ADSL - Asymmetric DSL the ISP can send you lots of data, but you can't send them much. SDSL - Symmetric DSL the ISP and you send at the same data rate IDSL - IDSN DSL you ISP and you have a pipeline of 144kps (uses ISDN transmission coding) hope this helps. The "A" stands for "Asymmetric", meaning the phone company can send lots of data to you, but you can't send much to them. Originally, only a tiny uplink of 16 or 64kbps was supported; recent flavors of ADSL support up to ten times that much.