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Basic Rate Interface 135
The ISDN Layer 1 is concerned not only with physical connectivity, but also with how the bits
traverse the wire. To accommodate transmission, a framing method must be established to
enable communication between the NT and the TE as well as between the NT and the Local
Exchange (LE).
The framing between the NT and TE is defined in the ITU specification I.430. Figure 6-5
depicts the BRI frame.
Figure 6-5
ISDN BRI Framing Between NT and TE
Notice in Figure 6-5 that 16 bits from each B (B1 and B2) channel and 4 bits from the D channel
(D) are being time division multiplexed along with framing (F) and alignment (A) bits. Frame
transmission is constant regardless of whether data is actually being sent.
Between the NT and the LE, another framing convention is used, ANSI T1.601. The intricacies
of ANSI T1.601 are not discussed at this time because they are not covered on the CCNP
Remote Access Exam. For more information on this topic, check out www.ansi.org.
ISDN Layer 2
The Layer 2 processes of ISDN are defined in the ITU specifications Q.920 and Q.921. Q.921
defines the actual communication format. Obviously, Layer 2 communication does not take
place unless Layer 1 is properly installed and functioning.
1
1
8
1
1
1
1
1
8
1
1
1
8
1
1
1
8
1
1
1
NT
TE frame
F
L
B1
L
D
L
F
L
B2
L
D
L
B1
L
D
L
B2
L
D
L
1
1
8
1
1
1
1
1
8
1
1
1
8
1
1
1
8
1
1
1
NT
TE frame
F
L
B1
E
D
A
F
F
B2
E
D
S
B1
E
D
S
B2
E
D
S