Line cards, especially Sonet/SDH transport line cards, have traditionally performed very few functions: terminating the line, monitoring and perhaps some low-level protection switching. Functionality is usually limited intentionally to keep a lid on these cards' cost and complexity, not to mention size and power consumption.
However, technology has advanced to the point where system designers now have access to greater functionality in compact, low-power form. Rudimentary multiplexing is being added to some framers, distributed switch fabrics on line cards are possible due to advances in switch fabric design and pluggable optics are now affordable and ubiquitous.
The application of system-on-chip (SoC) technology to line card design will further enable features on a line card that up to now have been unimaginable. Flexible port rates and oversubscription of drop port bandwidth are now possible. Additionally, in system line card designs using dense wave division multiplexing (DWDM), integration of complete add-drop multiplexer (ADM) functionality on a single ASIC is now a reality.
SoC technology can be deployed in high-speed networking designs either in a centralized architecture, where the framers are integrated onto the central switch fabric, or in a distributed architecture, where the SoC ASICs are deployed on the line cards. In a centralized architecture, the line cards become very low-cost, low-complexity designs. In a distributed architecture, they can have additional functionality to enable advanced features or to offload some of the core functionality. Let's compare both approaches and look at the impact these approaches have on a DWDM line-card design.
The distributed method
Designing an SoC directly into a line card can enable significant functionality to be distributed away from the central switching core. For existing switching elements (ADMs and digital cross-connects) with a limited upgrade path, SoC line cards can be an enabling technology for adding more capacity, new interface types and new features.
SoC technology can be used to provide fully flexible tributary port rates on a line card. A single, multiport line card can be deployed with each optical interface programmable from OC-3 to OC-192, including all Sonet/SDH rates between. Data formats can also be managed with integrated cross-connects that can accommodate flexible and virtual concatenation schemes.
Since all the logic required to flexibly change rates is built into the SoC, these designs are not significantly more complex than "standard" single-rate line cards. However, the additional flexibility can offer huge benefits to network providers, especially when combined with the granularity offered by pluggable optics.
Through the use of fixed-rate optics and software-provisionable ports, the network provider need only deploy one line card version for all optical interface requirements and the lowest cost optics can be deployed only as needed. This capability has been demonstrated in real-world network designs to have a capital cost advantage of greater than 60 percent over less flexible designs requiring, for example, 16-port OC-12 cards to serve a single OC-12 demand. The use of multirate pluggable optics is more expensive, but can offer significant operational savings by reducing truck rolls when a customer requests a new service or a change to an existing one.
SoC integration onto a line card can also allow oversubscription of the traffic, which is useful in subtending ring and low-density subtending traffic applications. The integrated cross-connect in a SoC design can consolidate under filled optical lines to conserve backplane resources, allowing much more efficient use of the central resources. Pass-through traffic on subtending rings that terminate on an oversubscribed line card do not necessarily affect the central switch fabric, also increasing the total effective capacity of the element.
In a centralized architecture, SoC ASICs are integrated into a centralized switch fabric, where they can provide not only cross-connect features, but also framing, overhead processing, performance monitoring and other advanced features. By building more functionality into the central switch complex, functionality can be removed from the high-speed line cards.
The line cards become only transceivers and backplane drivers, which is the lowest possible cost for an optical interface. Since high-speed optical technology is constantly changing, improving and dropping in cost, there can be significant benefits to both the designer and the network provider to having the optics divorced from the electronics as much as possible.
For the designer, new high-speed optical technologies (advanced forward error correction, different wavelength plans, smaller and less expensive components) can be quickly and easily integrated into the overall system design. For the network provider, new functionality and lower costs can be realized soon after technology introduction, since the redesign time required is greatly reduced.
SoC technology can be deployed to enable complete ADM-on-a-blade functionality in DWDM line cards. ADM functionality on a DWDM line card allows the DWDM system to integrate seamlessly into an existing Sonet network. Features such as subrate switching, overhead processing, performance monitoring and protection switching can be fully realized without significant additional cost, power or size. In fact, combining all of the framers required for both the low- and high-speed optics onto a single card can make the SoC ADM-on-a-blade design less expensive than comparable, simple transmuxing interfaces.
An ADM-on-a-blade DWDM line card is similar to the design, but the backplane drivers are replaced with high-speed optics. An SoC with complete ADM functionality provides two high-speed interfaces that can be used to provide redundancy for protection. Since the SoC has a complete cross-connect and overhead processor built in, the protection can be as simple as unidirectional path-switched ring path-based switching or as complex as bidirectional link-switched ring or more advanced line- and message-based switching. The result is a very efficient network design with all of the benefits of a Sonet architecture integrated directly onto a DWDM wavelength.
Since SoC technology combines multiple functions onto a single chip, additional advanced features can also be enabled that would not otherwise be possible in a line card. The framers and the cross-connect are integrated, so flexible overhead handing can be combined with switching to provide true transparent transport.
Advanced SoC implementations can address every byte in the Sonet overhead for reading and writing. Most Sonet elements--even those that offer transparent transport services--can access only the first STS-1 in the overhead. On an OC-192 line, there are an additional 191 STS-1s of overhead, largely undefined, usually ignored and made available by SoC technology for use on a line card.
The additional overhead bytes on an OC-48, OC-192 or OC-768 channel can be manipulated by the SoC to provide several services, including a clear channel for Ethernet connections between elements, a higher bandwidth internode communications channel ideal for generalized-multiprotocol-label-switching applications and true transparent services where any and all overhead from tributary channels can be carried through the network and reassembled at the egress port.
Combining flexible framers and a cross-connect in an SoC on a line card can also enable mesh-restoration techniques such as Sonet Mesh, a Sonet-based mesh network that is guaranteed to restore in 50 milliseconds or less.
As SoC technology is applied to Sonet/SDH transport functions, significant advances in line card design are being enabled. Flexible port rates have been shown to provide network capital and operational savings. The integration of overhead processing and cross-connects facilitate features ranging from oversubscription to mesh restoration on existing or newly developed switching platforms. SoC implementations in DWDM platforms can put full ADM functionality onto a wavelength for integration into Sonet/SDH networks. In both centralized and distributed designs, the advanced functionality enabled by SoC technology is poised to revolutionize line card design and result in more flexible, more powerful and less expensive hardware options for network providers.
Partha Srinivasan is CTO and Scott T. Wilkinson is a consultant at Parama Networks Inc. (Santa Clara, Calif.).
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