ATCA Defines Multiple Data Transports Tailored to Their System Roles

The AdvancedTCA PICMG 3.0 specification contains multiple data transports. These
transports provide system management, control plane and data plane connectivity.
The electrical interconnect and topology of the data transports are different
and based on the needs of each particular transport. All of the transports in
ATCA are designed for high-availability systems. The multiple transports were
chosen to allow separation of control and data traffic, so each traffic type can
be separated on an independent transport. There is no single point of failure
that would bring down the transport. It is important, then, that integrators of
ATCA systems pay attention to the transport capabilities of the backplane, node
and fabric cards.

System Management

The system management transport is used to manage an ATCA shelf. This includes
power management, electronic keying and thermal monitoring of the shelf. The system
management is performed by the Shelf Management Controller (ShMC). The ShMC is
responsible for managing ATCA Field Replaceable Units (FRUs) such as boards, as
well as intelligent modules like power supplies, fan trays and thermal sensors.
The ShMC is able to read the status of the FRUs and command that FRUs enter different
power states. As an example, if the ShMC notices a rise in the shelf temperature
it might increase the speed of the fans. The ShMC is also responsible for managing
power in an ATCA shelf. When power is applied to an ATCA board, the only portion
of the board that is allowed to run is the system manager. The ShMC determines
which boards are allowed to fully power up and when.

The physical transport for the system management infrastructure is Intelligent
Platform Management Bus (IPMB), based in the I2C interface. The I2C bus is a two-wire
bus with one data and one clock signal. It is clocked at 100 Khz and uses 3.3V
signaling. To improve the availability of the system management subsystem, the
ATCA specification requires two IPMBs, which are identified as IPMB-A and IPMB-B.
The two busses combined are referred to as IPMB-0.

The IPMBs can be implemented as dual-bus or dual-star configuration. The IPMBs
are used to connect the ShMC to AdvancedTCA Boards and other FRUs in an ATCA
shelf. Both IPMBs can be used simultaneously to double the available bandwidth,
although the designer needs to take into consideration the impact of one IPMB
failing. Figure 1 shows the system management interconnects in a typical ATCA
backplane. In this backplane there are two ShMC slots, two fabric slots, 11
node slots and a connection to a fan tray. Note that the management bus connects
all of the node and fabric slots to a pair of redundant shelf management controllers.

Base Interface

The base interface is intended to provide an IP-based transport in an ATCA shelf.
It is a dual-star architecture that supports 10/100 and 1000 BASE-T Ethernet.
An ATCA shelf has two fabric slots, which contain the switches for the base interface.
Two 10/100/1000 BASE-T connections are provided between the two fabric slots for
switch communication and Failover—one connection between the node slot and
each fabric slot. A base interface channel (the connection between a board and
the fabric) is made up of 4 differential pairs. The ATCA specification mandates
that the backplane contains the base fabric interconnects. ATCA boards are not
required to support the base interface but are required to provide an IP-based
transport.

The specification also allows for a connection between the fabric and ShMC
slots. If the fabric interface supports an IP-transport then an ATCA board does
not need to support the base interface. The requirement for IP-based services
is intended to provide a base level of data transport in an ATCA shelf. The
IP-based services could be used for network booting, remote monitoring or high-level
system management (Figure 2).

Fabric Interface

The fabric interface is the main data transport in an ATCA shelf. The ATCA
specification defines up to 15 communication channels on each board, which can
be configured as a full-mesh or dual-star topology. In a dual-star topology
each board has a channel to two different fabric boards. The fabric boards act
as the hub in the shelf. In a full-mesh architecture every board has a connection
to every other board. (Figures 3a and 3b).

The PICMG 3.0 specification defines both the electrical and physical interconnects,
but does not define the protocol supported. PICMG 3.x subsidiary specifications
define the several protocols that can be used by the communications channels.
Each fabric communication channel is made up of 8 pairs of low voltage differential
signaling (LVDS) 3.125 GHz connections. A channel is made up of 4 ports with each
port containing 2 pairs.

Therefore, one port is two pairs of connections, and 4 ports (8 pairs) make one
channel. Each pair can support signaling rates of 3.125GHz, so one port can support
6.250 GHz and one channel can support 25 GHz of traffic. When used with typical
signaling encoding schemes, a channel can support 20 Gbit/s half-duplex or 10
Gbit/s full-duplex. Electronic keying is used to ensure that connections between
two ATCA boards are not enabled unless there are compatible technologies on both
ends of the interconnect. ATCA currently supports Ethernet, Fibre Channel, Star
Fabric and PCI Express fabrics.

Update Channel

The update channel provides 10 pairs of differential signals between two adjacent
ATCA boards. The transport implemented for the update channel is not specified
and is up to the designer. It is expected that two like boards will use the update
channel to share state information in redundant applications. Electronic keying
is used to ensure that the update channel is only enabled when boards with identical
capabilities are on each end of the channel. Backplanes are required to support
the update channel and board support is optional.

System management is based on a redundant two-wire serial interface (I2C)
that is bused between all slots. The system management messages are based on
IPMI and extended for use in PICMG 3.0. The control plane traffic is handled
by the base interface, which is a dual-star topology with redundant switch cards
and Ethernet BASE-T signaling. The base interface is intended to provide an
IP-transport for PICMG 3.0 boards and is very similar to PICMG 2.16 in terms
of architecture. The high-speed data transport in PICMG 3.0 boards is the fabric
interface, which is based on 3.125 Gbit/s SERDES signaling and is capable of
supporting 10 Gbit data rates in a star or full-mesh topology. The PICMG 3.0
specification defines the signaling and interconnects for the fabric interface;
subsidiary specifications define the protocols that are used on these transports
(Figure 4).

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