An opposing view to the PONmeisters:
(snipped from worldwidepackets.com use the link to see the figures)
Passive Optical Networks are access networks in which fiber trunks are fed toward end points and split into multipoint trees along the way, until reaching a termination of the fiber run. A PON consists of Optical Line Termination (OLT) and Optical Network Unit (ONU) equipment. It is deemed "passive" because the physical connection between the OLT and ONUs, referred to as the Optical Distribution Network (ODN), consists only of passive components such as optical fibers, connectors, splitters, combiners and splice points. One OLT typically supports up to 32 ONUs-the ITU recommendation states desired support for up to 64 ONUs.
The OLT is usually located at a head-end, where services originate. ONUs can exist in a variety of locations depending on the infrastructure model. For instance, in an FTTC scenario, ONUs could be located on curbsides, where the optical signal is converted into an electrical signal for delivery over the coaxial cable that feeds the homes.
In an FTTH model, the ONU is called an Optical Network Termination (ONT). The optical signal from the OLT is transported to the customer premises without being converted by active components. While this passive nature sounds unique, it should be noted that in any FTTS model where fiber connects the customer premises directly to a head-end system, the transport of the signal between the two is passive by the very nature of fiber optics.
At each ONU or ONT, optical to electrical (or electrical to optical on the return path) conversion is performed. In addition to the signal conversion, a multiplexing algorithm is performed, such as Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM), in order to combine multiple subscriber signals onto a single fiber. All of this equals one thing; complexity that rapidly increases as the network scales. For an example of the different topologies, refer to Figure 3.
PON utilizes ATM cells and other transport technologies to create logical connections between the ONUs and OLTs. The bandwidth on the ODN is typically 155 Mbps (OC-3) or 622 Mbps (OC-12). Each fiber trunk is split at various points in the network in order to service all ONUs on its system. This effectively reduces the available bandwidth per subscriber. For instance, in a 32 ONU PON, given an OC-12 trunk and an FTTH topology, the bandwidth per ONT (subscriber location) would be 622 Mbps/32 ONUs "19.5 Mbps. In the case of FTTC, the bandwidth per subscriber is further reduced, due to the hundreds of subscribers on the coax branch, typical in most neighborhoods. So, rather than having 622 Mbps/32 ONUs (~19.5 Mbps per ONU/ONT), the bandwidth is further divided by the number of homes sharing the services of one ONU (19.5 Mbps/100 homes = 195 Kbps). Figure 4 illustrates this bandwidth splitting from a logical viewpoint.
Part of the problem with PON networks is that the original bandwidth recommendations were made in 1996 (International Telecommunications Union (ITU)-T G.982), with a second recommendation made in 1998 (ITU-T G.983). The "future services" outlined in the specification have come and gone, being overrun by content that requires double and sometimes triple the bandwidth of its predecessor. Even if the ODN bandwidth is scaled to
OC-192 rates (~10Gbps), a cluster of 32 ONUs needed to service a typical neighborhood would have only 312 Mbps to distribute among 100 homes. In an FTTC environment, this would approximately equal 3.12 Mbps of effective bandwidth to each subscriber. Obviously, as the number of homes on the copper loop increases, per-subscriber bandwidth decreases.
These examples are, however, only downstream data rates. The upstream path is limited by the number of combiners it can traverse. This lack of consistency leads to complicated fiber plans, the need for more equipment in the community and asymmetrical bandwidth rates. As of yet, PON is not a standard, but the ITU is aggressively seeking a standards specification.
The largest deployment of PON technology to date is in Japan. Nippon Telegraph and Telephone (NTT), Japan's largest telecommunications provider, is installing fiber optic cabling throughout Japan in order to deploy a Full Service Access Network (FSAN). An FSAN system is a large-scale, broadband, optical access network that attempts to deliver existing and future broadband services using PON or ATM technology. The FSAN Forum, a group of 20 telecommunications giants from around the world, is driving the development and acceptance of an FSAN standard for delivering voice, video and data.
The reasons behind deploying PON beg a few questions. If streets are going to be torn up to implement fiber in the first place, why not lay enough fiber to reach all the subscriber premises rather than using the coax or twisted pair already in place? And, if fiber was installed all the way to the subscriber, is ATM the best underlying technology to connect them? Of course, the 20 phone companies in the FSAN believe that it is. After all, they gave birth to ATM a decade or so ago for use on their voice networks. The fact remains, however, that with its blend of bandwidth limitations, expensive head-end systems and over-complex operation, Passive Optical Networks simply don't allow for the kind of future-proof, broadband access required to deliver the content of today, let alone tomorrow. |
