This means more than simply connecting more and more networks and attaching more and more users. It is vital that the links between the networks grow to keep pace with the overall traffic load. Network users expect to use services offered by the network, and these services include providing connections to other networks via the uplinks. These include service-related costs such as access and traffic costs, hardware costs for routers and hosts, salaries for support staff and other costs such as rent. These costs must be covered by charges to the network's clients.
Network traffic flows Downlink load increases with the number of users, and with the load per user.
This is easy to estimate if users are throttled, e.g. by working through modems. For example, if users can only connect via a pool of 32 V.32 modems, the maximum possible load is just 32 times 28.2 kbps, i.e. about 4.6 Mbps. If the users have higher-speed connections, it is harder to estimate their bandwidth requirements. Most user sessions are bursty, and depend on what the user is doing. News reading, for example, requires from one to about 20 screens per minute, and file transfers using FTP can range from kB to MB per file.