Comment by davrosthedalek
18 hours ago
Yes, because at that time, a modem didn't actually talk to a modem over a switched analog line. Instead, line cards digitized the analog phone signal, the digital stream was then routed through the telecom network, and the converted back to analog. So the analog path was actually two short segments. The line cards digitized at 8kHz (enough for 4kHz analog bandwidth), using a logarithmic mapping (u-law? a-law?), and they managed to get 7 bits reliably through the two conversions.
ISDN essentially moved that line card into the consumer's phone. So ISDN "modems" talked directly digital, and got to 64kbit/s.
An ISDN BRI (basic copper) actually had 2 64kbps b channels, for pots dialup as an ISP you typically had a PRI with 23 b, and 1 d channel.
56k only allowed one ad/da from provider to customer.
When I was troubleshooting clients, the problem was almost always on the customer side of the demarc with old two line or insane star junctions being the primary source.
You didn’t even get 33k on analog switches, but at least US West and GTE had isdn capable switches backed by at least DS# by the time the commercial internet took off. Lata tariffs in the US killed BRIs for the most part.
T1 CAS was still around but in channel CID etc… didn’t really work for their needs.
33.6k still depended on DS# backhaul, but you could be pots on both sides, 56k depended on only one analog conversion.
56k relied on the TX modem to be digitally wired to the DAC that fed the analog segment of the line.