Cisco Catalyst 8540 MSR Configuration Manual page 684

Overview of IMA
differential delay compensation and recombines the cells into the original ATM cell stream while
allowing minimal cell delay variation (CDV). The IMA process of splitting and recombining the ATM
cell stream is as transparent to the layer above as a traditional single-link physical layer interface.
Figure 21-1
switches which create a virtual IMA group connection.
Figure 21-1 IMA Grouping Example
IMA groups terminate at each end of the IMA virtual link. The transmit IMA receives the ATM cell
stream from the ATM layer and distributes it on a cell-by-cell basis across the multiple T1 or E1 links
within the IMA group. At the far-end, the receiving IMA recombines the cells from each link, also on a
cell-by-cell basis, recreating the original ATM cell stream. The aggregate cell stream is then passed to
the ATM layer.
The IMA frame is the unit of control in the IMA protocol. An IMA frame is a series of consecutive cells.
Periodically, the transmit IMA sends special cells that permit reconstruction of the ATM cell stream at
the receiving IMA. These cells, defined as IMA Control Protocol (ICP) cells, provide the definition of
an IMA frame. The transmitter must align the transmission of IMA frames on all links (shown in
Figure
links. Based on this required behavior, the receiver can detect the differential delays by measuring the
arrival times of the IMA frames on each link.
The transmitting end sends cells continuously. If no ATM layer cells are sent between ICP cells within
an IMA frame, the transmit IMA sends filler cells to maintain a continuous stream of cells at the physical
layer. Filler cells, which provide cell rate decoupling at the IMA sublayer, are discarded by the receiving
IMA.
A new OAM cell is defined for use by the IMA protocol. This cell has codes that define it as either an
ICP cell or a filler cell.
Within the IMA frame, the ICP cell appears at the ICP cell offset position, which can vary among the
links.
0/0/1, the ICP cells have their cell offset set to 0 and are the first cells in each IMA frame. On interface
0/0/2, the ICP cells have the ICP cell offset set to 3 and are the fourth cells in each IMA frame. On
interface 0/0/3, the ICP cells have their ICP cell offset set to 1 and are the second cells in each
IMA frame.
ATM Switch Router Software Configuration Guide
21-2
illustrates the configuration of the T1 IMA port adapters (with eight ports each) on two
Single ATM cell stream
from ATM layer
3 2 1
Switch A
In slot 0/0
1
2
3
ATM interfaces configured as:
atm 0/0/1, ima-group 1
atm 0/0/2, ima-group 1
atm 0/0/3, ima-group 1
21-2) to allow the receiver to adjust for differential link delays among the constituent physical
Figure 21-2 shows an example of the transmission of IMA frames over three links. On interface
Chapter 21
Original ATM cell stream
passed to ATM layer
Switch B
In slot 4/1
Virtual
ATM interfaces configured as:
IMA group 1
atm 4/1/4, ima-group 1
atm 4/1/5, ima-group 1
atm 4/1/6, ima-group 1
Configuring IMA Port Adapter Interfaces
3 2 1
1
2
3
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