R2511-HP MSR Router Series Interface Configuration Guide(V5)

• Permanent virtual circuit (PVC)

• Per-VC traffic shaping

• User-to-network Interface (UNI)

• RFC1483, Multiprotocol Encapsulation over ATM Adaptation Layer 5

• RFC2225, Classical IP and ARP over ATM

• RFC2390, Inverse Address Resolution Protocol

• F5 end to end loopback OAM

• ATM adaptation layer 5 (AAL5)

IMA

Overview

The Inverse Multiplexing for ATM (IMA) technology distributes an ATM cell stream across multiple

low-speed links and reassembles the cells into the original stream at the far end. When the ATM cell

stream is distributed, a round robin mechanism is invoked. Each of the low-speed links is fed with the

ATM cells in a specific order. The ATM cells sent in each round forms an IMA frame. In addition, the IMA

interface sends ICP cells (IMA control protocol cells) periodically to identify the IMA frames and for the

receiving end to reassemble the distributed ATM cells received. Before reassembling the ATM cells

received of an IMA frame, the receiving end adjusts the differential delay of the link and eliminates the

cell delay variation (CDV) using the ICP cells received. Because the IMA frames are aligned on the

sending end before they are distributed to the low-speed links, the differential delays among the

low-speed links can be detected on the receiving end according to the time when the cells transmitted

along the low-speed links arrive. IMA requires that cells be sent continuously. If no ATM cells are to be

sent between two successive ICP cells, filler cells are inserted, which are simply dropped on the receiving

end.

Figure 6 An IMA implementation

IMA is achieved through IMA groups. An IMA group is a collection of physical links grouped to form a

higher-bandwidth logical link. The rate of the IMA group is approximately the sum of the individual link

rates. IMA provides you a cheap way to transmit high-speed ATM cell streams over low-speed links

multiplexed together while allowing for great flexibility.