Forward Error Correction aka as FEC but in cisco terminilogy it is forward equivalence class which is totally different. FEC is a technology which is well known to correct bit errors at the physical layer. But it is being adapted by network layer to improve the application performance across the wan link consequently wan link performance also increases. FEC mainly comes in picture where the statical multiplexing is used. When TCP senses the packet loss over the wan link, it actually reduces the amount of data transfer over the link. These type of problems actually back off the data transmission rate and companies never utilize the link properly. This all works according to the nagle's algorithm. To over these problems FEC is used to increase the performance and throughput of the wan link. FEC plays vital role in the wan acceleration.