 agreed to create the National ISDN 1 standard (NI-1). This standard, later superseded by National ISDN 2 (NI-2), ensured the interoperability of end user and exchange equipment. Two basic types of ISDN service are defined - Basic Rate Interface (BRI) and Primary Rate Interface (PRI). ISDN carries voice and user data streams on "bearer" (B) channels, typically occupying a bandwidth of 64 kbps, and control data streams on "demand" (D) channels, with a 16 kbps or 64 kbps bandwidth depending on the service type. BRI provides two 64 kbps B channels, which can be used to make two simultaneous voice or data connections or can be combined into one 128 kbps connection. While the B channels carry voice and user data transmission, the D channel is used to carry Data Link and Network layer control information. The higher capacity PRI service provides 23 B channels plus one 64 kbps D channel in the US and Japan, or 30 B channels plus one D channel in Europe. As for BRI, the B channels can be combined to give data bandwidths of 1472 kbps (US) or 1920 kbps (Europe). As noted above, telephone wires are not ideal as a digital communication medium. The ISDN PHY layer limits the effect of line attenuation, near- end and far-end crosstalk and noise by using Pulse Amplitude Modulation (PAM) technology (see the Section "Pulse Modulation Methods, p. 104") to achieve a high data rate at a reduced transmission rate on the line. This is achieved by converting multiple (often two or four) binary bits into a single multilevel transmitted symbol. In the US, the 2B1Q method Chapter Two Input bit stream Input "dibit" stream +2.5 V +0.83 V -0.83 V 2B1Q line voltage -2.5 V Figure 2-9: 2B1Q Line Code Using in ISDN is used, which converts two binary bits (2B) into a single output symbol, known as a "quat" (1Q), which can have one of four values (Figure 2-9 and Table 2-7). This effectively halves the transmission rate on the line, SO that a 64 kbps data rate can be transmitted at a symbol rate of 32 ksps, achieving higher data rates within the lim