NOTE 6. PROPERTY, PLANT AND EQUIPMENT
Property, plant and equipment is summarized as follows at December 31:
| Lives (years) | 2013 | 2012 | ||||||
| Land | - | $ | 1,523 | $ | 1,689 | |||
| Buildings and improvements | 10-44 | 31,485 | 28,939 | |||||
| Central office equipment1 | 3-10 | 86,370 | 86,185 | |||||
| Cable, wiring and conduit | 15-50 | 76,107 | 80,338 | |||||
| Other equipment | 3-15 | 67,887 | 61,387 | |||||
| Software | 3 | 8,150 | 7,957 | |||||
| Under construction | - | 3,276 | 4,412 | |||||
| 274,798 | 270,907 | |||||||
| Accumulated depreciation and amortization | 163,830 | 161,140 | ||||||
| Property, plant and equipment - net | $ | 110,968 | $ | 109,767 | ||||
| 1 | Includes certain network software. | |||||||
Our depreciation expense was $17,722 in 2013, $16,933 in 2012 and $16,368 in 2011. Depreciation expense included amortization of software totaling $2,142 in 2013, $2,130 in 2012 and $2,243 in 2011.
Certain facilities and equipment used in operations are leased under operating or capital leases. Rental expenses under operating leases were $3,683 for 2013, $3,507 for 2012, and $3,414 for 2011. At December 31, 2013, the future minimum rental payments under noncancelable operating leases for the years 2014 through 2018 were $3,003, $2,857, $2,694, $2,458, and $2,233, with $11,707 due thereafter. Certain real estate operating leases contain renewal options that may be exercised. Capital leases are not significant.