# Four-corner model

### &#x20;Two-corner model

The Two-corner model describes a traditional approach for setting up a communication network between several organizations. This model is rather straight-forward and easy to implement. Whenever two organizations want to exchange information, a new connection is made between these organizations. The organizations are together responsible for the definition of the communication, the maintenance and improvement. Traditionally, FTP or even e-mail is used as transportation protocols for the communication, but nowadays custom made REST APIs are more common.

![](https://2328835143-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LquTHQI9fx-hOEUQGiB%2F-LriR1_bCdGC9dHKQVV3%2F-LriREyz4pfzrIYotRAh%2Fimage.png?alt=media\&token=0aa1ca76-0871-4573-b051-c7c11ddeac36)

The advantage of this model is that it is very flexible. The disadvantage is that the organizations itself are responsible for setting up and maintaining the connection (which is often not their main focus as a company). Furthermore, whenever a new company C enters the network, both company A and company B have to setup a new connection to this new organization. Such a network results in extremely high numbers of connections between companies, which are often not even standardized, and is therefore not scalable.  

### &#x20;Three-corner model

The three-corner model acknowledges the major limitation of the two-corner model, namely its lack of scalability by introducing a mediator between all companies. This mediator, named Service Provider and is often an IT integrator, sets up a connection with all companies. This connection between the service provider and the company can be customized, e.g. Company A might sent messages via e-mail while Company B sends messages via FTP. The Service Provider is responsible for converting the different messages and transportation protocols such that Company A can read messages from Company B in its preferred format. This solves the major limitation of the Two-corner model, i.e. scalability, because each company only needs a single connection to the central service provider in order to communicate to all connected companies.

![](https://2328835143-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LquTHQI9fx-hOEUQGiB%2F-LriR1_bCdGC9dHKQVV3%2F-LriRCeCjzTF1lx9sUcy%2Fimage.png?alt=media\&token=b6e18bf9-1391-4052-bc50-b783eab66f4c)

However, the Three-corner model also has disadvantages. In this model, the service provider becomes an essential party for all companies and might thus become very powerful. Moreover, this model only works when there is only a single service provide which every company agrees on. In practise this rarely happens as multiple companies might start offering service provider functionalities. When there are multiple service providers, with each their own set of connected companies, it is still not possible to connect to all companies at once via a service provider. In order to connect to all companies, each company should connect to all service providers which limits the model’s scalability.  

### &#x20;Four-corner model

The Four-corner model aims to finally solve the scalability issues of the former two models by uniting the service providers in a network. Each company, again depicted in blue in the figure below, is free to choose the service provider which best meets its requirements. Each company can, similarly to the Three-corner model, setup a connection to its service provider with a communication method of choice (e.g. FTP, e-mail). The service providers are all connected to each other via the model’s inner-network, meaning that each service provider can communicate with all other service providers. The service providers are responsible for translating all incoming messages in a common format, in our case the SCSN communication standard. The translated SCSN message is then send to the service provider of the message receiver, who translates the message and finally sends the message to the receiver.

![](https://2328835143-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LquTHQI9fx-hOEUQGiB%2F-LriR1_bCdGC9dHKQVV3%2F-LriRAMh6m9PsBQwHubb%2Fimage.png?alt=media\&token=cb1df3a1-e25d-43d0-979d-2e31df982605)

The four-corner model is very scalable because each company only has to setup a single connection to a single service provider. The service providers are responsible for translating and sending the message to the receiver. This model is comparable to the telecom sector, where each customer is connected to only a single service provider. The service provider is responsible for sending messages to the receiver, regardless which service provider the receiver has chosen.

Additionally, it should be noted that besides the service providers another central component is required, namely the address book. This address book contains all members of the network and their corresponding service provider. The service providers need to access this address book in order to route the message to the right service provider.
