INTRODUCTION This Telecommunications Systems Builetin (TSB) applies to TiA/EIA/IS-637. It introduces the following modifications: ? 14.4kbps data rate support ? Service negotiation support ? IS-637 layering problem correction@ editorial corrections This TSB uses the same structure and section numbering as TIA/EIA/IS-637. Sections of this TSB that remain unchanged relative to TIA/EIA/IS-637 are represented using ellipses contained within brackets ([...])Modified sections are either reproduced in their entirety or use the [...] notation to represent unchanged portions. Within modified sections@ change bars indicate the specific paragraphs@ figures@ or table cells that have changed. Within modified paragraphs and table cells@ strike-through indicates deleted text and underline indicates new text.