8
1 Introduction to the AVM ISDN-Controller T1/T1-B
1.1.1 ISDN Line Types
There are two standardized types of access to ISDN:
- ISDN BRI (S
0
interface) with 2 B channels
(64 Kbit/s each) and one D channel (16 Kbit/s)
- ISDN PRI (S
2M
interface) with 30 B channels
(64 Kbit/s each) and one D channel (64 Kbit/s)
Control functions for communication between the user
and the network are handled over the D channel using
the same protocols for the S
0
and S
2M
interfaces. The D
channel protocol for lines provided by Deutsche Telekom
AG may be 1TR6 (national ISDN) or DSS1 (Euro-ISDN).
The D-channel signaling is the only difference between
national ISDN and Euro-ISDN. The hardware for all PRI
lines is the same, so that no hardware modifications are
necessary when changing from 1TR6 to DSS1. The
AVM ISDN-Controller T1/T1-B’s software supports
both D-channel protocols.
1.1.2 The Primary Rate Interface
The primary rate interface provides 30 user data
channels (B channels) and one signalling channel (D
channel). The data rate over each of these channels is
64 Kbit/s. In contrast to the BRI with just two B
channels, the PRI’s channels can be individually config-
ured for specific purposes: for incoming connections
only, for outgoing connections only, or for both incom-
ing and outgoing connections.
The actual data transmission takes place using a time-
multiplex technique. The data from each channel is
switched onto the physical line successively in a fixed
cycle. Each time frame is 125 µs in duration, and 256
bits are sent in this time. These 256 bits are composed
of 8 bits from each B and D channel plus 8 synchroni-
zation bits (30 B channels * 8 bits + 1 D channel * 8 bits
+ 8 synchronization bits = 256 bits).