Modem doubling is an inexpensive way for a user who wants a fast Internet connection, but can only connect with an analog telephone line, to use two 56kbps modems to double their bandwidth. Modem doubling makes it possible for the user to use one of two methods, modem bonding or modem teaming, to achieve a connection speed close to 112kbps.
Modem bonding, which is supported by over ninety percent of Internet Service Providers (ISPs), takes advantage of Multilink Protocol Plus (MP+). Data packets are "inverse-multiplexed" through the two modems, meaning that each of the modems receive half of the data packets as they are sent over the Internet and then they are recombined. An advantage of modem bonding is that if either of the modems gets disconnected for some reason, the other modem will take over so that the connection isn't lost.
Modem teaming, a good alternative for users whose IPS does not support MP+, allows the modems to work as separate connections by taking advantage of the "smart download" capability that most HTTP and FTP servers use in case a download is interrupted. Modem teaming requires a software application to instruct one modem to download the first part of the file while the other modem downloads the second part. A disadvantage of modem teaming is that it is not flexible if one modem should disconnect, nor is it useful when downloading streaming multimedia or conducting continuous sessions such as telnet or PCAnywhere.