Types of Passive Optical Networks

PON evolved from the telecommunications carriers' need to improve efficiencies and deliver more services to subscribers. In doing so, service providers needed to ensure that their workforce need not be re-trained on an entirely new technology. Consequently the variety of PON "flavors" that exist today are a reflection of the service provider's operational requirements as well as the subscriber's demand for more bandwidth and services.

Asynchronous Transfer Mode PON (APON)

The first PON systems were based on Asynchronous Transfer Mode (ATM) protocols. This is because the state of telecommunications networks in the early to mid nineties was still very much focused on circuit switched technologies such as T1 and Integrated Services Digital Network (ISDN). Carriers who were rapidly constructing the backbone of the internet saw ATM as a natural fit for the next generation of subscriber networks. Despite the fact that DSL and the broadband cable networks were now delivering more than ample throughput for the average subscriber, the core of the carrier's network was suffering from serious growing pains. The internet boom was providing huge quantities of fiber optic cable to lay the road bed for the information super highway. The protocol selected to utilize these new fiber networks was synchronous optical network or SONET. SONET networks provided quite a leap in performance as well as throughput relative to the legacy switched network protocols. So as the carriers developed their SONET backbone, it was a natural progression to develop a new fiber based solution to answer the call for more bandwidth in the "last mile" of subscriber service delivery. Carriers did not want to take on the expense of training their engineers and field technicians on an entirely new technology so an ATM-based PON service delivery was inevitable. Thus was born the APON standard which was first recommended by the International Telecommunications Union (ITU) in 1995. The standard, known as ITU-T G.983 included a number of line rate (throughput) variations but was almost exclusively implemented as a 622 Mbps downstream and a 155 Mbps upstream capability. It also included a churning cipher security mechanism which, even by mid-1990's standards was considered rather weak. Not quite encryption, the churning cipher actually amounted to more scrambling than encryption technology and was later replaced by a triple churning cipher functionality patented by PMC Sierra.

Broadband PON (BPON)

Most PON networks utilizing the ITU-T G.983 recommendations are based upon Broadband PON (BPON) technologies. BPON is comprised of more recent G.983 recommendations and is one of the most widely deployed PON standards in North America. Like APON, BPON also utilizes a 622 Mbps downstream and a 155 Mbps upstream line rate. Owing to its ATM roots, BPON is very much a TDM-centric technology. The upstream payload of a BPON frame consists of 53 timeslots, a familiar number if you ever worked worked with ATM. For every one of these BPON timeslots there is a 48 byte ATM cell which includes an additional 5 bytes of overhead information. This overhead information provides the separation between adjacent timeslots of ONT's on the same PON. This is where PON gets fascinating. The timing of the PON lasers shutting off and on is literally in the billionths of seconds. In fact the BPON timing calls for 154 ns (nano seconds) to allow for one ONT's laser to shut down followed by the next ONT's laser to be switched on as well as provide the correct synchronization with the OLT. The ONT's laser also has to use this overhead information to tune its power output (gain) to accomodate for varying optical losses due to splices, connectors, and fiber distances.

Much of the progression from APON to BPON had to due with the efforts of the FSAN group (Full Service Access Network). FSAN is a non-standards body focused on pushing existing standards toward new products and services in support of the PON industry and consumers. FSAN worked with the ITU to develop improved technical specifications in support of BPON technologies. One of the most notable BPON deployments in the US are the Tellabs BPON systems that make up much of Verizon's FiOS network. While Tellabs comprises most of the original FiOS footprint, the Verizon network is no longer deploying BPON in new implementations since adopting GPON in early 2008. So while there are millions of BPON units in use today, one should not expect to see continued proliferation of BPON. In the past few years GPON has become the PON of choice for major network implmentations in the Americas as well as Europe.

Gigabit PON (GPON)

Gigabit PON (GPON) is also an ITU-T standard (G.984) and is characterized by a 2.5 Gbps downstream and a 1.25 Gbps upstream line rate. G.984 also includes a 128 bit AES encryption standard for security making GPON a much more secure as well as a faster line rate technology. As described above, there are amazingly tight timing considerations in the upstream direction of PON networks. If BPON is considered amazing, then GPON is downright incredible in its ability to switch one ONT's laser off in preparation for turning on the next ONT's laser. With GPON, the timeslot allowed for this interaction is a tiny 13 ns. That is almost 12 times more narrow of a window than in BPON networks. These tight optical timing constraints led many in the industry to predict that GPON would never take hold due to the complexity and cost of the optics. But just as the proliferation of BPON lowered the costs of PON optics in the mid 2000's the rapid adoption of GPON in the last few years has lowered the costs to a point where the concerns for high-priced GPON networks never materialized. Today, GPON manufacturers are starting to release 10 Gbps PON hardware and the Passive Optical LAN (POL) market is really heating up around GPON. For more information on POL, see our tutorial focused on the topic.