All-optical networking in data communications is inching
closer to reality on the heels of new product introductions by system suppliers and new
deployments on the part of carriers.
So far, most of the action centers on the use of Internet
protocol-over-synchronous optical network (IP-over-SONET) capabilities built into
next-generation routers in conjunction with dense-wave-division-multiplexing technology in
long-haul data networks.
But even as this trend is just starting to build, a new
wave of applications is following close behind, involving DWDM at the metro level, in
conjunction with more advanced trafficking capabilities in the optical domain.
"Although the metro market has been slow to adopt
DWDM, we believe that 1999 will be the start of a tremendous market upswing," said
Ken Lewis, senior vice president of the transmission division at Alcatel Telecom's Alcatel
Toward that end, Alcatel last week demonstrated an
all-optical, cross-connect-employing, space-switching and optical-wavelength translation
that it said would be used in a complex multinetwork trial involving European carriers
"Our test results confirm our vision of a relatively
near-term all-optical layer," Lewis said.
Such capabilities -- currently offered by at least one
manufacturer, and soon to be offered by many -- are being driven by the growing support
for the last generation of commercial data-over-DWDM products by carriers such as Sprint
Corp., Frontier GlobalCenter, GTE Corp. and Qwest Communications International Inc., all
of which have begun deploying IP-over-SONET OC-48 (2.5-gigabit-per-second) systems.
"There are several large ISPs [Internet-service
providers], as well as carriers, that want to move in this direction by end of the year,
so that's where we're heading," said Tony Bates, director of marketing at the
optical-internetworking business unit of Cisco Systems Inc., the supplier of gigabit
routers in these deployments.
Competition for bragging rights among data carriers
exploiting this technology has become intense, with most claiming to have the biggest,
fastest network around.
For example, Guy Cook, vice president of data networks at
Qwest, said last week that his company was expanding its IP-over-SONET DWDM links to
connection of 125 major metro regions nationwide by year's end, putting it "in the
But Frontier claimed to be offering the highest density of
wavelengths in operation using IP-over-SONET, with five of its 23 markets slated to be
turned up by year's end, and the rest by the end of the first quarter. The company's
system supports 16 wavelengths in the current iteration, with upgradability to 32.
"We have more than 200 large-scale customers operating
some of the most popular Web sites -- companies like Yahoo! [Inc.], Netscape
[Communications Corp.], USA Today and Ziff-Davis [Publishing] -- that are telling
us that they need more high-speed network capacity for things like database replication
and system backups," said Frontier chief technical officer Jonathan Heiliger.
"As providers of Web content see ever more
profitability associated with local high-speed access, the need for distant caches to
facilitate rapid distribution of that content is going to become even more
significant," he said.
The IP-over-SONET capabilities of today's gigabit routers,
and of the terabit routers that are slated to follow next year, represent a means by which
the router eliminates the need for a SONET multiplexer between the local and long-haul
Today, this is done by delivering a high-level SONET
payload into a transponder at the carrier point of presence for conversion to a specific
wavelength on the DWDM long-haul network.
But as Mark Yin, manager for technical marketing at Ciena
Corp., noted, this step sets the stage for quickly moving to even greater efficiency by
using multiwavelength technology to off-load some of the traditional SONET-protection
mechanisms to the optical layer.
Ciena, supplier of the DWDM system for Sprint, is working
closely with Cisco to facilitate the router-DWDM-integration process.
"We don't have any way for the IP layer to talk down
to the SONET layer to negotiate operation commands," Yin said. "But there are a
lot of ways that you can make the transport layer-responsive, including defining
protection on a per-application basis."
While it's too early in the DWDM-implementation cycle for a
buildup in demand for fully meshed systems, such as what would be possible with
all-optical cross-connects, it's clear that the demand will soon be there, said Bill
Gartner, vice president for optical-networking-product development at Lucent Technologies.
"How many wavelengths will it have to handle to be
useful?" he asked. "We believe that it will have to be more than 128, with an
upgrade path to much higher levels. In fact, we have a customer that wants to see a
product with a migration path to 1,000-by-1,000."
Long before such products become feasible, key issues
surrounding the partitioning of management functions at various layers must be resolved,
One step in that direction involves
"path-switching" in ring architectures, as opposed to the standard SONET BLSR
(bidirectional line-switch ring) approach, said Pawan Jaggi, manager of the
optical-networking group at Fujitsu Network Communications Inc.
"The basic difference between line and path switching
is that path switching gives you the opportunity to remove the loop-back switch, which can
cut the number of regenerative sites on the ring by 50 percent," Jaggi said.
In the case of BLSR, a fiber cut results in regeneration of
the signal at the node and a switch in direction either over the same optical fiber or
wavelength, or over a separate fiber or wavelength, which, in either case, requires that
substantial capacity on the ring be reserved for such contingencies.
In path switching, an optical add/drop multiplexer provides
a means by which signals are switched to an open wavelength on the ring in the opposite
direction around the cut, avoiding any need to regenerate the signal.
As carriers move to the use of router switches pumping data
directly into the long-haul optical domain at OC-48 rates, the search for regional
solutions necessarily intensifies, Jaggi said.
Fujitsu last week began demonstrating an
"eight-by-eight" DWDM metro-fiber system, delivering eight OC-48 channels in
each direction over a single, unamplified fiber.
By mid-1999, the company plans to introduce a dynamic
add/drop multiplexer, which will allow its unidirectional, DWDM-metro system to evolve
into an all-optical ring, Jaggi said.
Cisco has already begun supplying routers with lower-speed
optical outputs suitable for long-reach interfaces of under 80 kilometers in the MAN
(metro-area network) environment, Bates said.
"Later, we'll add long-reach interfaces at OC-48,
allowing the router to drive the fiber directly at that rate," he added.
Over time, this will support direct integration of services
into the optical layer. "It doesn't make much sense to integrate full WDM into the
router at this point, given the cost of finely tuned lasers, but this will change with the
advent of truly tunable lasers," Bates said.