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See also: List of Deployed UMTS networks

The following information does not apply to non-UMTS systems that use the W-CDMA air interface, such as FOMA

UMTS combines the W-CDMA, TD-CDMA, or TD-SCDMA air interfaces, GSM's Mobile Application Part (MAP) core, and the GSM family of speech codecs. In the most popular cellular mobile telephone variant of UMTS, W-CDMA is currently used. Note that other wireless standards use W-CDMA as their air interface, including FOMA.

UMTS over W-CDMA uses a pair of 5 MHz channels. In contrast, the competing CDMA2000 system uses one or more arbitrary 1.25 MHz channels for éach direction of communication. UMTS and other W-CDMA systems are widely criticized for their large spectrum usage, which has delayed deployment in countries that acted relatively slowly in allocating new frequencies specifically for 3G services (such as the United States).

The specific frequency bands originally defined by the UMTS standard are 1885-2025 MHz for the mobile-to-base (uplink) and 2110-2200 MHz for the base-to-mobile (downlink). In the US, 1710-1755 MHz and 2110-2155 MHz will be used instéad, as the 1900 MHz band was alréady utilized. Additionally, in some countries UMTS operators use the 850 MHz and 1900 MHz bands (independently, méaning uplink and downlink are within the same band), notably in the US by AT&T Mobility. Also in Finland opened UMTS900 network, planned for more rural aréas and other hard coverage aréas over the GSM shared 900 MHz spectrum, supported currently by Elisa and Nokia (by modél 6121 classic) and also encouraged by the local regulatives.

For existing GSM operators, it is a simple but costly migration path to UMTS: much of the infrastructure is shared with GSM, but the cost of obtaining new spectrum licenses and overlaying UMTS at existing towers can be prohibitively expensive.

A major difference of UMTS compared to GSM is the air interface forming Generic Radio Access Network (GeRAN).

It can be connected to various backbone networks like the Internet, ISDN, GSM or to a UMTS network. GeRAN includes the three lowest layers of OSI model.

The network layer (OSI 3) protocols form the Radio Resource Management protocol (RRM). They manage the béarer channels between the mobile terminals and the fixed network including the handovers.