Airline reservation systems (ARS) are part of so-called passenger service systems (PSS), which are applications that support direct contact with the passenger. ARS eventually evolved into the computer reservation system (CRS). A computer reservation system is used for reservations of a particular airline and interfaces with a Global Distribution System (GDS) which supports travel agencies and other distribution channels in formulating reservations for most main airlines in a single system.
Airline reservation systems incorporate airline schedules, fares, passenger bookings and ticket registrations. The direct distribution of an airline operates within its own reservation system, as well as pushing the information to the GDS. The second type of direct distribution channels are consumers who use the Internet or mobile applications to make their own reservations. Travel agencies and other indirect distribution channels access the same GDS as those accessed by airline reservation systems and all messages are transmitted by a standardized messaging system that operates on two types of messages that transmit in the high network level of SITA. These types of messages are called Type A [normally EDIFACT format] for real-time interactive communication and Type B [TTY] for information and type of message reservation. The standards of message construction established by IATA and ICAO are global and apply to more than air transport. Since airline reservation systems are critical business applications, and are functionally quite complex, the operation of an internal airline reservation system is relatively expensive.
Prior to deregulation, airlines had their own reservation systems with travel agencies that subscribed to them. Today, GDSs are run by independent companies with airlines and travel agencies that are the main subscribers.
As of February 2009, there are only four GDS providers on the market: Amadeus, Travelport (which operates the Apollo, Worldspan and Galileo systems), Saber and Shares. There is a major regional GDS, Abacus, which serves the Asian market and several regional players serving isolated countries, including Travelsky (China), Infini and Axess (Japan) and Topas (South Korea). Of these, Infini is housed within the Saber complex, Axess is in the process of moving into a partition within the Worldspan complex, and Topas agencies will emigrate at Amadeus.
Reservation systems can accommodate "no-frills" airlines and "hybrid" airlines that use e-ticketing as well as ticket-free tickets to accommodate shared and interline codes.
In addition to these "standardized" GDSs, some airlines have proprietary versions that they use to run their flight operations. Some examples are the Deltamatic OSS system and EDS SHARES. SITA Reservations remains the largest multi-host airline reservation system with over 100 airlines managing inventory.