iXBT Labs - Computer Hardware in Detail






Arapahoe and
HyperTransport Data Buses

The overall performance of a computer system mainly depends on three components (if you don't take into account hard discs): a processor, memory and a video system. Each one affects productivity in particular applications. But what if the performance of these components is so great that a bus connecting them fails to operate so efficient? Exactly this issue made the manufacturers to change the PCI bus specs. This bus is a bottleneck now in lifting the overall performance. The developers are currently working on new standards on system buses so that they can be realized in 2 years. Some specifications are already finished, and soon you will see products based on them. Of course I mean Arapahoe and HyperTransport technologies which are meant to replace the PCI bus which is not able to meet the up-to-date requirements anymore.

The problem of increasing a PCI bandwidth (Peripheral Component Interconnect) has been being faced for a long time already. The AGP specification is aimed at a higher performance by a faster transfer of graphics data. The ISA bus has already completely delivered its functions to the PCI one. Some functions are, however, implemented by the USB bus (Universal Serial Bus).

A modern system bus means, first of all, a good protocol which controls data exchange. Increasing of processors' clock speeds, development of such memory types as DDR RAM and Rambus with a higher bandwidth has contributed into computer operation. A bus is no more able to manage such loads and, therefore, doesn't allow the performance grow at the expense of a processor, video card and memory.

You can see how the priorities of the industry of high technologies change. When Intel and AMD optimized their CPUs, they turned to the chipsets. The next was memory. It resulted in Rambus DRAM and DDR RAM specifications. After that they tried to lift the system memory bandwidth. Later we will see which architecture will finally win.

A problem of increasing the bus bandwidth was earlier dealt with by an unprofitable organization PCI Special Interest Group (PCI SIG) which implemented development, realization and support of the PCI bus. Today there are two groups promoting their own standards. The first one - HyperTransport Technology Consortium (HTTC) is headed by AMD. It promotes the HyperTransport standard. The other group headed by Intel, Arapahoe Working Group promotes the Arapahoe standard which is meant to replace the PCI-X bus (see Table 1). The competition between these two has extended from the processor and chipset markets to the bus architecture sphere. Currently these standards are positioned as open, but the situation can change. If an open standard turns into a closed one, the components makers will have to make payments under the licence agreements.

Processor makers won't let these standards pass by. First of all, the specification will bring a lot of money on conclusion of the licence agreements. And secondly, optimization of the architecture for their CPUs will let to play more successfully on the processor market. But there can arise more problems than advantages. According to Gabriele Sartori, President of HyperTransport Technology Consortium, promotion of the Arapahoe by Arapahoe Working Group and of the HyperTransport by HTTC can result in dividing the computer market between these architectures.

Table 1. Companies which form the groups supporting the Arapahoe and HyperTransport standards

Arapahoe SIG HTTC
  • Intel
  • Compaq
  • Dell
  • IBM
  • Microsoft
  • AMD
  • API Networks
  • Apple
  • Cisco Systems
  • NVIDIA Corporation
  • PMC-Sierra
  • Sun Microsystems
  • Transmeta

In fact, such situation might result in the fact that components makers will support a platform of only one giant (one chipset will obviously support either one or the other bus), and therefore, will have to refuse from the other. A video card from NVIDIA designed for the HyperTransport bus won't be compatible with an Intel's platform or will require an adapter. This might lift the price of a computer system and even reduce the overall performance. However, it is still too early to speak about an equal struggle of the architectures. AMD is ready to release products with the HyperTransport support this year. Intel hasn't finished the development works, and the Arapahoe will be launched probably at the end of 2003. But let me now compare the standards.


The standard of the system bus promoted by Arapahoe Working Group is also known as 3GIO (3D Generation Input/Output). And the group led by Intel is also called Arapahoe Special Interest Group (Arapahoe SIG). Apart from Intel it includes Compaq, Dell, IBM and Microsoft. These companies were also members of PCI SIG and took part in development works on the PCI bus. Roger Tipley, President of PCI SIG, stated that the transition from the PCI bus to the Arapahoe one must be as gradual as the transition from the ISA to the PCI. So, let's look at the Arapahoe bus and its advantages.

  1. Arapahoe is a symmetrical bi-directional bus ensuring 2.5 GBytes/s data rate which is almost 2.5 times more than the PCI-X bandwidth and 9 times faster than the PCI bus (The PCI speed is taken as 266 MBytes/s as an average of 133 MBytes/s for a 32-bit 33-MHz one and of 512 MBytes/s for a 64-bit 66-MHz one).
  2. The technology of peripherals connection includes a host bridge and several terminal points to connect peripherals with a switch. A switch can be either a separate logic element or can be integrated into the host bridge. The switch's main function is to transfer data streams between peripherals without using the host bridge; it is a peer-to-peer connection. Data, when transferred from one peripheral to another, are not kept in the cache, and this allows reducing the overall load on the system.
  3. The Arapahoe bus has a scalable bandwidth, unlike the PCI one. The manufacturers can, thus, either lift the bandwith or reduce it by changing the number of lines.
  4. 32- and 64-bit addressing will be supported. Each data burst will have one out of three priority levels so that the system can divide a data stream from peripherals according to their priority levels and process them by turn.
  5. The architecture will feature 3 levels of organization: a physical level, a data level and a transaction level. The latter one will transfer requests for reading and recording of data from peripherals and back and form data bursts to transfer to the data level.
  6. One of the advantages is DDR RAM and QDR RAM (Quadro Data Rate RAM) support which means twice (four times) faster operation than it was earlier.

The Arapahoe is positioned primarily as a competitor to the architectures of AMD (HyperTransport) and Motorola (RapidIO). The Arapahoe does not try to be a uniform bus. Louis Burns, Vice-President and Chief Manager of Intel's Desktop Platforms Group, named InfiniBand, IEEE 1394b (FireWire), USB 2.0, serial ATA and 1/10-Gb Ethernet among its "mates".

The technology meant to extend the PCI bus possibilities can fail to appear because of a tough competition in this sphere. There are still two years before this bus will be realized on a hardware level, and the competitors are ready to release their products already today.


The standard promoted by HyperTransport Technology Consortium is currently developed by 150 large and small companies. The consortium was founded in 1997 to develop an architecture of a system bus. A number of companies joined the consortium after NVIDIA announced its support for its nForce chip. The most famous members are API NetWorks, Apple, Cisco Systems, NVIDIA Corporation, PMC-Sierra, Sun Microsystems and Transmeta (the most of them are open architecture supporters). Let's take a gander at the advantages of this technology in comparison with the PCI and PCI-X and with the oncoming buses. At http://www.hypertransport.org you can get the particulars.

  1. HyperTransport, former Lightning Data Transport (LDT), is positioned as addition to the InfiniBand technology at the telecommunicational and integrated systems market. According to HTTC, the technology can be successfully used both in server systems and desktop and mobile devices. As a result, the computer architecture will change a bit. Controllers of peripherals will be connected with a HyperTransport bus (fig.1).
  2. Like Arapahoe, this technology allows changing the number of signal lines, and, therefore, the number of outputs on the board if required. This will reduce power consumption as additional outputs require additional feeding. That is why this technology may become widely popular in mobile systems. Besides, the HyperTransport is also a peer-to-peer bus, i.e. it allows exchanging data between peripherals without enabling a processor and memory. The protocol uses a burst transfer, the bus controller is in charge of data exchange between devices. On Figure 2 you can see how the controller is connected in a dual-processor systems.

  3. 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 GBytes/s when transferring two 8-bit words per clock. Let's compare this performance with the current technologies. As compared with the InfiniBand bus (1.25 MBytes/s in a 4-channel realization), the HyperTransport bus is 10 times faster, the PCI-X one (1 GB/s) is 12 times faster, and when compared with the PCI bus (266 MBytes/s), the HyperTransport is 48 times faster.
  4. Unlike the Arapahoe, the HyperTransport allows transferring asymmetrical data streams from (to) peripherals. A symmetrical bandwidth is not always necessary in a computer. For example, in systems displaying graphics information, or in systems that actively send requests to a net to receive large amounts of data.

Last month NVIDIA announced the first nForce chipset with the HyperTransport support. The most of the consortium participants declared that the products supporting this bus will be launched at the end of the current month - beginning of the next one. It means that the specification is ready for realization, unlike the Intel's one, and some parameters are not worse or even better than that of the Arapahoe.

Table 2. Comparison characteristics of the Arapahoe and HyperTransport standards

Parameter Arapahoe HyperTransport
Symmetrical/Asymmetrical symmetrical asymmetrical
Bidirectional/Unidirectional bidirectional bidirectional
Data rate 2.5 GBytes/s 12.8 GBytes/s
Peer-to-peer connection + +
Scalable bandwidth + +
Addressing 32- and 64-bit 64-bit
Estimated release date end of 2003 end of 2001


We have examined only two, the most interesting system buses from Intel and AMD. It doesn't mean that they are the only possible candidates for a leading position in future computers. They are just supported by the majority of manufacturers at the moment. It is possible that the future of each system bus will depend on the number of hardware manufacturers supporting it. Both specifications do not differ much from each other (Table 2), but the fact that products with HyperTransport support will appear much earlier may become a determining factor.

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