Re: Big Blur- TV Land, ATM/PON or APON, VDSL, ALA, SBC Project Pronto and 99-238
Thread- Sorry for all the acronyms above, but there was a lot covered in this article. It's a very interesting article mainly about PON and it's future possiblities. I had no idea PON was originally a TV architecture! I picked this off a website where I think you have to register. -MikeM(From Florida)
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Optics is hot, but the hottest topic in optics isn’t even much recognized by the media. It’s passive optical networking, or PON. There’s a good chance that most businesses will be touched by PON in the next five years, and almost a unity probability that PON will touch every business by the end of the decade.
PON is a fiber architecture in transition, its original application having been largely displaced by newer ones. Still, both the PON of old and the PON of today are alive and well, and there’s a good chance that PON will eventually influence not only business communications, but residential voice and data, and even entertainment video.
The original PON objectives were more related to broadcast television than to two-way communications. Starting in the late 1980s, various trials were conducted on the use of fiber architectures that served multiple consumers using passive splice technology (hence the name) rather than a network of active devices. This would cut the cost of fiber deployment sufficiently to permit its use as an alternative to CATV cable in provisioning television services.
In application, PON creates what might be called a “tree”, a tributary structure that involves splice-based “forks” that eventually terminate in consumer locations. The tree is fed at the trunk or head end with enough laser power to not only accommodate the optical dispersal along multiple paths, but to account for the loss in the splices.
A PON tree can be used to carry virtually any signal that can be converted into optical laser drive, including analog and digital video. However, the nature of PON is that the tree works fine from trunk to branch and less well from branch to trunk. The reason is that traffic originating at the branches would collide at each splice point, contention.
Solutions to the PON contention problem have been proposed for some time, but only one has really been broadly accepted. APON, or ATM PON, is a cell-based architecture that employs TDM multiplexing that some will find reminiscent of the old SMDS slot structure. The architecture allows the optical tributaries on the tree to receive a common feed (the downstream direction) and to share an upstream feed to the head end.
APON has been standardized (G.983.1), and large-scale trials are already underway. APON is also apparently the architecture that most of the RBOCs will deploy to provide for broadband access to business and residential services. It’s suitable as a direct business service (ATM is then the payload to the user) and as a feeder for both ADSL and the higher-speed (up to 58 Mbps) VDSL.
The G.983.1 standard for APON is a fairly conservative one. The architecture consists of a tree with a total of 32 splits (64 endpoints) and a total length of no more than 20 km (12 miles). One mode supports symmetrical 155 Mbps (OC-3) service, and another has a 622 Mbps downstream feed (OC-12) and a 155 Mbps upstream feed. The electrical layer supports SVC capability as well as PVCs.
A more aggressive approach is being taken in the so-called “SuperPON” trial, in which Alcatel is the main player. SuperPON increases the service data rate to 2.5 Gbps (OC-48) downstream and 311 Mbps (OC-6, if you can imagine) upstream. The topology of SuperPON isn’t entirely passive; there’s a feeder and feeder repeater structure feeding amplified splitters. The feeder-to-splitter distance can be as much as 90 km (54 miles), and the terminations can be up to 10 km (6 miles) from the splitter (the 90/10 topology is also supported for APON, but not by everyone).
Those interested in how APON and SuperPON work at the electrical and physical layer can refer to the relevant standard and other documents from the vendors for the details. At the high level, the downstream feed is TDM-divided and its upstream feed is controlled by TDMA (time division multiple access). Broadcasting downstream is easy, and redundancy can be supported through the use of doubled fiber strands. Studies generally show that PON is more fault-tolerant than SONET, because the components in PON aren’t active and can’t lose power. The MTBF of a splice is also clearly lower than that of a node.
The international Full Service Access Network Consortium (FSAC) has declared APON to be the most effective approach to generalized broadband access services, which it clearly is. That it’s also a player in the new-gen RBOC infrastructure has been noted already, though the exact role and timing of PON there may be still undecided.
For the RBOCs, PON may solve a knotty problem; how to get fiber infrastructure into the field at a large scale without an unacceptable wholesale risk. ATM infrastructure is shielded from unbundling, our readers will know, but customer loop isn’t. Here’s a critical question, then; is PON loop shielded? Even if the answer is that it is not, what’s the wholesale price and what are the restrictions on access? You can’t co-locate with an optical splice, after all.
The RBOCs appear to have decided that at the very least, APON offers no more risk of unbundling than other fiber remote strategies, and SBC has specifically indicated that its Project Pronto infrastructure will serve large businesses (and presumably multi-tenant sites) with APON. Less clear is the nature and timing of the use of PON in the residential market. The newer Alcatel Litespan 2012s have PON capability, and thus could support the deployment of APON-based residential services.
If SBC intends to shield their business infrastructure from unbundling, as we’ve suggested, it’s our view that ATM (with or without PON) isn’t enough. There’s no statutory basis for claiming exemption for pure business infrastructure, since the FCC ruling involved (99238) draws its authority from the Telecom Act’s mandate of the FCC to promote universal service—meaning residential service. Thus, SBC must associate residential service with any given PON tree to claim exemption from unbundling.
But this means that any business PON tree must have residential subscribers, so some neighborhood gateway remotes must be based on APON. Will they all do so?
We think most will, perhaps all. The problem for SBC and the other RBOCs is that one never knows when and where a business user will pop up. Deploy specialized residential fiber remotes with no PON capability and you risk deploying something you can’t leverage without rebuilding it. PON also lets SBC and others create more use for access fiber that might be lying around in dark fiber form. The 10Q Bell Atlantic filed with the SEC this spring notes that the same FCC order that creates the unbundling exemption also mandates dark fiber be unbundled. Why do that unless you plan to light it?
In any event, it should be clear to all that PON in general, and the APON/SuperPON family in particular, are things to be watched for the future. |
