Chapter 1. System Overview and Modes of Operation
39
Figure 1-17. Dual Channel Bank Operation
Loop Timing
System loop timing is provided by clock recovered from the incoming T1 signals. If the incoming
T1 reference clock fails, then timing is derived from the internal timing source. This works like
drop-and-insert mode in which the T1-1 Receive clock sources the T1-2 Transmit clock, and the
T1-2 Receive clock sources the T1-1 Transmit clock.
The three loop timing clock sources used to set up the backup clock status include the following.
• T1-1 receiver
• T1-2 receiver
• T1-1 and T1-2 receivers
The T1-1 receiver and T1-2 receiver selections determine which span “faces” toward the clock
source. The “downstream” location (i.e., away from the clock source) must be set to recovered
timing.
The T1-1 and T1-2 receivers selection compensates for clock frequency variations between two
similarly timed T1 paths, as may be found between two different carriers. Note that while this
may imply that there are two clock sources here there are not
. The D/I Mux III can only have one
clock source per multiplexer.
Local Timing
Local timing clock source for the T1 outputs is derived from the system's internal 1.544 MHz
oscillator. This oscillator can run freely or be synchronized to one of two different clock sources
as follows:
• DB-9 Clock - The DB-9 Clock is an 8 Kbps to 1.544 Mbps TTL or V.35 external clock brought
into the SYNC I/O connector on the back of the D/I Mux III system shelf.
• Slot 2 Synchronization - The Slot 2 Sync is an 8 Kbps to 1.544 Mbps clock recovered from a
data line card in slot 2 of the D/I Mux III shelf.
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