Sippy B2BUA is a RFC3261-compliant Session Initiation Protocol (SIP)
Back-to-back user agent (B2BUA) server software. 

The B2BUA is a SIP call controlling component. Unlike a SIP proxy server,
which only maintains transaction state, the B2BUA maintains complete call
state and participates in all call requests. For this reason it can perform
number of functions that are not possible to implement using SIP proxy, such
as for example accurate call accounting, pre-paid rating and billing, fail
over call routing etc. Unlike PBX-type solutions such as Asterisk for example,
the B2BUA doesn't perform any media relaying or processing, therefore it
doesn't introduce any additional packet loss, delay or jitter into the
media path. 

Features:

 o 5,000-10,000 simultaneous sessions per server;

 o 150-200 call setups/tear-downs per second;

 o Realtime calls control and call data records (CDRs) generation;

 o Optional ability to use Sippy RTPproxy for media relaying;

 o Optional ability to perform Cisco-compatible RADIUS AAA (Authentication,
   Authorization and Accounting);

 o RFC3261 compliance;

 o Seamless compatibility with majority of popular SIP software and hardware on
   the market today;

 o Robustness and Resilience;

 o Simple and clean, yet flexible, internal design making implementing new
   features and services easy;

 o B2BUA could be easily combined with other Open Source software, such as SIP
   Express Router / OpenSIPS to build complete softswitch solution.