Cisco 2600 Series Operation Manual page 80

4-Port and 8-Port ISDN BRI U Network Modules
In addition, BRI U modules have three LEDs for each port. Two of these LEDs indicate call
activity on the two ISDN BRI B channels, as listed in Table 7-3. The third LED, labeled
NT1, indicates synchronization status of the NT1.
Table 7-3
LED
B1
B2
NT1
7-10 Cisco Network Modules Hardware Installation Guide
BRI U Network Modules LEDs
Meaning
Activity on B1 channel
Activity on B2 channel
Synchronous status of NT1