Large businesses typically use point-to-point connections with DSU/CSUs to connect two sites.
In turn, these are connected to low- and high-speed serial interfaces on routers—usually Cisco
routers. The router backplane and the number of interfaces the router can handle determine
how well it supports WAN connections. The Cisco 7000 series of routers supports the Fast
Serial Interface Processor (FSIP), which provides either four or eight serial ports, permitting
the four or eight point-to-point connections to remote offices. Other Cisco routers support the
Multichannel Interface Processor (MIP), which furnishes support for two full T-1/E-1 ports in
the 7000 series and one port in the 4000 series.
ISDN T-1s, which are called Primary Rate Interfaces (PRIs), run at 1.544Mbps. These use 24
channels in contrast to E-1s, which use 31 channels and run at 2.048Mbps. E-1 is mainly used
in Europe, and both T-1 and E-1 are considered wide-area digital transmission schemes.
Each port in the MIP can support 24 DS0 channels of 64Kbps each when using a T-1 interface,
and 31 DS0 channels when using an E-1 interface. The MIP refers to each serial interface
as a channel group; this enables each channel or DS0 to be configured individually. Each channel
has the same characteristics and options as regular serial interfaces.