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The following table further describes our stock options outstanding as of December 31, 2013: 

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## Restricted Stock Program Description 

On November 6, 2008, the Compensation Committee of the Board of Directors approved the Performance Shares Agreement under the 2006 Plan which sets forth the terms and conditions of awards of performance-based restricted stock units (RSUs). Of the 13.0 million shares of common stock authorized for issuance under the 2006 Plan, we may grant up to 5.0 million shares of common stock for issuance to certain employees and officers for awards other than stock options, which would include RSUs. Under a proposal that was approved by the Board of Directors and shareholders at the 2010 annual meeting, the number of shares available for awards other than stock options under all stock plans was reduced to 3.3 million. The number of shares of common stock earned by a recipient pursuant to the RSUs is subject to a market condition based on ADTRAN’s relative total shareholder return against all companies in the NASDAQ Telecommunications Index at the end of a three-year performance period. Depending on the relative total shareholder return over the performance period, the recipient may earn from 0% to 150% of the shares underlying the RSUs, with the shares earned distributed upon the vesting of the RSUs at the end of the three-year performance period. The fair value of the award is based on the market price of our common stock on the date of grant, adjusted for the expected outcome of the impact of market conditions using a Monte Carlo Simulation valuation method. A portion of the granted RSUs also vest and the underlying shares become deliverable upon the death or disability of the recipient or upon a change of control of ADTRAN, as defined by the 2006 Plan. The recipients of the RSUs receive dividend credits based on the shares of common stock underlying the RSUs. The dividend credits are vested and earned in the same manner as the RSUs and will be paid in cash upon the issuance of common stock for the RSUs. 

The following table is a summary of our RSUs and restricted stock outstanding as of December 31, 2012 and 2013 and the changes that occurred during 2013: 

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At December 31, 2013, total compensation cost related to the non-vested portion of RSUs and restricted stock not yet recognized was approximately $1.7 million, which is expected to be recognized over an average remaining recognition period of 1.9 years. 

## Valuation and Expense Information 

We use the Black-Scholes option pricing model (Black-Scholes Model) for the purpose of determining the estimated fair value of stock option awards on the date of grant. The Black-Scholes Model requires the input of certain assumptions that involve judgment. Because our stock options have characteristics significantly different from those of traded options, and because changes in the input assumptions can materially affect the fair value estimate, existing models may not provide reliable measures of fair value of our stock options. We use a Monte Carlo Simulation valuation method to value our 

