By Motty Anavi
Power utilities with mission-critical applications are now migrating their communications infrastructure to packet-switched networks. Many of these mission-critical applications are delay-sensitive, requiring the same level of high reliability that is expected from the electrical grid.
Because the electrical grid relies on communications, a power utility's operational network requires the highest level of resiliency. Traditionally, resiliency was based on a single technology. If a utility wanted to provide resiliency for their Synchronous Optical Networking- (SONET-)/ Synchronous Digital Hierarchy- (SDH-) based network, for example, it would use a redundant SONET/SDH link as a fallback.
Likewise, redundancy schemes for the newer packet networks-such as G.8032 and Open Shortest Path First- (OSPF-) based networks-relied on a similar network to achieve the desired redundancy and resiliency.
There is, however, a new dual-network approach that allows utilities to maintain their legacy networks for as long as they need, while at the same time moving to newer packet-switched networks such as Carrier Ethernet. This approach is known as Traffic Duplication.
Traffic Duplication allows networks with mission-critical applications to enhance reliability and performance. It can be used to minimize delay for critical utility applications-such as Teleprotection-by capitalizing on Carrier Ethernet's reduced latency at higher speeds. Mission-critical traffic can be transported over a new Carrier Ethernet network running in parallel with the existing SONET/SDH network while the utility prepares for full service migration. See Figure 1.
|Figure 1. Dual Ring Architecture, with SONET/SDH and Ethernet transport|
The SONET/SDH network architecture sends communications traffic over two different directions on a ring, to assure reliability. A similar function is provided through packet-switched networks with Carrier Ethernet using, for example, the G.8032 protocol. With Traffic Duplication, equipment on the network can use the SONET/SDH circuits and packet networks.
This dual mechanism provides the highest level of resiliency because it enables alternative transport in case of failure. It exceeds the resiliency of conventional SONET/SDH schemes, where the ring can survive a single fiber cut but is not as effective in the event of multiple cuts. Traffic Duplication technology adds, in effect, a third path through the packet network, enhancing resiliency by an order of magnitude. See Figure 2.
|Figure 2. Dual Star Connection Architecture with separate TDM and Ethernet engines|
An advantage of Traffic Duplication technology is its ability to use existing networks to implement this added resiliency. Traffic Duplication does not require a custom-built overlay but can leverage an existing infrastructure. It requires a modest investment in communications equipment to use available operational networks and facilities.
With traditional SONET/SDH networks, the length of time it takes to transmit traffic over a link is always fixed, regardless of the speed of the interface. This happens because of how SONET/SDH reserves "space" on the network to transmit traffic, but with packet networks, the faster the link, the lower the transit time.
What might take a full second to transmit over an OC-12 SONET link would take, at most, 1.6 milliseconds, and perhaps as little as two-tenths of a millisecond on a packet network, depending on the network's speed. This represents about a 100,000 percent reduction in transmission delay and helps illustrate the promise that packet technology can provide a reduction in transmission times for utility communications networks.
Another benefit of Traffic Duplication is that it permits a controlled transition to an operational core alternative. In this approach, the packet network can originally be used solely as a redundant mode of transport. Over time, and as the utility's confidence in packet networking increases, this network can become the primary one. The dual operations of the transport equipment over both networks can highlight the inherent reliability and benefits of a packet network. Once any uncertainties are allayed, the utility can migrate to the packet network, retaining its legacy SONET/SDH network strictly as a rarely-needed backup to the new infrastructure.
One of the most mission-critical and delay-sensitive applications for utilities is teleprotection, and here Traffic Duplication adds a clear improvement in reliability. In the traditional teleprotection scenario, the network is protected on the SONET ring through the ring's inherent redundancy. Again, this can withstand a single fiber cut-but, through Traffic Duplication, the network can withstand multiple fiber cuts and still deliver the teleprotection commands. See Figure 3.
|Figure 3. Differential Teleprotection over Dual Plane Architecture|
Utilities depend on two critical components for reliable operations: resiliency and speed. Traffic Duplication yields notable improvements in network resiliency by employing a redundant transport method. It accomplishes this with minimal investment by using any existing packet network to provide this resiliency. And, because of the physics of packet network technology, this can be leveraged to provide shorter network delay and improve critical application performance in teleprotection and other areas.
About the author: Motty Anavi is vice president of the Utilities Line of Business at RAD Data Communications, an internationally recognized manufacturer of communications equipment for utility and telecommunications applications.