GlobespanVirata's Nitro XM
I spent over an hour with numerous GlobespanVirata folks getting the pitch and poking at a demo they had set up in their hotel suite. But before I get to the demo, let me share what I learned about what makes Nitro XM tick.
Nitro XM builds on Nitro's packet bursting mechanism, adding Compression, Concatenation and something GlobespanVirata calls Direct Link (Figure 1).
Figure 1: Nitro XM Direct Link explained
Direct Link had me confused when I first read the company's press release because of this statement:
PRISM Nitro XM reduces costly overhead throughput loss by creating an automatic, secure, high-speed link directly between wireless clients when they are within communication range of each other. This allows clients to communicate directly with each other at speeds of up to 140 Mbps while maintaining a simultaneous network connection with the AP.
After my time with GlobespanVirata, I'm now unconfused. DirectLink is not the same as 802.11 AdHoc mode. Direct Link Stations (STAs) must associate with a Nitro XM-enabled AP and stay that way even when Direct Linking. (STAs listen to 802.11 beacons and receive WEP or WPA key information from the AP - that's the "simultaneous network connection"). But when it comes time to move data, the transfer does take place directly from STA to STA, not retransmitted through the AP.
Nitro XM delivers enhanced throughput between any two radios, including the radio in a Nitro XM-enabled AP. So wireless STA to STA throughput will be the same as STA to (Ethernet) LAN - assuming equal signal conditions. This "one-hop" method gives Nitro XM an advantage over all other throughput enhancement methods, which require all communication to be retransmitted through an AP or wireless router, costing a "hop" (and throughput loss) in STA-to-STA transfers.
The other not-so-obvious benefit of Direct Link comes from the higher throughput that should be obtained between two closely-spaced STAs, especially when they are both located some distance from their associated AP. Since the main data flow is directly between STAs, they should be able to communicate at a higher speed instead of having to fall back to a lower transmit rate. Although it's an unusual approach, this allows Direct Link to provide effective range (more accurately throughput vs. range) enhancement.
Nitro XM's weakness is that a major portion of its throughput enhancement comes from the compressibility of the data being sent. GlobespanVirata made it clear that the demo used a best-case-compressible data file. In real multimedia applications, which use highly compressed data, Nitro XM will provide little to no throughput gain over normal Nitro. You'll also see no throughput gain when sending encrypted data or statically compressed (zipped, etc.) files over the network. Then why do they call it Xtreme Multimedia? I'm still trying to figure that one out... Note that Atheros' Super-G technology also relies on data compression and will provide less enhancement with already-compressed files.
It's important to note (and I would get a nasty-gram from GlobespanVirata if I didn't) that Nitro XM doesn't use any form of channel bonding, and so shouldn't interfere with neighboring WLANs. Nitro XM STAs and APs also constantly look for non-XM STAs and skip using the fancy stuff when they're associated with a Nitro XM-enabled AP.
Got all that? On to the demo then...