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'''IL2P''' (Improved Layer 2 Protocol) is a [[Data link layer|data link layer]] protocol originally derived from layer 2 of the [[X.25]] protocol suite and designed for use by amateur radio operators. It is used exclusively on amateur packet radio [[Computer network|networks]].
IL2P occupies the [[Data link layer|data link layer]], the second layer of the [[OSI model]]. It is responsible for establishing link-layer connections, transferring data encapsulated in [[Frame (networking)|frames]] between [[Node (networking)|nodes]], and detecting errors introduced by the communications channel.

The Improved Layer 2 Protocol (IL2P) was created by Nino Carrillo, KK4HEJ, based on [[AX.25]] version 2.0 and implements Reed Solomon [[Forward error correction|Forward Error Correction]] for greater accuracy and throughput than either [[AX.25]] or [[FX.25 Forward Error Correction|FX.25]]. Specifically, in order to achieve greater stability on link speeds higher than 1200 baud.

IL2P can be used with a variety of modulation methods including AFSK and GFSK. The [https://github.com/wb2osz/direwolf/tree/dev direwolf] software TNC contains the first open source implementation of the protocol.

== IL2P Specification ==
The IL2P draft specification v0.5<ref>{{Cite web|url=http://tarpn.net/t/il2p/il2p-specification-draft-v0-5.pdf|title=IL2P protocol description|website=tarpn.net|access-date=2022-06-15}}</ref> was published via the [http://tarpn.net/t/il2p/il2p.html Terrestrial Amateur Radio Packet Network (TARPN)] on June 10, 2022.

As of version 0.5, the Weak-Signal-Extensions were added which adds several features to the protocols, intended primarily for SSB links. The automatic ID transmission for the FM/1200-baud and faster modes is still in place, but is not enabled if these lower speed weak-signal SSB modes are selected.

== Implementations ==
IL2P was first implemented in the closed source and proprietary ninoTNC<ref>{{Cite web|url=http://tarpn.net/t/nino-tnc/nino-tnc.html|title=TARPN -- NinoTNC info for Builders|website=tarpn.net|access-date=2020-01-14}}</ref> to solve for lossy network links due to low [[Physics:Signal-to-noise ratio|Signal-to-noise ratio]] or weak [[Engineering:Signal strength in telecommunications|signal strength]].

The specification itself outlines several design goals including:

* [[Error correction code#Forward error correction|Forward error correction]]
* Eliminating bit-stuffing
* Streamlining the [[AX.25]] header format
* Improved packet detection in the absence of Decode (DCD) and for open-[[Engineering:Squelch|squelch]] receive
* Produce a bitstream suitable for modulation on various [[Physical layer|physical layer]]s
* Avoid bit-error-amplifying methods (differential encoding and free-running [[Linear-feedback shift register|LFSR]]s)
* Increase efficiency and simplicity over [[FX.25 Forward Error Correction]]

== See also ==
*[http://ncpacket.net/north_carolina_packet_radio_network.html NCpacket group]

== References ==
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[[Category:Link protocols]]

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