Power utilities worldwide are modernising their existing communications infrastructure to support new IP-based grid monitoring, control and automation applications.
To succeed, they need to address the challenge of migrating the traffic of legacy time-division multiplexing (TDM)-based applications from their existing transport networks, which are rapidly approaching end of life.
Legacy applications such as SCADA use TDM to transmit data over V.24/RS-232 and four-wire analogue interfaces. These applications play a critical role in grid operations and require resilient connectivity. They rely on the network to provide consistent, on-time delivery for every byte of application data.
Packet technologies like internet protocol (IP) are mainly architected to carry best-effort IP traffic, so utilities are understandably cautious, or even wary, about transitioning TDM traffic to new, packet-based networks.
This is where IP/Multiprotocol Label Switching (MPLS) comes in. By supporting protection features such as fast reroute (FRR), MPLS brings synchronous digital hierarchy/synchronous optical networking-like resiliency to IP networks.
When these features detect a network failure, they quickly switch the impacted traffic to an auto-computed, pre-established backup path within 50ms. FRR protection also works when multiple failures occur, providing the utmost protection for critical grid communications
Network jitter and utility pain points
Another top concern is network jitter, which is inherent in packet switching technologies.
Jitter causes problems when TDM traffic is carried over packet networks because it disrupts the necessary steady, on-time network transport.
Some packets arrive with a larger delay and some with a smaller one. This impairs the emulated TDM circuits, driving them to error states such as loss of frame (LOF) or simply a high bit error rate. These errors cripple critical applications and substantially impact grid operations.
The circuit emulation service provided by IP/MPLS, also known as TDM pseudowire, uses a playout buffer at the network egress node to accommodate jitter (see figure). During initial start-up, the buffer gathers and stores TDM data until half of the buffer is full and only then transmits data out.
This ensures that TDM data playout is never disrupted if packets arrive late or early due to jitter. Moreover, the MPLS network supports deterministic quality of service (QoS), so jitter amplitude is bound with a limit. Utilities can dimension the playout buffer size to completely mitigate the impact of jitter.
Synchronisation as a barrier to TDM migration
Transporting TDM data requires end-to-end network synchronisation. IP networks are not architected for that.
IP/MPLS addresses this need by supporting seamless integration with synchronisation technologies including GPS, synchronous Ethernet and IEEE 1588. It brings precise synchronisation to the network with complete flexibility.
Of course, migration is more than network engineering. There are important operations issues to think about:
- How can IP/MPLS network interoperate with my legacy applications?
- How do I plan and carry out the migration? What are the pitfalls?
- What are the benefits of migration? Is it worth the effort?
Are you bothered by these questions, among others? If so, I would like to invite you to register for the Engerati webinar, ‘Modernising communications without compromising critical legacy systems’ on 26 September.
I will be joined by John Alberti, a Telecom/Network Architect working at Oncor Electric Delivery. Together, we will be exploring a converged IP/MPLS architecture that bridges the past to the future. John will also be sharing his experience gained in planning and executing the migration.