3. Revenue

We generate revenue primarily through three broad revenue streams: 1) SDB, 2) Software, and 3) Training and Consulting Services. We recognize revenue from SDB and software contracts mainly through the Performance Improvement Solutions segment and the training and consulting service contracts through both the Performance Improvement Solutions segment and Nuclear Industry Training and Consulting segment.

The following table represents a disaggregation of revenue by type of goods or services for the years ended December 31, 2018 and 2017, along with the reportable segment for each category:

(in thousands)

	Twelve Months Ended December 31,
	2018		2017⁽¹⁾
Performance Improvement Solutions segment
System Design and Build	$	25,948	$	29,217
Software		2,883		3,409
Training and Consulting Services		14,123		7,273

Nuclear Industry Training and Consulting segment
Training and Consulting Services		49,295		30,981

Total revenue	$	92,249	$	70,880

⁽¹⁾ Prior period amounts have not been adjusted under the modified retrospective transition method for the adoption of ASC 606.

SDB contracts are typically fixed-priced, and we receive payments based on a billing schedule as established in our contracts. The transaction price for software contracts is generally fixed. Fees for software are normally due in advance of or shortly after delivery of the software. Fees for PCS are normally paid in advance of the service period. For Training and Consulting Services, the customers are generally billed on a regular basis, such as weekly, biweekly or monthly, for services provided. Contract liability, which we classify as billing in excess of revenue earned, relates to payments received in advance of performance under the contract. Contract liabilities are recognized as revenue as performance obligations are satisfied.

The following table reflects the balance of contract liabilities and the revenue recognized in the reporting period that was included in the contract liabilities from contracts with customers:

(in thousands)

	December 31, 2018		December 31, 2017
Billings in excess of revenue earned (BIE)	$	10,609	$	14,543
Revenue recognized in the period from amounts included in BIE at the beginning of the period	$	11,275		-

For an SDB contract, we generally have two main performance obligations: the training simulator build and PCS. The training simulator build generally includes hardware, software, and labor. We recognize the training simulator build revenue over the construction and installation period using the cost-to-cost input method. In applying the cost-to-cost input method, we use the actual costs incurred to date relative to the total estimated costs to measure the work progress toward the completion of the performance obligation and recognize revenue accordingly. Estimated contract costs are reviewed and revised periodically as the work progresses, and the cumulative effect of any change in estimates is recognized in the period in which the change is identified. Estimated losses are recognized in the period such losses are identified. Uncertainties inherent in the performance of contracts include labor availability and productivity, material costs, change order scope and pricing, software modification and customer acceptance issues. The reliability of these cost estimates is critical to the Company's revenue recognition as a significant change in the estimates can cause the Company's revenue and related margins to change significantly from the amounts estimated in the early stages of the project.

For the year ended December 31, 2018, the Company recognized revenue of $2.0 million related to performance obligations satisfied in previous periods.

As of December 31, 2018, the aggregate amount of transaction price allocated to the remaining performance obligations of SDB, software and fixed-price training and consulting services contracts is $41.6 million. The Company will recognize the revenue as the performance obligations are satisfied, which is expected to occur over the next twelve months.

Part of the training and consulting services contracts are T&M based. Under a typical T&M contract, the Company is compensated based on the number of hours of approved time provided by temporary workers and the bill rates, which are fixed by type of work, as well as approved expenses incurred. As part of our adoption of ASU 2014-09, we have elected to use the optional exemption under ASC 606-10-50-14(b) Revenue from contracts with customers, pursuant to which we have excluded disclosures of transaction prices allocated to remaining performance obligations under such contracts and when we expect to recognize the revenue.