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Bandwidth on Demand

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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.

Carrier Offerings

The telephone companies and other providers offer bandwidth on demand as part of their service offerings. Both ISDN and frame relay provide the services and have the potential to replace expensive dedicated leased lines such as T1 lines. As mentioned, basic rate ISDN has two 64-Kbit/sec B channels that can be combined into a single 128-Kbit/sec channel using bandwidth-on-demand techniques. For corporate users, AT&T provides worldwide switched digital services over an ISDN backbone that provides bandwidth on demand in increments of 64 Kbits/sec up to T1 rates (1.544 Mbits/sec).

A common telephone company offering called Multirate ISDN requires that you call the phone company in advance of needing the bandwidth and "demand" the extra bandwidth. This service is often used for videoconferencing where users need a specific bandwidth at a specific time.

An option for handling LAN traffic is MLPPP (Multilink PPP), an IETF recommendation. This protocol dynamically allocates bandwidth as needed and is supported in most vendors' routers. If you use MLPPP to get on the Internet, your ISP must have MLPPP equipment to support your dial-in connection. MLPPP is well suited for traffic bursts and overflows caused by backup sessions, conferences, large file transfers, or start-of-day traffic spikes. The protocol supports many different types of connections, including ISDN, frame relay, and analog lines. It operates in software, making it more efficient for on-the-fly allocation of lines. BAP/BACP is an extension to the protocol that defines a way for devices from different vendors to negotiate bandwidth.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.