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ISP, whether that be by DSL, cable, or even dial-up. For a mobile user, it's the connection via their mobile network. This section of the connection between user and server is the most inefficient and variable, and it will add latency onto any connection. To illustrate this, in 2013 the FCC released research that showed that a top-speed fiber connection would add 18ms latency—and that was the best-case scenario—a DSL connection would add 44ms, and dial-up was considerably slower. For mobile users, the story was even worse: a 4G connection had a latency overhead of 600ms on new connections, a 3G connection had a latency of over 2s on new connections, and even existing open connection had a latency as high as 500ms. The "Last Mile" or the "First Mile"? Although the last mile is the traditional name of the first stage of the connection between a user and the server, it may be more appropriate to think of it as the first mile or the on ramp, as the delay is often in establishing the connection in the first instance, particularly in mobile networks. Mobile connections have to communicate with the network to vali‐ date that a connection is allowed and to define the speed at which they can connect before anything can be opened. For 4G networks, this exchange happens with the local cell tower, but for 3G net‐ works, the exchange takes place with the core network; therefore, 3G networks have much higher latency on newly opened connec‐ tions. This is a high-impact area of the delivery of any website, and it's the one area where there is genuinely little to be done about the issue. Nevertheless, it's important to be aware of the variations that are possible and actually being experienced, and to ensure that your website's functionality is not affected by them. Performance Risks Unreliable delivery of content The variability in connection speed of the last mile means that it's hard to determine how fast content will be delivered to users. This 1. The Last Mile | 3

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