RTC Magazine

The AdvancedTCA (ATCA) architecture is rapidly gaining acceptance among telecom equipment providers and users of that equipment. This architecture’s mandatory shelf manager, typically implemented in a dual-redundant configuration by a pair of boards, is a crucial strength of ATCA. Many ATCA shelves implement shelf managers as dedicated field replaceable units (FRUs) connected to the in-shelf Ethernet network. These are also typically implemented in a dual-redundant configuration by a pair of Ethernet hub boards.

Unfortunately, the original AdvancedTCA specification only defines a standard way for each shelf manager to connect to one of these Ethernet hub boards, not to both of them. This limitation can reduce the effectiveness of both the dual-redundant shelf managers and the dual-redundant hub boards, since a failure in just one of each pair may require both pairs to switch over in order to preserve shelf manager connectivity to the in-shelf network. It is more difficult, and much less desirable, to arrange such a coordinated switchover of two separate types of FRUs. In fact, the single shelf manager link to these Ethernet hub boards is the only known architectural single point of failure in ATCA. In contrast, all normal node boards in a shelf have the ability to connect with both hub boards.

Recently, PICMG adopted an Engineering Change Notice (ECN) to PICMG 3.0 R2.0—the formal specification defining the AdvancedTCA architecture—that fixes this limitation, while preserving backward compatibility with pre-existing equipment.

Before ECN: Each ShMC Links to One Hub

In the original AdvancedTCA specification, through revision 2.0 adopted in March 2005, each Ethernet “Base Interface” hub board is connected by a 10/100/1000 Mbit/s link to one of the dedicated shelf manager FRUs, also called Shelf Management Controllers (ShMCs) (Figure 1). This link is provided on the backplane and reaches the hub boards via their Zone 2 high-speed signal connectors. Other external or auxiliary connections may also be available to each ShMC and to the hub boards. Each 10/100/1000 Mbit/s link requires four differential signal pairs or a total of eight wires.

At the architectural level, there are several consequences of linking each ShMC to one hub. One is that a failure in a ShMC may cause the hub boards to switch over if the interconnects between the two hubs are not used. Another consequence is that a failure in a hub board will definitely cause the ShMCs to switch over. In general, the original architecture reduces the effectiveness of dual redundancy in ShMCs and hub boards due to these effects. AdvancedTCA vendors have developed various ad hoc approaches to remove this single point of failure in their shelf products. However, there has been no generic solution that all vendors can use that preserves interoperability so that a hub board can be installed in any shelf and still implement a solution to these problems.

After ECN: Each ShMC Links to Both Hubs

The recently adopted ECN 3.0-2.0-001 provides an optional way to remove this single point of failure. It defines dual 2-pair 10/100 Mbit/s ShMC cross-connects that can connect each ShMC to both hubs, as well as map these dual 2-pair links into the single 4-pair connection (the ShMC port) that was originally defined in the AdvancedTCA specification on a hub board for use as an ShMC link. Furthermore, ECN 3.0-2.0-001 adds this option while allowing full backward compatibility for products that conform to the pre-ECN specification. Each 10/100 Mbit/s link requires only two differential signal pairs or four wires.

Figure 2 shows the connections enabled by ECN 3.0-2.0-001, including how the two 2-pair ShMC cross-connects are mapped into a single 4-pair ShMC port on each hub board. When two 2-pair connections—which can each support a 10/100 Mbit/s link—are mapped into a 4-pair connection—which could otherwise support a single 10/100/1000 Mbit/s link—the tradeoff is a reduced data rate for improved availability. However, few, if any, existing shelf manager products support or need a 1000 Mbit/s link to the outside world. For most ATCA users, the availability improvement enabled by the ShMC cross-connects is much more important than the higher data rate.