The present invention relates to a PON (Passive Optical Network) system in which a plurality of subscribers' devices share optical transmission lines.
The PON generally comprises one OLT (Optical Line Terminal) and a plurality of ONUs (Optical Network Units) and transforms signals from terminals such as PCs (Personal Computers) connected to ONUs into optical signals before sending them to the OLT via optical fibers. The optical fibers from the plurality of ONUs are connected together by an optical splitter which optically multiplexes (time-division multiplexes) the optical signals and sends them to the OLT.
The length of optical fibers between ONUs and OLT is specified by Chapter 8 and 9 in ITU-T Recommendations G.984.1 to be in the range of, for example, 0-20 km, 20-40 km or 40-60 km, and each ONU is installed at any desired distance from the OLT within the above ranges. Therefore, transmission delays of optical signals between OLT and respective ONUs differ depending on the length of optical fiber. Without considering the transmission delays, there is a possibility of the optical signals output from ONUs colliding or interfering with one another when they are optically multiplexed by the optical splitter.
To deal with this problem, the OLT uses a ranging method as defined in Chapter 10 in ITU-T Recommendations G.984.3 to adjust the delays of output signals from ONUs to make the respective ONUs appear to be located at equal distances, for instance 20 km, from the OLT in order to prevent the optical signals from ONUs from interfering with one another. Further, to allocate a communication band of one optical fiber to as many ONUs as possible according to user requests, a DBA (Dynamic Bandwidth Allocation) method whereby the OLT allocates an upstream bandwidth for each ONU (data transmission position/time) is also defined in ITU-T Recommendations G.983.4. A transmission band control based on this method is also being practiced.
For example, according to the specifications of Chapter 8.2 of ITU-T Recommendations G.984.3, signals transmitted from a plurality of ONUs to the OLT are called upstream signals and comprised of a preamble, a delimiter and a payload signal. As shown in FIG. 8-2 of Chapter 8 of the same recommendations, a guard time is set immediately before the upstream signal to avoid a possible collision with a preceding burst signal. According to the specifications of Chapter 8.1 of the same recommendations, signals sent from the OLT to the plurality of ONUs are called downstream signals and comprised of a frame synchronization pattern, a PLOAM area, a US Bandwidth MAP area and a frame payload.
As shown in Chapter 8.1.3.6 of the same recommendations, an area called a US Bandwidth MAP is used to specify a send permission timing for the upstream signal from each ONU. The US Bandwidth MAP area has a Start value representing a start of the send permission timing and an End value representing an end of that timing, both specified in bytes. These values are also called grant values as they permit the transmission. A difference between the End value and the next Start value is an area where there is no upstream signal, and corresponds to the guard time. Each ONU can be assigned a plurality of bandwidth allocation units called T-CONT and the send permission timing for the upstream signal is specified for each T-CONT.