Space Link Extension (SLE) Transfer Services




An overwhelming majority of our ever-expanding Universe is occupied by outer space, the massive realm pieced together by an estimated 2 trillion galaxies. Despite its billions of years of existence, exploring space as a practice only extends back about 60 years through efforts made to understand just a minuscule fragment of the matter outside of our world. Sending technology and people into outer space leads to innumerable benefits; other than learning more about the universe, space exploration can trigger advances in science and technology. However, getting this equipment into space and managing it properly is challenging.

Other than the obvious, such as giving technology the capability to exit the exosphere, a major struggle with space exploration is communication. It is absolutely necessary for the individuals within a space shuttle to be able to communicate with mission control groups back on the Earth’s surface. The standard for sending data under this communication stream is Space Link Extension (SLE) Transfer Service.

SLE Transfer Services enable missions to send forward space link data units to a spacecraft and to receive return space link data units back. These exchanges are understood as taking place between an SLE user and an SLE provider; typically, the provider is the ground antenna, while the user is on the side of the control center. In this transfer, the user requests a service and the provider complies, giving the user important data needed to take action, such as the status of a spacecraft’s instruments.

The actual services of Space Link Extension exist as software operating on the computers of a ground communication network. As a standard practice, SLE Transfer Services should be fully interoperable with all parties involved, whether they are part of the ground network or the SLE users out in the known universe.

The ISO 18440 series of standards details common practices for Space Link Extension, striving to make them more interoperable and non-prescriptive, so that the user of the standards can incorporate any suitable type of technology. While not suggesting any specific design or technology for implementation, the standards follow a layered model of a system supporting the SLE protocol. This is visualized as:


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The higher layers in this model demonstrate the functionality and are split into sub-levels, which include the authentication layer, responsible for generating and analyzing the credentials specified, the data encoding layer, responsible for encoding of SLE protocol data units received from higher layers and decoding of protocol data units received from the peer application, and the transport mapping layer, handles the interface to the Transmission Control Protocol (TCP).

Overall, these standards cover the operations necessary to comprehend and/or utilize the transfer service, the parameter data associated with each operation, the behaviors that result from the invocation of each operation, and the relationship between and valid sequence of the operations and resulting behaviors.

The standards in the 18440 series, which detail Space Link Extension, include:

ISO 18440:2016 - Space data and information transfer systems - Space link extension - Internet protocol for transfer services

ISO 18441:2016 - Space data and information transfer systems - Space link extension - Application program interface for transfer services - Core specification

ISO 18442:2016 - Space data and information transfer systems - Space link extension - Application program interface for return all frames service

ISO 18443:2016 - Space data and information transfer systems - Space link extension - Application program interface for return channel frames service

ISO 18444:2016 - Space data and information transfer systems - Space link extension - Application program interface for return operational control fields service

ISO 18445:2016 - Space data and information transfer systems - Space link extension - Application program interface for the forward CLTU service

ISO 18446:2016 - Space data and information transfer systems - Space link extension - Application program interface for the forward space packet service

It is important to note that, while these documents do identify a set of recommended SLE transfer services, they deliberately avoid specifying the methods and technologies required for communications. This non-prescriptive nature prevents unrelated constrictions on the part of the user and widens the ISO 18440 series’ applicability.


1. International Organization for Standardization (ISO), ISO 18440:2016 - Space data and information transfer systems - Space link extension - Internet protocol for transfer services (Geneva: ISO, 2016), 11.
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