Supporting ATM functions over multiple interfaces in the IMA group

IMA’s ability to preserve ATM cell order is only part of the reason why ATM attributes such as traffic
management and QoS control continue to function reliably over IMA links. The other key element is IMA’s
link management and framing capabilities. This section provides a conceptual overview of IMA framing
and how IMA uses the cells within IMA frames to regulate cell transmission rates (and cell delay) while
managing the IMA group and its constituent links.

Understanding IMA framing

An IMA process resides at each end of an IMA link. The two IMA processes communicate using IMA frames
to control data flow and manage the logical IMA group as well as its constituent DS1 (or E1) physical
links. Each IMA frame contains a fixed number of ATM cells. There are typically 128 cells in each IMA
frame, although operators can also set the frame size to 32, 64, or 256 cells, depending on the
specific IMA implementation.

As the ATM layer hands cells to the IMA process for transmission over the WAN, the IMA process
distributes them in round robin fashion among the constituent links, counting them, and grouping them
into IMA frames that span all of the constituent DS1 or E1 links. When the IMA process reaches the IMA
frame size limit, it begins creating a new IMA frame. Figure 1 illustrates this process for an IMA group
of three DS1 links.

Figure 1 — The two IMA processes associated with this IMA group communicate using IMA frames that
span across the three DS1 lines used to create the virtual 4.5 Mbps connection.

Regulating cell transmission rates and cell delay

Each IMA frame contains ATM data cells, ATM idle cells, and a few ATM cells marked as IMA Control Protocol
(ICP) cells. When an IMA process receives a frame, it hands off the ATM data cells to the ATM layer. The
IMA process uses the ICP cells, which are identified by a special code in the cell header, to communicate
with each other, control data flow, and manage both the IMA group and its constituent links.

Each IMA frame contains primarily ATM data cells. IMA frames include ATM idle cells only when there are no
ATM cells ready to transmit — that is, when the cell rate offered by the ATM layer for transmission over
the WAN is less than the virtual link cell rate. This use of idle cells is known as cell rate decoupling.
Because the WAN transmits cells at full speed even when the real ATM cell traffic is at a lower speed, the
two rates are decoupled.

Each IMA frame also contains one ICP cell per constituent DS1 or E1 link. The ICP cells are always
transmitted in the same location on a given DS1 or E1 link; however, the ICP cells can be transmitted in
different locations on different links. For example, one constituent link may always transmit the ICP cell
as the fourth cell in the IMA frame, while another link in the same IMA group may always transmit its ICP
cell as the twelfth cell in the IMA frame. In Figure 1, the DS1 WAN link 1 always transmits its ICP cell in
the second cell of the IMA frame, while DS1 WAN link 2 transmits its ICP cell in the first cell of the IMA
frame, and DS1 WAN link 3 transmits its ICP cell in the last cell of the IMA frame. (Figure 1 shows the ICP
cells as highlighted cell boxes.)

Because it inserts one ICP cell on each DS1 link per frame, the IMA process can introduce some delay into
the cell flow. However, the standard includes smoothing buffers that remove the delay and support virtual
circuits of all QoS classes, including those requiring minimal cell delay and predictable cell delay variation.
In addition, the IMA standard includes bit stuffing and other techniques that allow predictable cell delay
variation even when the different DS1 links use different clocks and have different delay variations.

Managing individual links

The IMA specification relies on ICP cells to monitor the status of the links in the IMA group and
dynamically adjust the bandwidth available on the IMA link. This function is critical to IMA’s ability
to bundle multiple DS1 or E1 links into a persistent virtual connection that can transmit ATM cells in
both directions.

During normal operations, individual links in the IMA group can be added, removed from service, or
simply fail — affecting the amount of bandwidth available for the IMA group. The IMA specification
requires the IMA group to dynamically adjust to these changing conditions without requiring operators to
stop and restart the overall traffic flow. The two IMA processes use ICP cells to communicate the status
of the constituent links as well as the IMA group as a whole. The IMA processes use the ICP cells to
remove failed links from the IMA group, and to restore recovered links to the IMA group.

This dynamic response to changing network conditions improves the fault tolerance on the IMA link. If
one constituent link in the IMA group fails, the throughput on the link will drop, but the IMA group will
automatically shift the traffic to the remaining links. The IMA group continues to transmit traffic as
long as at least one of its constituent links remains operational. Similarly, if an operator restores a
failed link to the IMA group or adds a new link, the IMA processes automatically adjust to the IMA group’s
new maximum bandwidth rate — again without interrupting the existing traffic flow.


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