Cisco Systems OL-7396-01

21-2

ATM Switch Router Software Configuration Guide

OL-7396-01

Chapter21 Configuring IMA Port Adapter Interfaces

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 illustrates the configuration of the T1 IMA port adapters (with eight ports each) on two

switches which create a virtual IMA group connection.

Figure21-1 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 21-2) to allow the receiver to adjust for differential link delays among the constituent physical

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 th at 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. Figure 21-2 shows an example of the transmission of IMA frames over three links. On interface

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.

Switch A Switch B

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

ATM interfaces configured as:

atm 4/1/4, ima-group 1

atm 4/1/5, ima-group 1

atm 4/1/6, ima-group 1

123 123

Single ATM cell stream

from ATM layer

Original ATM cell stream

passed to ATM layer

In slot 0/0 In slot 4/1

Virtual

IMA group 1

24337

PWR

FAIL

8T1-IMA

PWR

FAIL

8T1-IMA