Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Bandwidth on demand is a data communication technique for providing additional capacity on a link as necessary to accommodate bursts in data traffic, a videoconference, or other special requirements. The technique is commonly used on dial-up lines and wide area networks (WANs) to temporarily boost the capacity of a link. Some call it "rubber bandwidth" because the capacity can be increased or decreased as needed. It is also called dynamic bandwidth allocation or load balancing. A similar technique is bandwidth on time of day, which refers to providing additional capacity at specific times of the day.

A network administrator who cannot be sure of traffic patterns between two sites can install routers that provide bandwidth-on-demand features. Such routers can automatically establish links on demand (dial-up, ISDN, or other switched services) to provide more capacity, and then bring the line down when traffic demands diminish. Home users with ISDN connections can aggregate two 64-Kbit/sec lines into a single 128-Kbit/sec line on demand.

Bandwidth on demand is both economical and practical. It makes sense to use a switched line and only pay for services as they are needed, rather than lease an expensive dedicated line that may go underused part of the time. Networks such as frame relay can automatically provide more capacity without the need to add additional lines, but the capacity is limited by the size of the trunk that connects a customer to the frame relay network.

Inverse multiplexing is a technique that combines individually dialed lines into a single, higher-speed channel. Data is divided over the lines at one end and recombined at the other end. Both ends of the connection must use the same inverse multiplexing and demultiplexing techniques. A typical dial-on-demand connection happens like this: A router on one end makes a normal connection, and then queries the router at the other end for additional connection information. When traffic loads are heavy, the additional connections are made to accommodate the traffic requirements.

The Lucent/Ascend Pipeline 75 remote access device determines when to add or subtract channels as follows. A specified time period is used as the basis for calculating average line utilization (ALU). The ALU is then compared to a target percentage threshold. When the ALU exceeds the threshold for a specified period of time, the Pipeline 75 attempts to add channels. When the ALU falls below the threshold for a specified period of time, it then removes the channels.

As an aside, a technique called trunking or link aggregation is like bandwidth on demand, but the bandwidth is usually made permanently available. Trunking is manually configured on internal network links to create additional bandwidth to high-volume servers and other devices. The usual configuration consists of two or more bonded Fast Ethernet channels between a switch and a server farm. See "Link Aggregation" and "Load Balancing" for more information.