ISDN - Computer Busses - page 535

Table G.1 CCITT standards on ISDN

CCITT standard number Description

I.1 XX ISDN terms and technology

I.2 XX ISDN services

I.3 XX ISDN addressing

I.430 and I.431 ISDN physical layer interface

I.440 and I.441 ISDN data layer interface

I.450 and I.451 ISDN network layer interface

I.5 XX ISDN internetworking

I.6 XX ISDN maintenance

The great advantage of an ISDN connection is that the type of data transmitted is irrele-
vant to the transmission and switching circuitry. Thus, it can carry other types of digital data,
such as facsimile, teletex, videotex and computer data. This reduces the need for modems,
which convert digital data into an analogue form, only for the public telephone network to
convert the analogue signal back into a digital form for transmission over a digital link. It is
also possible to multiplex the basic rate of 64 kbps to give even higher data rates. This multi-
plexing is known as N 64 kbps or broadband ISDN (B-ISDN).

Another advantage of ISDN is that it is a circuit-switched connection where a permanent
connection is established between two nodes. This connection is guaranteed for the length of
the connection. It also has a dependable delay time and is thus suited to real-time data.

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G.2 ISDN channels

ISDN uses channels to identify the data rate, each based on the 64 kbps provision. Typical
channels are B, D, H0, H11 and H12. The B-channel has a data rate of 64 kbps and provides
a circuit-switched connection between endpoints. A D-channel operates at 16 kbps and it
controls the data transfers over the B channels. The other channels provide B-ISDN for much
higher data rates. Table G.2 outlines the basic data rates for these channels.
The two main types of interface are the basic rate access and the primary rate access.
Both are based around groupings of B- and D-channels. The basic rate access allows two B-
channels and one 16 kbps D-channel.
Primary rate provides B-ISDN, such as H12 which gives 30 B-channels and a 64 kbps D-
channel. For basic and primary rates, all channels multiplex onto a single line by combining
channels into frames and adding extra synchronisation bits. Figure G.2 gives examples of the
basic rate and primary rate.

Table G.2 ISDN channels

Channel Description

B 64 kbps

D 16 kbps signaling for channel B (ISDN)

64 kbps signaling for channel B (B-ISDN)

H0

384 kbps (6 × 64 kbps) for B-ISDN

H11

1.536 Mbps (24 × 64 kbps) for B-ISDN

H12

1.920 Mbps (30 × 64 kbps) for B-ISDN