The hybrid approach: mixing satellite with land-based solutions - to back-up systems

THE HYBRID APPROACH

A hybrid approach to backup--using satellite and terrestrial lines--lets the user quickly redirect vital traffic, select the highest priority traffic, and avoid outages.

One advantage to this approach is cost-effective use of backup facilities during normal operations.

Fires, floods, hurricanes, and earthquakes exact their worst financial penalties not in loss of capital equipment but in service outages, ranging from days to several weeks.

Today's terrestrial service-restoral alternatives employ techniques such as ring architecture, portable electronic switches, and microwave-link detours. Various companies, notably SunGard in Philadelphia and Comdisco in Chicago, offer:

* hot sites, with modems, muxes, TI facilities, and other data-processing equipment;

* or cold sites, empty rooms where customers relocate their own equipment and management in temporary quarters.

In addition, public carriers have developed complex restoral plans. Many plans involve alternate routing of cables and transmission lines.

Yet these backup options can be compromised. Alternate paths share a common right of way for some portion of their span; also, these options still rely on a central office to maintain services.

Wasted Capacity

Until disaster strikes, recovery centers and alternate cable routes lie dormant and unused.

The high cost of standly strategies is borne by the user. Stakes are even higher when alternate paths and emergency equipment deployed in recovery centers are not for voice and low-rate data but for T1, T3, and videoconferencing services.

A hybrid approach using satellite communications provides a reliable alternative, avoiding the pitfalls of diverse terrestial topologies.

Satellite links, for example, that originate and terminate on custome premises provide complete terrestrial bypass. Moreover, satellite paths can be overlaid on existing terrestrial networks.

The large footprint coverage of the satellite and the flexibility of the earth-terminal switching equipment offer cost savings over and above the primary purpose for service restoral, for the following reasons.

* The satellite channel is a single-pool telecommunications resource that can be shared by many sites and temporarily allocated to one or more "failed" locations.

* Satellite links are overflow and expansion avenues when not in use during emergencies.

* Flexible switching equipment can be configured, via user-friendly software control, to reroute only high-priority services during disasters.

Terrestial/satellite hybrid networks require that each of the load-sharing communications links emulate the technical and operational characteristics of the other.

Host Center Backup

Of the two major hybrid architectures that are emerging, the dominant--yet less flexible--strategy is host-center backup, which focuses on nodal failure.

Host-center backup is limited to VSAT (very-small-aperture terminal) and point-to-point networks, since satellite links are deployed to provide backup for catastrophic loss of a primary data center.

Network operation can be easily and quickly redirected to a secondly center.

This strategy includes "electronic vaulting"--frequent storage and updating of the primary-host database into a secondary location, ensuring a hot-standby secondary host site.

Small, portable remote satellite terminals can be rapidly deployed to the scene.

The large hub is a permanent installation.

Most business telecommunications needs, however, are neither primarily point to point nor exclusively computer data.

Voice, videoconference, and high-speed T1/T3 represent the larger percentage of your traffic.

Intelligent Network

The other major hybrid is an intelligent satellite network superimposed on existing terrestial services.

Satellite capacity serves a single pool. Any one site can access any part of this capacity to communicate with any other site on the network.

All sites can receive any single node's transmission.

During normal operations, the network is an integrated resource sized to carry the aggregate capacity.

The partial capacity carried by the satellite links, however, is not dedicated.

Unlike fixed point-to-point terrestrial links, the satellite channel is shared by all sites.

The advantage is apparent in voice services.

Exactly emulating terrestrial telephone switches, the multiple-access equipment of the satellite subnetwork can be sized less than the sum of all voice circuits, since not all circuits will be simultaneously busy.

So the satellite subnet can carry voice traffic more efficiently than can point-to-point terrestrial links.

Similarly, satellite subnet equipment can be configured to share the satellite channel for some data services.

Dynamic Balancing

Pooling access to a common satellite channel allows dynamic balancing of traffic carried among the sites by the satellite.

When one site experiences peak-traffic periods, it may borrow more satellite channels than another site in a low-traffic period.

For example, assume the terrestrial portion of the network is the primary path and carries a mix of data and voice traffic. The secondary-satellite portion of the network also carries its share of data, voice, and video traffic.

If the primary terrestrial network is subjected to temporary or permanent traffic overloads, circuits can be transferred to the unused satcom capacity (if any), or circuits can preempt lower-priority satellite circuits.

New services can be added in minutes. The satellite's broad coverage allows the network to extend quickly to new sites, even if adequate terrestrial services are not available at these sites.

Using The Backup

The backup subnet need not go unused during normal periods. Reacting to link or nodal failure becomes a matter of preempting selected low-priority satellite services as top-priority services are routed to the backup satellite subnet. Quick reaction to failures is inherent in the equipment's switching components at no additional cost to the user.

The switching "nerve center" can be at any protected site (not all sites must be protected), either at the end user's premises or the local central office.

A terrestrial switch allocates circuits to another terrestrial network or the satellite subnet.

This is done automatically by programming the switch, either via a local software console or remotely via a centralized control console at some other site. Once failure detection occurs, the switch also can use automatic route selection to redirect selected circuits upon detection of failure on either satellite or terrestrial subnets.

The second switch is in the satcom earth station emergency terminal. This switch redirects satellite circuits to their destination nodes or redirects all its traffic from a failed node to a predesignated alternate termination. Moreover, this switch is configured using preprogrammed "connectivity maps" via local console or remote centralized-control console at some other site.

Both switches together allow this site to maximize its use of the primary and secondary telecomm resources, yet remain flexible enough to accommodate each site's varying traffic patterns.

COPYRIGHT 1990 Nelson Publishing
COPYRIGHT 2004 Gale Group