Always bear in mind that your connection is constrained by the slowest component of the network and the amount of data being transmitted across it at the time. Bottlenecks on the other end of the link and even en route may also affect transmission times, and imperfect connections can lead to errors and delays. Some transmission devices and software compress files, reducing the amount of data and transmission time, but additional data is added to the file size by network and transmission protocols. A standard telephone line and a modem is the entry-level connection to the Internet. The modem translates the analogue telephone signal into a digital signal recognised by a computer, and the connection is initiated by dialling an ISP's Internet access telephone number.
Data can flow both ways, but only when a connection is live.
So e-mail sent to you will only be received the next time you connect to your ISP, and similarly remote Internet users will not be able to access your computer or network unless you have initiated a connection. Modems may be either an internal card or an external box. ISDN connections are also generally more stable. The ISDN equivalent of a modem is a terminal adapter, either an internal card or an external box, which can be plugged directly into an ISDN telephone socket. With a permanent connection e-mail can be delivered and received instantly, and other applications such as website hosting and videoconferencing become possible options.