The popularity of gateways that interwork between the PSTN (PublicSwitched Telephone Network) and SIP networks has motivated thepublication of a set ofcommon practices that can assure consistentbehavior across implementations. This document taxonomizes the usesof PSTN-SIP gateways, provides uses cases, and identifies mechanismsnecessary for interworking. The mechanisms detail how SIP providesfor both 'encapsulation' (carriage of PSTN signaling across a SIPnetwork) and 'translation' (protocol mapping).