Definition: From the user's perspective, upstream network traffic flows away from the local computer toward the remote destination. Conversely, downstream traffic flows to the user's computer. Traffic on most networks flows in both upstream and downstream directions simultaneously, and often when data flows in one direction, network protocols often send control instructions (generally invisible to the user) in the opposite direction.
One way to generate upstream traffic is to upload files to a server or send an email message. Conversely, downloading files and receiving email generate downstream traffic. Typical Internet users create much more downstream than upstream traffic.
Examples: The Web browser sends HTTP requests upstream to the Web server, and the server replies with downstream data usually in the form of HTML pages.
Asymmetric DSL (ADSL) services provides less bandwidth in the upstream direction in order to reserve more bandwidth for downstream traffic.
DSL Crib Sheet
This multi-part article discusses the nuts and bolts of DSL technology, service availability, and more.
These directory pages link to the best online resources for DSL.