Protection

Written on 3:21 PM by ooe

It has been identified that PON systems might not give the level of reliability required for certain applications. Whilst the PON electro-optics are generally reliable, it has been found that a major cause of failure is the physical media, i.e. the optical fibres in the access network themselves, and in many cases faults are caused by external influences. Point-to-point systems can allow for protection using back up or standby fibre pairs.

It is not the aim of protection scenarios to restrict the architectures possible in a PON architecture but to support and enhance whatever architecture is chosen for a particular application. For this reason, the protection scenarios offered are not exhaustive, but are representative of those it is considered will be of interest. These must be carefully tailored to the applications for which they are required.

Generally it is only FSAN, and through them, the ITU-T that have considered protection of PON systems in some detail and an ITU-T recommendation is due shortly. This will be known as ‘G.983.5: A broadband optical access system with enhanced survivability’.

The main requirements of a protection system shall be that:

• It should be possible to have several C protection configurations on the same OLT.

• It should be possible to duplicate the OLT-PON-interface and the fibres between the OLT and splitter and to duplicate the entire fibre path between the OLT and ONU for a set of ONUs.

• It should be possible to have a mixture of protected and unprotected ONUs on one B-PON interface.

• The addition or removal of a protected ONU on a PON should not affect other ONUs on the same PON.

• It should be possible to have automatic switching, which would be triggered by a fault detection such as loss of signal, loss of cell delineation, signal degrade (e.g., BER becomes worse than the pre-determined threshold), etc.

• It should be possible to have forced switching, which would be activated by administrative events such as fibre re-routing, fibre replacement, etc.

• It is necessary to avoid unnecessary switching. Because unstable switching affects service quality, unnecessary protection switching and unnecessary revertive protection switching should not occur.

• It should be possible to realise switching without connection loss of the ATM connections.

• It should be possible for the operator to choose between a revertive and a nonrevertive switching mode.

• The service halt time should be less than 50 ms if the extra traffic option is not used.

• The events or conditions that trigger automatic switching should be chosen among the G.983.1 OAM parameters.

• The chosen protocols and mechanisms must apply to the B-PON section layer.

• Extra traffic should be carried over the protection entities while the working entity is active and would not be protected. This capability will provide effective usage of bandwidth on the protection entities.

• It must be possible for an operator not to activate the extra traffic option (e.g., to achieve a lower service interruption time).

Extra messages are introduced into the base set of PLOAM messages from G.983.1 to cope with the additional functionality required for protection switching but generally it is the OLT that initiates the switch-over.

There are two basic protection schemes that can be used and the authors do not attempt to judge failure mechanisms within this chapter and make no recommendations as to the better protection mechanism to use. The first is a partial protection scheme and is designated type B protection. It is useful when it is the feeder fibre(s) that are considered at most risk of failure. The more fully protected scheme is known as type C protection.