Revenue recognition We generate revenue primarily from service subscriptions, including premium offerings, fixed fee service arrangements, customized engineering fees and software licenses. We also generate revenue from the delivery of search and display advertising impressions. We recognize revenue when persuasive evidence of an arrangement exists, delivery of the service or product has occurred, the fee is fixed or determinable, and collectability is reasonably assured. We have derived our revenue primarily from mobile navigation services, which are generally provided through our wireless carrier customers that offer our services to their subscribers or through application stores. Our wireless carrier customers pay us based on several different revenue models, including (1) a revenue sharing arrangement that may include a minimum fee per end user, (2) a fixed fee for any number of subscribers (up to specified thresholds) receiving our services as part of bundles with other voice and data services, (3) a monthly or annual subscription fee per end user, or (4) based on usage. Our end users who subscribe to our services through application stores pay us a monthly or annual subscription fee. We recognize monthly fees related to our mobile navigation services in the month we provide the services. We defer amounts received in advance of the service being provided and recognize the deferred amounts when the monthly service has been provided. We recognize revenue for fixed fees for any number of subscribers receiving our services as part of bundles monthly on a straight-line basis over the term of the agreement. Our agreements do not contain general rights of refund once the service has been provided. We also establish allowances for estimated credits subsequently issued to end users by our wireless carrier customers. We recognize as services revenue the amount our wireless carrier customers report to us as we provide our services, which are net of any revenue sharing or other fees earned and deducted by our wireless c