Information Technology Reference
In-Depth Information
0 (the
N
bit) it will not be confused with the
F
/
L
pair. The start of the frame can thus be
traced backwards to find the
F
/
L
pair.
10
-
6
)
There are 16 bits for each B-channel, giving a basic data rate of 64 kbps (16/250
×
10
-
6
).
The
L
bit balances the DC level on the line. If the number of zeros following the last bal-
ancing bit is odd then the balancing bit is a 0, else it is a 1. When synchronised the NT in-
forms the TEs by setting the
A
bit.
and there are 4 bits in the frame for the D-channel, giving a bit rate of 16 kbps (4/250
×
G.4 ISDN data link layer
The data link layer uses a protocol known as the link access procedure for the D-channel
(LAPD). Figure G.7 shows the frame format. The unique bit sequence
01111110
identifies
the start and end of the frame. This bit pattern cannot occur in the rest of the frame due to
zero bit-stuffing.
The address field contains information on the type of data contained in the frame (the
service access point identifier) and the physical address of the ISDN device (the terminal
endpoint identifier). The control field contains a supervisory, an unnumbered or an informa-
tion frame. The frame check sequence provides error detection information.
Start
delimiter
End
delimiter
Address
field
Control
field
Network
data
Frame check
sequence
01111110
01111110
Service access
point identifer (SAPI)
Terminal endpoint
identifier (TEI)
C/R
EA0
EA1
Figure G.7
D-channel frame structure
G.4.1 Address field
The data link address only contains addressing information to connect the TE to the NT and
does not have network addresses. Figure G.7 shows the address field format. The SAPI iden-
tifies the type of ISDN service. For example, a frame from a telephone would be identified as
such, and only telephones would read the frame.
All TEs connect to a single multiplexed bus, thus each has a unique data link address,
known as a terminal endpoint identifier (TEI). The user or the network sets this; the ranges of
available addresses are:
0-63
non-automatic assignment TEIs
64-126
automatic assignment TEIs
127
global TEI
