Wireless LANs are becoming an important piece of the mobile computing infrastructure, but for enterprise users, they have a serious shortcoming.

James Kobielus: Smooth roaming depends on directories

Above the Cloud

Wireless LANs are becoming an important piece of the mobile computing infrastructure. But for enterprise users, they have a serious shortcoming that could prevent them from becoming a mainstream network technology: they only operate within a narrow geographic range, about 150 feet from the radio frequency transmitter. When users take their portable computers outside the wireless LAN’s limited coverage area, they lose the signal, the connection and the computing session.

Given this limitation, it’s no surprise the industry is exploring various approaches to enabling transparent roaming between wireless LANs and carriers’ wireless data services. The most encouraging sign is that the Wireless Ethernet Compatibility Alliance, the vendor consortium that manages the Wireless Fidelity (WiFi) standard (known as 802.11b), is defining a standard for roaming between wireless LANs and public wireless data networks. Draft specifications from the Wireless Internet Service Provider Roaming (WISPr) group are expected by year-end.

It’s not yet clear what functional range the WISPr specifications will address, but the group needs to pay attention to directory interoperability requirements. A well-connected, federated, multivendor directory environment should be central to any future WISPr standard.

For starters, the WISPr group should specify a roaming architecture that interoperates with the “location registry” directory features already embedded in today’s cellular telephony service environments. In a mobile computing environment, we would want our subscriber account information to follow us to the next wireless LAN or carrier environment in which we power up our PDA. We would also want the ability to personalize the mobile client user interface in keeping with identity and profile information maintained in network-connected enterprise and service provider directories.

The WISPr group should also pay attention to Lucent’s recently announced development of an architecture for interoperability among the location registry infrastructures of heretofore incompatible cellular protocols. Lucent’s “Common Operations” architecture defines what might be called a “metadirectory” service for bridging location registers between Global System for Mobile Communications (GSM), Code Division Multiple Access and other cellular protocols.

Another wireless LAN roaming approach, introduced by Nokia, also has implications for the distributed directory infrastructure. Essentially, Nokia’s Operator Wireless LAN product turns the WiFi-based LAN into a premises-based microcell on the carrier’s GSM network, interoperating with the worldwide GSM location registry infrastructure.

This product requires portable computers to use a Nokia-developed WiFi network interface card (NIC) that includes an onboard GSM Subscriber Identity Module (SIM) card. The WiFi/SIM NIC authenticates the WiFi client to a Nokia Authentication Server installed on premises in the wireless LAN environment.

For this product to work, WiFi LAN operators must have established interconnection and roaming agreements among themselves and with GSM carriers, integrating their Nokia Authentication Servers with carrier-based location registries and billing systems.

The principal shortcomings of Nokia’s approach are that it’s proprietary, works only in GSM environments and doesn’t support multimode handoffs between the WiFi and General Packet Radio Service (GPRS) airlink protocols. But it points to a future in which WiFi is the standard, in-building, mobile-data microcell; SIMs are embedded in client devices in all wireless environments; and the SIM (or the similar WAP Identity Module) is the standard carrier of credentials for authenticating to all cellular location registry services.

Another important feature missing from Nokia’s product is support for wireless VPN functionality. Addressing this concern are vendors such as NetMotion and Ecutel, whose wireless VPN products let networks support client mobility from WiFi to GPRS networks, locate the mobile client’s new IP address on the GPRS network and maintain session persistence when wireless clients are out of range.

Amid all this activity, one thing is clear: transparent roaming without well-connected directories, registries and other identity repositories is not feasible. Directories will be a critical component of whatever ubiquitous mobile-data roaming environment emerges from this period of frenzied innovation.

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