15.4 Fiber Network Restoration

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The major carriers in the United States operate extensive fiber networks and have implemented architectures with sophisticated protection mechanisms to ensure uninterrupted voice and data services. Many of the smaller carriers with fiber networks, on the other hand, ran out of investment capital while expanding the reach of their backbones and did not have time to build much redundancy into their networks. Companies with mission-critical applications, therefore, should exercise due diligence when considering such carriers.

AT&T is one carrier that provides extensive restoration services for its fiber network. The carrier’s Fast Automatic Restoration (FASTAR) system provides automated facilities restoration for all types of services (special services and switched) traveling over AT&T’s fiber-optic transmission systems. FASTAR is designed to restore 90% to 95% of network circuits within 2 minutes, while FASTAR II is capable of rerouting circuits within 60 ms of a failure.

Specifically, FASTAR is a routing algorithm used to instruct DACS systems to reroute traffic around failed or congested routes. In the event of a fiber-optic cable cut in the core network, FASTAR automatically locates the exact site of the cut and transfers the affected circuits to spare capacity going around the cut. In this way, 72 T3 circuits can be rerouted by FASTAR within 5 minutes. Before FASTAR was introduced in 1992, T3 circuits had to be rerouted manually at patch panels, which could take hours.

When a facilities problem occurs, such as a cable cut, the following activities are typically performed by the FASTAR system:

• The problem is identified;

• The exact location of the problem is determined;

• The amount and location of currently available protection or backup/spare facilities is determined;

• A substitute route is constructed from the available spare facilities;

• The substitute route is tested to ensure it is operational and of high quality;

• The traffic on the damaged route is moved to the substitute route.

The FASTAR system goes through all these facilities restoration steps outlined but at computer speed and on a fully automated basis.

FASTAR II operates in conjunction with a network consisting of more than 50 double-interconnected SONET rings with ATM switching at crossover points to create a ring and mesh architecture for data services. Using overlapping, self-healing rings, FASTAR II can restore certain types of network failures, such as simple cable cuts, in milliseconds. With this type of outage, clients often do not even notice that traffic was interrupted.

Many other carriers also use SONET for disaster recovery, including RBOCs and CLECs. Fiber is deployed in redundant rings around major metropolitan areas and high-traffic corridors between major cities. SONET fiber facilities are typically configured in a dual counter-rotating ring topology, as illustrated in Figure 15.1. This topology makes use of self-healing mechanisms in SONET-compliant equipment [i.e., add-drop multiplexers (ADMs)] to ensure the highest degree of network availability and reliability. In the event of a break in the line, traffic is automatically switched from one ring to the other, thus maintaining the integrity of the network. In the unlikely event that both the primary and secondary lines fail, the SONET-compliant equipment adjacent to the failures automatically loops the data between rings, thus forming a new C-shaped ring from the operational portions of the original two rings. When the break is fixed, the network automatically returns to its original state.

Figure 15.1: Self-healing SONET-compliant fiber-ring topology. In this scenario, if the inner ring is cut or fails, traffic is rerouted in the opposite direction on the outer ring. The SONET equipment at node D changes the direction of the traffic.

SONET’s embedded management channels give carriers and users alike more capabilities for continuous monitoring and preemptive corrective action to impending trouble conditions. In private SONET networks, network managers can reconfigure channels and facilities without the involvement of telephone companies. Through software programming, it is even be possible to map SONET circuits so that they can be automatically rerouted to alternate carrier facilities should a failure occur on the primary circuit(s).

A new generation of optical systems have become available that offer much more bandwidth than SONET. WDM uses the different colors in the light source as separate high-speed channels. Each channel that can support a particular service, such as T-carrier for private lines, Gigabit Ethernet for LAN interconnectivity, Fibre Channel for storage-area networking, or ESCON for IBM mainframe connectivity. WDM-equipped fiber links can also transport SONET payloads. The WDM systems carry SONET’s embedded overhead channels transparently, which perform link supervision and gather performance statistics and allow SONET’s fault-recovery procedures to operate as normal to ensure network availability. With a 50-ms recovery time, WDM-matches the recovery performance of SONET in case of link failures, allowing both technologies to play complementary roles. And with embedded supervisory channels, WDM systems can report on a number of performance metrics to help diagnose problems with individual channels, as well as with the fiber link.

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