Issue 2 - 07/00 A3-9
Appendix 3—Introduction to ISDN
Basic Rate Layer 1 S bus operation
Layer 1 transfers information in frames across the S bus
between the terminals and the NT1.
Each frame is 48 bits long and lasts for 250 microseconds—its
structure depends on the direction of transmission. It contains
two 8 bit groups of channel B1, two 8 bit groups of channel
B2, plus bits for channel D and other functions.
D Channel Contention
If two TEs attempt to make a call simultaneously, a procedure
known as D Channel Contention prevents a collision.
Activation and deactivation
TEs and NTs can be deactivated to reduce power
consumption, then reactivated to normal power. Both the TE
and NT can receive activation messages, but only an NT can
instruct a TE to deactivate.
‘Info’ signals
Activation and Deactivation messages are transmitted using
‘Info’ signals.
Info 0 idle state—neither the TEs or the NT are operating.
Info 1 TE to NT—requests activation.
Info 2 NT to TE—requests activation/responds to Info 1.
Info 3 TE to NT—contains operational data.
Info 4 NT to TE—contains operational data.
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