Growing consensus within the rival cable and telco camps
about how they will move to broadband is inspiring suppliers to come up with fresh
approaches to fiber distribution in local and regional networks.
On the telco side, unanimity among major local-exchange
carriers in the United States and abroad on the use of ATM (asynchronous transfer mode)
for end-to-end broadband connectivity is inspiring new initiatives at the interoffice
level.
For example, telcos like BellSouth Corp. and Japan's
Nippon Telegraph and Telephone Corp. are taking new looks at dense wave-division
multiplexing for the last mile in a new push for fiber-to-the-home solutions.
Meanwhile, in cable, a new trend toward the direct
insertion of IP (Internet-protocol) packets from routers into SONET (synchronous optical
network) multiplexers for high-speed backbone transport is inspiring thoughts of going one
step further: adding SONET-like capabilities to the layer-3 domain of IP-packet
management.
The foundation for the new BellSouth/NTT FTTH initiative,
officials said, is agreement on the fundamental architecture for next-generation
fiber-rich access networks within an international carrier group -- the Full Service
Access Network initiative.
That architecture entails the delivery of ATM-formatted
signals over fiber from central office or remote digital terminals to passive optical
splitters, which distribute the signal into as many as 32 fibers for the final run of
fiber to termination points known as optical network units. The signal speeds range from
OC-3 (155 megabits per second) bidirectionally to OC-12 (622 mbps) downstream and OC-3
upstream
This architecture can be applied in several configurations,
said Hank Kafka, executive director at BellSouth's Advanced Technology and Systems
Engineering Center. It uses time-division multiplexing at each ONU to extract the portion
of the SONET OC signal meant for the user or users served by that connection.
These include:
Fiber-to-the-node, where signals are sent over VDSL
(very high-speed digital-subscriber-line) copper links at anywhere from 13 mbps to 25 mbps
to customers, depending on the distance of the node from the premises;
Fiber-to-the-curb, which is a deeper extension that
uses VDSL to operate at up to 52 mbps; and
FTTH, where the ONU is located at the customer
premises.
In all cases, the design calls for single-fiber links,
using two wavelengths (1310 nanometers and 1550 nm) to carry downstream and upstream
signals over the same fiber.
"Basically, the way that we're approaching access
technologies is to have a full set of tools that we can bring to bear in various
situations, depending on what works best," Kafka said. "We expect that FTTH will
prove in economically in a number of different applications."
Those include newbuild areas, where BellSouth has routinely
been constructing FTTC systems for supply of voice services over the past several years,
and possibly some rebuild situations, Kafka said. In addition, the company is considering
FTTH as a video-transport overlay to existing copper, he noted.
"There is high interest within the industry overall in
the FSAN architecture, but the focus on different versions varies from one company to the
next," Kafka said. "Our initial focus is more on FTTH than on VDSL."
With NTT sharing that interest, BellSouth hopes that the
pooling of efforts to resolve specific design issues that are unique to the FTTH aspects
of the FSAN architecture, particularly at the ONU, will lead to industrywide consensus on
an FTTH model, Kafka said.
"The key aspect to getting costs down is to get volume
up, which requires agreement on the specifics," he added.
NTT, which is already using the FSAN model for all-fiber
systems serving business centers, has been informally sharing information with BellSouth
for some time. Now, the two companies hope to test FSAN-compliant FTTH systems starting
next year, with the intention of making the technology commercially viable for deployments
by 2001.
The FSAN initiative -- which includes the six major U.S.
LECs and eight other large telcos in Canada, Europe, Asia and Australia -- has made SONET
a fundamental part of the architecture. Some suppliers, however, believe that the
widespread concurrence on the use of ATM end-to-end opens an opportunity to dispense with
the use of SONET multiplexers. This would occur by using the power of ATM protocols in
conjunction with all-optical add/drop multiplexers to perform the optical-network
configuration, restoration, monitoring and management tasks that are now assigned to
SONET.
"Direct feed of ATM into the optical layer is
something that we're taking a hard look at," said a senior executive at Bell
Communications Research, which has been tapped to supply the system-integration software
for Sprint Corp.'s new Interactive On-Demand Network (ION).
While Sprint's data-networking unit has begun
deploying IP-over-SONET solutions, bypassing ATM altogether, the new ION architecture
requires new thinking about how to exploit the cost-savings potential of DWDM technology
in the backbone, the Bellcore official said. The design tightly integrates IP-over-ATM in
new edge switch/routers to be supplied by Cisco Systems Inc.
The first vendor to publicly tout such a solution is
Ericsson Inc. The U.S. unit of the Swedish supplier recently demonstrated the concept of
SONET-less long-haul WDM transport over a 650-kilometer link.
In this case, the packet format employed was Ethernet LAN
(local-area network). Here, a gigabit Ethernet router directly fed the Ericsson Optical
Networking (Erion) DWDM system. But the idea will work with ATM, frame relay and even IP,
said Roselyne Genin, vice president of optical networks and transport solutions at
Ericsson.
"What we've shown is that WDM can be a bridge to
multiservice transport in native formats that gives carriers great flexibility in network
configuration," Genin said. "We replace the restoration capabilities with the
optical-layer protection and scaleable capacity provisioning that's built into the
Erion system."
This approach uses optical add/drop multiplexers as access
points to a ring architecture, where a wavelength segment of the ring can be flexibly
shifted around through optical-amplifier toggling to create a "flexing bus"
catch-all for any failure that might occur, she said.
Other vendors are looking at the native formats themselves
-- specifically IP and ATM -- as the key to eliminating SONET, rather than relying solely
on the protections afforded by solutions like Ericsson's. "SONET offers direct
interface with network-management systems, as well as the optical-layer protection, which
requires more than an optical-layer solution," said an executive at a leading
competitor to Ericsson, who asked not to be named.
From a cable perspective, the possibility that IP could be
developed to supplant the network-management components of SONET opens a route to much
tighter integration of the high-speed-data architecture on an end-to-end basis.
"This is where some important synergies come into play
that could benefit cable-data providers," said Karl May, vice president and general
manager of broadband technology at Bay Networks Inc., which is set to be acquired by
Nortel.
"Things like IP-over-SONET or ATM-over-SONET are
continuations of a trend, and not the type of inflection point that we believe is possible
in conjunction with the growing expansion of IP technology," May said. "We
haven't reached the point of engineering a product yet, but we've been
discussing internally the idea of what you could do if you had access to the DWDM
technology that we will now have access to through the merger with Nortel."
May added, "There may be a lot of overhead in the
intervening layers that we can get rid of. That would really be an inflection point in the
evolution of network architecture."
