Both BPON and GPON architectures were conceived by the FSAN group, which is driven by major incumbent telecommunications operators. Most of the operators are heavily invested in providing legacy TDM services. Accordingly, both BPON and GPON are optimized for TDM traffic and rely on framing structures with a very strict timing and synchronization requirements.
In BPON, an upstream frame consists of 53 timeslots, where each timeslot is comprised of one ATM cell and 3 bytes of overhead. When two consecutive timeslots are given to different ONUs, these 3 bytes or approximately 154 ns of the overhead should be sufficient to shut down the laser in the first ONU, turn it on in the second ONU, and perform gain adjustment and clock synchronization at the OLT.
Similarly, very tight timing is specified for GPON. For example, in GPON with a 1.244 Gbps line rate, only 16-bit times (less than 13 ns) are allocated for the laser-on and laser-off times. Such short intervals require more expensive, higher-speed laser drivers at the ONU.
A very tight bound of 44-bit times (less than 36 ns) is allotted for the gain control and clock recovery. In many cases, the dynamic range of the signal arrived from different ONUs will require a longer AGC time than the allotted overhead (guard interval). To reduce the range of necessary gain adjustment, BPON and GPON perform a power-leveling operation, in which the OLT instructs individual ONUs to adjust their transmitting power, so that the levels of signals received at the OLT from different ONUs are approximately equal.
The IEEE 802 work group has traditionally focused on enterprise data communication technologies. In EPON, the main emphasis was placed on preserving the architectural model of Ethernet. No explicit framing structure exists in EPON; the Ethernet frames are transmitted in bursts with a standard interframe spacing. The burst sizes and physical layer overhead are large in EPON. For example, the maximum AGC interval is set to 400 ns, which provides enough time to the OLT to adjust gain without ONUs performing the power-leveling operation. As a result, ONUs do not need any protocol and circuitry to adjust the
laser power. Also, the laser-on and laser-off times are capped at 512 ns, a significantly higher bound than that of GPON. The relaxed physical overhead values are just a few of many cost-cutting steps taken by EPON.
Another cost-cutting step of EPON is the preservation of the Ethernet framing format, which carries variable-length packets without fragmentation. In contrast, both BPON and GPON break the packets into multiple fragments. BPON uses AAL5, discussed above, to break a packet into cells at the transmitting end and to reassemble multiplecell payloads into a complete packet at the receiving end. GPON employs the GPON encapsulation method (GEM) to enable packet
fragmentation. This method uses a complicated algorithm to delineate variable-size GEM segments and reconstruct the packets at the receiving device.
Several operators have deployed BPON systems; however, the foretold mass deployment and corresponding equipment cost reduction have never materialized. At the time of this writing, there are no announced GPON field trials, let alone commercially deployed systems. Given the level of complexity of the GPON or tight specification for various physical-layer parameters, it is very doubtful that the cost of GPON equipment can match that of an EPON.