It's no surprise that Tropos would offer multiple-radio mesh nodes for its metro-scale networking equipment, but how they're using the second radio is unusual: It has been abundantly clear for many months that for Tropos Networks to compete effectively for metro-scale network deployments they would need to add models of mesh network routers that had two or more radios. Tropos has now done so, with a model slated to ship in October that contains both a 2.4 GHz and a 5.8 GHz radio. What surprised me is how they chose to use that second radio, and how relatively little they're charging for it. (This announcement was slated for Thursday, and News.com broke the embargo this evening.)

Tropos has chosen to upgrade its MetroMesh operating system to support mesh routing across bands. The company said in a wide-ranging interview that their new approach will choose the most efficient route to move packets among clusters of mesh nodes. In an extreme example, if the user-facing 2.4 GHz Wi-Fi network is busy, then Tropos routers would move all intra-nodal traffic to 5 GHz hops; more typically, however, any given packet will transit over the least loaded connection, which could be hopping among a combination of 2.4 GHz and 5 GHz links.

Combined with a recent improvement that allows per-packet power level controls, this change could produce a substantially more efficient use of spectrum; lack of efficiency has been one of the biggest criticism of single-radio mesh networks in general and Tropos's system specifically. True mesh networks require all nodes in a cluster to use the same frequencies, which results in a single packet occupying network time slots usually at least once, but possibly several times, to reach a connection out of the cluster. With a second radio, packets could transit the mesh-only 5 GHz connections as need be, offering more time slots to clients. (In future software releases, Tropos might support clients on 5 GHz as well as mesh traffic.)

Their first two-radio router doesn't solve the problem of backhaul, and will still require an capacity injection connection to equipment like that from Alvarion or Motorola, which Tropos points out is typically substantially less expensive than a mesh node.

Saar Gillai, vice president of engineering, said Tropos approached the multi-radio problem differently than their competitors, because there's a high cost in "closing" 5 GHz links--or providing full coverage in every part of a network. Tropos has opted for omnidirectional 5 GHz antennas, which dramatically reduces a 5 GHz radio's potential range, but fits in neatly with Tropos' desire to distribute traffic across a cluster of nodes. In some clusters, 5 GHz radios will only cover 50 to 80 percent of the same area that 2.4 GHz radios cover.

This approach allows less dense areas to use single-radio routers and more densely deployed areas to employ dual-radio routers, rather than requiring every device have at least two radios. (Clusters could number from about three to eight units in which the 2.4 GHz and 5.8 GHz radios are using the same channels.) Tropos says this helps them maintain a cost advantage over competitors.

Tropos says that they can increase capacity 50 percent above their current levels with the same density of access points as they currently recommend for single-radio deployment. This gives them 50-percent closure, as they describe it, meaning that their 5 GHz links will only cover about 50 percent of the area. For a 20 to 30 percent higher cost, they suggest they could reach 80-percent closure, and nearly twice the capacity of their existing gear.

Bert Williams, senior director of marketing, said that deployment costs would be less because each 5 GHz radio wouldn't require alignment with other radios, as in point-to-multipoint systems, and adding capacity could involve simply upgrading their single-radio unit to a dual-radio model rather than building a new backhaul topology.

Not above some mild trash talking, Gillai stressed Tropos's development of their own radio technology, using industry-standard Atheros chips, but building their own radio boards and systems which reduced cost and allowed them to create what they characterize as more consistent performance from their radios. "Some of [our competitors] are using directional [antennas] because they don't have such a great radio," Gillai said. "If they're buying a whole radio in a reference design it's going to be more expensive for them. We invest much more in software." (I have heard this as well from wireless experts outside of the metro-scale networking business.)

The first hardware version of this technology in Tropos routers appears in model 5320, which supplements the existing single-radio 5210. Gillai said, "This is completely transparent to the users. You can add these routers to an existing network, or you can add these routers to a new network" as existing single-radio routers can receive software upgrades. He also said that existing networks under construction using Tropos gear would not see a shift in orders-in-progress, and that their partners had been aware for some time of the features and timetable of the new routers. Newer routers could replace existing ones in areas of congestion, although Tropos expects that busy areas would involve a denser deployment and dual-radio routers.

Tropos competitors--all of them--use a second band primarily as capacity injection, or the layer at which traffic from the user-facing radio in 2.4 GHz is offloaded to aggregated backhaul through point-to-multipoint connections. Now BelAir will say that they use intelligent switching in 5 GHz when they have two or more 5 GHz radios in a single box, and that's just fine. Strix has similar statements. SkyPilot uses extremely high-gain antennas to create point-to-point limits that are isolated in frequency and time from adjacent networks.

In their competitors' models, this allows substantially greater density at lower power of Wi-Fi nodes on nonoverlapping channels to provide a lot more bandwidth per square mile; a true mesh network like Tropos's requires all radios on the same channels, and thus you can only shrink or grow the area of a cluster of nodes (or reduce the number of nodes in a cluster) to improve bandwidth. However, the cost is prohibitive for high densities in any of the currently envisioned metro-scale projects using non-Tropos gear because each competing radio includes at least one 5 GHz radio for backhaul--what Tropos has described as the "5 GHz tax." This makes it difficult for competitors' advantage in a honeycombed Wi-Fi deployment to be expressed in the marketplace.

What Tropos is saying with their multi-radio approach is that 2.4 GHz is typically not fully loaded at any given time and that 5 GHz isn't just a directionally appropriate band. Rather, they're trying to combine the best attributes of mesh networking--closely placed nodes with frequency reuse in which the best node responds with the strongest necessary signal to a client--with the advantages of routing across dissimilar networks. The real question is whether in the field this amounts to the performance advantage that they cite, and whether the increased cost of their new units is justified by that advantage.

Tropos stated that their new 5320 would cost about 30 to 40 percent above their single radio units' cost, or more than 30 percent less than their competitors' two-radio units. It is nearly impossible to obtain pricing for metro-scale equipment--Tropos declined to provide dollar figures--and bulk orders and special partnerships also skew the numbers. Tropos also claimed in a briefing that their competitors substantially undercut equipment prices or even give equipment away to have a lower bid than Tropos-based projects.

The 5320 will ship in October, and the company expects versions with other technology, such as 4.9 GHz public safety band radios, to ship within the year. Future devices could include their own backhaul radios with directional antennas, too, and the company is watching MIMO closely as a tool to improve coverage areas. They are also watching WiMax, both fixed and mobile, to see whether there's enough service appearing in the market to offer either a client radio for backhaul or a base station radio as part of a mesh.