Most DSL customers primarily choose Fast mode due to the higher efficiency of data transfers and less frequent ping times. Although Fast mode provides more data throughput, there will be a decreased amount of error correction for each packet that is sent. This mode is ideal for connections with little to no interference on the line or a close loop that does not exceed 5.5 km.
Interleaved mode on the other hand, has a slower transfer rate when compared to fast mode allowing it to have a higher capacity for error correction.
Interleaved mode increases ping times and possible latency. This reduces speed but in exchange, increases reliability and error detection. Interleaved mode is ideal for poor wiring conditions or in cases where the CPE is close to the max limit of the xDSL loop. This mode also improves VoIP quality as well.
Both CO and CPE must mirror each other for this setting.