RTC Magazine

Platform Overview

A platform is a suite of hardware and software building blocks that can serve as the basis for many different types of network elements. By beginning with the mature designs of these building blocks and adding some applications-specific elements, the telecommunication equipment manufacturer (TEM) can construct an entire portfolio of network elements at a fraction of the development cost of the traditional development mode (where all elements of each system are custom built from scratch). Some of these platform building blocks will undoubtedly be purchased ready-made in the COTS marketplace. Other platform building blocks will be designed and built using the TEM’s traditional development processes and factories, but built to conform to the Advanced Telecom Compute Architecture (ATCA) standard. The ability to freely mix and match COTS and TEM-produced building blocks is a significant advantage.

The TEM’s customers, the network operators, also benefit from the platform-based approach. If many or most of the elements that make up their networks all rely on a common hardware base and a small core set of platform building blocks, they can realize significant operational cost savings. For example, they would only need to have a single, very small pool of spare parts to cover a large network wire center. Their expense for training their maintenance personnel and operating the network would be greatly reduced. Because of these benefits, it has been suggested that the various tenders that the network operators will be fielding in the coming years may actually express a strong preference for equipment based on ATCA.

Integration is an advantage to ATCA-based platforms that is applicable to both the TEMs and network operators. In highly integrated systems, the functions that were served by many different network element boxes are combined into a single, multi-function box. For example, many of the access and edge functions currently performed by elements like Digital Subscriber Line Access Multiplexer (DSLAMs), Fiber to the Home Terminals (FttH), Cable Modem Termination Systems (CMTS), edge switches and firewalls could all be combined into a single ATCA-based universal edge element. The high-performance, density and reliability inherent in the ATCA standard greatly simplify the creation of this sort of highly integrated box. The TEMs benefit from the development and manufacturing efficiencies of integration. The network operator benefits because they have far fewer boxes to install, configure, service and maintain.

ATCA is an ideal choice to serve as the standards base for universal platforms. It has a number of attributes that make it ideally suited to the needs of telecommunications network elements, including:

• • Scalable capacity to 2.5 Tbit/s
• • Scalable availability to 99.999%
• • Large (but volumetrically efficient) board form-factor—355 mm x 280 mm x 30 mm
• • Large cooling capacity—200W per board
• • Multi-protocol support for interfaces up to 40 Gbit/s
• • High levels of modularity and configurability
• • The ability to host large pools of DSPs, NPs, processors and storage
• • Convergence of telecom access, core, optical and datacenter functions
• • Advanced software infrastructure providing APIs and OAM&P
• • World-class element cost and operational expense
• • A healthy, dynamic, multi-vendor, interoperable “eco-system”

These attributes greatly enhance ATCA’s ability to serve as the basis for a universal telecommunications platform and will be available on the platform in 2003, with a design-in life through 2009.

To achieve the ATCA-based universal platform vision, several key building blocks must be created. These building blocks are used again and again on many network element designs (potentially with widely different functionalities). A surprisingly small number of shelves, boards and mezzanine boards can accommodate a significant portion of many different network element designs. With the addition of some application-specific building blocks (for example, implementing application-specific transport technologies or algorithms), a suite of ATCA-based hardware is created that can be drawn from to assemble nearly any network element type in a major TEM’s product portfolio.

AdvancedTCA Shelves

ATCA shelves contain four important structures. First, they include the mechanical means of rigidly holding the boards into the proper position, for example, mounting flanges, subracks and cardguides. Second, they include the backplane that provides high-speed serial electrical interconnect between the boards. Third, they contain the cooling apparatus that forces a sufficient flow of air up through the boards to remove up to 200 Watts of heat from each board. Finally they distribute the power for the system; shelves also often contain shelf management processors, various power conditioning modules and fan control.

Several variations of ATCA shelves will be common in platform applications. Some shelves will need to fit in 19-inch racks due to the legacy environment in which they will be installed, and those shelves are limited to 14 board positions. Other installations can use the 600 mm wide ETSI frames, or 23-inch racks, and these shelves can accept up to the ATCA maximum of 16 active boards. The 16-slot shelves often have significant density and system cost advantages over 19-inch systems, so they are becoming the preferred solution.

Interconnect topology represents another variation to the ATCA shelves. In dual-star topologies, fabric slots one and two form the hub of a dual-star network, with the higher order slots (numbers 3-14 or 3-16) providing the nodes of the dual-star network. The theoretical bandwidth of ATCA dual-star systems will be 240 or 280 Gbit/s, respectively. Alternatively, the ATCA spec supports a very high capacity full-mesh interconnect, where every board has a dedicated path to all other boards, and there are no centralized fabric resources. Fourteen-slot full-mesh backplanes have a theoretical bandwidth of 1.82 Tbit/s, while sixteen-slot full-mesh systems support 2.4 Tbit/s. Full-mesh backplanes make the most sense for applications where huge bandwidth, pay as you grow scalability or higher fault tolerance is required. Conversely, dual-star backplanes are the choice for more cost-sensitive or lower-power systems.

Carrier Boards

In some cases, ATCA boards will come with specific functions (I/O, processing, DSPs, etc.) built directly on the board. This often leads to an absolute cost minimum for high-volume designs. However, there are significant advantages to providing the system’s important functions in a more modular approach. A carrier board holds several (nominally four) mezzanine boards to perform the system functions and provides the basic services needed to interconnect these mezzanine boards with the ATCA backplane. In this way, it is possible to mix and match mezzanine functions to provide exactly the correct processing and I/O features required by a particular application. As technologies change, there is a good chance that the system can be updated by simply swapping out the mezzanine boards, without the need to change the carrier boards or backplanes.

Two types of carrier boards are required. The first one, called a mesh carrier board, serves two purposes. First, it acts as the main fabric of a dual-star system. However, it can also be used in all slot positions of a full-mesh implementation. The second carrier board design is called a node carrier board. It is used in the 12- or 14-node slots in dual-star systems. Node carrier boards are similar in design to, but simpler and less expensive than mesh carrier boards. Their backplane bandwidth is only 20 Gbit/s compared with the 150 Gbit/s theoretically available on the mesh carrier boards.

Figure 1 is a block diagram of a reference design for a mesh carrier board. Its primary function is to adapt the interfaces of the four mezzanine boards it carries to the ATCA backplane and mechanical specifications.

The backplane interface/switching fabric block connects the bandwidth of up to fifteen 10 Gbit/s backplane fabric channels—implemented as four lanes at 2.5-3.125 Gbits/s each—to the local clients on the carrier board. The switching capability can also direct packets from any of the fifteen backplane channels to any other backplane channel. This capability is needed if the mesh carrier board is used as a dual-star hub, and is also valuable in full-mesh implementations to provide additional system throughput or fault tolerance.

The switch fabric drops local bandwidth through its sixteenth port to a network processor complex. This may be a traditional network processor chipset, or an FPGA providing programmable interconnect and bandwidth management. This block performs protocol conversion, bandwidth management, queuing, QoS, security and packet address translation functions.

The local CPU complex performs board-level control functions. It includes a microprocessor, a large memory and support peripherals. It performs control functions like initialization, bandwidth management, table updates and the service and maintenance HMI. In some implementations, this CPU will also perform core network control and call processing functions, while in others a higher performance CPU will be installed in one or more mezzanine positions on the carrier board.

The power infrastructure block accepts the duplicated -48V power feeds from the ATCA backplane and produces the lower voltages used by the various chips on the carrier board and all the mezzanines. It must combine the two -48V feeds into a single DC bus, and isolate the board power system from the backplane power system. Often power ORing diodes are used here, but for high-availability systems they can have problems, and a pair of load-sharing DC-DC converters perform much better.

The board management processor block is a very small CPU that accepts the two shelf management buses from the backplane and performs elementary board control functions like power control, inventory, keying and reset.

Synchronization of the board is accomplished in the clock logic block. It can accept up to six backplane distributed clock signals, manipulate their frequencies and drive the various clock nets on the carrier board. Optionally, the clock block can also accept a timing reference signal from the board (perhaps derived from a received transmission facility, or a high-quality board-mounted oscillator) and drive it out onto the backplane. This permits the construction of a distributed clocking infrastructure for the ATCA shelf.

Finally, the carrier board has four mezzanine positions that are supplied with interconnect bandwidth, control signals and power. These mezzanine boards are where the real I/O and processing functions of the platform take place.

The node carrier board’s architecture is nearly identical to the mesh carrier board. The major difference is that instead of the fifteen backplane interface links and the complex switching fabric, only two links (one each to the hubs of the two stars) need to be terminated. The other blocks are the same as the mesh carrier board.

Mezzanine Boards

Using the ATCA modular platform approach, nearly all of the important system functions are provided by highly programmable mezzanine boards that are plugged into the two types of carrier boards described above. Some systems will have nearly all of the 64 maximum mezzanine slots on an ATCA shelf filled with the same types of mezzanine boards, while other systems will contain a rich mixture of different mezzanine types.

Initially, the mezzanines will use the PMC spec extensively. Once the emerging AdvancedMC specification is ratified by PICMG, it is anticipated that many of the mezzanine functions will migrate to that hot-swappable, and much higher performance standard. Figure 2 contains example block diagrams of four of these mezzanine boards.

The CPU mezzanine contains a carrier board interface, one or more high-performance microprocessors, a large bank of DRAM and a peripheral chipset. It may be based on Pentium, PowerPC, SPARC or other CPU architectures. The primary function of the CPU mezzanine is as a compute server, performing call processing, network control or web server functions.

A pool of perhaps a dozen DSP chips can be configured on the DSP mezzanine card. They are tied together with an aggregator device that distributes the bandwidth from the carrier card to each DSP chip in the farm. The aggregation function may also perform protocol conversions, for example, converting packet-based voice traffic to TDM timeslots. The DSP mezzanine is used for signal processing tasks like voice processing, echo cancellation and video compression.

Although the carrier board contains some network processing capabilities, for many platform applications supplemental network processing is required in conjunction with a transmission facility interface. The mezzanine-mounted NPs support higher bandwidths, more complex protocols, deeper layer packet inspections and can convert between different protocols (like IP to ATM).

The mezzanine holds a network processor chipset, a large buffer RAM, a table RAM and a facility interface (often optical). Special variations of the NP mezzanine may add CAM chips, network search engines or encryption engines to supplement the basic functions of the NP. The network processor mezzanine performs packet processing functions like routing, protocol conversion, Quality of Service, queuing and address translation.

All the processing and I/O provided by the above building blocks must be complemented with a high-performance storage capability. This storage is hierarchical; the first level consists of a large (multi-gigabyte) DRAM array, acting as a cache. The second level is an interface to an array of disk drives, typically using redundant array of inexpensive disks (RAID) techniques. The storage mezzanine consists of a large DRAM bank and control logic, as well as an interface to an external disk farm. It is used for storage-intensive applications like e-commerce, voicemail, video servers and web caches.

Some algorithms of interest are not amenable to running on CPUs, DSPs or NPs, and these are best implemented as custom hardware designs loaded onto FPGA mezzanines. The FPGA mezzanine consists of a backplane interface along with approximately six FPGA chips (providing millions of user programmable gates, and some optional RAM banks). These mezzanines are ideal for running applications like special protocol processors, encryption and video processing logic.

Software Platform Building Blocks

Software is at least as important a platform component as the hardware discussed above. Benefits exactly analogous to the hardware story exist if standards-based, multiple source software infrastructures are chosen as the basis of the platform’s software. The software infrastructure requires several layers. Device drivers for each of the hardware building blocks constitute the lowest layer. Next, an operating system is required. Carrier Grade Linux is often cited as an appropriate choice for the OS.

A layer of middleware is above the OS. Middleware provides the common API that isolates the application code from the often-changing details of the hardware, and provides important services like fault tolerance and resource management in consistent ways. The Service Availability Forum’s standard middleware is a reasonable choice here. Finally, the application program runs on top of the entire stack, providing the mainline functions of the application. By standardizing the lower layers, development expense can be greatly reduced, time-to-market can be enhanced and the overall customer perceived desirability of the platform improves.

Using the ATCA platform building blocks, a TEM can construct a large percentage of the products in a full service portfolio. The platform approach saves development effort and provides operational expense benefits. By basing the platform building blocks on an industry standard like ATCA, the TEM is able to optimize a sourcing strategy by selectively buying building blocks from leading vendors or building them in house.

Lucent Technologies
Murray Hill, NJ.
(908) 582-8500.
[www.lucent.com].