There are several standards for cable TV data networks in the U.S., but that should not be a problem for a consumer. Cable companies follow business models much like those of their cable TV services, and they choose and install the equipment you need in order to use their data services.
Symmetrical modems send and receive data at the same speed.
Asymmetrical modems receive data faster than they can send it. Cable modem performance varies across the country, and most companies are reluctant or unable to give exact performance statistics. That's because the data bandwidth on a cable modem network is shared by groups of users. As the number of users online increases, each user's share of bandwidth goes down. You can realistically expect peak speeds of up to 2 million bits per second, compared with 128,000 bps for ISDN or 28,800 bps for a modem.