end (OLT) and a remote terminal at some location near the customer premises depending on the type of FTTx system being used (e.g. FTTP, FTTC fiber to the cabinet, FTTDP fiber to the distribution point).
A schematic depiction of a conventional PON (10) is depicted in FIG. 1, in which an OLT (14) includes one or more core network linecards each of which has a plurality of core network optical ports, and one or more linecards (40) each of which has a plurality of optical PON line terminals or ports (typically 4, 8, or 16, also not shown). Data is transmitted to the customer end via a PON port or line terminal (LT) along a connection established over an optical distribution network (ODN) in the access network. This comprises a primary PON feeder fiber connection (34), an optical splitter (42) and a number of optical distribution fiber links (46) to an optical network unit (ONU) (also known as an optical network terminal ONT), which can be located at different points in the network close by or inside the customer premises (32). For ease of description, FIG. 1 depicts only one PON and the relevant section of the network, but it is known that an OLT can support many PON systems via multiple ODNs and serve many more ONUs.
Networks are susceptible to equipment or plant damage or failure and have limited resilience in the event of equipment or plant failure. The adverse effects can be made worse by the network branching architecture. For example, if the optical feeder fiber link (34) connecting the OLT to an optical splitter (42) is broken, or if a PON linecard serving multiple ONUs was to develop a fault, then all of the customers served by that PON port or the entire PON linecard will lose service. Thus, a problem with the OLT (14) could lead to a total loss of service, or at the least reduction in service quality, for all of the customers served by the affected OLT.
“Dual parenting” (also known as “dual homing”) is a network design approach addressing the need to provide operational protection or resilience in optical networks in the event of loss of service to customers. Here, a customer is connected to head ends in two different exchange buildings to increase the resilience of the network.
WO2009/050459 describes a dual parenting implementation in which a PON is configured with two virtual local area network (VLAN) connections between a switch and the customer terminal, using two different communication paths provided by dual parenting. In operation, one of the VLAN connections is used to transmit traffic under “normal” network conditions and the other provides a back-up path in the event of problems (such as cable breaks, equipment malfunction, or damage or destruction of the exchange building housing the primary OLT) affecting operation of the first connection. For the purpose of this description, “normal” network conditions refers to the situation where the network or part thereof is operating as expected in the absence of damage, failure or faults affecting network plant and equipment affecting the network or section under consideration. The provision of redundancy in this way at the network layer and the service layer enables a faster customer reconnection in the event of network plant or equipment failure. In this document, this approach is also referred as “protecting” the network or providing it with operational resilience.
Another dual parented implementation is described in EP13250023.2, in which a dual parenting network topology is deployed in a fiber to the cabinet (FTTC) network, in which the cabinets are connected to the exchange via point-to-point optical fiber links. Here, the standby network has a point-to-multipoint layout.
Providing redundancy in this manner significantly increases the number of standby line cards, network plant and equipment needed and thus the overall cost of building the network.
Redundant plant and equipment are typically not used to their fullest extent, if at all, under normal network operation conditions until there is a need to switch operations away from the primary OLT. Failure events are rare occurrences and so significant capital and operation expense is incurred in providing a standby OLT and its associated PON ports.
Other protection schemes are known in the prior art, such as that described in US2012/0251108, which discloses an optical network comprising two multiplexer/demultiplexers (mux/demux), the first being comprised in an OLT, and the second forming a wavelength routed wavelength division multiplexed PON. The network further comprises working feeder fibers and protection backup feeder fibers connected to the mux/demux units comprising an N×N or 1×N arrayed wavelength gratings (AWGs). The output ports of the second mux/demux are allocated as a working path output port or a protection path output port depending on: an identity of a second mux/demux, an identity of the output port, an identity of a free spectral range (FSR) of the first mux/demux, the number of working fibers, the number of input ports of the first mux/demux, and an identity of the backup feeder fiber.
The problem is also addressed in “Flexible and scalable PON protection architecture using N:M redundancy toward next generation access network” (Mitsui T; NTT Access Network Service Syst. Labs, NTT Corp, Yokosuka Japan; Sakamoto T; Hara K; Yoshimoto N, The 17th Asia-Pacific Conference on Communication, 2011), which describes a system having an optical switch between the splitters situated between the ONUs and the OLTs. If one of the OLTs fails, the fibers directed to the failed OLT are switched to another OLT, using an optical switch in order to implement an N:M protection (i.e. N PONs protected by M OLT ports). This application is restricted to use in FTTC networks only, where there is already an active element in the field, i.e. the DSLAM. The idea is not usable in the FTTP technology where the fiber plant and equipment is passive. It is also not suited for use in a dual parenting resilience scheme where the working and protecting OLT's are sited in remote locations.
Yet further approaches seek to protect the fibers between the remote node and the ONUs in specific configurations, such as those described in US2006104638 and MY140726.