Info:
Tellium, Ciena eye hybrid switches
for optical transport
By Loring Wirbel
EE Times
(08/11/00, 12:18 p.m. EST)
BURLINGAME, Calif. — Tellium Inc. and Ciena Corp. revealed details of
their upcoming hybrid optical/electronic telecom switches at the
Opticon conference last week. Both were on a crusade at the show to
convince analysts that the all-optical networks touted by some
startups are neither optimal for service transport nor possible, given
the state of optical switching components.
True all-optical switching will not be required until systems hit
20-terabit/second data rates, said Tellium chief technical officer
Krisha Bala. In the meantime, Bala and Ciena chief technical officer
Steve Alexander agreed that the broadband switching architectures
that matter will combine elements of medium-density photonic
switching and electronic control of wavelength management. In
upcoming systems, both companies will use electronic control points
as mediating switch fabrics to manage medium-density optical fabrics.
"It is so important that vendors right now define their terms when
they speak of transparent optical networking," Bala said. "Photonic
switching of reasonable density can play a role in systems today, but
a true all-optical switching system is not deployable today and will not
be until further component development makes it feasible and the
speed of the core network makes it necessary."
The companies have good reason to wish to clear the air. Investors
have wildly escalated the price of recent public stocks in the optical
domain — such as that of Corvis Inc. — where companies bandy
about the term "all-optical networking" while refusing to reveal where
optoelectronic conversions take place. But in speech after speech at
Opticon, developers warned that intelligent control of optical packet
switching requires the development of optical memories, optical
clock/data recovery circuits, optical header analyzers and other
devices to transform the current analog control of wavelengths into
true digital all-optical networks.
Tellium (Oceanport, N.J.) will add to its Aurora line of optical
cross-connect products, in which electronic switch fabrics are used
to control wavelength path assignment, with a new family called
Aurora Full-Spectrum. The switch product line will integrate true
photonic switching fabrics, based on three-dimensional gimbal-based
micromirrors, with an electronic switching fabric to control light paths.
Bala said the aim of the new product line is to allow broadband switch
architectures to support OC-768 (40-Gbit/second) signals at wire
speed, while sending only those packet flows requiring a true
optoelectronic conversion to the secondary electronic switching
fabric.
Significant break
Tellium, which grew out of Bellcore/Telcordia's work in the Project
Monet (Multiwavelength Optical Networks) optical switching
experiments, does not have a particular legacy environment to
support other than existing users of the Aurora cross-connect. Vice
president of marketing Grace Carr said the concept of a two-tiered
optical/optoelectronic switching fabric was a significant enough break
from the past to justify debuting Full-Spectrum as a new product line
in early 2001.
Ciena (Linthicum, Md.) came to similar conclusions based on different
motivations. The company moved from passive dense wave-division
multiplexing (DWDM) systems to active optical switching when it made
simultaneous acquisitions of cross-connect developer LightEra Corp.
and Sonet specialist Omnia Communications Inc.
CTO Alexander said that Ciena's interest in Omnia did not come from
Omnia's presence in traditional Sonet add-drop architectures but from
its work on a 622-Mbit/s asynchronous-transfer-mode switching
fabric. Unfortunately, the Omnia window of opportunity "passed Ciena
by," Alexander said, and Omnia developers were redeployed to
LightEra teams working on different speed interfaces for the switch
that became known as CoreDirector.
In contrast to Omnia, the LightEra project gained immediate success
as a core switch for highly dense long-haul applications. The company
extended CoreDirector to smaller central offices in metropolitan and
regional rings, unveiling the CoreDirector CI switch at the Supercomm
show in June.
Alexander said that a factor as important as the hardware was the
LightWorks dynamic service provisioning environment that Ciena
launched for CoreDirector, which allows efficient real-time assignment
of services to wavelengths.
Intelligent control of that process virtually requires some
electronic-domain control of wavelength assignment for the present,
Alexander said. LightWorks also makes it easier for Ciena to link
topological meshes, which will be the logical architecture for most
core broadband fiber networks for the foreseeable future, with the
physical rings that are becoming more predominant in metro and
regional networks, Alexander said.
In an Opticon panel, Alexander agreed with several points raised by
Cisco Systems Inc. product manager John Adler, who said that moving
to an all-optical "transparent" network would remove any possibility of
monitoring optical transmission for operations, administration,
maintenance and provisioning problems. Alexander said that many
analysts today do not grasp the difficulty of moving from analog
DWDM wavelength assignment to an all-digital photonic switching
system based on true optical packets.
Conversion requirement
"Distance is not the issue as much as connectivity, which turned out
to be a key problem in Monet and other early experiments," Alexander
said. "Today, you need wavelength-selectable switches and
wavelength converters in order to scale to thousands or tens of
thousands of connections, and that requires an optoelectronic
conversion."
Alexander described a three-tiered model for optical transmission
equipment to link to terabit routers, using a space-domain optical
cross-connect at the top layer, a frequency-domain cross-connect in
the middle and an underlying electronic fabric for control of packets.
Alexander said that the general rule of thumb should be, "if you have
to touch bits, you need to go electronic."
Bala said that Tellium is applying this kind of philosophy to the new
Full-Spectrum switch. If all-optical systems try to handle OC-768 links
carrying a mix of services, "the devil is in the demuxing."
Tellium began with a strategy of retaining electronic-domain switching
for traffic grooming and mux/demuxing, and using optical-layer speed
advantages only where signal regeneration clearly was not necessary.
Creating intermediate nodes to handle regeneration will give carriers a
best-of-both-worlds approach where optical transparency is used for
transmission speed and bit transparency, while hybrid opaque
conversion to electronic signals intelligently controls wavelengths. |
