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ATM Networks: Little Cells Doing
Big Things What's in the core of 80% of
today's telecom carriers might also make sense for your networking
By: John Shepler
Think of ATMs and you think of technology that
dispenses cash. In networking terms, that's very true. ATM is
something of a cash generator, but it's not something the consumer
is likely to come in contact with.
What I'm talking about is a core transmission
protocol called Asynchronous Transfer Mode or ATM. ATM is in
the family that includes Ethernet, Frame Relay and good old Time
Division Multiplexing or TDM. At one time, ATM was thought to
be the networking technology to end all others. That hasn't happened
and is unlikely to. Even so, as much as 80% of telecom carriers
are running ATM on their networks right now.
Why ATM? ATM was planned to be able to carry just
about any voice, video or data protocol that you can digitize
into packets. To do that, the datastream is constructed of small
uniform packets of 53 bytes or octets each. Five bytes are assigned
to the header and 48 to the payload. That's it. If your original
data comes from longer frames, it is split up among multiple
ATM cells as they are called.
A network that only has to deal with small
cells of uniform size can be streamlined. ATM packet handling
can be implemented in hardware instead of software. That makes
it easier to speed up the packet switching to optical carrier
speeds. In fact, that's where you'll find most ATM networks are
running. While the standard can be used on T1 and T3 lines, it
is most popular for OC3 and OC12 carriers. It can be scaled up
to at least OC48 and probably beyond.
Network interfaces at the core and edge
are all known as ATM switches. There are no routers. The reason
for this is that ATM packets are carried over virtual circuits
on virtual paths. All cells travel exactly the same route from
source to destination and stay in order. That is a big advantage
for voice and video because it results in properly sequenced
packets with very little jitter to garble the voice or smear
IP routing, on the other hand, can send
sequential packets over different paths because there is no connection
or circuit involved. That makes for a variable packet arrival
time which causes delay and jitter. If a voice or video packet
arrives out of sequence you might as well throw it out because
there is no going back to re-insert it in a real time data stream.
On the other hand, if somebody cuts a wire then routers can re-route
the packets to get to their destination via another path. That's
part of the robustness of the Internet which was designed to
workaround disasters of all types.
So why isn't ATM likely to rule the networking
Originally it was thought that Ethernet's 10 Mbps speed
limit would run out of capacity and that optical networks running
ATM would be installed to PCs on the desktop. Ethernet responded,
however, with higher speeds and other enhancements. It continues
to be the dominant LAN protocol with no end in sight. ATM was
adopted by the carriers for metropolitan and wide area networks.
Right now that interface between LAN and WAN is where the network
changes from Ethernet to ATM.
In some cases the push for a common IP
standard is creating end to end Ethernet. Having one network
standard to manage instead of two makes life easier for network
managers. The big question will be if the big WAN trunks convert
to a native IP format or if they simply carry IP over ATM. ATM
may also see a resurgence as digitized voice and video become
a larger part of network traffic.
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