Packet optical transport systems: The new POTS

The proliferation of triple-play and mobile services has put a new spin on that familiar acronym. Packet-optical transport systems enable carriers to transition from legacy SONET/SDH networks to architectures better able to provide both greater bandwidth and better quality of service.

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By Stanton Zeff

The proliferation of triple-play and mobile services has put a new spin on that familiar acronym. Packet optical transport equipment enables carriers to transition from legacy SONET/SDH networks to architectures better able to provide both greater bandwidth and better quality of service.

For more than 100 years, plain old telephone service, or POTS, has been a driver for ubiquitous telephony access. The proliferation of triple-play and mobile services has put a new spin on that familiar acronym, making “packet optical transport systems” the new POTS.

This technology offers the benefit of a converged infrastructure able to support demanding data traffic from innovative applications. The new packet optical transport equipment–which, to avoid confusion with the old acronym, we’ll call packet optical networking platforms, or PONPs–combines the features and functions of SONET/SDH, WDM, and Carrier Ethernet into a single cost-effective network node, bridging legacy TDM systems into the all-packet optical transport world to support IP-based services. This article will assume basic familiarity with the structure of packet optical transport network elements (as illustrated in Fig. 1) and focus on network strategies in the metropolitan-area networks space.

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FIGURE 1. PONP characteristics.
The benefits of packet optical transport
To support ever-increasing bandwidth requirements, operators are faced with a difficult balance: rising capital expenditures (capex) and operational expenditures (opex) on the one hand, and lagging revenues due to increasing traffic costs on the other. The transport network is evolving to a scalable packet-based infrastructure augmented with photonic support. The combination of TDM, Ethernet, IP, and WDM technologies with an integrated approach to operations, administration, and maintenance (OAM) provides operators the best mix to meet their needs for traffic scalability, operational efficiency, and service-level agreements.
PONPs are network transformation tools driving operators’ strategic evolution from SONET/SDH to packet-based transport networks. A universal switching architecture allows concurrent support for a mixture of native TDM and packet traffic, enabling operators to cap and grow their SONET/SDH infrastructure to 100% packet aggregation in line with service demands. Eventually, the paradigm shift to packet transport offers the opportunity to leverage the best of both worlds by combining the benefits of packet-based statistical multiplexing and quality of service (QoS) differentiation with the connection-oriented robustness typical of modern transport networks.

Packet transport is available today with International Telecommunication Union (ITU) approved Transport Multiprotocol Label Switching (T-MPLS) standards, which will smoothly align to MPLS-Transport Profile (MPLS-TP) as standardization efforts are harmonized between the ITU and Internet Engineering Task Force (IETF).

Packet optical transport is a key enabler of services like triple play and mobile backhaul. Operators around the world are responding to customer demands with bundled voice, video, and data services. While video is a bandwidth driver, IP services and Ethernet are critical to cost-effective delivery. Packet optical transport provides efficient traffic aggregation, regardless of the access methods employed, for delivery to the appropriate service points within the network.

Mobile applications are also a fast-growing segment in the industry. Innovative applications drive bandwidth and require exacting, real-time performance. From the cell site to the mobility core, networks are growing in size and sophistication. Whether mobility providers deploy their own backhaul network or purchase service from a third party, the mobile transport network must evolve to furnish increased bandwidth at a diminishing cost per bit. Packet optical transport provides the flexibility to serve today’s predominantly TDM-oriented backhaul with a smooth migration to packet-based backhaul as Ethernet interfaces become increasingly available on mobile network equipment.

The great debate

The question of “QoS versus more bandwidth” is one whose outcome is being altered by the enhanced functionality of PONPs. From one viewpoint, the integration of colorless CWDM/DWDM and tunable/reconfigurable optical add/drop multiplexer capability now allows the network operator to deploy reconfigurable terabit capacity in a network node.

From another perspective, the ability to implement QoS levels within switching fabrics provides levels of flexibility not previously available in broadband aggregation devices. As Ethernet speeds have advanced from Fast Ethernet to Gigabit Ethernet to 10-Gigabit Ethernet, network demand has driven the deployment of bigger, faster transport pipes. In addition, it is quite likely that end users will use QoS algorithms to ensure real-time traffic–such as VoIP and IP video–doesn’t get lost due to increased network congestion.

Operators must employ a combination of techniques from both sides of the QoS/bandwidth equation to meet the volume and service demands of emerging applications. It is critically important that the transport network and the higher-layer service networks operate seamlessly to provide the QoS and bandwidth necessary for robust, high-performance service in the most cost-efficient manner. PONPs are designed to provide this linkage of QoS and bandwidth.

Another advantage that packet optical transport brings to the table is the integration of CWDM and DWDM functionality. WDM support enables efficient switching of traffic at the optical level. Connection-oriented packet transport, embodied in the MPLS-based transport standards–T-MPLS and MPLS-TP–leverages the flexibility of MPLS with transport-oriented OAM features to assure reliable and robust traffic.

In response to the growing demand for Ethernet services, SONET/SDH platforms evolved into multiservice provisioning platforms (MSPPs), offering Ethernet over SONET/SDH. MSPPs can support Carrier Ethernet transport with the SONET/SDH layer providing carrier-class capabilities for OAM, manageability, and resilience. These MSPPs enjoyed tremendous success in the market as service providers were able to leverage their installed SONET/SDH networks. The MSPP model works well when TDM traffic is dominant, but it has scaling limitations with Ethernet traffic growth.

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FIGURE 2. Deployment plans for PONPs.

Packet optical transport represents the next step in the evolution of the transport network as traffic is increasingly dominated by packet services, principally Ethernet. Well-designed PONPs, with the ability to switch TDM and packet traffic with equal efficiency, enable service providers to migrate smoothly from TDM-optimized SONET/SDH and MSPP transport networks to packet-optimized transport networks.

No debate

Not up for debate is the fact that network element functionality is converging once again; we see Ethernet interfaces on transport equipment and SONET/SDH/WDM interfaces on switching equipment. Figure 2 illustrates deployment plans for packet optical transport equipment based on a survey conducted by Heavy Reading published in August 2008; nearly two-thirds of respondents indicated they would definitely be deploying some form of POTS equipment over the next two to four years. In that same report (“Packet-Enabled Optical Network: A Multi-Client Study”), respondents indicated their top three drivers in metro aggregation networks were (in order):

  • bandwidth and services scalability
  • manageability
  • convergence of packet and optical requirements

In fact, these drivers are critical to the TDM-to-IP network transformation taking place in telecommunications networks today. In a converging world, packet optical transport is ideally suited to transition from legacy (traditionally TDM) networks to next-generation packet-based networks.

Stanton Zeff,P.E. (, is a senior marketing manager with Alcatel-Lucent’s Transport Competency Center. The author wishes to thank Chip Pratt for his assistance in writing this article.

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