Note
Access to this page requires authorization. You can try signing in or changing directories.
Access to this page requires authorization. You can try changing directories.
The purpose of the Proximity Service Discovery Protocol is to convey service discovery information, such as service advertisements, as part of Beacon frames, as specified in [IEEE802.11-2007]. Beacon frames are periodically broadcast by access points and stations (STAs), as specified in [IEEE802.11-2007], that operate in ad hoc network mode. According to the IEEE802.11 protocol, stations in radio range, that is, in proximity, receive and process Beacon frames during a normal channel scan operation.
The Beacon frame can contain single or multiple proprietary information elements (IEs) that carry discovery information pertaining to the services that the device offers. Proprietary information elements are identified by their Element IDs and are further distinguished by an IEEE-administered Organizationally Unique Identifier (OUI) and a predefined OUI Type value.
A format identifier describes the format of the information carried in the information element. To ensure uniqueness while circumventing the need for central administration of format identifiers, a string in the form of a URI, as specified in [RFC3986], could be used to distinguish the format. However, because the transmission must be efficient and space in the information element is limited, the string is not actually transmitted; instead, its hash is transmitted. On the client, which is the receiving side of the beacon, the hash is matched against a known set of format identifiers.
Figure 1: PSDP client/server communication
The preceding diagram illustrates the relationship between a service-bearing device, which is an AP or ad hoc network station, as specified in [IEEE802.11-2007], and the client that acts as a station, as specified in [IEEE802.11-2007], in ad hoc network or infrastructure mode.
A client that is in discovery mode (that is, searching for a service in its physical proximity) picks up the beacon during its regular scan intervals. It processes the beacon for known discovery information elements based on the OUI and OUI Type,<1> and it notifies the application if the format identifier that is represented by the hash matches any known format identifiers. Data carried in the information element is opaque to the protocol.<2> It is the responsibility of the application to resolve possible hash collisions. The application can do so by examining the data carried in the information element or by re-issuing a discovery request at a higher layer by using the full format identifier string after a connection has been established.
The inclusion of service discovery information in broadcast messages enables the discovery of services before connecting to the service-hosting device.<3>