The access piece; an ATM passive optical network increases bandwidth to the home - PON - Technology Information

The explosive growth of the Internet and the increasing popularity of data transport to the home has end users screaming for more bandwidth. But as applications continue to surge, so does the wait time. This problem is being solved by many access systems.

Fiber-to-the-home (FTTH) offers users big bandwidth improvements beyond the emerging asymmetric digital subscriber line (ADSL) technology. Better still, small businesses, home offices, and residential users in new development areas can, and should, begin reaping the benefits of FTTH with an asynchronous transfer mode (ATM) passive optical network (PON).

The driving force for FTTH, of course, is the growing need for bandwidth. ADSL can effectively provide from 1.5 Mbps capacity to about 8 Mbps, but this is the limit of copper. Based on today' s user needs, this data rate should be sufficient until about the year 2004. Then, multimedia and video services will drive the need for a new network architecture, such as fiber-to-the-cabinet (FTTC) or FTTH.

GOING ALL THE WAY

While FTTC allows for very high-speed DSL (VDSL), which will increase the data rate to about 13 Mbps, this architecture is merely an interim step to FTTH. Running fiber to the cabinet (moving the opto-electronics to within 3,000 feet of the home) could increase band width to up to 20 Mbps, but this bandwidth will also be consumed by about 2010 at today' s rapid growth rate. Eventually, the need for bandwidth will demand the capabilities of FTTH. That is why telephone companies are considering running fiber all the way to the home as soon as possible.

In major metropolitan areas and commercial areas where copper or SONET rings are readily available, it may not be practical for network providers to put in an ATM PON right away. However, in the outlying residential areas where strip malls and other small businesses exist, an ATM PON architecture makes sense.

ATM PON allows end users to gain the benefits of FTTH. It provides the missing piece--the access piece--of the all-fiber network.

While the cost of laying fiber is an immediate consideration for network providers, it is a solution that is economically justifiable in new development areas, where the ground is already being excavated and laying fiber is relatively simple. In these areas, fiber can be put in as an overlay to copper, so subscribers will still receive traditional phone service.

In areas where development is complete, the need for more bandwidth will eventually require FTTH. Telcos will also consider FTTH there as well, particularly where there is already fiber in place or as a higher-speed alternative to a possible VDSL service offering.

HOW ATM PON WORKS

ATM PON is a point-to-multipoint system that uses two wavelengths--one downstream and the other upstream--to transfer information between the optical line terminal (OLT) at the central office and the optical network termination (ONT) at each end-user site. The OLT allocates a timeslot and a certain amount of bandwidth--established by the telco--to each ONT (downstream and upstream).

In the downstream direction, the OLT broadcasts information to the ONTs via ATM cells using a wavelength that is transmitted over a single fiber. The fiber is split at a passive optical splitter, which can be directed to up to 32 ONTs.

Along with each ATM cell that is sent downstream, an ID field is sent. This field identifies the destination ONT. Technically, all the information is sent to all the ONTs, but the ONTs only take the information addressed to them.

The OLT is always physically broadcasting information downstream, but only one ONT at a time can decode the information based on an encryption code. Architecturally, it is similar to a print server setup, in which various printers are ready to receive information, but only one is addressed and prints.

Upstream, the ONTs wait for their designated timeslot, then they send control information (such as alarm notifications) back to the OLT using a second wavelength that is transmitted over the same fiber.

WHERE IT CAME FROM

ATM PON was first presented as an FTTH solution in the early 1990s by the Full-Service Access Network (FSAN) group, made up of 14 telephone companies around the world. FSAN is an effort to set common requirements among all operators globally. The requirements were forwarded to the International Telecommunications Union for standard G.983, which is expected to be released by the end of this year.

The first working ATM PON system that uses both the ONT and OLT, developed by Fujitsu, began commercial operation in Japan in September 1997. North American service trials of FTTH are planned for next year.

FILLING UP WITH BANDWIDTH

The most obvious benefit that ATM PON offers to end users is a vast increase in the amount of bandwidth delivered to the home. This opens the door to more bandwidth-intensive applications such as video-on-demand and will eliminate any network-induced wait time experienced when surfing the Web.

Right now, the ATM PON data rate is 155 Mbps downstream and upstream. An asymmetric plan calls for this rate to be increased to 622 Mbps downstream, which can be shared among several individual subscribers. Using video-on-demand as an example, a dozen subscribers each would be able to order a movie at 6 Mbps. When the movie starts, their service would be increased to the higher bandwidth amount. After the movie is finished, service would return to the usual bandwidth level.

While the higher bandwidth that ATM PON provides should not increase the cost to subscribers, it probably won't lower the cost either. In some cases, such as for SOHOs in residential areas, ATM PON may provide the opportunity for a lower-tariffed service. The service costs may not be reduced, but the cost per bit should improve.

So why would end users choose FTTH over cable modern service, which already offers a lot more bandwidth for the dollar compared with ISDN and ADSL services?

Like ADSL, ATM PON provides end users with a more secure environment than cable modern service. Casual residential users who surf the Web may not care if someone can see what they are doing, but SOHO users with a home office connection to the corporate LAN do care. FTTH is a dedicated connection that is secure all the way to the central office.

Even though ATM PON is a broadcast medium, the passive optical splitter is directional, so attenuation is low upstream. Coaxial cable (used for cable modern service) is also a broadcast medium, but attenuation is the same in both directions. When attenuation is the same in both directions, the environment is less secure.

ATM PON uses a security mechanism called "churning" to provide data encryption. In this environment, the ONTs generate and send "keys" upstream to the OLT, which manipulates the data and then sends it back downstream. These keys change dynamically, making them very difficult to access.

ICING ON THE CAKE

With the OLT at the central office and the ONT in the subscriber' s home or business, there are no active electronics (such as digital loop carrier or FTTC electronics) outside the plant--just fiber to the splitter and then to the home. Because the optical splitter is passive, it has no power or electronics associated with it. This greatly reduces the chances of a problem occurring to the end user' s connection or service.

Another advantage of ATM PON is that once the network is in place, new subscribers can be easily added by installing a splitter and running fiber through it. For subscribers, this means quicker service delivery and no delay in the implementation of new services.

WHAT'S NEXT?

The demand for bandwidth will continue to grow at an incredible rate--mostly due to the Internet, but also due to video services and data transport to the home. Subscribers can receive maximum bandwidth from an all-fiber network setup. This calls for providers to begin running fiber all the way to the home, particularly in newly developed areas, rather than stopping at the cabinet.

ATM PON allows small businesses, home offices, and residential users to reap the benefits of FTTH. Very soon, subscribers will begin to enjoy the security of a dedicated connection all the way to the home. Perhaps even more importantly, they will delight in the huge increase in the amount of bandwidth delivered.

Advancements will allow ATM PON to permit dynamic bandwidth allocation, so end users can request more bandwidth as needed. This capability will fulfill end users' bandwidth requirements well into the future.

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Smith is access product planning manager and Grimes is principal strategic planner at Fujitsu Network Communications, Inc., Richardson, Texas.

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