GRF offers IP over SONET i/f
Cell Taxes': A Necessary Evil?
By Pankaj Chowdhry
December 12, 1997 2:39 PM PST
PC Week
With the rise of IP throughout global networks, many
high-volume links are trying to sidestep the "cell tax" that
burdens ATM. But, as with most taxes, there is no way to
completely avoid these costs.
Most carriers use SONET (Synchronous Optical Network)
as the OSI Layer 1 specification for data transmission over
optical fibers in the public network, and ATM is then used
as the OSI Layer 2 link. ATM's cell tax--the extra data
that must be added to a packet to traverse an ATM
link--can eat up more than 10 percent of ATM's raw
bandwidth. IP over SONET is gaining popularity as a way
to reduce the ATM cell tax and squeeze more bandwidth
out of a link.
IP over SONET removes ATM from the transmission
picture, forgoing the traditional method of encapsulating IP
packets into an ATM cell, which is then mapped into a
SONET frame. Instead, the technology maps the IP packet
straight into a SONET frame.
The nominal data throughput rate of an OC-3 link is
155.52M bps. The first "tax" to be paid on this link is the
management overhead inherent in SONET, which reduces
the data rate to 149.76M bps. The major hit comes when
ATM is placed on the link whose overhead reduces the
data rate to 119M bps. When IP packets are placed within
the ATM cells, the data rate is reduced even further, to
123M bps.
Without ATM, IP frames are mapped straight into the
STS-3c frames of a SONET link, which, when combined
with the overhead of IP, provides a transmission rate of
135M bps. On expensive wide-area links, this 12M bps
can mean a lot: Using ATM instead of IP over SONET is
basically throwing away 10 T-1 lines' worth of data
transfer time and money.
However, not all networks can reap the benefits of IP over
SONET. First, all data traversing the link must be IP.
Because 155M bps of IP data is not easy to come by, this
requirement is a major drawback of using IP over SONET.
ATM can be more easily statistically multiplexed than IP
over SONET, however, and ATM can be run on
lower-speed links. |
