This article describes the method of timing signal used to synchronize a hierarchy of network tier devices. This is only used by large-scale providers such carrier
providers and ISPs.
Overview
The timing I am referring to in this article is specific to carrier devices that are used by ISPs The timing is important because it provides an alignment of data traveling through multiple devices that send and receive data. "Timing" refers to systematic electrical pulses that keep the "bit rate" of data constant. If the timing is incorrect, the data will not move through these devices correctly. The most accurate clocks we have today are generated and controlled by GPS satellites. Central offices and data centers receive data from these satellites to maintain a "correct" timing value for network devices. These devices are also used with wireless towers and DSLAMs. These signals are received using roof-mounted antennas (and sometimes small satellite dishes) that send the signal to a receiver. See the image below:
After the signal is received from the antenna, it is transmitted to a highly accurate timing device know as a BITS (Building Integrated Timing Supply) clock. Some devices (such as multiplexers) will contain their own BITS. In other cases, a master BITS will receive all the timing data and distribute it among the rest of the timing dependent devices. This is a master-slave relationship know as loop timing. SONET and other connections will use the timing provided by a BITS. The image below shows a BITS card used in Nortel, Lucent, and Alcatel equipment:
Timing devices are classified by the type of clock they use. These special clocks using crystal oscillators are known as Stratum clocks. The composition and design of these clocks are quite complicated. You can find very technical specs here.
BITS play a key role in the operation of a SONET network. Without accurate timing, SONET would be unable to correctly multiplex
the data traveling through it.
