The cable industry's attempts to get as much as
possible out of its commitment to data services keep getting easier, thanks to the growing
effectiveness of Internet-protocol technology in network architecture.
Over the past few months, major players in data networking
-- including some cable entities -- have switched to a new data-backbone design that uses
very high-speed routers to directly transfer IP packets to and from SONET (synchronous
optical network) links.
This technique eliminates any need for intermediary
technologies between IP hubs and long-haul optical links, such as asynchronous transfer
mode or high-speed Ethernet. Further, it vastly expands the usefulness of "layer
three" -- the trafficking protocols contained in IP packets -- as the fundamental
paradigm in switching and aggregating data end-to-end.
"Today, [IP over SONET] is a point-to-point
technology, but there will be an evolution to more complex architectures, starting with
rings, and then moving to mesh topologies that use layer-three routing [at all network
levels]," said Graeme Fraser, vice president of marketing at Cisco Systems Inc.
The ring implementation of IP over SONET is already under
way in the new upstate New York regional backbone being installed by Time Warner
Cable's Road Runner, chief technical officer Mario Vecchi said.
"Packet over SONET is a pretty easy call for us to
make at this point," Vecchi said.
And other entities -- including GTE Corp.'s
Internetworking unit and America Online Inc. -- are already moving in the direction of the
"mesh" scenario mentioned by Fraser, where IP over SONET is used as a mechanism
for handling traffic among routers within a data center.
"We're committed to packet-over-SONET
technology," said Steven Blumenthal, vice president and general manager at GTE
Internetworking, which has been assigned the responsibility of spearheading GTE's
transition to a nationwide data-backbone infrastructure.
"We're relying on the router's ability to
deliver class of service and quality of service; to be able to mix real-time traffic, like
voice, video and multimedia; and to give that an express lane through the network while
mixing that traffic with lower-priority data, all at the IP layer," Blumenthal said.
Such a commitment is made possible, Blumenthal added, by a
rapidly evolving set of protocols that allows things to be done in native IP that were
once the purview of the switching domain.
"We're very comfortable with the ability of IP to
differentiate various types of packets in the network and to provide an express
[circuitlike] service, just as you might be able to do with ATM," he said.
"In terms of traffic engineering, you have to do this
at the IP level, anyway," Blumenthal said. "And we see technologies emerging,
such as MPLS [multiprotocol layer switching], that will be able to help with that."
MPLS, also known as "tag-switching," is an
emerging standard that relies heavily on algorithms developed by Cisco. It uses a simple
label attached to an IP packet to tell the router which class of service is involved and,
therefore, which priority to assign the packet in the routing process.
GTE has been working "pretty closely with Cisco to
understand MPLS technology," Blumenthal said. "We have some concerns with the
current tag system, and we're working very actively within the MPLS process with the
goal of deploying it in a more standard form."
Nobody has gone further than AOL in migrating IP-networking
power in conjunction with SONET to the local level.
"We've gotten to the point where we're using
IP over SONET really as a data-center technology, replacing LAN [local-area network]
technology," said Victor Parente, chief network architect at AOL.
"When I started looking at IP-over-SONET technology,
what we were looking for was a replacement for the current DS3 [45-megabit-per-second]
technology and ATM that we were using to aggregate intersite data-center traffic,"
"There are a lot of problems with [the ATM approach]
in terms of the lack of maturity of the software, and also in the overlap or inconsistency
between routed IP architectures and switch architectures," he said.
The company took out the ATM complexes and replaced them
with "clear-channel" connections between the data centers, using IP over SONET.
"All of that complexity was replaced by a simple pair
of routers running back-to-back," Parente said, adding that from there, it becomes a
relatively simple step to use IP as the LAN technology within each data center.
"We differ significantly from other ISPs in that we
run very large data-center networks, as well as transit networks," he said.
AOL is wrestling with issues that other providers of
mass-market services -- such as high-speed cable and ADSL (asymmetrical digital subscriber
line) providers -- must deal with as they ramp up to millions of customers.
"How do I bring many gigabits of Internet capacity
into a data center?" Parente asked. "How do I design a new network that's
going to allow AOL to peer with multiple ISPs [Internet-service providers]?"
AOL must handle in excess of 10 gigabits per second, per
data center, and it must sustain high-port densities connecting up to 64 routers, Parente
"We needed redundancy and reliability, and what I call
LAN/WAN [wide-area network] transparency, which, I think, is one of the really critical
values of IP over SONET," he said.
"This means that when I'm moving bits from one
computer room or data center to another, I want to be able to take that data transfer on a
continuous path, across the road or across the country, without changing data rates and
protocols," Parente said. "I can use the same capacity without having to deal
with bottlenecks at some point in my network."
Circuit management is made easier, as well, Parente said,
noting that the ATM-mesh array, to fit the size of AOL today, would require
troubleshooting 1,200 circuits, as opposed to the eight circuits that he has to watch
using IP over SONET.
Parente said he also looked at gigabit Ethernet, but he
decided that it "doesn't scale very well." A gigabit may sound like a lot,
he added, "but my job is to plan further into the future, and IP over SONET scales a
Adding force to the layer-three agenda is a new initiative
within the Internet Engineering Task Force that's focusing on a header byte that was
originally designated for differentiation of service, but that was never standardized. The
effort, which only got under way earlier this year, is moving quickly toward establishing
a standardized approach to setting quality-of-service parameters within the header itself.
This effort dovetails nicely with the efforts of the DOCSIS
(Data Over Cable Service/Interoperability Specification) group at Cable Television
Laboratories Inc. to define extensions to the standard cable modems that will support such
services as IP telephony and real-time video, said Gerry White, chief technical officer of
the broadband division at Bay Networks Inc.
"The extensions will allow the CMTS
[cable-modem-termination system in the headend] to look at IP level three and to determine
how that is to be implemented in the layer-two MAC [media-access control] of the
cable-data-distribution system," he said.