Evolution of next-gen SDH

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The market slowdown and shift from new operators back to incumbents is allowing SDH to evolve as an alternative to totally new platforms.

By Mark Telford

April's "Evolving SDH" conference in London focused on demand for next-generation networks versus the need for carriers to consolidate and get value from existing investments in legacy synchronous digital hierarchy (SDH) networks.

Dave Johnson of BTexact Technologies says that, of the alternative technologies, an Ethernet infrastructure could remove the SDH cost base, but it needs help replacing SDH functions such as traffic engineering and resilience.

Alternatively, one big IP pipe can replace multiple leased lines for data, but quality of service (QoS) is needed.

So, adds Johnson, if a new technology can't do everything SDH can, then why build yet another network when SDH is there anyway and can be added to?

Though optimised for circuits rather than data, SDH can evolve to accommodate Ethernet, IP, MPLS and WDM in next-generation multi-service networks designed for packet-over-optics transport. Ethernet-over-SDH allows an integrated solution, since there are Ethernet cards for existing SDH kit including switches for Ethernet rings on SDH rings. SDH support for management of infrastructure, reliability, errors and multiple services is retained, while enabling an evolution for incumbents.

For both Ethernet and IP, MPLS can add QoS, security and fast restoration, while Resilient Packet Rings (RPR) add resilience and performance, although this makes them more complex and expensive.

RHK Inc's Frederique Dame, analyst, Optical Networks, Europe & Global, says that, with the survival of new pan-European carriers in jeopardy and more mergers between incumbents likely, the dominance of legacy SDH for transport will be prolonged by the slowdown. Carriers are comfortable with traditional SDH platforms, rely on incumbent vendors and prefer to limit the number for network management purposes. Also, DWDM hasn't swept metro applications, and carriers are slowing evaluation of new platforms, while SDH platforms are evolving.

However, some persisting challenges for SDH in next-generation networks include:

  • complex network planning;
  • scalability and sub-network growth issues;
  • network management;
  • overcoming the "old technology" perception; and
  • identifying where this additional layer adds value.

SDH's presence in transport networks will be defined by traffic granularity and availability of technology features:

  • IP/Optical, high traffic volumes and lambda management will push SDH towards the transport edge; and
  • higher TDM rates (10Gbit/s and 40Gbit/s) and new features will push SDH towards the transport core (see Figure 1).

SDH is losing relevance in the core, since it has the wrong granularity: wavelengths, not bit streams, are the "managed objects"; while SDH is being pushed to the edge as aggregation devices will have SDH trunk-side and sub-lambda grooming remains a requirement.

However, further SDH evolution enhances its capabilities for next-generation networks, with 10 and 40Gbit/s transmission, 64/1 ADMs, larger more efficient cross-connects with multiple ring closures at the highest TDM rates, and multi-service features of ADMs with packet-over-SDH enhancements, especially Gigabit Ethernet ports.

So trade-offs are between flexibility of bandwidth and routing (greatest for Ethernet and IP) and delivery of everything, managed and robust (greatest for SDH and ATM), with MPLS/RPR providing a compromise.

Dave Lewis, Marconi's EVP Optical Networks, adds that "Operators' capital and operating expenditure is coming under significant pressure, so providers are sticking with what they know best [SDH]. The technology is well proven, it provides carrier-class reliability and functionality and network managers know how to drive it to best advantage".

Dame concludes that SDH will continue to be widely deployed in Europe. It is a resilient, well-understood protocol offering sub-lambda grooming, cross-connection, consolidation, and alternative approaches to by-pass SDH are not proven - especially as SDH is still best for voice.


In-Stat and MDR's report "SONET/SDH: The Struggle Toward Recovery" forecasts growth regaining momentum in 2003 to USD21.4bn in 2006, including USD7bn from next-generation solutions. New entrants are touting next-generation SONET/SDH almost exclusively for metro core and access. However "a wave of acquisition activity will occur in coming months, as established companies purchase new capabilities." About a dozen major vendors have a base in long-haul and have moved aggressively down to the metro core - while a handful of challengers are moving up from metro access.

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