Verilink AS2000: The Basics (880-502981-001) Product Manual - Manual (Page 26)

System Information
2-10 Verilink Access System 2000: The Basics
The NCM stores circuit information for the entire node, enables the
creation of inter-module circuits, and ensures all circuits are viable
from port to port. The NCM stores the configuration of all the
modules in the node for restoration tasks, and provides a single IP
address for the entire node.
Redundant NCMs
An ACP-based node can have more than one NCM. If the additional
NCM is located in the same shelf, it is called a redundant NCM. The
ALM LED is not lit on a redundant NCM.
Timing
Proper operation of a carrier channel network depends on timing.
All of the equipment in a point-to-point network must refer to a
single digital master clock.
In AS2000, timing is handled differently between TABS-based
nodes (NCC and SCC) and ACP-based nodes (NCM). This section
describes the general timing configuration for each type of node.
Timing Source
CSU Timing The CSU uses a master clock to synchronize DSU timing (in MUX or
drop-and-insert modes) and to transmit signals to the network.
The following options inform the CSU where to look for the master
clock signal. Since the options are not relevant to all CSU modes,
applicable modes are shown in parentheses.
Figure 2-6 CSU Timing Options
Level 1
Equipment
CSU
Clock
TIU
External
422
External
TTL
DSU
NSP
Receive Clock
Transmit Clock
Terminal Timing