1. Technical Field
The embodiments of the present disclosure relate to communications over an optical link and, more particularly, to multiple level signaling when communicating over a passive optical network.
2. Description of Related Art
Various wired communication systems are known today to provide communication links between devices, whether those devices are endpoint devices, intermediate routing devices or bridging devices. However, one of the limitations of wired systems is the amount of data such wired systems are capable of transferring. For example, wired links to homes and businesses may be capable of reaching bit transfer rates of 100 Mbits per second, but further increases on speed may require overly complex coding to transfer data at much higher data rates. Since the demand is continually increasing to transfer more data and at faster speeds, other mediums and techniques are being pursued to obtain the higher bandwidth.
One medium that is being pursued to obtain higher data transfer rates is optical medium, such as optical fibers. Although optical links exist today in providing the backbone infrastructure to cellular systems, optical fiber infrastructures have not yet been implemented in large scale to provide communication links in urban environments. With a few exceptions, most urban settings still rely on cable or telephone lines to transfer signals from a service provider for such services as television, Internet, video and movie downloads, etc., as well as uploading of subscriber information. These services are being taxed further as various functions are moved into the cloud environment.
An optical infrastructure is now being implemented in certain urban environments, but such systems suffer from the need to lay in a complete new infrastructure to businesses and residences. The equipment cost is also costly in that high intensity light sources, such as lasers and LEDs (Light Emitting Diodes) are needed at each end to provide the signal transmission. Thus, point-to-point systems are not desirable for such an infrastructure, since point-to-point systems typically require one-to-one connection between the two transmission ends. One way to achieve a reduction in light emitting sources and accompanying circuitry is to use Passive Optical Networks (PONs). In a typical PON structure, a service provider connects to multiple users or subscribers (businesses, homes, etc.) through a passive network where the light signal from the service provider is split by passive splitters. Thus, a service provider may use one light source to transmit to a number of end users. Instead of requiring 2N light source transceivers to have point-to-point communications between a service provider and N subscribers, a PON system may provide the connection with N+1 transceivers, since the service provider could provide the connection with one light source.
Current PON systems rely on data transmissions that are based on non-return-to-zero (NRZ) detection. That is, a first of a binary signal state of a bit (e.g. “1” state of a bit) is indicated by one signal state and a second of the binary signal state of the bit (e.g. “0” state of the bit) is indicated by a second signal state. NRZ coding does not rely on a rest state, such as a zero-crossing detection, for return-to-zero (RZ) coding. The NRZ technique provides for a two-state (0 or 1) serial transmission in which the state change of a bit is detected with the state change of the signal at the receiving end. Although the NRZ technique in PON systems still allows for 1 Gigabit (Gbit) per second data transfer rates and may also allow for 10 Gbit per second transfer rates, it may not be adequate for data transfer requirements in the future when higher data rates are anticipated.