• _{2 wire is usually half duplex}
  • _{however, CCITTs V.32 standard can transmit full-duplex over 2 wire using echo cancellation}
  • _{the latest modems support full-duplex transmission}
• _{4 wire supported full duplex without echo cancellation}
  • _{4 wire is more expensive}
  • _{no longer needed for full duplex modem transmission}