GEPON became one of the most popular FTTx technology last years. The protocol itselfs is dervied from Ethernet technology which is standard, even in the big networks. This is the reason that the GEPON integrates very well with other Ethernet-based technologies, and became technology of choice in the FTTx world.
GEPON is a part of family of the Passive Optical Networks technologies. What means passive? How it differs from other fiber-based solutions? We will try figure it out.
Passive Optical Networks#
The GEPON network base on four main elements: Optical Line Terminal, Optical Network Unit, passive optical splitter and of course the fiber (multimode in this case). Optical Line Terminal (or OLT) is a central point of the network. It forward data to and from the PON part to the core network. ONUs (Optical Network Units) are on the client side - as there are number of ONU types, we can decide that we will establish FTTH or FTTB network.
Each ONU is connected through the single fiber to the POS (Passive Optical Splitter) - POS multiplexing the optical signal from all the ONU and transmit it to the OLTs GEPON interface. Basic GEPON topology is shown on the picture below.
So what about the word passive? As you can deduct from the picture - the only active parts, parts which needs to be powered, are OLT and ONU - everything else is just a fiber and POSs. The point-to-multipoint topology reduces the investments in coverage and results in clear, hierarchical topology. As the whole distribution part is passive it decrase the possibility of the network failure.
Passive Optical Splitter#
One more very important thing can be seen on the picture above. To distribute the data to your customers you need just one
multimode fiber. It's a huge benefit against the optical switch solutions. Fiber divides, when it is needed, to multiple fiber by POS. There are number of POSs with different split - the most typical are 1:4, 1:8, 1:16 or 1:32 so you can make a choice according to the requirements. This approach implicates the low level frame distrbution method - every downstream frame is flooded to the all Optical Network Units - the frame is accepted only by the particular ONU, the rest of it ignores it. On the upstream situation is more complicated. The OLT assign the short time periods for transmission to all the ONUs so, as you can see, on upstream traffic the TDM approach is used. If we are talking about the TDM - the very important mechanism is DBA - Dynamic Bandwidth Allocation - the OLTs can assign more or less of time for the transmission based on amount of traffic reported by the ONU.
Basicly GEPON offers 1Gbps of symetrical bandwidth
on a single GEPON port (in fact it's 1,25Gbps but 250Mbps is used for OAM signaling and other low level protocol functions). The maximum distance beetween ONU and OLT is 20km which covers most of the FTTx needs. The maximum split ratio of the port is 1:64
(however because of signal loss the distance is decrased to around 10km) - the most widely used is 1:32. At this split ratio the bandwidth per ONU can be 31Mbps which is enough for triple-play services. Now we should noticed that the bandwidth is not all what we need for such services - there are VoIP enabled ONUs so we don't need to place additional SIP gateways on customer premises.
From technical point of view the OLT is just a switch. We can find most of the switching related functions supported by OLT - VLAN tagging including QinQ, IGMP related functions (another very important technique in tripple play services), LACP port aggregation, QoS functions etc. GEPON offers centralized management of all connected ONU. Optical Network Units can be divided into the two groups of products - most of the ONUs acts as simple, transparrent bridge devices (however simple doesn't mean we can't for example assign VLAN to the port!). The second group are the more complicated devices such as WLAN routers, SIP enabled gateways or even managed switches with uplink PON ports which can be used for FTTB implementations.
GEPON offers scalable, easy to maintain and relative cheap FTTx solutions. 1Gbps of throughput per port with 1:64 split ratio give enough bandwidth for triple-play services with Internet access, VoIP and IPTV distribution. As on the L2 it's a well known Ethernet protocol it's easy to mantain, don't force the administrators to learn new technology and it ensures easy integration with the existent network. There is no any frame encapsulation into the native, priopretary protocol which can reduce the performance of the whole network. The clear, hierarchical, centraly managed topology reduces the costs of network maintenance.