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1141 Optical signal-to-noise ratio
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The optical signal-to-noise ratio is the single most important quantity when describing the optical signal in a transmission system The optical signal-to-noise ratio, or OSNR, gives the ratio of the optical signal power to the noise content It is a statistical value, often calculated deterministically The OSNR determines the actual useful content of a signal In PON systems, the OSNR is only somewhat important as there are no optical ampli ers (the chief source for noise) However, OSNR-based designs are equally valid for power budget analysis of PON systems
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115 Components for PON systems
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Generically, the PON system consists of the OLT and the ONU We shall, in this section, describe the subsystem level con guration of the OLT and the ONU Amongst the three types
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of PON system described earlier, we will keep the OLT and ONU discussion to a generic level, signifying that the following discussion is valid for APONs, EPONs and WDM PONs, with some minor modi cations Shown in Figure 113 is the subsystem level layout of the OLT As can be seen, the OLT is a 2 2 port device, in the sense that it transmits and receives data from a carrier termination point, as well as transmiting and receiving data to multiple ONUs The OLT takes in information frames from the service provider feed (SP feed) and further deciphers these frames It may either reframe the information that is to be transmitted to the ONUs, or simply tag the information frames with some protocol identi cation information The information from the SP feed is correlated to the protocol stack, so that downstream broadcast and upstream communication are linked The relaying of downstream communication is buffered, and ow control is carried out through buffers in the protocol stack The buffers then control the ow of information The buffers also add control information to the downstream frames This control information is critical to arbitrate communication in the upstream This is primarily done by the control information being piggybacked on the downstream frames to all the ONUs The ONUs can then transmit in the upstream, depending on the information that they receive Hence, this way, the OLT controls upstream transmission through a piggyback procedure The information frames plus piggybacked control signals are then broadcast downstream to the ONUs, through a burst mode transmitter For cases where the OLT uses a synchronous protocol, as in the case of a BPON system, etc, the transmitter need not be burst mode but can be a simple direct modulated laser diode In the other direction of communication (the upstream), the OLT receives optical signals from the ONUs The OLT receives the upstream frames through a burst mode receiver The burst mode receiver is critical, irrespective of the communication methodology, as the upstream is often bursty in data ow Figure 114 shows the ONU subsystem layout The ONU has a single ber interface connected to the network side, to the OLT It also may have multiple local interfaces to connect to multiple physical devices at customer premises If we envision an ONU located at a curb site (in the case of FTTC ber-to-the-curb), then the ONU can be connected to end users via wireless, copper or even multimode ber lines, depending on the system needs In the downstream, the signal from the OLT enters the ONU and is ltered optically to ensure upstream and downstream communication is separate The optical signal is then converted to electric bit streams through a burst mode receiver and clock recovery circuit The burst mode receiver here is the most critical aspect of the ONU Since the downstream data is not continuous, a normal detector cannot detect the randomly varying stream of information properly This is because, for a typical detector to detect the information frames it needs to know the amplitude and frequency/phase of the incoming signal beforehand However, if the data is not continuous, as in the case of downstream transmission, a normal photodetector will soon lose lock with the incoming bit stream, and hence generate garbled frames To avoid this, a burst mode receiver is used The burst mode receiver is able to detect information frames without knowing the amplitude or phase/frequency of the incoming signal There are multiple technologies known for burst mode receivers The most common, however, is to decipher the amplitude and phase information through an embedded string of bits within the preamble of an incoming frame Other methods of deciphering the amplitude and phase are by using nonlinear elements that act as square law loops, enabling multiple harmonic components of the frequency and phase to be generated
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