## 2 Mobile Social Networking
### Methods for Deriving Location or Proximity Information  
We highlight the primary methods that context-aware mobile social networking services use to derive location or proximity information. Proximity information for a device refers to the knowledge of neighboring devices as locally discovered through short range communications. **Self reporting**: CAMSN services of the self reporting type allow end-users to report their physical location to the service, for example by typing it in on their cell phone's keypad. This often cumbersome method of deriving location information relies on users to accurately (e.g. non-maliciously) report their location. We find that this method of deriving location information is rarely used, possibly due to the prevalence of GPS hardware on end-user platforms (e.g. cell phones). BrightKite is notable in that it allows users lacking a GPS enabled cell phone to self report their location [8]. **Self-derived location**: CAMSN services in this category use trustworthy, externally provided information to localize themselves. If the hardware platform used by such a CAMSN service is a modern cellular phone, the service might use GPS or Cellular Assisted GPS (A-GPS) to determine its location in real time (or at specified time intervals). Loopt and Google Latitude, for example, update A-GPS derived location information in real time, automatically dispersing that information to chosen friends [9; 10]. FourSquare uses GPS derived location to suggest locations which users might _check in_ to, thereby allowing them access to that location's content [7]. Where GPS coverage is not available, most of the A-GPS based services fall back to using cell tower triangulation to provide location information. We find that self derived location was not used in peer-to-peer CAMSN services, as gossiping GPS location data between devices associated with such a service would include neighbor discovery - effectively polling (see below). **Network-derived location**: For CAMSN services in this category, the infrastructure or network is responsible for calculating a device's location and reporting this location back to the device. We did not encounter any CAMSN services that explicitly utilize network derived localization. This observation is likely because most CAMSN services to date have utilized cellular phone platforms, and such platforms generally include GPS receiver hardware. On the other hand, such platforms often use cellular-assisted GPS (A-GPS). Some implementations of A-GPS involve the wireless cellular network _pushing_ additional information to receivers (e.g. cell phones) to aid in the localization process. **Peer polling and broadcast**: CAMSN services utilizing polling continually look for neighbors over short range communication channels while simultaneously announcing their own presence over those channels. This method does not find location information for those peers but rather identifies which peers are nearby. Serendipity and WhozThat both utilize polling for neighbor discovery. Serendipity nodes broadcast and looks for Bluetooth IDs (BTIDs), whereas WhozThat (an augmenting CAMSN) utilizes user names from traditional social networks in place of BTIDs [5; 12]. Other polling based CAMSN services are Jambo, which utilizes 802.11 WiFi rather than Bluetooth, and MobiSN [11; 14]. Polling has the advantage of being inherently compatible with CAMSN services utilizing a peer-to-peer architecture, as polling does not require fixed infrastructure to work. Hybrids of self reported location, self derived location, and proximity based mobile social networking services are possible. \"Hybrid\" as used here does not refer to a hybrid network architecture (see subsection 2.1), but rather to how a service derives location or proximity information. As mentioned, BrightKite is able to switch between self reporting and GPS derived location information. MobiLuck allows users to either _check in_ to their locations or enable real-time GPS based updates, depending on their hardware platform and personal preferences [15].