Cisco High-Speed - Expansion module - HDLC/ RS-232/ PPP/ RS-530/ X.21/ V.35/ RS-449/ SLIP/ RS-530A - 4 ports - for Cisco 1841/ 1921 4-pair/ 1921 ADSL2+/ 19XX/ 28XX/ 28XX 2-pair/ 29XX/ 38XX/ 39XX