56k half-duplex? Sounds about right.
PC industry modems were all full duplex, upto 33.6kbps, but with 56k (V.90 / V.92) the speeds were limited on the return path. This worked as the ISP ran modem banks generally fed with ISDN trunks (ISDN30) and the home user just had PSTN.
Some non PC systems used half duplex but then ran over "PC industry style" Hayes modems, so the actual PSTN part of the connection was full duplex.
James BT Infinity 2 19/09/2012 - Estimate 44.6/6.5 - Install 52/12 - Actual 46 / 8 Mbps
13 years of broadband - 1999 ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(19M/16M)/BT FTTC(46M)