Advanced Switching For PCI Express: The Future Looks "Fabric" Fast

June 23, 2003
Now nearing completion, the Advanced Switching (AS) specification brings switch-fabric speed and scalability to PCI.

Now nearing completion, the Advanced Switching (AS) specification brings switch-fabric speed and scalability to PCI. AS shares PCI Express Base's physical and data link layers, but new applications need to take advantage of AS features like source path routing and peer-to-peer support where all nodes are hosts. The AS virtual-channel-based fabric eliminates the memory-oriented PCI architecture, so AS switches can be very simple and still deliver high performance (Fig. 1). Larry Chisvin, vice president of marketing for PLX (, explains that a more advanced switch can recognize a Base connection (switch, host, or node) and will tunnel Base packets using the PI8 protocol interface (PI). Every packet has an associated PI. A dozen provide the basic AS functionality. Frank Fitzgerald, director of engineering for Aurora Technology (, notes that it's possible to utilize applications without change within an AS environment, but without access to the advanced features found in AS. This approach takes advantage of the scalability of AS.

PCI and PCI Express Base utilize a tree architecture with a single host at the root (Fig. 2), so PCI Express Base can easily replace PCI. Yet migration to AS requires a change from a host-centric memory architecture.

AS offers major advantages in throughput and functionality only available in a fabric-oriented architecture. The AS specification will soon be available, but hardware isn't expected until later in the year. On the other hand, the PCI Express Base standard has been out for quite some time and chips are becoming available now. PCISIG ( handles PCI Express Base, while AS will probably wind up with its own special interest group (SIG), predicts Tim Parker, director of technology initiatives for Intel ( Designers needing AS features now can check out StarFabric. Features of StarFabric, like source routing, were incorporated into AS. According to Tim Miller, vice president of marketing for StarGen (, AS is the migration path for StarFabric. Needless to say, developers will have some heavy reading when the AS spec goes public. By late next year, though, AS will have made a substantial impact on system design.

Compatible with PCI Express L1/L2
Uses same physical interface (L1)
2.5-Gbit/s link, up to 32 links/connection
Uses same data link layer (L2)
Different protocol layer than PCI Express Base
AS uses peer-to-peer architecture
PCI Express AS requires new applications
PCI Express Base supported through PI tunnels
Source Routing
Switches don't require unicast routing tables
Peer-to-peer doesn't require central route management
Switches can notify source of delivery failure
Protocol Interfaces (PI)
Allow high-performance tunneling of any protocol, including PCI Express Base (PI8)
Used for AS features like multicast and congestion management
Virtual Channels
Support in-order and bypass queuing model
Implement class-of-service (CoS) support
Handle congestion and power management
In-band high-availability support like hot-swap
Standard PIs
Spanning tree generation/Multicast
Congestion management (CM)
Segmentation and reassembly (SAR)
Device management
Event reporting
Companion PIs
PI8 - PCI Express Base
Simple load/store (SLS)
Simple queue packets (SQ)
Secure data transport (SDT)
30 reserved PIs
255 PIs total

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