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IMA (Inverse Multiplexing over ATM)

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IMA is a specification defined by the ATM Forum that provides a way to combine an ATM cell stream over two or more circuits (i.e., T1 lines), thus allowing an organization to lease just the bandwidth it needs. When more than T1 and less than T3 is required, IMA provides a solution. It lets an organization purchase just the bandwidth it needs to transmit ATM cells across a carrier's network to its remote sites.

As shown in Figure 1, IMA distributes traffic across multiple T1 circuits. The T1 lines act as a single circuit rather than multiple separate circuits. This optimizes bandwidth. For example, if separate ATM virtual connections were carried over separate T1 lines, some of the bandwidth on those lines might go unused. If each of the circuits only required 1 Mbits/sec of bandwidth rather than 1.544, the .544 Mbits/sec of bandwidth would go unused. IMA also allows bigger virtual connections. For example, if three T1 lines are aggregated, a virtual circuit can be as big as the three T1 lines combined.

Figure 1 (see book)

A round-robin approach is used to distribute cells across the lines. Note in the figure that cell 1 is placed on line 1, cell 2 is placed on line 2, and so on. At the receiving end, the cells are recombined in their proper order to maintain ATM's quality-of-service characteristics.

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