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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Stock-Based Compensation Expense Related to Stock Options, PSUs, RSUs and Restricted Stock | The following table summarizes stock-based compensation expense related to stock options, PSUs, RSUs and restricted stock for the years ended December 31, 2023, 2022 an d 2021:
(In thousands) |  | 2023 |  |  | 2022 |  |  | 2021 |
Stock-based compensation expense included in cost of revenue |  | $ | 1,293 |  |  | $ | 2,876 |  |  | $ | 543 |
Selling, general and administrative expenses |  |  | 10,701 |  |  |  | 20,844 |  |  |  | 4,571 |
Research and development expenses |  |  | 4,022 |  |  |  | 4,602 |  |  |  | 2,366 |
Stock-based compensation expense included in operating expenses |  |  | 14,723 |  |  |  | 25,446 |  |  |  | 6,937 |
Total stock-based compensation expense |  |  | 16,016 |  |  |  | 28,322 |  |  |  | 7,480 |
Tax benefit for expense associated with non-qualified stock options, PSUs, RSUs and restricted stock |  |  | (3,837 | ) |  |  | (5,152 | ) |  |  | (1,849 | )
Total stock-based compensation expense, net of tax |  | $ | 12,179 |  |  | $ | 23,170 |  |  | $ | 5,631 |
Summary of PSUs, RSUs and Restricted Stock Outstanding | The following table is a summary of our PSUs, RSUs and restricted stock outstanding as of December 31, 2022 and 2023 and the changes that occurred during 2023:
|  | Number of shares (In thousands) |  |  | Weighted Average Grant Date Fair Value (Per Share) |
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2022 |  |  | 1,086 |  |  | $ | 17.54 |
PSUs, RSUs and restricted stock granted |  |  | 1,509 |  |  | $ | 16.43 |
PSUs, RSUs and restricted stock vested |  |  | (501 | ) |  | $ | 14.67 |
PSUs, RSUs and restricted stock forfeited |  |  | (152 | ) |  | $ | 17.04 |
Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2023 |  |  | 1,942 |  |  | $ | 17.46 |
Summary of Weighted-Average Assumptions and Value of Options Granted | The weighted-average estimated fair value of stock options granted to employees during the years ended December 31, 2023 and 2022 was $2.99 and $5.81 per share, respectively, with the following weighted-average assumptions:
|  | 2023 |  |  | 2022 |
Expected volatility |  |  | 51.78 | % |  |  | 45.00 | %
Risk-free interest rate |  |  | 4.13 | % |  |  | 3.00 | %
Expected dividend yield |  |  | — |  |  |  | 1.77 | %
Expected life (in years) |  |  | 5.8 |  |  |  | 2.4 |
Summary of Stock Options Outstanding | The following table is a summary of stock options outstanding as of December 31, 2023 and 2022 and the changes that occurred during 2023:
|  | Number of Options (In thousands) |  |  | Weighted Average Exercise Price (Per share) |  |  | Weighted Average Remaining Contractual Life in Years |  |  | Aggregate Intrinsic Value (In thousands) |
Stock options outstanding, December 31, 2022 |  |  | 3,148 |  |  | $ | 14.37 |  |  |  | 3.42 |  |  | $ | 16,251 |
Stock options granted |  |  | 1,330 |  |  | $ | 5.27 |  |  |  |  |  |  |
Stock options exercised |  |  | (23 | ) |  | $ | 7.02 |  |  |  |  |  |  |
Stock options forfeited |  |  | (74 | ) |  | $ | 11.20 |  |  |  |  |  |  |
Stock options expired |  |  | (487 | ) |  | $ | 22.74 |  |  |  |  |  |  |
Stock options outstanding, December 31, 2023 |  |  | 3,894 |  |  | $ | 10.32 |  |  |  | 5.25 |  |  | $ | 3,087 |
Stock options exercisable, December 31, 2023 |  |  | 1,478 |  |  | $ | 12.47 |  |  |  | 1.62 |  |  | $ | 254 |
Stock Options Outstanding | The following table further describes our stock options outstanding as of December 31, 2023:
|  | Options Outstanding |  |  | Options Exercisable |
Range of Exercise Prices (Per Share) |  | Options Outstanding at December 31, 2023 (In thousands) |  |  | Weighted Average Remaining Contractual Life in Years |  |  | Weighted Average Exercise Price (Per Share) |  |  | Options Exercisable at December 31, 2023 (In thousands) |  |  | Weighted Average Exercise Price |
$5.23 – $5.23 |  |  | 1,320 |  |  |  | 9.92 |  |  | $ | 5.23 |  |  |  | — |  |  | $ | — |
$5.24 – $8.58 |  |  | 716 |  |  |  | 2.01 |  |  | $ | 7.37 |  |  |  | 559 |  |  | $ | 7.45 |
$8.59 – $12.17 |  |  | 773 |  |  |  | 3.72 |  |  | $ | 11.49 |  |  |  | 195 |  |  | $ | 9.49 |
$12.18 – $15.33 |  |  | 415 |  |  |  | 2.20 |  |  | $ | 15.33 |  |  |  | 362 |  |  | $ | 15.33 |
$15.34 – $19.08 |  |  | 670 |  |  |  | 2.93 |  |  | $ | 19.02 |  |  |  | 362 |  |  | $ | 18.97 |
|  |  | 3,894 |  |  |  |  |  |  |  |  |  | 1,478 |  |  |  |
Adtran Networks [Member] |
Summary of Stock Options Outstanding | The following table summarizes Adtran Networks stock options outstanding as of December 31, 2022 and December 31, 2023 and the changes that occurred during the year ended December 31, 2023:
|  | Number of Options (In thousands) |  |  | Weighted Average Exercise Price (Per share) |  |  | Weighted Average Remaining Contractual Life in Years |  |  | Aggregate Intrinsic Value (In thousands) |
Stock options outstanding, December 31, 2022 |  |  | 81 |  |  | $ | 8.58 |  |  |  | 4.00 |  |  | $ | 1,222 |
Stock options exercised |  |  | (50 | ) |  | $ | 7.64 |  |  |  |  |  |  |
Stock options forfeited |  |  | (12 | ) |  | $ | 10.45 |  |  |  |  |  |  |
Stock options expired |  |  | (1 | ) |  | $ | 9.60 |  |  |  |  |  |  |
Stock options outstanding, December 31, 2023 |  |  | 18 |  |  | $ | 11.04 |  |  |  | 4.48 |  |  | $ | 193 |
Stock options exercisable, December 31, 2023 |  |  | — |  |  | $ | — |  |  |  | — |  |  | $ | — |
Stock Options Outstanding | The following table further describes Adtran Networks' stock options outstanding as of December 31, 2023:
|  | Options Outstanding |  |  | Options Exercisable |
Range of Exercise Prices (Per Share) |  | Options Outstanding at December 31, 2023 (In thousands) |  |  | Weighted Average Remaining Contractual Life in Years |  |  | Weighted Average Exercise Price (Per Share) |  |  | Options Exercisable at December 31, 2023 (In thousands) |  |  | Weighted Average Exercise Price |
€10.00 - €10.00 |  |  | 18 |  |  |  | 4.48 |  |  | $ | 11.04 |  |  |  | — |  |  | $ | — |
|  |  | 18 |  |  |  |  |  |  |  |  |  | — |  |  |  |
Market-Based PSUs [Member] |
Summary of Weighted-Average Assumptions and Value of Options Granted | The following table details the significant assumptions that impact the fair value estimate of the market-based PSUs:
|  | 2023 |  |  | 2022 |  |  | 2021 |
Estimated fair value per share |  | $ | 19.26 |  |  | $ | 24.01 |  |  | $ | 26.07 |
Expected volatility |  |  | 51.52 | % |  |  | 45.77 | % |  |  | 53.27 | %
Risk-free interest rate |  |  | 3.93 | % |  |  | 4.28 | % |  |  | 0.85 | %
Expected dividend yield |  |  | 2.55 | % |  |  | 1.76 | % |  |  | 1.63 | %

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