In computing, network traffic is often described as either upstream or downstream traffic. These terms are rarely used by most Internet users, but they are important to network administrators, who monitor the bandwidth usage of networks and individual systems.
The term “Upstream” and “Downstream” are very relative in its roots. Consider yourself throwing a ball to your friend. For you the ball is moving away from you, however, for your friend the ball is moving towards him. You can consider the ball moving upstream, whereas for your friend the ball is moving downstream. Similarly, in a network, upstream data for one device could be downstream for another device. The data packets are bidirectional in nature. The packets keep “flowing” in the network between two or more points. The packets send by one network are the packets received by another. Consider yourself downloading a file from a device. The packets you download will be travelling downstream in relation to you. But with respect to the device it would be sending data upstream.
- Upstream traffic refers to data that is sent from a computer or network. This includes all types of outgoing data, such as sending an e-mail message or uploading a file. It may also include data sent over the Internet while playing an online multiplayer game.
- Downstream traffic refers data that is received by a computer or network. This includes receiving e-mail messages, downloading files, or simply visiting Web pages. Online games also generate downstream traffic.
Some ISPs (Internet Service Providers) place monthly limits on upstream and downstream traffic for each user, collectively referred to as "bandwidth usage." However, these bandwidth limits are usually very high, which means most users don't need to monitor their monthly network traffic. Network administrators, on the other hand, usually keep a close eye on upstream and downstream traffic to make sure no single system is transferring an abnormal amount of data.
The server- client relation refers to the connectivity between the server and client. The remote hosts called clients located at one part of the world can access and indulge in data transfer to and from the servers that are situated at large distances from them.
Imagine downloading files such as images, videos or mail on a daily basis from your preferred websites. These websites are hosted on server located somewhere distant. Now, these files travel through the network in the form of packets. When you are extracting something from the server it is generally referred to a downloading or downstream traffic. On the other hand, if you are sending data to the server it is referred to as uploading or upstream traffic. In case of Server-Client paradigm, the terms “Upstream” and “Downstream” are used only with respect to the server.
Another way of looking at upstream and downstream is with reference to the network devices. There are a lot of network devices connected to each other in a network. The networks can be as complex as thousands of routers and switches or as basic as two computers. In a network the terms upstream and downstream are used with respect to the position of the devices.
For instance, the flow of packets between three kinds of switches: access, distribution and core switches. The flow of packets from the access switches to the distribution and core switches is upstream. Whereas, from core level switches to access switches, the flow of packets is downstream traffic. With respect to distribution switches, the flow to packets to core switches is upstream traffic while the flow to access switches is downstream traffic.