|
Next-Gen Backplanes: Change At The
Molecular Level
BY MELISSA HECKMAN AND JUSTIN MOLL
In the battle for market share in the communications arena, there are
several up-and-coming backplane developments. In the last few years,
CompactPCI has been a huge success. Growing from humble beginnings in
1994, the architecture has grown from board sales of less than $20 million
worldwide in 1997 to over $500 million in 2000. It is projected to hit
nearly $2 billion in revenues by 2004 (Venture Development Corporation,
2001). With its hot-swap capabilities and computer telephony
implementation, it has been an excellent architecture for communications
applications.
As the communications market converges, the requirements for
applications broaden. The ability to handle packet-based telephony will
become increasingly important for systems. High Availability (HA), having
a system up and running 99.999 percent of the time, will be a must. With
the convergence of voice, data, and video, Quality of Service (QoS) is an
important issue. CompactPCI backplanes are evolving to handle the need for
more bandwidth, scalability, reliability, and to keep costs low.
TODAY'S CompactPCI BACKPLANES
Today's CompactPCI backplanes are expanding into more applications. The 6U
8-slot cPCI backplane has been popular because it allows more
functionality within a system. But this bus architecture has been
traditionally limited to 8 slots. Options in smaller chassis and the
evolution of horizontal chassis opened up 6, 5, 4, 3, and 2-slot
CompactPCI backplanes. The "pizza-box" style stackable 1U
chassis are becoming increasingly popular. In some communications
applications, the trend has been to start with the smallest, simplest, and
least expensive chassis and scale up as more functionality is required.
The cPCI backplane offers flexible routing in the P3-P5 area, with P4
used for H.110 applications. The power connections leave a lot of choice.
An inexpensive and simple backplane design has power bugs or taps allowing
easy access cabling. Another popular option is hot-plugable power supply
connectors for hot-swap applications.
Bridge chips have allowed the expansion of the traditional 8-slot
limitations. Backplanes in 16 slots, 21 slots, or other variations provide
solutions for current telecom applications (Photo 1). These applications
tend to require hot swapability, so these backplanes tend to have
redundant CPUs, plugable power supplies, etc.
THE NEXT STEP: cPSB ETHERNET BACKPLANES
Bustronic is on the PICMG 2.16 subcommittee that is developing the cPSB (CompactPCI
Packet Switched Backplane) standard. Originally adopted by Performance
Technologies, the IP backplane should become quite popular with its
compatibility to the omnipresent Ethernet protocol. The technology looks
to improve the theoretical 533 Mbps over the 8 slots the 66 MHz version of
cPCI supports. The cPSB standard defines two fully independent packet
busses, which provides theoretical backplane throughput rates up to 5 Gbps.
Performance, scalability, and reliability of CompactPCI are improved with
cPSB for IP traffic.
Essentially, cPSB overlays an embedded Ethernet switching network on
the cPCI backplane, accessed via the J3 connector. An Ethernet switching
card interconnects all slots in the chassis. All cards plugged into the
backplane operate as standalone systems, interfacing with each other
through a network stack on top of Ethernet. As a standalone system, each
card has its own processor, memory, and operating system. As long as the
card supports Ethernet protocol, it can have any operating system
installed and run in any link slot.
A single fabric slot can support up to 19 node slots. Redundant,
hot-swap capable Ethernet switch slots support up to 19 node slots in one
chassis. The links are on P3 of the node slots and P3 and P5 of the cPSB
fabric slots. The cPSB fits into the existing mechanical, power, and hot
swap structures of CompactPCI for 10/100 Ethernet applications. The
backplane supports the CompactPCI bus and H.110 telephony bus, allowing
the system's design to evolve into the cPSB framework.
The group is working a scheme to provide redundant point-to-point
Ethernet links over P3 running separately from each slot to two slots that
hold redundant Ethernet switch cards. The goal is to have the standard
finalized in the early summer.
THE NEXT STEP: STARFABRIC BACKPLANE
Point-to-point connections across a switched fabric appear to be the
future of backplanes. Ethernet backplanes will address some of the next-gen
backplanes issues like scalability and reliability for IP traffic. Along
with the StarFabric working group, Bustronic is developing the backplane
utilizing StarGen's switched fabric technology, addressing higher speeds
(multiple Gbps per slot), enhanced high availability (HA) and QoS
requirements, and more.
The StarFabric Working Group is a partnership of leading companies
dedicated to developing the next-generation communications system. Initial
members include Agere Systems (formerly Lucent MicroElectronics),
Bustronic, Elma, FCI/Berg, Motorola Computer Group, Natural MicroSystems,
Pigeon Point Systems, StarGen, and Sun Microsystems.
StarFabric is a high-bandwidth, scalable technology utilizing switched
fabric across the backplane compatible with existing technology. It can
connect at the chip-to-chip, board-to-board, or chassis-to-chassis level.
It allows a few endpoints, or up to thousands. The point-to-point
connections isolate faults to single endpoints, enhancing reliability, and
are friendly to device insertion and removal. The StarFabric technology,
like cPSB, fits into the existing IEEE 1101.10 mechanical framework. It is
compatible with CompactPCI bussing and supports the H.110 telephony bus,
providing an easy upgrade path for the system design. The CompactPCI form
factor cards are hot swappable and the switched fabric technology lends
itself to high availability applications.
The initial products that will run in the backplane include a
PCI-to-StarFabric bridge and a 6-port switch manufactured by StarGen.
Agere Systems has also announced a H.110-to-StarFabric bridge. The serial
links on each device consist of four 622 Mbps LVDS, bi-directional
differential pairs. The completed system will support hot swap, error
detection, fault isolation, system notification, and support for automatic
fail over.
In HA systems, redundancy and hot swap capability are requirements.
Therefore, the backplane will need allowances for plugable fan trays,
power supplies, and system management and monitoring. For power supplies,
the PICMG 2.11 Power Supply Interface specification defined a 47-pin
Positronic connector for plugable supplies. Power supplies are hot
swappable by the OR-ing diodes in the supplies. Power supplies monitor the
health of the voltages on the backplane through either third wire or droop
current sharing methods. Current droop regulates to within 10-15 percent
and third wire regulates to 5 percent. The power supplies and fans have
indicators in case of failure for ease of diagnosis and replacement.
Different Traffic And The Protocol
Data, voice, and video are increasingly being transported as digital data
moving across a common network. The StarFabric architecture provides
deterministic delivery of isochronous traffic (voice) and asynchronous
traffic across the backplane. It makes a distinction between the data
payload, whether it's packet-oriented, IP traffic, or QoS-oriented traffic
requiring real-time delivery, like voice or TDM traffic.
The StarFabric protocol supports three routing methods -- address, for
full compatibility with PCI and H.110; path; and multicast. Redundant
routing and hardware failover are provided for high availability.
Ethernet backplanes and the StarFabric backplane will probably have
hybrids in the real-time/isochronous traffic space with speeds of less
than 100 MB per link or so. But for higher speeds, StarFabric may be the
way to go with its bandwidth capabilities and other special features.
Hybrid Backplane Features
The hybrid backplane will be used for proof-of-concept and working
demonstrations. With its expertise in complex, custom, and high-speed
backplanes, Bustronic has been developing the backplane for the StarFabric
Working Group.
The 21-slot hybrid backplane is currently being tested in a 7U form
factor. (Final versions will likely come in 10U, with room for hot-plugable
components.) It is made up of five CompactPCI segments utilizing standard
cPCI 2mm hard metric connectors. Two segments are two slots wide and each
segment has a system slot for supporting existing process cards and a
fabric-native node slot. Three segments are five slots wide. The system
slot of these segments supports a StarFabric system/node slot and four IO
slots compatible with existing line cards. Two of the fabric native node
slots are connected to CompactPCI standard power and ground. Three of the
CompactPCI segments are routed with H.110 per PICMG 2.5 on P4 and P5 for
future applications.
The StarFabric links are redundant between the fabric-native node slots
and the system/node slots. Point-to-point links are between fabric-native
node slots (with CompactPCI bussing) and the non-bussed fabric-native node
slots. The links are on P3 and P5. Each link is made up of four transmit
pairs and four receive pairs.
Layers
The StarFabric backplane has a 12-layer controlled impedance stripline
design (the layer count may increase in later stages). The outside layers
are ground for EMI protection and suppression. The signal layers are
alternated with power or ground layers for controlled impedance and to
minimize crosstalk. Vias are not used on signal traces because they
disrupt the impedance of the trace. Power and ground planes are 2 oz.
copper. High and low frequency decoupling capacitors are distributed
generously across the backplane.
The CompactPCI bussing on P1 and P2 and the H.110 bus on P4 can be
routed in eight layers. Some of the StarFabric links can be routed on the
same eight layers and the remainder on the other four layers. The
differential pairs are routed as close together as possible and kept on
the same layer.
Power Considerations
Finally, StarFabric Bridge chips are low power consumption and will not
require special power or cooling considerations. This is an important
feature as it helps keep system costs low.
Melissa Heckman is an electrical engineer and Justin Moll is the
marketing manager with Bustronic. Bustronic is an industry expert in
standard and custom design backplane applications. For more information,
please visit www.bustronic.com, or
call 510-490-7388.
[ Return
To The May 2001 Table Of Contents ]
|
