When we talk about standard internet connections, we are usually talking about Digital Subscriber Line (DSL) connections. DSL allows data to be transferred over existing wired telephone lines and is allocated to higher frequency bands to avoid conflicts with data from analog calls.
The term DSL most commonly refers to Asynchronous Digital Subscriber Line (ADSL) technology. In this type of connection, more bandwidth is allocated for downloads from the internet service provider to the user, making downloads quicker than uploads.
ADSL works like this because most users download more than they upload. In fact most common activities online, including web browsing and media streaming, only require a downstream data flow, making ADSL ideal for these use cases.
SDSL is different from ADSL in that bandwidth is split evenly between uploading and downloading flows.