2.2. Baseband Transmission
Baseband codes are distinguished from passband codes (See Section 2.3) in that baseband codes can transmit energy at DC (f = 0), while passband codes transmit at a frequency spectrum translated away from DC. Baseband line codes in DSLs have N = 1 and appear in the earliest DSLs: Examples are T1, ISDN, and HDSL.
2.2.1. The 2B1Q Line Code (ISDN and HDSL)
The 2B1Q baseband line code is heavily used in early DSLs. The 2B1Q line code ideally uses a basis function
 The sinc function is
which leads to the independence of ...