ISDN/LAPD
Frame relay can be connected over ISDN but ISDN can be used without frame relay as well.
ISDN can be used over existing telephone lines to transmit voice and data. It uses B-channels (bearer) and D-channels (data) to maintain the ISDN connection.
There are two basic service levels for ISDN:
- PRI (primary rate interface) which provides up to 1.544 mbps over 23 B-channels and one, 64 kbps D-channel
- BRI (basic rate interface) which provides two B-channels at 64 kbps each and one 16 kbps D-channel for a combined 128 kbps rate.
The protocol specification that defines how the D-channel manages communications is called LAPD.