The AXD301 ATM Switch
The AXD301, a telephony-class 10–160 Gbps ATM switch, was designed and imple-
mented from scratch in less than three years. At the heart of the AXD301 are more than
1.5 million lines of Erlang code, handling all the complex control logic, and overseeing
operations and maintenance. This integrates with about half a million lines of C/C++
implementing low-level protocol and device drivers, much of it coming from third-party
sources.
This ATM switch has been installed in networks all over the world, but the installation
that shot to prominence was used by British Telecom to build what was at the time the
largest “Voice over ATM” backbone in the world. According to an Ericsson press re-
lease issued at the end of the trial period, “Since cut-over of the first nodes in BT’s
network in January 2002 only one minor fault has occurred, resulting in 99.9999999%
availability.” The director of Ericsson’s Next Generation Systems program, Bernt Nils-
son, confirmed that “the network performance has been so reliable that there is almost
a risk that our field engineers do not learn maintenance skills.”

Experiences with the AXD301 suggest that “five nines” availability, downtime for soft-
ware upgrades included, is a more realistic assessment. For nonstop operations, you
need multiple computers, redundant power supplies, multiple network interfaces and
reliable networks, cooling systems that never fail, and cables that system administrators
cannot trip over, not to mention engineers who are well practiced in their maintenance
skills. Considering that this target has been achieved at a fraction of the effort that
would have been needed in a conventional programming language, it is still something
to be very proud of.
How did Erlang contribute to the success of the AXD301? It supports incremental
development, with the absence of side effects, making it easier to add or modify single
components. Support for robustness and concurrency is built into the language and
available from the start.
Erlang was very popular with the programming teams that found they were building
much more compact code, thus dramatically improving their productivity. Experience
from the project, although not scientifically documented, suggests that the Erlang code
was 4 to 10 times shorter than similar systems written in C/C++, Java, and PLEX,*
while the fault rate per thousand lines of code was the same.
Ericsson has gone on to use Erlang on other projects across the company, including a
SIP telephony stack, control software for wireless base stations, telephony gateway
controllers, media gateways, broadband solutions, and in GPRS and 3G data trans-
mission. And these are just a few of the many we are allowed to talk about.