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Would you consider using a cellular data network service as a last-mile access alternative in small or temporary locations? If so, check with your carrier about the conditions under which this is allowed.

There’s a gizmo on the market called a cellular router (or 3G router). This device - as you might suspect - aggregates multiuser LAN traffic and forwards it over a cellular last-mile WAN connection. In such a configuration, the cellular data connection functions in place of a DSL, cable modem, dial-up, fractional T-1, ISDN or other access link.

A sampling of 3G router makers includes Digi International, D-Link, Junxion, Kyocera, and Linksys/Cisco. Some of these devices support firewalling and VPN encryption.

SonicWall has also announced a device aimed at the business market that couples cellular routing with the gamut of unified threat management protection tools: deep-packet inspection firewalling, VPN, intrusion detection/prevention, endpoint security (antivirus, antispyware) and content filtering.

Usage policies vary around the world. You may or may not be able to use the network for this application at all, for example. Or you might have to buy a specific service, priced separately, to do so. And in cases where you’re allowed to run 3G as a multiuser access link, you’ll likely need to use a router certified by the carrier.

In the US, the major carriers' usage policy descriptions can be confusing. For example, here’s an excerpt from Verizon Wireless’ policy for its NationalAccess (1XRTT) and BroadbandAccess (EV-DO) data services, indicating that last-mile usage is a no-no: “Unlimited NationalAccess/BroadbandAccess services cannot be used… as a substitute or backup for private lines or dedicated data connections.”

Still, the carrier offers a stand-alone service called Broadband Access Business Continuity, priced separately, that allows you to use a Verizon-certified cellular router and Verizon’s EV-DO network as a backup link.

Here’s a similar excerpt from Cingular’s 2006 policy: “Prohibited uses include, but are not limited to, using services… in conjunction with… applications or devices which aggregate usage from multiple sources prior to transmission.” Be aware, though, that this verbiage applies to Cingular’s DataConnect individual-subscription service.

Sprint Nextel is perhaps the most flexible in its cellular last-mile usage policy in that you don’t have to buy a special service package. You do have to use a 3G router that is Sprint-certified (Junxion, Kyocera or Linksys, for now). But, contrary to what you might deduce based on information on the company’s Web site, there’s no policy that restricts the number of users you can support beyond the limits of the equipment and network access bandwidth, says Sprint spokeswoman Amy Schiska-Lombard.

Usage rules outside the US tend to be more generous than those from the big US incumbents, notes SonicWall senior product line manager John Gordineer. So far, the company says it has no formal relationships with US operators, though Gordineer indicates he’s hoping that his company’s router/security appliance, called the TZ 190, will inspire US mobile operators to create new router-based services.

Scheduled to ship next month, the TZ 190 connects to any carrier’s EV-DO or UMTS/HSDPA 3G network service (speeds of up to about 700 kbit/s) via PC Card. It targets branch offices, retail stores, small businesses and temporary network setups, such as construction sites, for a better-performing alternative to dial-up, fractional T-1, and ISDN access. It can also be used in locations where these or higher-speed DSL and cable modem options aren’t available.