Alcatel-Lucent 7950 XRS Series, Configuration Manual

Page: 254 / 502

Administrative Tasks
Page 254 7950 SR OS Basic System Configuration Guide
Network Timing
In Time Domain Multiplexed (TDM)-based networks (for example, SONET or SDH circuit-
switched networks), the concept of network timing is used to prevent over-run or under-run issues
where circuits are groomed (rebundled) and switched. Hardware exists in each node that takes a
common clock derived from an internal oscillator, a specific receive interface and provides it to
each synchronous interface in the system. Usually, each synchronous interface is allowed to
choose between using the chassis-provided clock or the clocking recovered from the received
signal on the interface. The clocking is used to drive the transmit side of the interface. The
appropriate configuration at each node which defines how interface clocking is handled must be
considered when designing a network that has a centralized timing source so each interface is
operating in a synchronous manner.
The effect of timing on a network is dependent on the nature of the type of traffic carried on the
network. With bit-wise synchronous traffic (traditional circuit-based voice or video), non-
synchronous transmissions cause a loss of information in the streams affecting performance. With
packet-based traffic, the applications expect and handle jitter and latency inherent to packet-based
networks. When a packet-based network is used to carry voice or video traffic, the applications use
data compression and elasticity buffering to compensate for jitter and latency. The network itself
relies on appropriate Quality of Service (QoS) definitions and network provisioning to further
minimize the jitter and latency the application may experience.