Until now, we have discussed only analog circuits. Gradually, however, analog distribution is being replaced by digital distribution. Initially, this process was confined only to the phone companies. Today, even users can take advantage of digital circuits, i.e., ISDNs.
Telecom companies offer the following two types of ISDN connections:
- Basic Rate: This type of connection consists of one line (a twisted-pair) containing two B data channels, each with a capacity of 64 Kbps and one signalization D channel with a capacity of 16 Kbps.
- Primary Rate: This type of connection consists of the following:
- In North America: 23 B data channels, each having a capacity of 64 Kbps, and 1 D signalization channel with a capacity of 64 Kbps.
- In Europe: 30 B data channels, each having a capacity of 64 Kbps and 1 D signalization channel with a capacity of 64 Kbps.
Figure 3.14: Basic Rate and Primary Rate
The D channel is used in ISDN for signaling, i.e., for establishing a virtual...