Information Technology Reference
In-Depth Information
G.3.1 Alternative mark inversion (AMI) line code
AMI line codes use three voltage levels. In pure AMI, 0 V represents a '0', and the voltage
amplitude for each '1' is the inverse of the previous '1' bit. ISDN uses the inverse of this, i.e.
0 V for a '1' and an inverse in voltage for a '0', as shown in Figure G.3. Normally the pulse
amplitude is 0.75 V.
Inversion of the AMI signal (i.e. inverting a '0' rather than a '1') allows for timing in-
formation to be recovered when there are long runs of zeros, which is typical in the idle state.
AMI line code also automatically balances the signal voltage, and the average voltage will be
approximately zero even when there are long runs of zeros (this is a requirement as the con-
nection to the network is transformer coupled).
+V
0V
-V
1
0
0
1
1
0
1
1
0
0
0
0
Figure G.3
AMI used in ISDN
G.3.2 System connections
In basic rate connections, up to eight devices, or items of termination equipment (TE), can
connect to the network termination (NT). They connect over a common four-wire bus using
two sets of twisted-pair cables. The transmit output (
T
X
) on each TE connects to the transmit
output on the other TEs, and the receive input (
R
X
) on each TE connects to all other TEs. On
the NT the receive input connects to the transmit of the TEs, and the transmit output of the
NT connects to the receive input of the TEs. A contention protocol allows only one TE to
communicate at a time.
An 8-pin ISO 8877 connector connects a TE to the NT; this is similar to the RJ-45 con-
nector. Figure G.4 shows the pin connections. Pins 3 and 6 carry the
T
X
signal from the TE,
pins 4 and 5 provide the
R
X
to the TEs. Pins 7 and 8 are the secondary power supply from the
NT and pins 1 and 2 the power supply from the TE (if used). The
T
X
/
R
X
lines connect via
transformers, thus only the AC part of the bitstream transfers into the PCM circuitry of the
TE and the NT. This produces a need for a balanced DC line code such as AMI, as the DC
component in the bitstream will not pass through the transformers.
G.3.3 Frame format
Figures G.5 and G.6 show the ISDN frame formats. Each frame is 250
µ
s long and contains
10
-
6
) made up of two 64 kbps B chan-
nels, one 16 kbps D-channel and extra framing, DC balancing and synchronisation bits.
The
F
/
L
pair of bits identify the start of each transmitted frame. When transmitting from a
TE to an NT there is a 10-bit offset in the return of the frame back to the TE. The
E
bits echo
the D-channel bits back to the TE.
48 bits; this give a total bit rate of 192 kbps (48/250
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