I/O bus standards like Intel's Arapahoe and AMD's Hyper
Transport promise to make data shuffling between processor
chips and PC peripherals faster. How do they compare?
by Ong Boon Kiat
of the most crucial yet understated metric about computing
performance is the I/O bus. The peripheral connection
interface (PCI) remained an immovable standard for the
last ten years while processor clock speeds and memory
latencies improved by leaps and bounds. Now it looks
like the industry will soon receive a major bus upgrade.
Not surprisingly, the two key, next-generation bus technologies
are being led by chip giants and rivals Intel and AMD.
Probably because of this, the media has touted the developments
a platform war that will splinter the industry into
Intel's offering is the 3GIO, recently dubbed Arapahoe.
Based on the PCI standard, Arapahoe has the backing
of key PC makers like Dell, Compaq, IBM, Microsoft and
Intel. The AMD-led technology is called Hyper Transport,
with key supporters Apple Computer, Transmeta, Cisco
Despite media hype, however, it appears that talks of
PC vendors splitting ranks over I/O buses may have been
greatly exaggerated. In the last few months, both Intel
and AMD have loudly declared that both technologies
will be friends, not rivals.
On Intel's website, Arapahoe is called a "general-purpose
I/O technology optimized towards the constraints of
connecting external components and device sharing".
Hyper Transport, on the other hand, was alluded to having
an architecture that will "solve specific application
needs." In other words, Hyper Transport can conceivably
tether to Arapahoe in the same I/O chain, with each
operating in its own space.
In reality, it may be hard to tell if both technologies
can continue to operate with such cleanly-defined boundaries
as the standards take shape. Bear in mind that overlapping
features in both technologies probably means that both
will have the capacity to take over some of each other's
function should their proponents choose to do so. For
example, both Hyper Transport and Arapahoe can serve
as a chip-level interconnect.
If we look at the raw specs, Hyper Transport, with an
oft-quoted data rate of 12.8 Gbps, appears to overwhelm
Arapahoe's 2.5 Gbps. Raw numbers, however, rarely tell
the full story. Bob Gregory, director Desktop Platform
Initiatives, Intel, pointed out that the ongoing comparison
of specs by the media may be misleading, since the numbers
may not reflect similar operating conditions.
He said: "The published Hyper Transport for I/O
applications is the one to compare to Arapahoe since
it is designed for I/O applications and runs at a signaling
rate of 0.8 Gbps (16-bit versions will have performance
of 1.6 Gbps).
this to Arapahoe, which will run at 2.5 Gbps signaling
(a 16-bit version will have performance of 5 Gbps).
This is different than Coherent Hyper Transport whose
numbers are often quoted but is targeted as a multi-processor
bus, not I/O. An important metric of bandwidth delivered
is bandwidth per pin. Arapahoe is 100 Mbps/pin and Hyper
Transport for I/O devices is 29 Mbps/pin."
Other differences between the two technologies are less
clear-cut. One apparent difference is the industry support
that both have garnered so far, which will affect the
standards' eventual adoption. Not surprisingly, Gregory
touted Arapahoe's openness and "evolution(ary)"
approach (it is essentially PCI 3.0) as its key advantage.
But just as Intel has its usual list of impressive backers,
Hyper Transport has garnered good support too-from Apple
Computer, Transmeta, Cisco and NVIDIA. And according
to many industry watchers, one of the reasons that many
vendors chose to support AMD is precisely because they
do not want to become too reliant on Intel.
Intel, of course, has another I/O standard in waiting,
called Infiniband. This technology is often touted as
a PCI-replacement, but for many, Infiniband's ultimate
purpose is to connect servers as part of a processing
cluster, which is more of a data centre application.
As such, it doesn't compete with PCI or Arapahoe, which
are PCI-based bus standards.
two buses explained
Intel is priming Arapahoe
to hit the shelves before the end of next
year. Like PCI, Arapahoe links up peripherals
with the memory and video subsystems, which
are two of the most important PC subsystems
on the motherboard.
Arapahoe's first order of design was to
have a peripheral bus bandwidth that can
keep up with the next few conceivable iterations
of processor clock speed increases. Based
on a totally new architecture, the bi-directional,
symmetrical Arapahoe has more than twice
the bandwidth of PCI-X, or 2.5 Gbps. It
will be 9 times faster than PCI.
Besides improved speeds, Arapahoe features
a switch-enabled architecture. It lets chipset
makers configure I/O as part of a switch,
with peer-to-peer connections to components
like mobile docking, Gigabit Ethernet ports
and other add-ins.
The Arapahoe Special Interest Group (Arapahoe
SIG) includes major PC vendors like Dell,
Compaq, IBM, Microsoft and of course, Intel.
AMD's Hyper Transport has supporters in
Apple Computer, Cisco Systems, NVIDIA, Transmeta
and others. Like Arapahoe, it is also a
peer-to-peer bus, i.e. it allows exchanging
data between peripherals without invoking
processor and memory, which lessens latency
and reduces the overall load to the system.
The bus transfers data at 800 MHz on the
front and the rear pulse gates so that the
total bus speed is around 12.8 Gbps when
transferring two 8-bit words per clock cycle.
This makes it 12 times faster than the PCI-X
specifications (1 Gbps).
Hyper Transport, unlike Arapahoe, allows
transferring asymmetrical data streams to
and from peripherals. (A symmetrical bandwidth
is not always necessary in a computer, like
when displaying graphics).
NVIDIA announced the first nForce chipset
with the Hyper Transport support.
It is at the other I/O scale at the silicon level that
AMD envisions using Hyper Transport. This is where processors
needs to be connected together both for signaling and
data transfer. For example, Cisco could use Hyper Transport
inside its switch to improve reliability and connection
speeds between silicon devices on system boards. According
to the head of the Hyper Transport consortium, Gabrielle
Satori, the Hyper Transport technology is meant for
implementation up to the I/O link.
But support or not, Hyper Transport is likely to face
competition from Arapahoe in the chip-to-chip interconnect
stakes simply because both technologies can operate
there. In addition, Hyper Transport will see competition
from another technology: RapidIO.
RapidIO is targeted to the embedded market, and receives
firm support from Motorola and IBM. Both companies plan
to use the standard in the front-side bus of new PowerPC
processors that will be shipped in new RS/6000s and
Apple Macintoshes. Like Hyper Transport, RapidIO is
scheduled to splash into market this year. One early
adopter is US-startup Red Switch, which said that it
will release an eight-port RapidIO switch by the middle
of this year.
Actually, both RapidIO and Hyper Transport proponents
eye a far bigger market beyond the PC space. That is
to provide embedded interconnect silicons between chips
in communications and mobile devices. But both may have
to duke it out with each other in the coming years.
As for PCI, MIS can expect to see PCI 2.3 continue to
be implemented in home and corporate desktops in the
next two years, before Arapahoe wades into the fray
in 2004. For workstations and servers, PCI-X is expected
to be the I/O staple until 2003 when Arapahoe is expected
to get early adoption.
The next bus
The need for faster buses is obvious to chip makers
like Intel, which saw the futility of cranking up clock
speeds in their microprocessors only to have laggard
PCI buses slow down its data links to critical computing
components like memory, disk and video. But replacement
of legacy bus technologies and finding a symbiotic existence
with new rivals is as much a political issue as it is
a technical issue.
For now, the pieces appear to fall into neat places.
To take a simple analogy, Hyper Transport can be seen
as a next-generation rail service that operates within
municipals, while Arapahoe is a train service that plies
between different states. Infiniband? Think of it as
ultra-fast, exclusive freight trains that have the potential
to link between states.
Well, at least that's the plot tacitly agreed on by
all working parties so far. Time will tell if the script