ERTMS has two components: GSM-R a radio system for enabling communication between the driver and the traffic management centre and ETCS; an automatic control system helping to control the  train speed limits by communicating with the driver.
There are three different levels of ERTMS/ETCS application, L1, L2 and L3, based on the railway infrastructure progressive requirements.

• L1 is as an add-on system designed for conventional lines equipped with line side signals and trains detectors. Balises are installed on the trackside adjacent to the side signals  for transmitting  information to the control centre and the train.
• L2 does not require line-side signals. The movement communication occurs directly from a Radio Block Centre (RBC) to the on board unit using a GSM-R. Continuous communication system of the L2 allows the train to reach its optimum or maximum speed while maintaining a safe braking distance.
• L3 is based on moving blocks technology. It involves the use of special equipment on the train to continuously supply data on the train's position to the control centre via satellite systems (Galileo), rather than by track based detection equipment. The train thus continuously monitors its own position.

The European ERTMS Deployment Plan foresees installation of the ERTMS equipment to more than 10,000 km of railway lines by 2015 and 25,000 km by 2020. A number of key European freight lines have been identified for ERTMS deployment. While most of the ongoing programs are targeting L1,  ERTMS is designed in a flexible way allowing  for smooth migration from one level to  the other. It is fully interoperable implying that any train fitted with the system can run on any other line equipped with a similar one. The trackside infrastructure from an ERTMS supplier can be used by any train equipped  by other ERTMS suppliers.
From the infrastructure capacity perspective the benefit of ERTMS level 1 is limited. Substantial benefits come from escalating to L2 and L3  with capacity  increase of up to 40% versus conventional system and more according to qualified sources. Extension of L1 and systematic escalation to L2  are actions to  be addressed for the period 2020-2030. In the following decades the actions need to go on according to the capacity requirements and upgrading  technology for  L3. Level 3 must be the ultimate solution. It may take decades to have it fully operational.