Upstream indicates the transmission direction of the signal flow between the signal-generating and the signal-receiving station. Upstream is the backward direction from the subscriber towards the server or central office( CO) of a network. Inclient-server architecture, uplink is the direction of transmission from the client to the server, or from the browser to the web server.
The terms upstream and downstream, which is the opposite direction, are mainly used for Internet access, since these are usually asymmetrical in terms of data rates. This is true for most DSL methods, but also for cable TV, satellite transmissions and for the various mobile technologies.
The upstream usually supports a much lower data rate than the downstream. In the case of the Internet, this is due to the fact that control commands are sent from the web client, the browser, to the web server, but the websites are downloaded in the opposite direction.