In order to increase transmission efficiency in an EPON system, the upstream and downstream transmission channels should ideally be separated. One simple solution is to utilize space division multiplexing, where two separate optical fibers and passive couplers are used for upstream and downstream transmissions. However, there is a more cost-effective solution. This is by using different wavelengths for upstream and downstream transmissions. Typically, a 1550 nm wavelength is used for downstream transmission and a 1310 nm wavelength is used for upstream transmission . In the upstream direction of an EPON system, multiple ONUs transmit data packets to the OLT through a common passive combiner and share the same optical fiber from the combiner to the OLT.
The passive combiner has a directional property so the data packets from an ONU can only reach the OLT but not the other ONUs. Conventional contention-based multiple access such as the carrier sense multiple access with collision detection (CSMA/CD) protocol, is difficult to implement because the ONUs is unable to easily detect a collision that may occur at the OLT. Although the OLT is able to detect a collision and inform the ONUs by sending a collision message, the transmission efficiency is reduced because of considerable propagation delay between the OLT and the ONUs.
To address this problem, an optical looping-back technique was proposed in order to achieve high channel efficiency with CSMA/CD . With this looping-back technique, a portion of the upstream signal power transmitted by each ONU is looped back to the other ONUs at using a 3: N coupler and connecting two ports of the coupler together through an isolator.
However, to implement the optical CSMA/CDprotocol, each ONU has to use an additional receiver operating at the upstream wavelength and a carrier sensing circuit, which would largely increase the network cost. This solution also is unable to provide guaranteed bandwidth to each ONU and thus is difficult to support any form of quality of service (QoS).
Please type your essay title, choose your document type, enter your email and we send you essay samples