This is the right answer; I’m not sure if bit = baud for say… a OC-192 line though.
As far as I know, OC-192 systems (10 Gb/s) generally use simple binary symbols, typically either NRZ (non-return to zero) or RZ (return to zero). Thus baud = bits-per-second. Advanced modulation formats have only recently become practical for lightwave systems, since they require coherent systems and that is much harder to do with light than with electronic signals. The lightwave folks are now going crazy packing enormous data rates into the fiber bandwidth using coherent photonics and sophisticated modulation formats.
This thread is making my head hurt. A simple question: How much faster does my “stuff” get downloaded with a modern broadband connection versus an old 2400 baud modem?
This is a 2400bps modem.
Let’s call modern DSL 1500000bps.
So a 1.5mbps DSL line is approximately 625 times faster on the download side. Upload on DSL is generally capped at 1/8 the upload. So approximately 78 times faster on the upload side.
Lots.
Cable speeds are typically 20Mb/sec vs 2,400b/sec for a 2400 baud modem, so 8,333x faster.
Much!
Data compression is quite a different technique to modulation schemes such as phase shift keying. The compression employed in some modem standards was the same basic idea that is used in .zip files and the like. What they did was compress each chunk of data before transmitting it, which the receiving modem would uncompress. But you could already achieve much the same thing by zipping a file before sending it, and indeed this technique is of little use with data that has already been compressed, such as jpeg images. PSK, in contrast, fundamentally adds more capacity to the transmission medium, by providing a greater number of distinct signals.
Why are download and upload speeds so drastically different?
Because a lot more data is downloaded than is uploaded. So the cable companies allocate more bandwidth to the downloads.
Thanks.
But the question remains, why do they feel the need to allocate some of the bandwidth to downstream and some to upstream?
In the days of modems, it was possible to use half-duplex communication, in which the entire channel was used either for upstream or downstream, but not both at the same time. Twice as fast as full duplex (two-way) communication, but a bit awkward - as Saint Cad says above, who wanted to do that?
Since modern schemes employ multiple parallel channels rather than a single channel, full duplex has less of an impact - you can always use some channels for upstream and some for downstream. But with current technology, each channel is designated as upstream or downstream. Since most users are downloading much more than they upload, asymmetric standards such as ADSL favour downstream.