A great number of Nokia mobile phones are supplied with Bluetooth® hardware and software which allow wireless usage of other devices, such as headsets. But the main idea is to exchange information in full measure efficiently as it is possible today by wired networking technologies. To do that it is needed to transport present-day networking protocols (such as TCP/IP) using Bluetooth® media.
The BNEP (Bluetooth Network Encapsulation Protocol) protocol defines how currently popular protocols payloads can be encapsulated into Bluetooth® packets. And then be transmitted inside networks of Bluetooth® powered devices. The transmission goes over L2CAP protocol which is wireless data link layer.
The Bluetooth® PAN profile would then use BNEP protocol to form networks.
As a protocol, BNEP has a header format and a lot of packets types to determine standard, extended, filtered, compressed and other transmissions.
You can assist in development of open source BNEP class for Symbian OS while participate in BluSPAN project on Nokia Developer.