There are several types of broadband communications.
Integrated Services Digital Network (ISDN) is a digital transmission technology that carries both voice and data over digital phone lines or PSTN wires. Connections are made on demand by dialing another ISDN circuit’s telephone number. ISDN and DSL are very similar technologies because they both use existingphone lines to transmit digital signals. However, ISDN technology predates DSL and has largely been superseded by DSL for the home and small busi- ness market. ISDN requires a specialized client adapter called a Terminal Adapter, which DSL does not. ISDN is also slower than DSL, being limited to a data rate of approximately 128 Kbps for basic rate ISDN, and thus barely qualiﬁes as broadband. (Primary rate ISDN, which was commonly used for network backbone communications before ﬁber optic cable, provides more bandwidth and has higher speeds.)
Digital Subscriber Line (DSL) is a broadband technology that transmits digital signals over existing phone lines. It has become a popular way to connect small businesses and households to the Internet because it is affordable and provides a relatively high download speed—a typical maximum is 1.5 Mbps for basic DSL and 7 Mbps for high-end DSL. However, distance from the phone company’s switching station and the quality of the lines affect the total bandwidth available to a customer.
Cable or cable modem transmissions use a cable television connection and a specialized interface device known as a cable modem to provide high-speed Internet access to homes and small businesses. Cable access arranges users in groups around nodes that split the television and data signals at the cable provider’s end.
The speed of the network varies depending on how populated the group on each node is. Download speeds can vary by more than 1 Mbps in different areas. Most cable companies try to guarantee at least a 768-Kbps download speed; however, speeds of 3.0 to 7.0 Mbps are common, and speeds of 20 Mbps or more are possible.
Orbiting satellites provide extremely long-range wireless network transmissions. For broadband network access, satellites can be employed to relay network signals from the network service provider to individual customers. This can be a way to provide broadband network communications in rural or remote areas where cable service or viable DSL lines are not available. How-ever, satellite Internet communications are comparatively expensive and some only operate one-way; satellite transmissions provide the downstream communication, but another media (typically telephone) is needed for upstream communication. For remote or mobile locations, true two-way satellite-only communication is also available, but aligning a dish for upstream communication to a satellite requires a certain degree of precision and skill.