Chapter 6. Network Architectures

Initially, private digital networks were arranged for point-to-point, single trunk service (see Figure 2). A channel bank at one end provided the source of timing for the distant end terminal bank, known as the slave. The slave terminal derived or extracted timing from the transmitted data. With no intervening transmission equipment to provide a source of timing, this arrangement seldom experienced synchronization problems.

When more than one trunk was connected point-to-point, all equipment at one end used the same clock, which was established through a clock distribution system (see Figure 5). The other end was slaved as before, and timing problems were not encountered.

The public switched network gets more complex every day, comprising numerous long distance carriers, local operating companies, as well as the competitive access providers. In addition, networks are no longer point-to-point, or point to multi-point. They now include add and drop, as well as ring configurations.

Designating the location of master and slave terminals becomes quite difficult when point-to-point circuits are used for add and drop services, switched services, or services passing through certain customer multiplexing equipment. However, reliable network timing can be achieved if one location is designated as the master, and Stratum 2 or 3E clock systems are installed at all other sites. In ring systems, using the traffic carrying DS1 to drive the timing systems is not generally recommended, but has been found to work well and solve many problems when Larus STS 5400 or STS 5800 systems are used.

While today's point-to-point services and tariffs are typically untimed, telephone companies and long distance carriers can install equipment to administer, operate, and provide service observation (such as digital access and cross-connect system [DACS]). Without network timing, the timing is that of the customer's equipment, and may create jitter and wander problems with SONET transmission equipment in the network.

The advantage of having the local or long distance carrier pass the DS1 circuit through a DACS is that the trunk is timed by the carrier. This then provides a source of timing directly traceable to a Stratum 1 reference. In a network with carrier imposed timing on the trunk or trunks, all private network sites should be configured as slaves, and equipped with Stratum 3E (Larus 3E Enhanced) clocks to provide timing to the equipment.

SONET networks are designed to operate in a maintenance mode when not timed. In this mode, the jitter may be larger than that of a properly timed network. It has been noted by many that for voice traffic, and even for digital traffic between switches and digital cross-connects, no impairments are noticed even when the SONET system is untimed. However, it is recommended that all SONET networks be timed to minimize phase noise (jitter and wander).

With SONET, the multiplex equipment itself must be timed. If the timing of the dropped DS1s is different from the multiplex timing, there will be unsatisfactory jitter and wander performance, which may or may not affect the connected equipment. By timing all network sources of DS1 signals from the same reference, and making sure that the reference chosen is the same as the SONET reference, problems can be minimized.

A typical source of Stratum 1 timing could be a STS 5800 which integrates the GPS receiver with a Stratum 3E holdover oscillator. The Larus STS 5400 system includes a GPS receiver integral with the 5410 GPS Track and Hold Card. The unit integrates the GPS receiver and a Stratum 2 clock function for optimal performance and integrated system management.

A Stratum 2 clock system should not be used in networks using carrier derived timing. This arrangement may not work when the transmission system timing is in its holdover condition at the Stratum 3 level at the serving central office, or long distance terminal. In all cases, careful examination of the network and equipment is required.

A Stratum 1 clock system may always be used, since it should be identical to the network providers' clocks. Only in the case of satellite networks will a Stratum 1 remote clock lead to problems. Consult Larus Application Engineering for satellite network synchronization applications.