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 needs.

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.

Wait a second. All the latest buzz is about Ethernet, Fast Ethernet, Gigabit Ethernet and 10 Gigabit Ethernet. 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 the picture.

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 universe? 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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