are billed in arrears and are recognized based upon minutes of traffic processed when the services are provided in accordance with  contract terms.  We recognize other revenues as follows: (i) one-time subscriber connection fees (on fixed-line services) are deferred and  recognized over the average expected customer service periods, (ii) monthly fees (on fixed-line services, mobile and internet and  data services) are accrued every month, and (iii) prepaid services (fixed line, mobile and internet and data services) are recognized  as income based upon actual usage by customers or when the right to use those services expires. Where we enter into transactions  which involve both the provision of air time bundled with products such as 3G data card and handset, total consideration received  from handsets in these arrangements is allocated and measured using units of accounting within the arrangement based on relative  fair values limited to the amount that is not contingent upon the delivery of other items or services. Relative fair values are based on  the selling prices of 3G data cards and handsets sold on a standalone basis and the monthly fees on the subscription contracts.  Where we sell products to third party cellular phone stores we record the direct sale of the products, typically handsets, as  gross revenue when we are the primary obligor in the arrangement and when title is passed and the products are accepted by the  stores. We maintain allowances for doubtful accounts for estimated losses that result from the inability of our customers to make  required payments before January 1, 2011. We assesses the probability of collections of accounts receivable by examining the aging  analysis of the outstanding receivables and assessing the value of the collateral provided by customers. On January 1, 2011, we adopted the third-time revised Statement of Financial Accounting Standards No. 34, “Financial  Instruments: Recognition and Measurement.” One of the main revisions is that the impairment of receivables originated by us  should be covered by No. 34. Accounts receivable are assessed for impairment at the end of each reporting period and considered to  be impaired when there is objective evidence that, as a result of one or more events that occurred after the initial recognition of the  accounts receivable, the estimated future cash flows of the asset have been affected. There was no effect on the net income and after- tax basic earnings per share for the year ended December 31, 2011 as a result of the adoption of SFAS No. 34. The amount of the impairment loss recognized is the difference between the asset carrying amount and the present value of  estimated future cash flows, after taking into account the related collateral and guarantees, discounted at the receivable’s original  effective interest rate. A significant portion of our total assets consists of long-lived assets, primarily property, plant and equipment and definite- lived intangibles. We estimate the useful lives of property, plant and equipment and other long-lived assets with finite lives in order  to determine the period of time over which depreciation and amortization expense should be recorded. The useful lives are estimated  at the time assets are acquired and are based on historical experience with similar assets as well as the anticipated technological  evolution or other environmental changes. If technological changes were to occur more rapidly than anticipated or in a different  form than anticipated, the useful lives assigned to these assets may need to be shortened, resulting in the recognition of increased  depreciation and amortization in the relevant periods. Alternatively, technological obsolescence could result in a write-down in the  value of the assets to reflect impairment. We review these types of assets for impairment quarterly, or when events or circumstances  indicate that the carrying amount may not be recoverable over the remaining life of an asset. In assessing impairments, we use  estimated cash flows that take into account management’s estimates of future operations. We did not have significant impairment  losses of long-lived assets in 2009 and 2011, but had NT$66 million impairment loss in 2010. 71 Estimated Useful Lives of Long-Lived Assets  Impairment of Accounts Receivable P AR T I