1. GSM: 2G Europian standard and the first digital mobile system. It offers only circuit-switched traffic (i.e voice calls, SMS...etc) and data transfer at very low bitrates 9.6kbps.
2. GPRS: also reffered to as phase 2+. It offered packet-switched traffic with new network components integrated over the existant GSM core network. This allowed data bitrates to increase to 171kbps. It is also considered as the first step towards 3G networks.
3. EDGE: It was made to cover the delay of delivery of 3G/UMTS network. It supported data rates upto 384kbps.
4. UMTS: the first 3G Europian standard network. It is the optimal solution for GSM operators to support 3G services. The Air interface technique used for this network is WCDMA. This technique allowed data rates to reach 2Mbps.
5. HSDPA: It is actually a software upgrade to the UMTS network using the same air interface technique used in UMTS (WCDMA) and supporting data rates upto 10Mbps.
Source: Posted by h_alswidi @edaboard
Q: Why is my internet connection intermittent despite full HSDPA signal?
A: The service that you receive on your connection depends partly on how busy the network is. Voice and text users take priority over dongle (data) users, so when the network is particularly busy, data speeds may be slow even though signal strength is high. Conversely when the network is not busy, you may get fast data speeds although you barely have a signal. Unfortunately you can only influence this aspect of service by connecting at less sociable hours.
Source: theGR8Rgood @caravantalk