Other features include high immunity to background noise, enabling transmission of multiple HDSL lines via the same physical cable, and the use of equalisation, adaptive filtering, echo cancellation and 2B1Q
line coding to compensate for line impairments and mixed cabling.
Mindspeed's CX28975 and CX28985 solutions are backward-compatible with the company's large installed base of original ZipWire SDSL modem solutions that operate in 2B1Q
High-data-rate DSL (HDSL) also uses 2B1Q
modulation to provide a symmetric service of 1.544 Mbps in each direction, but over two twisted-pairs.
The early market entry by AT&T Microelectronics with full production of a 2B1Q transceiver, coupled with requests for products from the operating companies, form the catalyst for the initial deployment of both 2B1Q CO line cards and NT1s.
The NT1 is used in the basic rate interface portion of the ISDN network to connect four-wire S/T interface ISDN terminals to the two-wire 2B1Q U interface network.
Users planning to build their own NT1 can use the AT&T Microelectronics T7252A four-wire transceiver, the T7262/63 2B1Q chip set, and the T7270 to interconnect the subscriber to the network.
Although there are several line code schemes which could provide the basic two-wire transport mechanism for user data, the 2B1Q scheme has been adopted by ANSI and major European PTTs, and has been acknowledged by the CCITT and European Telephone Standards Institute (ETSI).
In another ISDN development, AT&T announced a single-chip 2B1Q
transceiver that connects ISDN users to public network facilities.
The result, achieved at the end of 1988, is a new transmission standard called 2B1Q: the missing link in ISDN.
As presently adopted by ANSI, 2B1Q is the transmission line code for the basic-rate interface (BRI).
2B1Q is guaranteed to run on 99% of the worst wires surveyed by Bellcore in the telephone-loop plant in 1983.
Due to an unfortunate collision of names, 2B1Q and 2B+D are frequently confused.