15.2 Redundant Carrier Systems

---

The networks of the major carriers are built as redundant systems, meaning that there is a duplicate or backup system immediately available to overcome outages that may occur virtually anywhere on their networks. While the large carriers offer a high degree of redundancy in their networks, smaller competitors may not. Many CLECs, for example, may have fiber rings in the cities they serve, which move traffic between their regional or national data backbones. But not all the CLECs have dual-ring architectures that can route traffic in another direction if a cable cut occurs. Given the high cost of building resilient networks and the shortage of capital for infrastructure enhancement, telecom and IT managers must factor these considerations into their decision-making when selecting carrier services.

15.2.1 Switching Systems

For voice services and low-speed data, local exchange carriers (LECs) operate central offices, which use voice- and data-switching equipment from vendors such as Lucent Technologies, Nortel Networks, Fujitsu, Ericsson, and Siemens. Typically, the goal for these systems, including failure and both scheduled and unscheduled maintenance, is 99.999% (five 9s) availability. This works out to 5 minutes of downtime per year. An exception is Lucent’s 5ESS, which performs at 99.9999% (six 9s) availability. This equates to only 10 seconds of downtime per year. To achieve these high levels of performance, each switch is equipped with dual processors, so that if one processor fails, the second one can take over automatically. In essence, the switch can be viewed as two computers running simultaneously, with the backup ready to take over the full processing load instantly if a problem is detected.

The near 100% availability of these switches is also achieved with redundant subsystems and continuous internal testing. If an internal test reveals that one or more of a subsystem’s performance metrics fall below an established baseline, the backup subsystem takes over while the primary subsystem undergoes a full suite of diagnostic tests to pinpoint the problem. So even though the primary system is not in service, the availability of the switch is not diminished. The switches themselves are closely monitored by on-site technicians as well as remotely from one or more NOC.

In selecting the services of a CLEC, however, telecom and IT managers should be aware that these carriers’ switches may not be provisioned in the same way as those of the Regional Bell Operating Companies (RBOCs). Many CLECs did not purchase central office switches in the volumes that would qualify them for discounts. Others lacked the negotiating skills to obtain feature parity with the RBOCs. As a result, they do not always have the redundant subsystems and features to provide their equipment with the highest level of reliability. To make a bad situation worse, some CLECs leveraged the reputation for reliability of their vendor’s equipment, while not actually having a configuration that would provide that level of reliability.

15.2.2 Signal Transfer Points

The carriers also operate signal transfer points (STPs), which are the computers that route network inquiries into their signaling networks. These signaling networks are separate from the networks that carry the voice and data traffic of customers; they are packet-switched data networks that use messages to set up calls and support intelligent services. The STPs are configured as mated pairs with separate processors. The load-balanced STP pairs are not collocated but are usually hundreds of miles away from each other and operate at just under 50% capacity. With this architecture, if something happens to one STP, its mate can pick up the full load and operate until repair or replacement of the damaged STP can be made.

15.2.3 Network Control Points

NCPs are the customer databases for advanced services such as 800 number routing. The NCP nodes process 800-number call-routing requests received from telephone switches in the carrier’s network. They have dual processors, but if the second processor should fail, there is a backup NCP that is called into operation, thus protecting the customer’s intelligent services information. With several levels of redundancy, there is little chance that customer information regarding services and features will be lost. By way of comparison, AT&T alone has 310 NCPs in its network—50% more than its nearest competitor—enabling it to provide the highest level of redundancy to cope with virtually any disaster scenario.

15.2.4 Digital Interface Frames

Digital interface frames (DIFs) provide access to and from Class 4 central office switches for processing calls. The DIFs that handle this work have spare units available to take over immediately should a problem occur. Guiding the overall work of each DIF are two controllers running simultaneously, so that if one experiences a problem, the backup controller can take over without the customer noticing.

In addition, certain switched business services such as 800, which entail large-quantity egress (traffic flowing off the carrier’s network) can make use of an optional capability. This feature sends a customer’s traffic to another DIF at another switch location if the customer’s primary switch encounters a problem.

15.2.5 Power Systems

Carrier switching systems derive power from the local utility companies. Power lines come into the building to provide direct current (dc) to redundant rectifiers, which distribute power to the switching equipment and battery banks. If commercial power fails, batteries, which are kept charged by the rectifiers, provide backup power. The power levels of the batteries are monitored to ensure readiness in case a commercial power outage should occur. An additional stage of redundancy is provided by diesel-fueled generators, which can replace commercial power for days or weeks at a time.

15.2.6 Cable, Building, and Signaling Diversity

A carrier’s network facilities (i.e., cable routes) are built as a series of circles or loops that touch one another to form an interconnecting grid. Should any particular loop be cut, such as by a backhoe operator hitting a fiber cable, a fair amount of traffic can be sent over one or more adjacent fiber loops. Construction of new facilities in recent years has focused on making these loops smaller and smaller to reduce the magnitude of problems when they occur.

Generally, in larger metropolitan areas, carriers are able to offer business customers building diversity. By being able to reach the carrier’s network at two distinct geographic locations, business customers can enhance reliability for their high-capacity switched and/or special services applications.

This is how STPs are protected as well. Each pair of STPs is connected to every other pair of STPs by multiple data links. To ensure that connectivity will always be available, these links are established through three geographically separated routes. Should something happen on one route to disrupt signaling message transfer, the other two routes remain available to keep the carrier’s signaling system operational.

Within each central office switch, there is a device that permits the switch to interface with the carrier’s common channel signaling system to send and accept information that is used to set up and deliver calls. Should this interface device malfunction, the switch can use special data links that are directly connected to one or more “helper” switches to gain access to the signaling network via their interface. In this manner, central office switches can continue to process long-distance calls while a repair is made. AT&T calls this backup signaling capability the alternate signaling transport network (ASTN).

The central office switches can also make use of ASTN should both halves of a mated pair of STPs fail. Each switch normally uses a particular mated pair of STPs to handle call setup. If something happens to the STP pair on which the switch normally relies, the switch can use ASTN to access the signaling network through helper switches that use a different STP pair.

15.2.7 Real-Time Network Routing

Some of the larger carriers employ very sophisticated call-routing schemes. The network of switches belonging to AT&T, for example, routes calls through a system known as real-time network routing (RTNR). This software system enables every switch within the AT&T network to know the available resource capacity of every other switch in the network on a real-time basis. Since AT&T has more than 130 switches in its U.S. network, customer calls will have more than 130 ways to be routed across the network. This path diversity enables high call-completion rates despite regional congestion and resource constraints. Together, the redundancy and alternate routing features of AT&T and other public networks enable AT&T to offer customers special restoration services.

---