Fiber Optic Splitter for GEPON Network
GPON (Gigabit ethernet Passive Optical Network) - EPON is Fast Ethernet (100 Mbps) PON, while GPON is Gigabits Ethernet (10 Gbps) PON. Ethernet PONs build on the ITU standard G.983 for ATM PONs and seek to bring to life the dream of a full services access network that delivers converged data, video and voice over a single optical access system.
There are several primary components of a last-mile PON -- the OLT, the fiber and splitters, and the ONU:
* OLT (Optical Line Terminal) - located at the CO, the OLT interfaces with the metropolitan network. The main functionality of the OLT is to adapt the incoming traffic (Voice/Data/Video) from the metropolitan rings into the PON transport layer.
* ONU (Optical Network Unit) and ONT (Optical Network Termination) - ONU and ONT are basically the same device – ONT is located at the customer premise, and ONU is located outside the home. ONU receives optical signal and converts it into an electrical signal for use in the customer premises.
* PON Splitters - With a single PON splitter, 32 subscribers can be served with two-way ATM. This way, it is not necessary to include a lot of add/ drop multiplexers and install the dreaded OSP cabinet. The PON splitters can be arranged in star, ring, or tree configurations to increase reliability.
A PON network may be designed with fused fiber splitter, or it can have two or more splitters cascaded together. Since each optical connection adds attenuation, a single splitter is superior to multiple cascaded splitters. One net additional coupling (and source of attenuation) is introduced in connecting two splitters together.
A single splitter is shown in the GPON network diagram below. Note that the splitter can be deployed in the Central Office (CO) alongside the OLT, or it may be deployed in an OutSide Plant (OSP) cabinet closer to the subscribers. A splitter can also be deployed in the basement of a building for a Multiple Dwelling Unit (MDU) installation (not shown).
An interesting (and strange) fact is that attenuation of light through an optical splitter is symmetrical. It is identical in both directions. Whether a splitter is combining light in the upstream direction or dividing light in the downstream direction, it still introduces the same attenuation to an optical input signal (a little more than 3 dB for each 1:2 split).
There are two basic technologies for building passive optical network splitters: Fused Biconical Taper (FBT) and Planar Lightwave Circuit(PLC). Fused Biconical Taper is the older technology and generally introduces more loss than the newer PLC splitters, though both PLC and FBT splitters are used in PON networks.
A Fused Biconical Taper (FBT) splitter is made by wrapping two fiber cores together, putting tension on the optical fibers, and then heating the junction until the two fibers are tapered from the tension and fused together. FBT attenuation tends to be a bit higher than attenuation from PLC splitters.
A 1:8 PLC splitteris diagrammed in the figure below. A PLC splitter is made with techniques much like those to manufacture semiconductors, and these optical splitters are very compact, efficient, and reliable. 1x32 splitter may be no larger than 1 cm x 2 cm.
The loss to be expected from a 1:8 splitter like the one diagrammed above is less than one dB greater than what would be expected from a perfect splitter, which has exactly 9 dB of loss (3dB for each 1:2 split). A good 1:32 PLC splitterhas an attenuation in both directions of less than 17 dB or even 16 dB (a perfect 1:32 optical splitter would introduce 15 dB of loss).
