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Downlink is a telecommunication term pertaining to data which is sent out or downwards from a higher level or portion of a network.
Traditionally, it refers to a satellite communications process where data is sent from a satellite down to an earthbound terminal or device, hence the word "down." It is also used in other fields of networking like cellular and computer networking, where it is used in a similar manner but may or may not involve the directional meaning of up or down.
In satellite communications, downlink simply refers to the process when a satellite beams down information toward earthbound terminals or devices. The opposite of this is uplink, where the satellite receives data from an earthbound terminal.
In cellular networking, downlink can be seen from the user perspective as receiving some sort of message or data. A cellular device or phone receives direct communication from a cellular base station. An example of this is receiving a text message or an image from someone—that message is received the through a downlink communication process.
The term is used in the same way in computer networking. Edge terminals or nodes receive data from the network core or from higher-level network nodes such as routers and servers, and is commonly known as downloading. Therefore, the downloading of images and videos is done through the downlink process.