SDH and SONET – Is it time to turn the light off?

November 15th, 2018

As we enter the 5G era, it is finally time to turn off those old TDM networks

For well over a decade, IP has been the ubiquitous technology for all services and applications. And MPLS and Ethernet have become the defacto mechanisms for transporting IP. Over this time MEF produced their famous Carrier Ethernet criteria, and both MPLS and Ethernet (are used in networks offering Layer 2 Carrier Ethernet services (e.g. E-Line, E-LAN, E-Tree, EVP-Line, EVP-LAN, EVP-Tree) and for customers requiring Layer 3 connectivity IP/MPLS is used. For those services requiring deterministic behaviour from the MPLS network, operators were given a choice between the computation heavy RSVP-TE protocol or MPLS-TP which is a static approach offering Layer 2 connectivity.

The era of 5G requires an evolution to next generation packet, IP is still the ubiquitous technology for services and applications but MPLS transport is being supplemented with new technologies like Segment Routing, Ethernet VPNs and Flexible Ethernet (FlexE) that provide more flexibility, lower latency and improve determinism.

So in this era of next generation of packet transport, it is surprising to see that many operators still have large SDH/SONET networks offering SDH services and indeed some service providers are still offering old PDH services like E1/T1 and even the old n x 64kbit/s PDH dedicated digital private circuit services.

Is lack of functionality keeping the lights on for SDH/SONET?

In fact the case is quite the opposite and there are a number of proven migration paths that allow the SDH/SONET network to be turned off.

  • Circuit emulation: For service providers that want to continue to offer legacy SDH/SONET/PDH services to their customers, circuit emulation can be used. In this approach, SDH/SONET/PDH interfaces continue to be provided to the end customer however these legacy services are mapped into pseudowires for transport on the packet network. If the end customer demands deterministic performance and OAM, then MPLS-TP can be used in the packet network, with this approach, end users wouldn’t feel a change in the operational performance of their services. This process has been well proven in many networks over the last 10 years and indeed a few large Tier 1s have announced that they will force customers to move any legacy services they still want onto a circuit emulated transmission network
  • Migration to Carrier Ethernet: This approach is of course preferred by the Service Provider as customers are moved onto a more modern network and be provided with many value added capabilities and increased speeds.

Is the cost of migration keeping the lights on SDH/SONET?

Again, quite the opposite is true. The cost of the new packet equipment is paid for by huge savings made in OpEx. The cost of a migration project could be absorbed into the OPEX budget, as a project payback period of well under 12 months can be achieved for most migrations. The main OpEx savings are from:

  • Savings in power: newer equipment uses much less power per transported bit. This means a massive reduction in operations costs associated with powering and cooling the equipment. Has the added benefit of helping the Service Provider towards its carbon emission target
  • Huge savings in space: The size of telecoms equipment, per transported bit, has reduced by many orders of magnitude. Migrating to new equipment translates into substantial space savings
  • Spares and maintenance: Much of the old equipment is out of support and some of it is past end of life. This means that the cost of maintenance contracts is starting to grow and there is increasing difficulty in sourcing spares

What is keeping the lights on?

In my experience, the main thing keeping the SDH/SONET lights on, are that the legacy network just does not get the management attention and time that is required to run a migration project. Management attention is at a premium and migrating the old network is just not as important as an increased focus on customer churn, or understanding the impact of 5G and next generation packet. So for the last few years we have been running the SDH/SONET network in a “If it isn’t broke don’t fix it” mentality

So what changes?

Costs are on the increase, as much of the old equipment is out of support and some of it is past end of life. This means that the cost of maintenance contracts is starting to grow and there is increasing difficulty and expense in sourcing spares. In addition, given the age of the equipment, we are starting to enter the backend of the reliability bath-tub curve and it is inevitable that we will start to see and increased failure rate. This in turn will have an impact on customer satisfaction, with the potential for Service Providers to lose some long-term high value customers.

It is my belief that Service Providers now need to focus on turning the SDH/SONET lights off, before the legacy network gets management attention for the wrong reasons. With the agility available in today’s products, a new network platform can be deployed that can support the past, present and future. Allowing legacy services to be transported natively and/or with circuit emulation; current generation packet transport with IP/MPLS, MPLS-TP or Ethernet and even an evolution path to next generation packet.

In this way, evolving the past becomes a stepping stone to the future.