Most Internet service providers use
Primary Rate Interface (PRI)
ISDN to connect to the public
switched telephone network (PTSN). A PRI enables service to analog modem users, digital modem
users, and ISDN customers. The calls are routed to the appropriate modems after the access server
receives the calling number’s bearer capability. ISDN also provides a means to deliver calling line ID
(CLID), as well as called number or automatic number identification (ANI). These features can be
used to determine the correct authentication server for this customer.
PRIs have the following capacities:
A T-1–based PRI has 23 64Kbps B channels and one 64Kbps D channel, which equals a
bandwidth of 1.536Kbps. An 8Kbps channel for framing and synchronization is also used,
resulting in a total bandwidth of 1.544Mbps for a U.S. T-1/PRI. The last T-1 channel is
used as the D channel.
An E-1–based PRI has 30 B channels and one 64Kbps D channel. An E-1 uses channel 15
for signaling (D channel). An E-1 has 2.048Mbps of total bandwidth.