1. Field of the Invention
This invention relates to communications technologies. In particular, this invention relates to a multimode multicarrier modem.
2. Description of Related Art
The role out of broadband services over telephone lines using Digital Subscriber Lines (DSL) technology is occurring around the globe. In order to achieve reliable communications over DSL links, a technique known as Discrete Multitone (DMT) modulation is used. Discrete Multitone Modulation has been standardized for DSL transmission by the ANSI standards body for full-rate ADSL (T1E1.4/97-007R6 Interface between network and customer installation Asymmetric Digital Subscriber Line (ADSL) metallic interface, Sep. 26, 1997, i.e., T1.413 Issue 2, incorporated herein by reference in its entirety, and by the International Telecommunication Union (ITU) in the G.992.1 (full-rate ADSL) and G.992.2 (G.lite) standards, both incorporated herein by reference in their entirety. These standards specify that hundreds of 4.3125 kHz sub-channels are assigned for DSL transmissions between a telephone company Central Office (CO) and a Remote Terminal (RT), such as a home or business. Data is transmitted between the CO and RT in both the downstream direction, i.e., from the CO to the RT, and in the upstream direction, i.e., from the RT to the CO. The aggregate bandwidth, i.e., the bandwidth that is used by both the upstream and the downstream communications, of a full-rate ADSL system is over 1 MHz and that of G.lite is over 500 kHz. The systems typically transmit 1.5 Mbps (G.lite) or 6 Mbps (full-rate ADSL) data rates downstream.