Information Technology Reference
In-Depth Information
Layer 1
ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound (from
terminal to network) or inbound (from network to terminal). Both physical layer interfaces are shown in
Figure 16-2.
Figure16-2 ISDN Physical Layer Frame Formats
Field length
in bits
1
1
8
1
1
1
1
1
8
1
1
1
8
1
1
1
8
F
L
B1
L
D
L
F
L
B2
L
D
L
B1
L
D
L
B2
...
NT frame (network to terminal)
Field length
in bits
1
1
8
1
1
1
1
1
8
1
1
1
8
1
1
1
8
F
L
B1
E
D
A
F
F
B2
E
D
S
B1
E
D
S
B2
...
TE frame (terminal to network)
A = Activation bit
B1 = B1 channel bits
B2 = B2 channel bits
D
= D channel (4 bits x 4000 frames/sec. = 16 kbps)
E
= Echo of previous D bit
F
= Framing bit
L
= Load balancing
S
= Spare bit
The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer frame
are used as follows:
•
F
—Provides synchronization
•
L
—Adjusts the average bit value
•
E
—Is used for contention resolution when several terminals on a passive bus contend for a channel
•
A
—Activates devices
•
S
—Is unassigned
•
B1, B2, and D
—Is used for user data
Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions
can result if two terminals transmit simultaneously. ISDN, therefore, provides features to determine link
contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position.
The TE expects the next E bit to be the same as its last transmitted D bit.
Terminals cannot transmit into the D channel unless they first detect a specific number of ones
(indicating “no signal”) corresponding to a pre-established priority. If the TE detects a bit in the echo
channel that is different from its D bits, it must stop transmitting immediately. This simple technique
ensures that only one terminal can transmit its D message at one time. After successful D message
transmission, the terminal has its priority reduced by being required to detect more continuous ones
before transmitting. Terminals cannot raise their priority until all other devices on the same line have
had an opportunity to send a D message. Telephone connections have higher priority than all other
services, and signaling information has a higher priority than nonsignaling information.
