Nortel Networks 3500, NTRN10AN manual Timing modes

Operation, administration, and maintenance (OAM) features 2-149

Table 2-30 (continued)

Timing signal sources

DS1

EC-1x3

slots 3 through 10 (see Note 2)

slots 3 through 10

Note 1: Requires STX-192 circuit packs provisioned in slots 13 and 14.

Note 2: Supported on shelved equipped with VTX-series circuit packs in slots 13 and 14.

The best timing reference source is a high-level stratum clock.

Timing modes

The four possible timing modes for OPTera Metro 3500 network elements are:

•Free run. See Free run mode on page 2-149.

•Acquire. See Acquire mode on page 2-149.

•Normal. See Normal mode on page 2-149.

•Holdover. See Holdover mode on page 2-149.

Free run mode

Free run mode is a target mode that can be provisioned by the user. In free run mode, the voltage-controlled crystal oscillator (VCXO) clock in the module is not locked to a timing reference and runs at its natural frequency. Network elements with STX and VTX-series circuit packs provide timing references of ST3 quality.

Acquire mode

Acquire mode is not a mode that the user can provision. Acquire mode is the current mode when the VCXO clock in the module tracks a timing reference and the timing-mode-maintenance software quickly brings the clock frequency into approximate agreement with the timing reference frequency. That reference may be the 8-kHz timing signal derived from an incoming SONET signal, a DS1 signal, or BITS inputs. The signal format of the BITS input can be set to DS1 or composite clock.

Normal mode

Normal mode is a mode that can be provisioned by the user. When this is the current mode, the VCXO clock in the module locks to a timing reference. Normal mode is used during trouble-free operations.

Holdover mode

Holdover mode is not a mode that the user can provision. The module enters holdover mode automatically if the target mode is normal but all timing references have become unavailable. If the module enters holdover mode, the VCXO clock in the module holds within a certain frequency range of the last locked-in timing reference. When a timing reference becomes available again,

Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004