Overview

This overview explains how GPON optical protection works and defines the features and capabilities of the Tellabs Type B PON protection switching. The GPON system, and type B protection, are further defined in ITU-T G.984.1.

• PON – Passive Optical Networking. Networks using 1:N connections from a single OLT port to up to 32 ONTs on the PON. The infrastructure between the OLT and ONT is all optical and completely passive with no electronics in between the two.
• Primary PON – The PON that by default carries all the traffic when no failures exist in the system.
• Secondary PON – The standby PON is the designated PON that will carry the traffic in the event of a failure on the Primary PON.
• Active PON – The active PON is the PON which is currently carrying all of the traffic.
• Standby PON – The standby PON is the PON which is not carrying traffic and is currently providing standby protection for the Active PON.

PPG Protection Example

Type B PON Protection

Type B PON protection is a part of the GPON standard and allows the OLT, and OLT connections, to be redundant and allows high reliability connections over the PON.

Type B protection makes use of a 2:N splitter that allows two inputs on the OLT side of the circuit. There is a single fiber going to each of the ONTs on the PON. This gives both equipment and facility protection for the OLT side of the PON. With Type B PON Protection one can accomplish fiber path redundancy to the communication closet or zone box (e.g. 2:32 splitter) as well as full path protection to the user work area (using a 2:1 splitter).

The diagram shows the two PON interfaces being within the same OLT, but the protection interface can be configured as:

• Two ports on different PON cards within the same OLT
• Two ports on PON cards within different OLTs

This allows the user to control what level of redundancy they require based on what makes the most sense in terms of a trade off of cost/benefit, usually driven by the importance of the data flowing over the PON network.

Implementing Type B PON Protection on the Tellabs OLAN system extends system availability beyond 99.999%.

Tellabs Type B PON Protection

Tellabs supports the ITU-T G.984.1 Type B PON protection on all 1100 Series OLTs and ONTs. This allows the user to fully protect the PON side of the network in a cost effective manner.

The Tellabs product has been enhanced with Tellabs patented technology such that the standby PON link can detect the lack of messaging to or from the ONTs. The standby PON takes over if it detects this condition and starts driving its laser onto the PON. This is all done using the standard Tellabs QOIU7 PON cards without additional equipment other than the 2:N fiber splitter.

The 2:N fiber splitter allows both PONs to have the capability to inject light into the PON network and detect the traffic going to/from the other OLT. The Tellabs solution can support PON protection using two different PON cards within the same OLT, or utilizing two different OLTs.

Type B PON Protection Switching Timing

The current Panorama PON release is able to detect the failure of the PON within 500ms and is able to switch traffic in less than 5 seconds for the entire PON.

It should be noted that the protection switching is revertive, meaning when the primary link is returned to service the system switches back after the wait to restore (WTR) timeout has expired. The default WTR time is 60 seconds. The revert time is used to provide hysteresis and avoid thrashing if the Primary PON is transitioning up and down.

Type B PON Protection Implementation

The Tellabs 1100 Series Optical LAN uses a simple and effective method for arbitrating control of the PON. This patent pending technology improves the reliability and responsiveness of the PON Protection while not increasing the cost of the Optical Data Network (ODN). The technology supports standard GPON physical and data link layer signaling characteristics, as defined by ITU G.984.1 and G.984.2.

This solution does not require any specialized, custom, proprietary hardware, software, or other functions in the ONTs and is therefore a solution to maximize compatibility across the entire Tellabs 1100 GPON ONT product portfolio

This Passive PON Protection technology functions by utilizing the presence, or lack of an optical signal, rather than depending on encoding any specific messages in that signal. In ITU G.984.2, the ONT is a complete slave to the OLT that is commanding the PON and performing the Media Access Control (MAC) function for the ODN. As such, the ONT never transmits an optical signal without first being ordered by an OLT. Furthermore, when told to transmit, the ONT is given a very specific window of time in which it is allowed to transmit. This window, called a burst, is carefully coordinated by the OLT so that every ONT is occupying a non-overlapping period of time in which they are transmitting. This effectively becomes a Time-Division-Multiplexing (TDM) system in which the TDM "slots" or "bursts" are assigned by the OLT to the ONT. This assignment and burst process lasts for a small 125μsec period of time and then is repeated ad infinitum. During each epoch, called a frame, every ONT is assigned a specific allotment of the period in which it can burst. This includes no burst, a complete 125μsec burst, and some fraction therein that starts and stops at some arbitrary point within the frame. It should be emphasized that the resulting ONTs upstream optical signal is split by the passive optical 2:N splitter and received by both OLTs regardless if the OLT was actually transmitting a signal itself.

A second, and equally important, fact is in order for an ONT to receive its allocationed burst window, it must be able to successfully lock-to and recover the complete GPON encoded signal from a single OLT. The laws of optical physics dictate how much interference the ONTs receiver can suppress and continue to effectively decode the OLT's commands. In the presence of both OLTs commanding the ONT, each OLTs signal is destructively interfered with by the other to the extent that a discernible bitstream with valid CRC integrity checks cannot be obtained. As a result, the ONT has received no time slot allocations and therefore does not transmit any burst.

The significance of these two conditions is that the two OLTs can effectively infer the behavior of its peer OLT despite not being able to directly send signals and messages to it. From the perspective of one OLT:

1. If no upstream signal is seen, then my peer must not be performing its MAC function and can be deduced to be either off line or disconnected from the ODN.
2. If an upstream signal is seen that I haven't commanded to be sent, then my peer must be successfully performing its MAC function.
3. If I am sending commands to the ONTs for specific time slot burst assignments, but those bursts are not being received at the expected times and durations, then the ONT must be successfully re- ceiving my peer's allocation commands and not my own.

Using these rules, a simple and effective MAC arbitration protocol is realized.

1. Upon startup, the OLT can monitor for the existence of any signal received from any ONT. If present, it can assume the peer is active and therefore it should abstain from enabling its transmitter. Thus, that OLT enters standby mode.
2. An OLT, in standby mode can monitor for the loss of any signal received from any ONT for a representative amount of time corresponding to the tolerance threshold of failure detection. If no signal is received for that period, the standby OLT can infer that it must perform takeover actions to provide backup service to the PON. In doing so, that OLT enables it's continuous transmission laser and enters the active mode, providing media access control for all ONTs on the PON.
3. An OLT, if in active mode, detects that a representative number of bursts corresponding to the tolerance threshold for failures from ONTs are not recovered in the time slots it had allocated in the last Grant Map issued to the ONTs, that OLT can infer that it is no longer in control of the PON and a stronger signal from its peer OLT has won control of the PON. Therefore, that OLT disables its transmitter and enter standby mode.

Each OLT follows the essence of the following decision state machine:

RSTP Protected ONTs

One other simple and cost-effective method of providing full redundancy end to end is to install two ONTs in a location and place a inexpensive Layer 2 (L2) switch which supports Rapid Spanning Tree Protocol (RSTP). This allows for full end-to-end redundancy for selected users without burdening the entire system with the additional costs of protection.

Assuming there is a second OLT on site, the only additional equipment required for protection in this diagram is the L2 switch and an additional ONT. All the other equipment is deployed non-redundant and utilized to its full capacity. This method also only requires 1:N splitters rather than 2:N splitters.

One other advantage of this protection method is that it switches within the time period that RSTP can detect and changes the topology. The typical convergence time is in the 2-3 second range.

Switching Times with RSTP Protected PONS

The switching times for RSTP protected ONTs is driven by RSTP and typically would failover in less than 1 second. The GPON system would not be aware of the transition of traffic from one OLT to another and is not involved in the switch.

Optical Network Setup and Budget

There are two key points associated with the optical setup of a protected PON network:

• Secondary PON must have an optical level 5dB lower than the Primary PON.
• 2:N PON Splitters typically add 3dB loss to the optical budget.

This 8dB must be planned for within the optical budget of the system being deployed.

The system relies on the Primary Optical link being able to take over control of the PON. This is accomplished in the optical domain by engineering the optical network such that the Primary PON signal is 5dB above that of the Secondary PON signal. This allows the Primary PON to drive the signal on the PON and take control.

The typical way this is accomplished is via the insertion of a 5dB pad in the Standby PON optical path.