Link Fragmentation and Interleaving (LFI) provides the ability to interleave high priority traffic within a stream of fragmented lower priority traffic. This feature helps avoid excessive delays to high priority, delay-sensitive traffic over a low-speed link. This can occur if this traffic type shares a link with lower priority traffic that uses much larger frames. Without this ability, higher priority traffic must wait for the entire packet to be transmitted before being transmitted, which could result in a delay that is too large for the application to function properly
For example, if VoIP traffic is being sent over a DS-1 or fractional DS-1 which is also used for Best Effort Internet traffic, LFI could be used so the small (usually 64-128B) VoIP packets can be transmitted between the transmission of fragments from the lower priority traffic.
Figure 1 shows the sequence of events as low priority and high priority frames arrive and are handled by LFI.
A low priority frame arrives in the low priority queue. At this particular instant, there are no packets in the high priority queue so low priority frame is de-queued and passed to the fragmentation mechanism for MLPPP.
The original packet is divided into ‛n’ fragments based on the size of the packet and the fragment threshold configuration.
The fragments are then transmitted out the egress port.
After the transmission of the fragments has begun, high priority frames arrive in the high priority queue.
The transmission of the remaining fragments stops and the high priority packets are transmitted out the egress interface. Note that high priority packets are not fragmented.
When the high priority traffic is transmitted, the remaining lower priority fragments are then transmitted.
On the ingress side, LFI requires that the ingress port can receive non-fragmented packets within the fragment stream and pass these packets directly on to the forwarding engine and then continue with the reassembly process for the fragmented frames.