Travel

Airline reservation systems incorporate airline schedules, fare tariffs, passenger reservations and ticket records. An airline's direct distribution works within their own reservation system, as well as pushing out information to the GDS. The second type of direct distribution channel 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 the airline reservation systems, and all messaging is transmitted by a standardized messaging system that functions on two types of messaging that transmit on SITA's high level network HLN. These messaging types are called Type A  for real time interactive communication and Type B  for informational and booking type of messages. Message construction standards set by IATA and ICAO, are global, and apply to more than air transportation. Since airline reservation systems are business critical applications, and they are functionally quite complex, the operation of an in-house airline reservation system is relatively expensive.

Prior to deregulation, airlines owned their own reservation systems with travel agents subscribing to them. Today, the GDS are run by independent companies with airlines and travel agencies being major subscribers.

As of February 2009, there are only a few major GDS providers in the market: Amadeus, Travelport which operates the Apollo, Worldspan and Galileo systems, Sabre, InteliSys Aviation which owns ameliaRES PSS and Shares. There is one major Regional GDS, Abacus, serving the Asian market and a number of regional players serving single countries, including Travelsky China, OR Russia, Infini and Axess both Japan and Topas South Korea. Of these, Infini is hosted within the Sabre complex, Axess is in the process of moving into a partition within the Worldspan complex, and Topas agencies will be migrating into Amadeus.

Reservation systems may host ticket-less airlines and hybrid airlines that use e-ticketing in addition to ticket-less to accommodate code-shares and interlines.

In addition to these standardized GDS, some airlines have proprietary versions which they use to run their flight operations. A few examples are Delta's OSS and Deltamatic systems and EDS SHARES. SITA Reservations remains the largest neutral multi-host airline reservations system, with over 100 airlines currently managing inventory.

Inventory management

In the airline industry, available seats are commonly referred to as inventory. The inventory of an airline is generally classified into service classes  economy, premium economy, business or first class and any number of fare classes, to which different prices and booking conditions may apply. Fare classes are complicated and vary from airline to airline, often indicated by a one letter code. The meaning of these codes are not often known by the passenger, but conveys information to airline staff, for example they may indicate that a ticket was fully paid, or discounted or purchased through a loyalty scheme, etc. Some seats may not be available for open sale, but reserved for example for connecting flight or loyalty scheme passengers. Overbooking is also a common practice, and is an exception to inventory management principles. One of the core functions of inventory management is inventory control. Inventory control monitors how many seats are available in the different fare classes, and by opening and closing individual fare classes for sale.