Broadband services are commonly supplied over legacy telephony networks using digital subscriber line (DSL) technology. One of the limitations of DSL is that the maximum data rate is not that high (for example, the maximum bit-rate is around 20 Mb s−1) and that the available bit rate decreases as the distance between the customer and the telephone exchange increases. It is believed that future broadband services are increasingly likely to be offered over optical fibre infrastructure using passive optical network (PON) architectures (D. B. Payne and R. P. Davey, “The future of fibre access systems”, BTTJ, Vol. 20-4, pp 104-114, (2002)). There are a number of large scale PON deployments currently underway around the world using either IEEE EPON technology or FSAN/ITU BPON and GPON. There is an emerging trend from network operators to consolidate network nodes and to reduce the amount of real estate used, leading to reductions in operational costs. A consequence of centralizing network equipment into a smaller number of network nodes is that the network is more vulnerable to large scale outages in the event that a catastrophic fault (for example fire, earthquake, etc.) renders a node out of service.
One approach to this problem is to use dual-homed PONs to enable the rapid restoration of communication services. FIG. 1 shows a schematic depiction of a conventional dual-homed PON architecture. A PON 100 comprises a first plurality of optical network terminals (ONTs) 10 and a second plurality of optical network terminals (ONTs) 15. The first plurality of optical network terminals (ONTs) 10 are connected to a first optical line terminal (OLT) at network node 20a via optical fibers and a passive optical splitter. The first OLT 16a is then connected on to a core network 40 for the subsequent routing of traffic. To provide resilience, the first plurality of ONTs is also connected to a second OLT 16b at network node 20b, which is also connected to the core network 40. Similarly, the second plurality of ONTs 15 are connected to a third OLT 16c, which is located at the second network node 20b and to a fourth OLT 16d which is located at a third network node 20c. Conventionally, the first plurality of ONTs will communicate with the first OLT but in the event of a network fault, or other contingency, then the first plurality of ONTs will switch to the second OLT. Similarly, the second plurality of ONTs are dual homed to OLTs at network nodes 20b & 20c. That is, under normal conditions the second plurality of ONTs are in communication with the third OLT 16c but if the network were to be damaged or otherwise disturbed then they would switch to communicating with the fourth OLT 16d. All of the OLTs at the various network nodes 20a, 20b & 20c are connected to a PON management system 30.
Whilst the GPON (Gigabit PON) standards defined in the ITU-T G.984 series define how a PON can switch traffic to the appropriate OLT in the event of a network failure, there is no definition in those standards as to how the higher network levels need to switch traffic in the event of a failure. J. Kang et al, “Restoration of Ethernet Services over a Dual-Homed GPON System—Operator Requirements and Practical Demonstration”, Proceedings of OFC/NFOEC 2008, discloses an Ethernet OAM based solution, in which an Ethernet edge switch in a communications provide (CP) point of presence (PoP) will check the connectivity status between itself and each individual ONT (or customer premises equipment (CPE) periodically. This solution requires each individual CPE to implement Ethernet OAM functionality, thus increasing the cost of the CPE devices.
D. Hunter and T. Gilfedder, “Routing and fast protection in networks of long-reach PONs”, BT Technology Journal, Vol. 24-2, p.p. 26-32 (2006), propose a protocol called FROTH (Fast Recovery for OLTs via Transmission of Hellos). This solution uses status packets to exchange network status information and requires either additional external FROTH routers at each CP PoP or additional functionality implemented in existing routers to support the proposed fault detection mechanism. EP 1 176 765 discloses a PON in which multiple network paths are provided to customer equipment, with protection switching being enabled in the event that a fault occurs in one of the network paths.
A yet further option is to run dynamic IP routing protocols between a communications provider's network and the customer premises equipment (CPE), which can detect the loss of connectivity when the protocol keep-alive timer times out due to loss of connectivity. However, this solution has very limited stability which may make it prohibitive to deploy in broadband access networks where millions of CPE may be connected. That is why most CPEs use static routing instead of dynamic routing.