(1) Field of the Invention
The present invention relates to a fiber-optic subscriber communication system using star type topology for fiber-optic networks, and more particularly to a fiber-optic access system in such a communication system.
(2) Description of the Related Art
A fiber-optic subscriber communication system, particularly that for telephone or like low transfer rate services, has used a passive double star (PDS) type network in order to minimize the cost of fiber-optic networks and optical interfaces in the central office (CO) side. In such a system, as shown in FIG. 1, an optical fiber 84 connected to a single optical interface (IF) 83 in the CO 81 is branched by a passive splitter 85 in the network and connected to a plurality of optical network units (ONUs) 86. Reference numeral 82 in the CO 81 denotes a switcher (SW). An optical interface 87 in each of the ONUs 86 is connected to a plurality of subscriber homes 88. Please refer to "Fiber Optic Subscriber Loop, Special Edition", IEEE Communications Magazine, February 1994, Vol. 32, No. 2. In this scheme, a plurality of ONUs 86 commonly use the CO-side optical interface 83 and also part of the transmission line 84.
In the system that provides wide band services, the PDS scheme has not been used but a single star type fiber network has been used as shown in FIG. 2, in which the transmission line 94 is laid from the single CO 91 to each of the ONUs 96 on a one-to-one basis. In FIG. 2, the reference numeral 92 denotes a switcher; 93, 97 denote an optical interface; and 98 denotes a subscriber home.
Where the PDS is used, however, a bottle neck problem is encountered if it is intended to accommodate high rate services because of the common use of part of transmission line by all the ONUs. More specifically, in the common part of transmission line, signals between all the ONUs and the CO are multiplexed in their transmission. This means that the transfer rate necessary for the common transmission line part is the product of one band required for the ONU multiplied by the number of ONUs. For this reason, for accommodating high rate services by using PDS, a super-high transfer rate has to be set for the optical interface, or resort has to be had to the provision of new channels by using the wavelength multiplexing technique. With the PDS, however, it is difficult to sufficiently increase the transfer rate because the upstream line (that is, data transfer from the ONU to the CO) is of a multi-access system in the burst mode. In addition, the above methods dictate very high cost.
Accordingly, the single star type scheme as described before is used to accommodate the high rate services. In this case, unlike the PDS, there is no bottle neck problem because there is no common transmission line part to the ONUs. However, on the CO-side, the same number of optical interfaces 93 as the number of ONUs 96 is necessary as shown in FIG. 2. Therefore, the merits of the PDS, i.e., the small scale and low cost of the CO, are lost.