What is a Passenger Service System (PSS)?

Travel Technology for Dummies: What Is a Passenger Service System?

Complicated topics explained in a simple way: Last time I explained what a booking reference or PNR is. Today let us talk about the Passenger Service System (PSS).

The PSS usually comprises of the Central Reservation System (CRS), in other words booked inventory, an airline inventory system (free inventory) and a departure control system (DCS). It is basically the technology an airline needs:

  • The CRS is the system that allows an airline to sell their inventory (seats). It contains information on schedules and fares as well as a database of reservations (or passenger name records) and of issued tickets.
  • The airline inventory system may or may not be integrated with the CRS. The system contains all the airline’s flights and the available seats. The main function of the inventory system is to define how many seats are available on a particular flight by opening or closing an individual booking class in accordance with rules defined by the airline.
  • The departure control system is the system used by airlines and airports to check in a passenger. The DCS is connected to the reservation system, enabling it to check who has a valid reservation on a flight. The DCS is used to enter information required by customs or border security agencies and to issue the boarding document. In addition, the DCS may also be used to dispatch cargo and to optimize aircraft weight and balance.

If we look at an image from my book Value Creation in Travel Distribution, we can see that a PSS and in this respect the CRS and inventory management portion are part of a central technology piece for airlines:

Travel distribution landscape
Travel distribution landscape (explanation below)

Travel distribution landscape

The travel market, like any other industry, is based on supply and demand. The supply side is differentiated into providers for air travel, cars, hotels, cruises, rail travel, etc. Demand arises from end customers who act via an agent for the most part in order to obtain an adequate overview of the multitude of offers. Thus, the value chain in the travel industry can be divided into five parts:

  1. Supplier: airlines, hotels, car rental companies, etc.
  2. Inventory management: administration of the inventory in a CRS
  3. Distribution: usually Global Distribution Systems (GDSs), but also aggregators, consolidators and organizers
  4. Sales: End customers can arrange their travel purchase through the agency/TMC, one of the online agencies (OTA, OTMC) or even the provider directly on the corresponding website (direct).
  5. Market: the traveler or the company

The Underlying Technology

Still today, PSSs and especially the CRS portion of it are usually built on mainframes running the Transaction Processing Facility (TPF) operating system. However, providers of CRSs are also moving (at least in parts) away from such mainframes to allow for more flexibility, open source and cloud-based solutions (compare The Company Dime [paywall]). Those mainframes along with TPF are often referred to as “antiquated technology” in the travel industry, which may be the case, however, they are very reliable, secure and crazy fast. IBM invented this technology in the 1960s and besides airlines, it is still used by banks, retailers, and insurers – in any field that requires the handling of high volumes of transactions by a large number of users. The operation of such mainframes is expensive though: It is hard to find developers, there is no community of developers providing the services to the public for free and the price for the hardware itself usually has an additional digit. This is the main reason that companies are looking to replace such technology. We have even created automated migration technology to allow (in this case banks, but possible for travel systems as well) to migrate off such expensive technology.

How to Change a PSS

If airlines change their PSS, it is usually a huge undertaking of several years: Southwest, for instance, operated for the better part of 45 years with clumpy technology systems, mainly built in-house. It was generally a cheaper approach that better fit the needs of its network, radically different from those of hub-and-spoke airlines. However, it was lacking crucial flexibility and Southwest could e.g. not follow the general trend of charging for bags, as their system couldn’t be modified to allow ancillary services (did not prevent them from starting a marketing campaign around that for their advantage “bags fly for free”).

In a three years project for $ ½ million dollar, in 2015 Southwest decided to move to a special version of Amadeus’s Altea system as their PSS. Operations schedule to start in the first half of 2017, followed by additional enhancements later that year. New capabilities include: red-eye flying, which Southwest negotiated with its pilots in June 2012 but they haven’t been able to take advantage of these overnight flights to the East Coast and Midwest. Less-than-daily service, so that Southwest does not have to fly the same schedule Sunday-Friday to all the cities in its domestic network any longer, with a separate schedule only on Saturdays. Day-of-the-week tweaks, so that Southwest will be better able to adjust for Tuesday or Wednesday, when airlines tend to suffer from leaner demand. It will also put an end to the old information-technology plumbing, which did not adequately support the airline’s commercial efforts, such as selling early boarding or innovating around other ancillary revenue products.

In the merger of American Airlines and US Airways, the combined airlines switched to Sabre, which was a blow for HP as the old bankrupt American decided for one of the two systems currently supported by HP: AXRes (the former Attraxis system based on old Swissair PARS-based system) and the SHARES multi–host system. However, during chapter 11 proceedings the new American decided for SabreSonic.

PSS changes can be disastrous or successful. United and Continental, combined in 2012, was considered an epic fail. American and US Airways are generally considered a huge success. Regardless, years of planning and preparation have to be taken into consideration.

But due to the fact that it is such a huge undertaking and risk to switch to a new PSS, it can be considered that providing the PSS to an airline is pretty much the license to print money. In the recent battle between airlines such as Lufthansa and GDSs, which are in most cases also the providers of the PSS, it remains to be seen if airlines can at all set themselves free or are locked into.

List of PSS providers

Name Vendor
AirCore Unisys
Radixx International Radixx
ACCELaero Information Systems Associates FZE
Altéa Amadeus
Crane PAX Hitit
Zenith Travel Technology Interactive
Open Skies / New Skies Navitaire (acquired by Amadeus)
HP Agilaire / HP Shares (former EDS Shares) HP
SabreSonic Sabre
Horizon SITA
ARCO Alitalia
Travelport Meridian Travelport
Astral Aer Lingus
Mercator (MARS) Mercator (Emirates Airlines)
AeroCRS Enoya-one LTD

Picture credit: Shutterstock / vlad09

27 thoughts “Travel Technology for Dummies: What Is a Passenger Service System?”

  1. Arun Chhikara

    Hi Micheal,
    Allow me to pass on my humble regards for the yeoman service you are doing to thousands of people who are out there in the dark by explaining so well the intricacies of travel technology.

    I am a Business Analyst working for a travel technology company and yet I am unable to connect all the dots, part of it is to be blamed on the vastness of the topic.
    Coming on to the topic now, when you say inventory management, what exactly do you mean? Do the airlines use GDS for inventory or the CRS? I read somewhere that different component of a flight search come from different systems for e.g. schedules come from OAG, seat availability from Airline CRS/GDS, and fares from ATPCo or GDS etc. What exactly does than this inventory management part does?


    1. Michael Strauss

      Michael Strauss Post author

      Dear Arun,

      I appreciate your kind words and will try to answer your questions:

      Question: Inventory management, what exactly do you mean?

      Answer: In the airline industry, available seats are commonly referred to as inventory. Inventory management is where the actual booking is hosted on behalf of the airline. This can be considered the real system, where there is one open spot for one seat on an aircraft. This inventory is handled by the Central Reservation System which is part of the PSS. In my new NDC article, I also have a set of pictures which may shed some additional light into the dark: http://www.travel-industry-blog.com/travel-industry/ndc/.

      On a side note: the PSS inventory is influenced by the airline’s yield management to support a permanent optimization of the offered booking classes in response to changes in demand or pricing strategies of a competitor. It may also include overbooking strategies (http://www.travel-industry-blog.com/travel-technology/travel-technology-for-dummies-what-is-overbooking/).

      GDSs mirror the inventory of all the airlines that they are allowed to distribute in their GDS system and thus have a pretty good overview of the truth of what is available, but in general (unless the co-operation between GDS and PSS supports access to the last seat in real time – called “last seat availability” or “last seat inventory”) not the full truth – as it is only a mirror and hence a cache. I’m sure you have experienced this when for instance you selected your seat on an Online Booking Tool that uses a GDS, you pay and all of a sudden, the system tells you that your seat is taken. This could have happened between your shopping and booking process, but much more likely the GDS cache was outdated.

      Question: Do the airlines use GDS for inventory or the CRS?

      Answer: The difference between a GDS and CRS is explained here: http://www.travel-industry-blog.com/gds/the-difference-between-crs-and-gds-in-the-travel-industry/. Airlines use the CRS as part of their PSS as their inventory management system. It is just as confusing because all of the major GDSs also ‘own’ a PSS and thus the distinction is not done correctly in many cases. A graphic of which company carries out which value creation step can be found in my book (https://amzn.to/mstrauss) as well as here: https://www.linkedin.com/pulse/siemens-uses-amadeus-cytric-hook-altea-bypassing-gds-save-strauss/. It doesn’t become easier when commercial aspects are included, such as the typical money flow for commission/overrides and tactics that airlines and GDSs may impose (http://www.travel-industry-blog.com/gds/multi-source-booking-tool-costs-and-restrictions/) – but that’s also a different topic.

      Question: I read somewhere that different components of a flight search come from different systems for e.g. schedules come from OAG, seat availability from Airline CRS/GDS, and fares from ATPCo or GDS etc. Then, what exactly does this inventory management part do?

      Answer: You are right about airlines filing fares with the Airline Tariff Publishing Company (commonly known as ATPCo). The users of ATPCo’s data indeed are the GDS such as Sabre, Amadeus, Travelport, along with their associated travel agents, but also Central Reservation Systems, online travel agencies (OTA) such as Expedia, Orbitz, and Travelocity and other service providers in the travel industry.

      Also, OAG indeed has an airline schedule database which holds future and historical flight details, according to the company from currently more than 900 airlines and over 4,000 airports. I’m not sure who uses OAG and who doesn’t.

      With NDC it may be that both fare filing and schedules will be handled by the airline itself again – which is a topic the GDS frequently bring up as a challenge for NDC: in a mixed configuration, where you have classical airlines using classical GDS interfaces and ATPCo fare filing, ticketing is handled by the GDS. In an NDC environment, in accordance with the question “Who is asking?” and tailored shopping results for the person requesting the flight, ticketing is handled by the airline.

      With regard to inventory management, I hope I already answered this question above.

      Let me know if these answers satisfy your great set of questions.

      Thank you so much,

  2. Software for travel industry

    Every industry is adopting new technology to improve its existence and traveled industry is not untouched with this. Every travel agent want to have Best tour operator software with them. Keep sharing about more technology in this industry. Great share.

  3. Arun Chhikara

    Hi Micheal,
    Thanks for that great answer, you perfectly answered all the questions I had.
    This blog is a like a gold mine of information on Air Trave Technology, enjoy reading it.

    Best Regards.


  4. Jitender

    Sir, please make me understand what is married segment? Please, explain it as simple as you can. I’m from India.

  5. Karthik Arthyadka

    Dear Micheal,

    As a new comer to travel industry, your blogs have been a great source of information to understand several fundamental concepts of the Travel world. Thank you so much for sharing this with the world.

    Reading through the ‘Travel Industry Landscape’, I wanted to ask you the differences between GDS, Online Travel Agencies (OTA), Aggregators and Meta Search. It would be great if you can provide examples for each and help understand how these pieces are interconnected.


    1. Michael Strauss

      Michael Strauss Post author

      Dear Karthik,

      There are numerous articles about exactly this topic on my blog. Pls. feel free to utilize the search button. A couple of links:

      And, I can also recommend my book “Value Creation in Travel Distribution” (http://www.amazon.com/author/strauss).

      If you cannot find your answers there, pls. feel free to post them again here and I will do my best to answer them.


  6. Vineet Kumar

    Dear Michael,
    Thank you so much for such an informative article. I have the following questions – if you could please elaborate on them
    1. I understood the difference between PSS and GDS theoretically however I am still not sure how it works from Technology point of view. For example Altea is PSS from Amadeus, does that mean airline has to kind of but license of Altea from Amadeus and set it up in their infrastructure or Altea will be more of a SAAS model?
    2. At the same time Amadeus is GDS as well which is more SAAS (software as a service), how does it help this airline in distribution as this airline does sell tickets online using Amadeus web services?
    3. If possible, could you please explain difference between Amadeus e-Retail and Altea? I doubt that Amadeus e-Retail is GDS but only facilitates easy selling of tickets which are otherwise difficult using Altea since in that case you will have to integrate with numerous web services. Could you please validate my thinking? Also can you please confirm if Amadeus e-Retail is also SAAS?
    4. If Amadeus e-Retail is not GDS and site is selling its tickets, how will I know if that airline used Amadeus as GDS or not? If I get to see this airline listings on skyscanner, is that good enough to assume that it uses GDS?
    5. As you rightly mentioned that moving away from PSS is risky, if airline does want to use NDC, what does it mean from tools/products point of view? Does this mean that airline will still use Amadeus Altea and e-Retail but will not use Amadeus as GDS like closing its EDIFACT pipe? What all things to consider in this move?
    6. As I am a software developer, if you could please help answer these in more technical terms or architecture diagrams, it will be very appreciated.

    Thanks Again for such a wonderful article. Loved it very much.

      1. Michael Strauss

        Michael Strauss Post author

        Dear Vineet,

        Sorry for the delay.

        1. PSS/CRS and GDS are two completely different systems. The article mentions what a PSS consists of: one portion is the central reservation system of one (1) airline. Obviously the similar/same software is used for another airline as well, but if you access the CRS of e.g. Lufthansa, you only see Lufthansa (maybe the whole Lufthansa Group). The system is also tailored to hold the inventory of that airline. It does not per se include fares for instance. A GDS accesses many CRSs for availability (up to 500 airlines are listed within the various GDSs). GDS also access other 3rd parties (such as ATPCo) for schedule and faring information. GDSs also usually have a shopping engine (such as Sabre Bargain Finder Max or Amadeus Masterpricer) but there are also other pricing engines such as SITA. GDSs also cache CRS data to ensure that not every of the (in consumer space 500:1) shopping requests hit the reservation system – you realize that when you get the message once you hit book like ‘the fare is no longer available’ or ‘the price has changed’ – which only means the cache of the GDS was outdated. Usually the provider of the PSS also hosts the system, so indeed it is usually a SaaS model. Pls. also take a look here: https://t.co/n3Y8O8vbKz
        2.GDSs bring a huge sales channel. Besides direct sales (= own website) a GDS provided indirect sales. If you are listed in a GDS, an airline is in front of many agents and other UIs and the airline will sell significantly more seats. In the litigation of US Air vs. Sabre, this was a major aspect, that Sabre delisted US Air for some time which damaged US Air significantly. A good article for you may be this one: https://www.travel-industry-blog.com/gds/multi-source-booking-tool-costs-and-restrictions/
        3. Amadeus e-Retail is another platform from Amadeus which is widely used as a booking system for airlines websites (= direct sales). I believe it powers 260 airline websites for 100 airlines. It is basically to be seen in parallel to the GDSs: A booking through Amadeus e-Retail will go directly to Altea not through the GDS.
        4. No, to my knowledge Skyscanner is not an authorized GDS developer (like us). Skyscanner goes directly into the airline systems – however to my knowledge not directly into the CRS system but rather to the newly designed APIs of an airline powered by IATA’s NDC (https://www.travel-industry-blog.com/travel-industry/ndc/). So, I would see Skyscanner rather as an aggregator/Metesearch engine. In my book I touched on Metasearch Engines (Kayak, etc.),.
        5. It will not use the EDIFACT pipe. It’s the GDSs decisions if the GDS wants to use NDC. The airline only provides an API and is in control of creating the offer – while historically in a GDS world, the GDSs created the offer based on schedule, fare and availability.
        6. Unfortunately, the comment section does not allow me to provide diagrams, but I hope I was able to answer some of your question. My book could also help: http://www.amazon.com/author/strauss. There will be a follow up on NDC shortly. Stay tuned.


  7. Vineet Kumar

    Thanks Michael. Your comments were really very helpful and made me think a bit. Have following queries based on how I understood your reply:
    1. Is there a Business Context Diagram (BCD; fuctional areas solved by them) for PSS vs. e-Retail vs. GDS? As you mentioned in #5, offer creation is by GDS (prior to NDC), how can e-Retail than use it if GDS will sit in parallel to e-Reatil?
    2. Specifically for Amadeus, they have come up with Digital APIs (https://amadeus.com/en/insights/press-release/finnair-transforms-its-booking-experience-with-amadeus-newest-digital-retailing-api) how will it impact the airline industry?
    3. The Digital APIs are a wrapper to e-retail solution or PSS? How does it impact NDC? Can Digital APIs be used as NDC? Is it a wise area to start leverage Digutal APIs?
    4. Could you please point me where I can read more about what is offer, Offer creation process, their delivery, who all are involved in the process? In case GDS is not there, which system will do it?
    5. Also why NDC requires to re-think of Offer and Order Management? What does it really mean? If possible, process flow of creation/management of what we get to see on Airline site?

    Thanks so much again and its been always a great learning going through your articles.

    1. Michael Strauss

      Michael Strauss Post author

      Dear Vineet,

      1. I’m sorry, Vineet, but this blog is intended to introduce people to travel distribution. Not to go too deep into it and replace documentation provided by IATA, GDSs and all the other players. Again, the diagram that comes closest is https://t.co/n3Y8O8vbKz (more elaborated in my book). E-Retail is a platform for the airline.com website. Airline.com sales are direct distribution and a GDS is not in between (shown in my picture above with that pipe on the right that goes through the distribution layer. Only offers for the indirect sales channels are created by the GDSs. Website offers are usually not created by the GDSs. Many airlines use e.g. ITA software (now Google) as a shopping engine, but there are others, too.
      2. + 3. Everybody will come out with Representational State Transfer (REST) interfaces coupled with JSON (JavaScript Object Notation), which are modern API practices that provide flexible and developer-friendly access to back-end resources. We are also debating it on a regular basis. XML is a language from around the year 2000. Developer today ‘run away’ when they hear XML (they probably rather stick around when they hear EDIFACT because they probably don’t know what it is as they were not even born when it came out. EDIFACT is a language still present in travel technology and just now being replaced with NDC). Coming back to REST interface: it is not so easy, because travel is fairly complicated. Especially with business travel lots of functionality is needed and while REST may be easy to be used on the UI (user interface), you still need the flexibility and compile a lot of functionality in the background to create for instance a corporation compliant PNR (or its newer version one order: https://www.travel-industry-blog.com/travel-technology/pnr/). To make it easier for UI developers, the API business logic needs to become more complicated in order to combine the front end with the backend. One large message (XML) has to be broken down in many small messages (Compare our FareSearch: https://xx1.pass-consulting.com/fileadmin/media/1.7.3/documentation/xsd/FareSearchRS_xsd/FareSearchRS.htm , or even worse the air shopping message from IATA NDC). So, somebody has to apply the logic from a few big messages to many small messages. GDS REST interfaces today are usually a ‘sandbox’ environment and their functionalities are limited – probably sufficient for leisure travel, most likely not for business travel. We are working on our iXX1 (https://www.pass-consulting.com/en/industries/travel/) which at some point should become a REST interface. It is supposed to speak traditional legacy backend language but modern front-end language. It is a transitional phase currently from XML to REST. How long it will take, I don’t know. However, it was one of my comment to IATA, that they are replacing 40-year-old technology (EDIFACT) with almost 20-year-old technology (XML) and call it “New” Distribution Capabilities.
      4. + 5. Pls. compare https://www.travel-industry-blog.com/travel-technology/fares-rates-and-tariffs/ and stay tuned for my next NDC article (NDC the next curtain)


  8. DTT

    Fully Fantastic Post. Your all post was always been great, Lots of thanks for sharing the best and useful information. Thanks again!

  9. Christian Diaz

    Hi Michael, if an airline needs to make a change in their PSS, let’s say seat cofiguration of an airplane, how much money that would cost?

    1. Michael Strauss

      Michael Strauss Post author

      I’m afraid, I can’t answer this. We are a technology provider and not a content provider. Also this blog is focused on technology. Pricing is solely up to the suppliers (airlines, etc. ).

  10. Ugen Tashi

    Dear Michael,

    Thank you for your insights on the Travel Technology. I have the following question.

    1. Is there such thing as a Web-Based PSS and CRS based PSS?
    2. I am also given to understand that PSS has Three Tier – For e.g All PSS from a GDS falls on Tier 1. PSS like SITA Horizon falls on Tier 2 and Web Based PSS falls under Tier 3. Is this correct?

  11. MAK Ripon


    Can you please give me an idea of development effort of a CRS part only like CRS of zenith? Technology Java or C#

    1. Michael Strauss

      Michael Strauss Post author

      Thank you for this question. However, I’m afraid I cannot answer it, as I do not have all the facts on the table. An integration of one or more GDSs usually takes about 1 year per GDS if you do it directly or 3-9 months (no matter how many GDSs) when you use an aggregation platform such as https://www.pass-consulting.com/en/industries/travel/travel-xml-api/. You may want to also take a look at https://www.travel-industry-blog.com/corporate-travel/ndc-for-dummies/ to learn more about direct access to an airline distribution system via NDC (not via GDS).

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