Previous Table of Contents Next


Tariffed Redundancy and Protection

Among the traditional methods for protecting WAN facilities are the tariffed redundancy and protection services offered by such interexchange carriers as AT&T, MCI Communications Corp., and US Sprint Communications Co.

A reliable method for minimizing downtime on the WAN is to have redundant lines ready and waiting. When a link goes down, the standby facility can be activated until the source of the failure is determined and appropriate action taken to restore service. Having duplicate facilities is a prohibitively expensive option for most businesses because monthly charges accrue whether or not the facilities are used.

To minimize the effects of failed facilities on the same route, AT&T, for example, offers two special routing methods in conjunction with its digital and analog service offerings: diversity and avoidance.

Diversity. Diversity is available for ACCUNET T1.5, ACCUNET Spectrum of Digital Services (ASDS), 56k-bps Dataphone Digital Service (DDS), and voicegrade private lines. With diversity routing, designated groups of interoffice channels (i.e., AT&T’s portion of the circuit) are furnished over physically separate routes. Each route entails installation and routing charges. A custom option for diversity furnishes the interoffice channels partially or entirely over physically separated routes when separate facilities are available. In this case, AT&T applies a special routing charge to each channel.

Avoidance. The avoidance option allows the customer to have a channel avoid a specified geographical area. The customer minimizes potential impairments, such as delay, that might be exacerbated by long, circuitous routes. It also enables the customer to avoid potential points of congestion in high-use corridors, which can block traffic. This option also gives customers the means to avoid high-risk environments that can be prone to damage from floods, earthquakes, and hurricanes.

Further Protective Capabilities

Although special routing can minimize the damage resulting from failed facilities by allowing some channels to remain available to handle priority traffic, special routing makes no provision for restoring failed facilities. AT&T has attempted to address this issue with its automatic protection capability and network protection capability.

Automatic Protection Capability. Automatic protection capability is an office function that protects against failure for a local channel or other access for the ACCUNET TI.5 and ACCUNET T45 services. Protection of interoffice channels is provided on a one-to-one basis through the use of a switching arrangement that automatically switches to the spare channel when the working channel fails. To implement this capability, a separate local access channel must be ordered to serve as the spare, and compatible automatic switching equipment must be provided by the customer at its premises.

Network Protection Capability. Whereas AT&T’s automatic protection capability guards against the failure of a local access channel, its network protection capability is designed to guard against the failure of an interoffice channel. Protection is furnished through the use of a switching arrangement that automatically switches the customer’s channel to a separately routed fiber-optic channel on failure of the primary channel.

For both ACCUNET T1.5 and ACCUNET T45, an installation charge is incurred for the network protection capability. For the amount of protection promised, however, it may not be worth the cost, because most, if not all, of AT&T’s interoffice channels are automatically protected, whether or not they use the network protection capability. When AT&T circuits go down, traffic is automatically switched to alternative routes.

Dial Backup

Over the years, dial backup units have come into widespread use for rerouting modem and digital data set transmissions around failed facilities. Dial backup units are certainly more economical than leasing redundant facilities or opting for reserved service or satellite-sharing arrangements.

This method entails installing a standalone device or an optional modem card that allows data communication to be temporarily transferred to the public switched network. When the primary line fails, operation over the dial backup network can be manually or automatically initiated. At the remote site, the calls are answered automatically by the dial backup unit. When the handshake and security sequence are completed and the dial backup connection is established, the flow of data resumes. On recovery of the failed line, dial backup is terminated in one of two ways: a central site attendant manually releases the backup switch on the dial backup unit, or, when in the automatic mode, the dial backup unit reestablishes the leased line connection and disconnects the dial network call upon detection of acceptable signal quality.

Rerouting on T1 Lines

To support applications requiring a full TI, dial backup over the public switched network is available with AT&T’s ACCUNET Tl.5 reserved service. With this service, a dedicated T1 facility is brought online after the customer requests it with a phone call. An hour or more may elapse before the reserved line is finally cut over by AT&T. This option may be acceptable under certain conditions, however, as a possible alternative to the loss of network availability for an indeterminate period. This is an effective alternative for the customer who has resubscribed to the ACCUNET T1.5 reserved service. If the customer has not resubscribed, this service is not a suitable alternative for routing traffic around failed facilities, if only because the local access facilities must already be in place at each end of the circuit.

Customer-Controlled Reconfiguration

Management capabilities, such as customer-controlled reconfiguration (CCR) available using AT&T’s Digital Access and Crossconnect System (DACS), can be a means to route around failed facilities. Briefly, the DACS is a routing device; it is not a switch that can be used for setting up calls (i.e., a PBX switch) or for performing alternate routing (i.e., a multiplexer switch). The DACS was originally designed to automate the process of circuit provisioning. With customer-controlled reconfiguration, circuit provisioning is under user control from an on-premises management terminal.

With customer-controlled reconfiguration, however, a failed facility may take a half-hour or more to recover, depending on the complexity of the reconfiguration. This relatively long period is necessary because the carrier needs time to establish the paths specified by the subscriber through use of a dial-up connection.


Previous Table of Contents Next

Copyright © CRC Press LLC