Revenue is measured based on the consideration we expect to receive in exchange for transferring goods or providing services to a customer and as performance obligations under the terms of the contract are satisfied. Generally, this occurs with the transfer of control of a product or service to the customer. For transactions where there are multiple performance obligations, we account for individual products and services separately if they are distinct (if a product or service is separately identifiable from other items and if a customer can benefit from it on its own or with other resources that are readily available to the customer). The consideration, including any discounts, is allocated between separate products and services based on their stand-alone selling prices. The stand-alone selling prices are determined based on the prices at which we sell the separate products and services and are allocated based on each item’s relative value to the total value of the products and services in the arrangement. For items that are not sold separately, we estimate stand-alone selling prices primarily using the “expected cost plus a margin” approach. Payment terms are generally 30 days in the U.S. and typically longer in many geographic markets outside the U.S. Shipping fees are recorded as revenue and the related cost is included in cost of sales. Sales, value-added, and other taxes collected concurrently with revenue-producing activities are excluded from revenue. Costs of obtaining a contract are capitalized and amortized over the period that the related revenue is recognized if greater than one year. We have elected to apply the practical expedient related to the incremental costs of obtaining contracts and recognize those costs as an expense when incurred if the amortization period of the assets is one year or less. These costs are included in selling, general and administrative expenses. Capitalized costs with an amortization period greater than one year were immaterial.

The following is a description of the principal activities from which we generate our revenue by reportable segment.

Network Solutions includes hardware products and software defined next-generation virtualized solutions used in service provider or business networks, as well as prior generation products. The majority of the revenue from this segment is from hardware sales.

Revenue from hardware sales is recognized when control is transferred to our customers, which is generally when we ship the products. Shipping terms are generally FOB shipping point. This segment also includes revenues from software license sales which is recognized at delivery and transfer of control to the customer. Revenue is recorded net of estimated discounts and rebates using historical trends. Customers are typically invoiced when control is transferred and revenue is recognized. Our products generally include assurance-based warranties of 90 days to five years for product defects, which are accrued at the time revenue is recognized.

In certain transactions, we are also the lessor in sales-type lease arrangements for network equipment that have terms of 18 months to five years. These arrangements typically include network equipment, network implementation services and maintenance services. Product revenue for these leases is generally recorded when we transfer control of the product to our customers. Revenue for network implementation and maintenance services is recognized as described below. Customers are typically invoiced and pay in equal installments over the lease term. In relation to these lease agreements, during the years ended December 31, 2018, 2017 and 2016 we recognized revenue of $13.7 million, $16.5 million and $2.7 million, respectively.

To complement our Network Solutions segment, we offer a complete portfolio of maintenance, network implementation, and solutions integration and managed services, which include hosted cloud services and subscription services.

Our maintenance service periods range from one month to five years. Customers are typically invoiced and pay for maintenance services at the beginning of the maintenance period. We recognize revenue for maintenance services on a straight-line basis over the maintenance period in services revenue as our customers benefit evenly throughout the contract term and deferred revenues are recorded in current and non-current unearned revenue.

We recognize revenue for network implementation, which primarily consists of engineering, execution, and enablement services, at a point in time when each performance obligation is complete. If we have recognized revenue, but have not billed the customer, the right to consideration is recognized as a contract asset that is included in other receivables in the Consolidated Balance Sheet. The contract asset is transferred to accounts receivable when the completed performance obligation is invoiced to the customer.

As of December 31, 2018, we did not have any significant performance obligations related to customer contracts that had an original expected duration of one year or more, other than maintenance services, which are satisfied over time.

The following table provides information about receivables, contract assets and unearned revenue from contracts with customers:

(In thousands)	December 31, 2018		January 1, 2018
Accounts receivable	$	99,385	$	144,150
Contract assets	$	3,766	$	374
Unearned revenue	$	17,940	$	13,070
Non-current unearned revenue	$	5,296	$	4,556

The decrease in accounts receivable is due to the collection of customer-specific payment terms that became due in the first quarter of 2018. The increase in the contract asset balance for the year ended December 31, 2018 is primarily attributable to revenue recognized that has not yet been billed to the customer during the period. The increase in the unearned revenue balance as of the year ended December 31, 2018 is primarily attributable to cash payments received or due in advance of satisfying our performance obligations, offset by $9.9 million of revenues recognized that were included in the unearned revenue balance as of December 31, 2017.

The following table disaggregates our revenue by major source for the year ended December 31, 2018.

(In thousands)	Network Solutions		Services & Support		Total
Access & Aggregation	$	301,801	$	57,069	$	358,870
Subscriber Solutions & Experience ⁽¹⁾		129,067		5,393		134,460
Traditional & Other Products		27,364		8,583		35,947
Total	$	458,232	$	71,045	$	529,277