What are GPRS and EDGE, and why are they sometimes called "2.5G" and "2.75G" instead of full 3G?

GPRS and EDGE were incremental upgrades to GSM that added faster data, but they weren't fast enough or different enough architecturally to qualify as a new generation.

GPRS (General Packet Radio Service), 2.5G:

• Introduced around 2000.
• First packet-switched data service on mobile, meaning you were billed by data volume, not connection time.
• Enabled always-on internet access, the first mobile internet.
• Theoretical: 171.2 kbit/s. Realistic in practice: about 56–114 kbit/s.
• Made basic mobile web browsing and email possible.

EDGE (Enhanced Data Rates for GSM Evolution), 2.75G:

• Used improved modulation techniques (8PSK instead of GMSK).
• Backwards-compatible with GSM. Only the base station firmware needed updating, not the entire network.
• Theoretical: 473.6 kbit/s (using all timeslots with MCS-9).
• In practice: up to about 263 kbit/s (versus GPRS's real-world 56-114 kbit/s).

Why the ".5G" and ".75G" labels? These were evolutionary upgrades to the existing GSM infrastructure, not revolutionary new systems. They kept the same radio interface (TDMA) and core network. True 3G would require a completely new air interface: W-CDMA.

Go deeper:

• General Packet Radio Service (Wikipedia) — why GPRS is "2.5G": the always-on packet-switched layer bolted onto GSM, with coding schemes and real-world rates.
• Enhanced Data rates for GSM Evolution (Wikipedia) — how EDGE's 8PSK modulation pushes GSM to "2.75G" without a new air interface.