Year Ended December 31, 2018 Compared to the Year Ended December 31, 2017
Our management reviews and analyzes several key financial measures in order to manage our business and assess the quality of and variability of our service revenue, operating results and cash flows. The following summary tables present a comparison of our results of operations with respect to certain key financial measures. The comparisons illustrated in the tables are discussed in greater detail below.
(1) Includes non-cash equity-based compensation expense of $895 and $604 for 2018 and 2017, respectively.
(2) Includes non-cash equity-based compensation expense of $16,813 and $12,686 for 2018 and 2017, respectively.
Service Revenue. Our service revenue increased 7.2% from 2017 to 2018. Exchange rates positively impacted our increase in service revenue by approximately $4.0 million. All foreign currency comparisons herein reflect results for 2018 translated at the average foreign currency exchange rates for 2017. We increased our total service revenue by increasing the number of sales representatives selling our services, by expanding our network, by adding additional buildings to our network, by increasing our penetration into the buildings connected to our network and by gaining market share by offering our services at lower prices than our competitors.
Revenue recognition standards include guidance relating to any tax assessed by a governmental authority that is directly imposed on a revenue-producing transaction between a seller and a customer and may include, but is not limited to, gross receipts taxes, Universal Service Fund fees and certain state regulatory fees. We record these taxes billed to our customers on a gross basis (as service revenue and network operations expense) in our consolidated statements of operations. The impact of these taxes including the Universal Service Fund resulted in an increase to our revenues from 2017 to 2018 of approximately $1.6 million.
Our net-centric customers tend to purchase their service on a price per megabit basis. Our corporate customers tend to utilize a small portion of their allocated bandwidth on their connections and tend to purchase their service on a per connection basis. Revenues from our corporate and net-centric customers represented 64.9% and 35.1% of total service revenue, respectively, for 2018 and represented 62.3% and 37.7% of total service revenue, respectively, for 2017. Revenues from corporate customers increased 11.8% to $337.8 million for 2018 from $302.1 million for 2017 primarily due to an increase in our number of our corporate customers. Revenues from our net-centric customers decreased by 0.4% to $182.3 million for 2018 from $183.1 million for 2017 primarily due to an increase in our number of net-centric customers being offset by a decline in our average price per megabit. Our revenue from our net- centric customers has declined as a percentage of our total revenue and grew at a slower rate than our corporate customer revenue because net-centric customers purchase our services based upon a price per megabit basis and our average price per megabit declined by 25.9% from 2017 to 2018. Additionally, the net-centric market experiences a greater level of pricing pressure than the corporate market and net-centric customers who renew their service with us expect their renewed service to be at a lower price than their current price. We expect that our average price per megabit will continue to decline at similar rates which would result in our corporate revenues continuing to represent a greater portion of our total revenues and our net-centric revenues continuing to grow at a lower rate than our corporate revenues. Additionally, the impact of foreign exchange rates has a more significant impact on our net- centric revenues.
Our on-net revenues increased 8.1% from 2017 to 2018. We increased the number of our on-net customer connections by 12.1% at December 31, 2018 from December 31, 2017. On-net customer connections increased at a greater rate than on-net revenues primarily due to the 5.1% decline in our on-net ARPU, primarily from a decline in ARPU for our net-centric customers. ARPU is determined by dividing revenue for the period by the average customer connections for that period. Our average price per megabit for our installed base of customers is determined by dividing the aggregate monthly recurring fixed charges for those customers by the aggregate committed data rate for the same customers. The decline in on-net ARPU is partly attributed to volume and term based pricing discounts. Additionally, on-net customers who cancel their service from our installed base of customers, in general, have an ARPU that is greater than the ARPU for our new customers due to declining prices primarily for our on-net services sold to our net-centric customers. These trends resulted in the reduction to our on-net ARPU and a 25.9% decline in our average price per megabit for our installed base of customers.
Our off-net revenues increased 5.2% from 2017 to 2018. Our off-net revenues increased as we increased the number of our off-net customer connections by 10.3% at December 31, 2018 from December 31, 2017. Our off-net customer connections increased at a greater rate than our off-net revenue primarily due to the 6.8% decrease in our off-net ARPU.
Network Operations Expenses. Network operations expenses include the costs of personnel associated with service delivery, network management, and customer support, network facilities costs, fiber and equipment maintenance fees, leased circuit costs, access and facilities fees paid to building owners and excise taxes billed to our customers and recorded on a gross basis. Non-cash equity-based compensation expense is included in network operations expenses consistent with the classification of the employee's salary and other compensation. Our network operations expenses, including non-cash equity-based compensation expense, increased 4.9% from 2017 to 2018 as we were connected to 11.9% more customer connections and we were connected to 170 more on-net buildings as of December 31, 2018 compared to December 31, 2017. The increase in network operations expense is primarily attributable to an increase in costs related to our network and facilities expansion activities and the increase in our off- net revenues. When we provide off-net services we also assume the cost of the associated tail circuits.
Selling, General, and Administrative Expenses (“SG&A”). Our SG&A expenses, including non-cash equity- based compensation expense, increased 4.6% from 2017 to 2018. Non cash equity-based compensation expense is included in SG&A expenses consistent with the classification of the employee's salary and other compensation and was $16.8 million for 2018 and $12.7 million for 2017. SG&A expenses increased primarily from an increase in salaries and related costs required to support our expansion and increases in our sales efforts and an increase in our headcount partly offset by a $1.1 million decrease in our legal fees primarily associated with U.S. net neutrality and interconnection regulatory matters and by the $1.3 million reduction in commission expense from the impact of the new revenue accounting standard which requires us to capitalize certain commissions paid to our sales agents and sales employees. Our sales force headcount increased by 7.8% from 574 at December 31, 2017 to 619 at December 31, 2018 and our total headcount increased by 4.8% from 929 at December 31, 2017 to 974 at December 31, 2018.
Depreciation and Amortization Expenses. Our depreciation and amortization expenses increased 7.0% from 2017 to 2018. The increase is primarily due to the depreciation expense associated with the increase related to newly deployed fixed assets more than offsetting the decline in depreciation expense from fully depreciated fixed assets.
Gains on Equipment Transactions. We exchanged certain used network equipment and cash consideration for new network equipment resulting in gains of $1.0 million for 2018 and $3.9 million for 2017. The gains are based upon the excess of the estimated fair value of the new network equipment over the carrying amount of the returned used network equipment and the cash paid. The reduction in gains from 2017 to 2018 was due to purchasing more equipment under the exchange program in 2017 than we purchased in 2018.
Interest Expense. Interest expense results from interest incurred on our $445.0 million of senior secured notes, interest incurred on our $189.2 million of senior unsecured notes, interest on our installment payment agreement and interest on our finance lease obligations. Our interest expense increased by 5.3% for 2018 from 2017 primarily due to the issuance of $70.0 million of senior secured notes in August 2018 and an increase in our finance lease obligations.
Income Tax Expense. Our income tax expense was $12.7 million for 2018 and $25.2 million for 2017. The decrease in our income tax expense was primarily related to an increase in deferred income tax expense for 2017 primarily due to the impact of the Tax Cuts and Jobs Act (the "Act"). On December 22, 2017, the President of the United States signed into law the Act. The Act amended the Internal Revenue Code and reduced the corporate tax rate from a maximum rate of 35% to a flat 21% rate. The rate reduction was effective on January 1, 2018 and may reduce our future income taxes payable once we become a cash taxpayer in the United States. As a result of the reduction in the corporate income tax rate and other provisions under the Act, we were required to revalue our net deferred tax asset at December 31, 2017 resulting in a reduction in our net deferred tax asset of $9.0 million and we also recorded a transition tax of $2.3 million related to our foreign operations for a total income tax expense of approximately $11.3 million, which was recorded as additional noncash income tax expense in 2017.
Buildings On-net. As of December 31, 2018 and 2017 we had a total of 2,676 and 2,506 on-net buildings connected to our network, respectively.
Table:
 | Year Ended December 31, |  | Change
 | 2018 | 2017 | Percent
 | (in thousands) |  | 
Service revenue | $520,193 | $485,175 | 7.2%
On-net revenues | 374,555 | 346,445 | 8.1%
Off-net revenues | 145,004 | 137,892 | 5.2%
Network operations expenses(1) | 219,526 | 209,278 | 4.9%
Selling, general, and administrative expenses(2) | 133,858 | 127,915 | 4.6%
Depreciation and amortization expenses | 81,233 | 75,926 | 7.0%
Gains on equipment transactions | 982 | 3,862 | (74.6)%
Interest expense | 51,056 | 48,467 | 5.3%
Income tax expense | 12,715 | 25,242 | (49.6)%


Question: What is the average service revenue earned by the company in 2017 and 2018?
Answer: 502684
Calculation: (485,175 + 520,193)/2