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Optical networks: Metro network design best practices

Optical networks in metro areas are being affected by many factors, including whether to use SONET, WDM or Ethernet, as well as the services, such as IPTV, that will be supported. Find out best practices for sifting through the complexity and designing the best metro network for your customers.

The trends in telecommunications today show clearly that the largest incremental amount of fiber deployed in the...

next decade will not be in the network core but in the access network and metro network. Content, the fuel of consumer broadband traffic growth, is an application that delivers a relatively small number of movies or programs to a large population of users. In most cases, this means that content will be cached at a metro level and that the greatest traffic growth created by content will be in the metro area.

Metro fiber today is based largely on SONET, which is 1310nm single-wavelength deployment. SONET networks are usually constructed as a series of protected rings that allow fast failover to the alternate "rotation" in the event of a fiber cut. Rings are connected via optical add/drop multiplexers (ADMs).

The advent of wavelength division multiplexing (WDM) -- coarse or dense -- deployed in the 1550nm range has added versatility to metro optics by providing multiple lightpaths per fiber and greatly increasing the capacity of a given fiber strand. At the same time, the increased volume of packet traffic, which does not require SONET's synchronous delivery behavior, has changed the traffic profile for the metro network of the future. Today, Ethernet is more likely to be the planned electrical layer of metro networks, and WDM the optical. This shift is changing the balance of tasks between electrical and optical components and the best practices for deployment.

SONET rings can be replicated in metro Ethernet and dense wavelength division multiplexing (DWDM) networks by simply using the same fiber and relying on wavelength separation or by running multiple SONET paths over WDM. Since there are probably no major metro networks worldwide without any traditional synchronous TDM traffic, planners should expect to use a hybrid of SONET and Ethernet technology. Where there is a large installed base of SONET equipment, no plans to eliminate PSTN switches, and major customers with direct SONET access, it may be advisable to plan a transition in the metro optical network from SONET-over-1310 to SONET/WDM and then to begin to integrate Ethernet-over-SONET, finally moving portions of the network to Ethernet-over-WDM.

The changing economics of WDM appear to be defusing the "SONET replacement" issue. Most operators now expect to maintain SONET for PSTN transport for as long as those services are offered, moving to non-SONET architectures only as packet voice displaces TDM voice. However, the gradual evolution is most likely to be compromised by exploding consumer broadband use, particularly by IPTV plans. Operators report voice traffic is stable while data traffic is growing at often triple-digit rates. The faster packet traffic grows relative to "circuit" or TDM traffic, the more likely it is that hybrids of SONET and Ethernet (Ethernet over SONET) will have too small a window of value to justify investment. This is probably the reason why more and more optical vendors are offering hybrid products with reconfigurable add-drop multiplexing (ROADM) and Ethernet.

A WDM issue that receives considerable media play is the way in which transit optical connections are handled. Most products have converted between optical and electrical (O-E-O) to perform a wavelength transit connection because pure optical cross-connect (O-O-O) has been expensive. While O-O-O products have been available for five years or so, most vendors still use O-E-O technology.

The primary issue today is still cost; service providers believe that future ROADM products will provide all-optical transit connections. The key issue for designers, regardless of the mechanism used, is that wavelengths can be "transcoded" to a different wavelength across the switch; a system that fails to provide this is too complex to manage because wavelength assignment on various fibers becomes interdependent, and some reconfiguration modes may not be available because of collisions.

Read the series on optical network design

Best practices for optical core design

Best practices for metro network design

Best practices for access network design

Metro optical deployment is affected by the service mix to be supported, but the service topology has an equal or greater impact. A primary question to be addressed is the amount of intra-metro traffic to be carried relative to the volume of traffic that will simply be connected to a metroPOP for transport outside the metro area.

In areas where consumer broadband traffic makes up the bulk of total traffic, most fiber deployment will focus on linking serving offices to a POP for core network interconnect. While these connections have to provide resiliency, they will rarely require the SONET standard of failover, 50ms, since they will support Ethernet traffic. The introduction of IPTV may change this picture because loss of connectivity will cause pixelization that may produce viewer dissatisfaction, particularly on pay-per-view systems. If user buffering is available, the failover time should be no more than about two-thirds of the buffer interval. Often, consumer broadband failover is best accomplished at the Ethernet level.

Where there is significant synchronous (TDM) traffic and significant corporate packet traffic, it may be necessary to provide optical failover at SONET 50ms levels, in which case SONET or resilient packet ring (RPR) may be required. As noted, WDM may allow metro optical designers to separate traffic according to optical failover requirements and provide improved failover only where needed.

Reconfigurability, meaning the ability to create variable metro optical topologies by interconnecting wavelengths in various ways, is most likely to be needed either to accommodate a large amount of business traffic (metro Ethernet services) or to support alternate routing between serving offices and metroPOPs where the core network connection is made. Where IPTV is delivered, this multi-homing may also be needed for content service points.

At the optical layer, reconfigurability and fast failover are very different things. ROADMs offer a great amount of topology flexibility to adapt to changes in traffic demands, to the point where wavelength services can be offered to metro customers and where even Gigabit Ethernet customers can be quickly accommodated. Adding rapid and multiple spanning trees to Ethernet can provide resiliency at the electrical layer for everything but the most stringent failover requirements.

Many believe that metro optics will, over time, migrate away from the 50ms failover standard of SONET as circuit-switched and TDM traffic become a smaller portion of network load. If this is true, then a pure ROADM-and-Ethernet solution, particularly one based on one-box optical/electrical approaches, may be the best long-term solution.

About the author:
Tom Nolle is president of CIMI Corporation, a strategic consulting firm specializing in telecommunications and data communications since 1982. He is a member of the IEEE, ACM and the IPsphere Forum, and the publisher of Netwatcher, a journal in advanced telecommunications strategy issues. Tom is actively involved in LAN, MAN and WAN issues for both enterprises and service providers and also provides technical consultation to equipment vendors on standards, markets and emerging technologies.

This was last published in July 2007

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