Information Technology Reference
In-Depth Information
Network
connection
ISDN node
Incoming
call
Data flow
TEI_1
Figure G.9
Example connection between a primary/secondary
G.4.4 D-channel contention
The D-channel contention protocol ensures that only one terminal can transmit its data at a
time. This happens because the start and the end of the D-channel bits have the bitstream
01111110
, as shown below:
11111
01111110
XXXXXXXXX...XXXXXXXX
01111110
1111
When idle, each TE floats to a high-impedance state, which is taken as a binary 1. To trans-
mit, a TE counts the number of 1s in the D-channel. A 0 resets this count. After a predeter-
mined number, greater than a predetermined number of consecutive 1s, the TE transmits its
data and monitors the return from the NT. If it does not receive the correct D-channel bit-
stream returned through the E bits then a collision has occurred. When a TE detects a colli-
sion it immediately stops transmitting and monitors the line.
When a TE has finished transmitting data it increases its count value for the number of
consecutive 1s by 1. This gives other TEs an opportunity to transmit their data.
G.4.5 Frame check sequence
The frame check sequence (FCS) field contains an error detection code based on cyclic re-
dundancy check (CRC) polynomials. It uses the CCITT V.41 polynomial, which is
1
16
12
5
G
(
x
)
=
x
+
x
+
x
+
x
.
G.5 ISDN network layer
The D-channel carriers network layer information within the LAPD frame. This information
establishes and controls a connection. The LAPD frames contain no true data as this is car-
ried in the B-channel. Its function is to set up and manage calls and to provide flow control
between connections over the network.
Figure G.10 shows the format of the layer-three signalling message frame. The first byte
is the protocol discriminator. In the future, this byte will define different communications
