Optical Inter Satellite Links: A Glimpse Into The Past

ALOHAnet, developed at the University of Hawaii in the 1970s, laid the foundation for modern wireless communication. Originally designed to provide data communication between computer systems on the Hawaiian Islands using radio frequency, it allowed multiple computer terminals to connect to a central mainframe computer and communicate with each other. One of its key innovations was the use of a random access protocol which enabled multiple users to share a common communication channel without centralized control. ALOHAnet became operational in 1971, demonstrating the feasibility of wireless networking and providing valuable insights into the design and management of shared communication channels.

Standing on the shoulders of ALOHAnet, Robert Metcalfe, along with his team at Xerox's Palo Alto Research Center (PARC), developed Ethernet as a means to connect computers and share resources in a local area network (LAN) using coaxial cables and the concept of packet switching, that is, breaking data into small packets for transmission. The original Ethernet operated at a speed of 2.94 megabits per second, and it utilized a bus topology, where multiple devices shared the same physical cable.

The simplicity that made Ethernet widely adopted hid a fundamental weakness: Ethernet was a rather rudimentary way of sending chunks of data from node to node in a small area network and within the same network context. Ethernet had (and still has) basic forward error correction mechanisms, it provides no retransmission mechanisms (corrupted frames are simply discarded), no routing, and has no native encryption as it assumes it will be difficult to physically tamper a cable to sniff the bits coming and going.

Alongside Ethernet, several other network protocols proliferated, and with the increasing diversity of protocols, it started to become evident that connecting dissimilar networks was not an easy task. To facilitate interoperability, the ISO formed a committee called the International Organization for Standardization's Technical Committee 97 (ISO/TC 97). The committee was tasked with creating a framework that would enable different computer networks to communicate effectively.
Said framework would establish a set of rules and guidelines for network architecture and communication protocols, allowing different networks to interact seamlessly.
The OSI model, published in 1984, consisted of seven layers, each responsible for specific functions in the network communication process. These layers include the Physical, Data Link, Network, Transport, Session, Presentation, and Application layers.
In time, these upper layers in the OSI model would contribute with all the functionalities Ethernet (and other data link layer protocols) were lacking: session management, authentication, encryption, file transfer, and a long et cetera.

Time For Lasers in Orbit

An uncomfortable fact: there is no such thing as a true point-to-point radio link. This means, one ideal transmitting antenna sending radio photons to be exclusively captured by a distant receiving antenna. Cable-like. In reality, there will be scattered photons all around the place. Statistically, a substantial number of photons will arrive at the intended receiving point and the signal will be hopefully reconstructed if the receiver is active and able of doing so, but what happens with those nomad radio photons that miss the target? They may eventually interfere other systems.

Because radio transmitters are lousy and inefficient, there is a need for international spectrum coordination, where every radio transmitter in most frequency bands requires approval from the International Telecommunications Union, or ITU. Satellites must also get approval to onboard transmitters.
In summary, radio links are great and are still getting better as technology matures, but the main downsides—notably interference and compliance in an increasingly contested spectrum—are still there and will be there. The alternative to this? Laser communications. But why lasers?

Laser communication is not regulated by the ITU, which means it can be used without restrictions and does not require licensing. The reason for this should be clear by now: laser-based links are more focused than radio links—they have small beamwidths, and comparatively higher bandwidths—which means that between transmitter and receiver there are less photons going astray. How? Lasers produce a narrow beam of light in which all of its composing waves have very similar wavelengths, and they travel together in phase—the emitted photons are "in step" and have a definite phase relation to each other—concentrating a lot of energy on a very small area.

As there is no such thing as a free lunch, adding laser communications to satellites does not come without challenges. Along with the technical challenges related to onboarding laser comms terminals (stringent pointing requirements, thorough vibration control, ample onboard storage) is the fact that their beams cannot easily pass through solid/opaque objects. Hence, manmade structures on earth as well as cloud cover can cause attenuation in earth-to-space communications. However, it is not a major deterrent, and can be overcome with careful planning and considerations. More importantly, there are no such hurdles in space-to-space communications and, hence, laser technology is ideal for inter-satellite links and other space-based communication. So far, so good.

What about the protocols used when it comes to OISLs?  

The Space Development Agency (SDA) has released its Optical Communication Terminal (OCT) Standard. From the website: “Version 3.0.0 of the OCT Standard is designed to ensure interoperability, enable a strong marketplace, and provide advanced communication capabilities to terrestrial, maritime and airborne warfighting elements.”

Said standard appears to be spearheading the protocol definition efforts across the OISL industry. Interestingly, this standard only addresses layer 1 (physical) and layer 2 (data link). In short, the standard addresses a protocol to carry chunks of data from node to node, Ethernet style. In fact, the OCT standard envisions the encapsulation of actual Ethernet frames in Free Space Optical (FSO) frames.

Illustratively, the standard seeks to ensure that LCT providers become like modern equivalents of an Ethernet NIC (Network Interface Controller) from back in the day. Network interfaces. But somewhat more complex NICs, because LCTs must ensure the acquisition of the channel by means of a joint spatial choreography between the two distant endpoints in order to establish the link and allow the transfer of data from satellite A to satellite B. Interestingly, the standard makes no mention about encryption or security. It appears to be left up to the upper layers.

But what upper layers exactly? Is there an OSI Reference Model of sorts for Optical Inter-satellite Links? Is there a standard specifying those? Nobody knows.

One could think that hooking LCTs to IP stacks on both ends should do the trick. But it will not. The existing protocol stacks were designed for the low-latency, ample-MTU networks we find here on the ground. Are optical inter-satellite links so well behaved that we could assume a ground based protocol such as the delay-sensitive TCP will just work in space? Definitely not. Space optical links depend on satellites keeping their three-axis attitudes under control for the data to keep flowing. In case of a star tracker reset, a faulty reaction wheel, a safe mode triggered, there is no link anymore. A slowly spinning satellite will provide an intermittently working link, like an artificial pulsar. Therefore, OISLs call for delay and disruption tolerant protocols, where line of sight and optical connection are never assured, and data integrity is always at stake. We are talking about protocols that must ensure determinism and reliability in spite of all the challenges. The risk of not having an OSI model for space networks is lack of interoperability, which will impact the network effect. The situation could be as absurd as two satellites having SDA-compliant LCTs while being uncapable to send a file to each other.

Let’s look back at the seventies and the eighties, and let’s inspire ourselves from the spirit back in those days when the Internet was still to be done. Let’s look at how mobile comms generations were built on top of the tireless work of standardization organizations like the 3GPP. We have a new Internet to design, an internet that will expand into the Universe beaming lasers in all directions. We better start soon.