Take Your Pick: Frame, ATM Options Abound (Inter@ctive Week)
For the corporation whose wide-area traffic growth is creating
increasingly cost-sensitive network technology decisions, enhancements
in both frame and cell relay access technologies are coming to the
rescue.
On the frame relay side, vendors are lowering the price and refining the controls of Permanent Virtual Circuits (PVCs), making it easier to
migrate and manage multiple traffic types over a single frame connection.
On the Asynchronous Transfer Mode (ATM) side, vendors are easing the
ability to bond multiple frame relay circuits into a single ATM circuit through Inverse Multiplexing over ATM (IMA). They're even promising low-bandwidth ATM circuits by next year.
According to Cathy Gadecki, senior broadband consultant for TeleChoice
Inc., wide-area users begin to face a decision point as their
1.5-megabit-per-second, or T1, frame access lines multiply beyond a
handful.
With IMA's ability to bond multiple T1s together as if they were a
single 3-Mbps, 4.5-Mbps or 6-Mbps link, she says, "some commercial users are moving to IMA because their T1 frame relay networks are saturated. Rather than buying one more frame relay T1 after another, which will run out soon, I can pay a little more and interface my router to the IMA mux, so every application gets access to the total capacity."
ATM's statistical multiplexing capability creates a variable, on-demand Switched Virtual Circuit (SVC) for each application, tailoring capacity to the application's requirements. With inverse muxing, the SVCs can manage that capacity across multiple T1 lines.
In contrast, with frame PVCs, an application is limited to fixed
capacity, and "it's very hard to load-balance across multiple routers," Gadecki says.
Yet ADC Kentrox (www.kentrox.com), Cabletron Systems Inc.
(www.cabletron.com), Memotec Communications Inc. (www.memotec.com),
Paradyne Corp. (www.paradyne.com) and others are adding frame access
devices that offer rudimentary traffic control and load-balancing.
Paradyne offers prioritization on a PVC-by-PVC basis.
"You can combine applications out of the router, prioritize each one at the frame layer, then recombine them over a single PVC," says Frame
Products Director Jim DesRosiers. "If an application starts outrunning
its capacity and performance degrades, the user can turn on compression."
Cabletron's frame access devices similarly accommodate more than 900
PVCs per physical connection, and each PVC can be assigned a committed
information rate, as well as a "burst window" for additional capacity
when it's available.
On the same track, Momotech's frame access gear applies compression,
prioritization and packet fragmentation -- so no small packet gets
congested behind a big packet -- to put more bandwidth management into
the user's hands.
SVC Advantage
Also, IMA products designed to consolidate growing T1 lines, while
making frame-ATM operation transparent to the user, are debuting from
Ascend Communications Inc. (www.ascend.com), Fore Systems Inc.
(www.fore.com), General DataComm Inc. (GDC) (www.gdc.com), Newbridge
Networks Inc. (www.newbridge.com) and other vendors.
Like frame suppliers, those ATM suppliers are pitching the relative ease and scalability of managing circuits rather than packets.
"Whether it's ATM to the desktop or to the workgroup, management of
bandwidth, class of service, security and other policies gets simpler
because it's circuit-based," says Alistair Crolle, TranscendWare
management products manager for 3Com.
Consequently, the new Carrier Scale Internetworking partnership of 3Com, Newbridge and Siemens AG intends to extend traffic prioritization and other policy capabilities from enterprise through service provider networks, regardless of the frame-ATM technology mix.
Giving the end user policy control over wide-area ATM SVCs "increases
trust between the customer and service provider," he says. "Whereas with frame relay growth, you're just managing more devices and links, with ATM, the service provider can help with things like applying circuits on demand."
Even without the bandwidth efficiency benefits of IMA, carriers may
succeed in selling managed IMA services on the basis of disaster
recovery.
For example, any IMA connection will require one local access provider
at each end and at least one long-distance backbone carrier in the
middle. With IMA, a corporation could assure itself a more fail-safe
network by multiplexing its multiple T1s across more than one
long-distance carrier.
Advancing Internetworking
"Before a failure, IMA is spreading cell throughput across perhaps three T1s, with each T1 provided by three different backbone providers," says Tom Murray, ATM core switch product line manager for Fore Systems. "Once one of the three carriers' T1s fails, IMA recognizes the lost carrier and begins to multiplex across the two remaining T1s at the reduced aggregate data rate."
At bottom, says Tim Krasky, marketing vice president for Ascend's core systems division, IMA works with both what is and what can be to advance wide area network-local area network internetworking. "There shouldn't even be a question anymore that ATM has won in the wide area, Ethernet has won in the enterprise, and IP [Internet Protocol] over frame, ATM or TDM [Time Division Multiplexing] wins everywhere."
Carriers may skin the internetworking cat with not only IMA but
usage-based T3 services and, eventually, direct T1 ATM services.
According to TeleChoice's Gadecki, at distances over 300 miles, carrier tariffs make an ATM T1 cheaper than a private dedicated line.
However, not until next year do vendors expect processor technologies to facilitate affordable port densities for T1 ATM.
"Expect significantly higher densities in 1998 from most vendors," says Bill Miller, associate vice president of ATM marketing for GDC. "That's where most of future ATM growth is seen -- in T1."
Further, the administration overhead required in ATM transport has so
far made its efficiency highly questionable at low bandwidths like T1.
However, Miller says, the emerging implementation of a new ATM
adaptation layer, AAL2, in variable bit-rate real-time service will
solve that problem.
Emerging Tools For WAN Capacity Management
Frame Relay:
Permanent Virtual Circuit multiplication -- assigns applications to
hundreds of PVCs per physical port.
Circuit-by-circuit compression -- squeezes more data into fewer frames.
Circuit prioritization -- puts critical traffic first in line between
router and WAN access port.
Burst window -- automates ability of applications to "burst" above its
assigned bandwidth when other capacity is available.
Asynchronous Transfer Mode:
Inverse Multiplexing -- enables enterprise to bond multiple
low-bandwidth access lines together to create a single broadband line.
T1 ATM -- promises to put low-bandwidth, on-demand switch circuits in
reach of branch offices and small businesses next year.
Usage-based T3 -- promises to provide on-demand access to broadband
Switched Virtual Circuits without the cost of monthly broadband line
charges. |
