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NE40E V800R010C00 Feature Description - WAN Access 01

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Basic IMA Principles

Basic IMA Principles

IMA performs inverse multiplexing of an ATM cell flow to multiple physical links and remotely restores the original cell flow on these physical links. The ATM cell flows are multiplexed on multiple physical links on a per cell basis. To know the IMA feature, you need to learn the basic concepts of IMA.

Basic Concepts

  • IMA group

    An IMA group can be considered a logical link that aggregates several low-speed physical links (member links) to provide higher bandwidth. The rate of the logical link is approximately the sum of the rate of the member links in the IMA group.

  • Minimum number of active links

    It refers to the minimum number of active links that are required when the IMA group enters the Operational state. Link faults may cause the number of active links for the IMA group in the Operational state to be smaller than the configured minimum value. As a result, the IMA group status changes and IMA may go Down. Two communication devices can be configured with different minimum numbers of active links, but both devices must be configured with at least the specified minimum number of active links to be able to properly send ATM cells.

  • ICP cell

    ICP is short for IMA Control Protocol. ICP cells are a type of IMA negotiation cells, used mainly to synchronize frames and transmit control information (such as the IMA version, IMA frame length, and peer mode) between communicating devices. The offset of ICP cells in IMA frames on a link is fixed. Like common cells, ICP cells consist of a 5-byte header and 48-byte payload.

  • Filler cell

    In the ATM model without an IMA sub-layer, decoupling of cell rates is implemented by Idle cells at the Transmission Convergence (TC) sub-layer. After the IMA sub-layer is adopted, decoupling of cell rates can no longer be implemented at the TC sub-layer due to frame synchronization. Therefore, Filler cells are defined at the IMA sub-layer to implement decoupling of cell rates. If there is no ATM cell to be sent, the sender sends Filler cells so that the physical layer transmits cells at a fixed rate. These filler cells are discarded at the IMA receiving end.

  • Differential delay

    Links in an IMA group may have different delays and jitters. If the difference between the greatest phase and the smallest phase in an IMA group exceeds the configured differential delay, the IMA group removes the link with the longest delay from the cyclical sending queue and informs the peer that the link is unavailable by sending the Link Control Protocol (LCP) cells. Through negotiation between the two ends of a link, the link becomes active and then rejoins the cyclical sending queue of the IMA group.

Features Supported by ATM IMA and Their Usage Scenarios

Table 2-1 shows the features supported by ATM IMA and their usage scenarios.
Table 2-1  Features supported by ATM IMA and their usage scenarios

ATM Feature


Usage Scenario


IMA divides one higher-speed transmission channel into two or more lower-speed channels and transports an ATM cell stream across these lower-speed channels. At the far-end, IMA groups these lower-speed channels and reassembles the cells to recover the original ATM cell stream.

An IMA group can be considered a logical link that aggregates several physical low-speed links (member links) to provide higher bandwidth. The rate of the logical link is approximately the sum of the rate of the member links in the IMA group.

When users access an ATM network at a rate between T1/E1 and T3 , using T3 lines is cost-ineffective for carriers. In this scenario, IMA can be used.

IMA transports ATM traffic over bundled low-speed T1 or E1 lines. It allows a network designer and administrator to use these T1 or E1 lines, not the expensive T3 lines, to implement ATM access.


Figure 2-1 shows inverse multiplexing and de-multiplexing of ATM cells in an IMA group.

  • The sending end: In the sending direction, IMA receives ATM cells from the ATM layer and places them in circular order onto member links of the IMA group.
  • The receiving end: After reaching the receiving end, these cells are reassembled into the original cell flow and transmitted onto the ATM layer. The IMA process is transparent to the ATM layer.
Figure 2-1  Inverse multiplexing and de-multiplexing of ATM cells in an IMA group

Figure 2-2 illustrates IMA frames.

The IMA interface periodically sends certain special cells. The information contained in these cells is used by the receiving end of IMA virtual links to recreate ATM cell flows. Before recreating ATM cell flows, the receiving end adjusts the link differential delay and removes the Cell Delay Variation imported by the IMA Control Protocol (ICP) cells.

When IMA frames are transmitted, the sending end must align these frames on all links. Depending on the arrival time of the IMA frames on different links, the sending end detects the differential delay between the links and makes adjustments.

Cells are consecutively sent out from the sending end. If no cells on the ATM layer can be sent between ICP cells of an IMA frame, the IMA sending end maintains consecutive cell flows on the physical layer by adding filler cells. These filler cells are discarded at the IMA receiving end.

Figure 2-2  Schematic diagram of IMA frames

Updated: 2018-07-04

Document ID: EDOC1100027168

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