Dueling Interconnects Unmasked

HyperTransport and RapidIO both aim to improve chip throughput: But what's the difference?

November 16, 2001

4 Min Read
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Recent news of the HyperTransport Technology Consortium draws attention to an increasingly public duel between two technologies aimed at speeding up throughput on today's high-speed networks (see 'HyperTransport' Consortium Grows).

Those two technologies are HyperTransport and RapidIO (which has its own banner group, the RapidIO Trade Association). Both are chip-to-chip interconnection techniques aimed at ensuring that the inner workings of network and storage gear won't wind up being a bottleneck as carriers move to ever faster services.

A quick backgrounder: Today, networking devices that deploy multiple processors typically rely on the PCI bus or similar techniques to process information, such as packet lookups, inside a box. These technologies have maximum data rates of about 1 Gbit/s today (with exceptions), and they don't support varying amounts of bandwidth in one device.

But speed and bandwidth flexibility will be required of mainstream storage and networking gear as networks move to 10 Gbit/s data rates and beyond. For example, OC192 network processors supporting functions for next-generation Sonet switches will need to get lots of data on and off their chips fast.

Enter HyperTransport and RapidIO, which were devised by different vendors to speed up devices based on their chips. Simply put, both are ways of ensuring that complex, multiprocessor boxes will perform as well as the high-speed networks they're meant to run on.Neither HyperTransport nor RapidIO are alternatives to Infiniband, which is designed to improve the I/O in servers and between subsystems instead of between chips (see InfiniBand Trade Association). Both HyperTransport and RapidIO proponents say their designs work with Infiniband.

Intel Corp. (Nasdaq: INTC) is often cited as having its own interconnect program, called 3GIO or Arapahoe. But by all accounts, this is not up to speed with HyperTransport or RapidIO. "Intel is a latecomer," says Bert McComas, founder and principal analyst at InQuest Market Research, a consultancy and market research firm. With less bandwidth and scaleability than either HyperTransport or RapidIO, by the time 3GIO is ready for market, about 2004, it's unlikely to be any competitive threat to the others, in McComas's view.

Meanwhile, both HyperTransport and RapidIO have a lot in common: They are designed to link chips, network processors, and other components inside multifunction devices based on silicon semiconductors. They are packet-based. They support varying bus widths and bandwidth rates. They are geared to support power and design efficiency. They both deploy so-called source-synchronous technology, where data and clock signals are sent over different connections that use the same voltage. They use similar signaling, based on the Low Voltage Differential Signaling specs approved by standards groups worldwide. And each claims to run at speeds above 60 Gbit/s chip to chip.

Table 1: Interconnect Techniques Compared



Main backers

AMD, API Networks

Motorola, IBM

Consortium URL



Clock speeds

200 MHz to 800 MHz

100 MHz to 1 GHz

Bus widths

2,4,8,16, and 32 bits

8 or 16 bits

Number of pins

24 for 2-bit to 197 for 32-bit

40 for 8-bit; 76 for 16-bit

Aggregate bandwidth

128 Gbit/s

3 to 60 Gbit/s

Power required

1.2 volts

2.5 volts

But HyperTransport and RapidIO differ in several key ways, starting with pedigree. HyperTransport's creator and main backer is Advanced Micro Devices (NYSE: AMD), and originally it was geared to MIPS processors used in high-end servers and similar gear. More recently, API Networks Inc. has assumed a leading role as well. Broadcom Corp. (Nasdaq: BRCM) and PMC-Sierra Inc. (Nasdaq: PMCS) also license the technology.RapidIO was originated by Motorola Inc. (NYSE: MOT) and IBM Corp. (NYSE: IBM) and is geared to Power PC processors, including those used in chassis-based equipment. This has given it early inroads with makers of networking gear. Alcatel SA (NYSE: ALA; Paris: CGEP:PA), Lucent Technologies Inc. (NYSE: LU), and Nortel Networks Corp. (NYSE/Toronto: NT) are all part of its steering group.

These early distinctions are getting increasingly blurry. Cisco Systems Inc. (Nasdaq: CSCO), for instance, belongs to both groups, as does Xilinx Inc.

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