- https://en.wikipedia.org/wiki/Link_layer - the group of methods and communications protocols that only operate on the link that a host is physically connected to. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a local area network segment or a wide area network connection.
Despite the different semantics of layering in TCP/IP and OSI, the link layer is sometimes described as a combination of the data link layer (layer 2) and the physical layer (layer 1) in the OSI model. However, the layers of TCP/IP are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures, data semantics, or networking technologies.
- https://en.wikipedia.org/wiki/MAC_address - or media access control address of a device is a unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet, Wi-Fi and Bluetooth. In this context, MAC addresses are used in the medium access control protocol sublayer. As typically represented, MAC addresses are recognizable as six groups of two hexadecimal digits, separated by hyphens, colons, or no separator (see Notational conventions below).
- https://github.com/alobbs/macchanger - GNU MAC Changer is an utility that makes the manipulation of MAC addresses of network interfaces easier.
- ModemManager - a DBus-activated daemon which controls mobile broadband (2G/3G/4G) devices and connections. Whether built-in devices, USB dongles, bluetooth-paired telephones, or professional RS232/USB devices with external power supplies, ModemManager is able to prepare and configure the modems and setup connections with them.
- Michael Kohn - tin can phone modem - Here's the popular "science experiment" of a phone made from tin cans and some kite string. This time instead of sending voice from one can to the other, data will be sent using a short beep for 0 and a long beep for 1 with a longer start beep to begin the transmission. A transmitter circuit on one end takes data from UART cable to a PC and relays the data through a speaker in one can. The receiver circuit has a microphone in the can and UART cable to relay the response back to the PC.
- https://github.com/wb2osz/direwolf - a software "soundcard" AX.25 packet modem/TNC and APRS encoder/decoder. It can be used stand-alone to observe APRS traffic, as a tracker, digipeater, APRStt gateway, or Internet Gateway (IGate).
- kitz.co.uk - DSL Broadband Information - one of the UK's largest DSL broadband resource & information sites.This site contains a wealth of information such as how ADSL & VDSL (fibre) broadband works.There's tips on choosing an ISP, help comparing ISP prices and ISP performance.A large section of the site is devoted to self help diagnostics and what to do if you run into problems with your broadband connection, such as how to get dsl line stats from your router to understanding how such things as SNR and attenuation can affect your connection. There's also a vast array of tutorials and FAQs about broadband.
- LibreSpeed - Speedtest - Free and Open Source Speedtest. No Flash, No Java, No Websocket, No Bullshit.
- https://en.wikipedia.org/wiki/ARCNET - or ARCnet, is a communications protocol for local area networks. ARCNET was the first widely available networking system for microcomputers; it became popular in the 1980s for office automation tasks. It was later applied to embedded systems where certain features of the protocol are especially useful.
- http://dtrace.org/blogs/rm/2016/09/15/turtles-on-the-wire-understanding-how-the-os-uses-the-modern-nic 
- https://en.wikipedia.org/wiki/RDMA_over_Converged_Ethernet - a network protocol that allows remote direct memory access (RDMA) over an Ethernet network. There are two RoCE versions, RoCE v1 and RoCE v2. RoCE v1 is an Ethernet link layer protocol and hence allows communication between any two hosts in the same Ethernet broadcast domain. RoCE v2 is an internet layer protocol which means that RoCE v2 packets can be routed. Although the RoCE protocol benefits from the characteristics of a converged Ethernet network, the protocol can also be used on a traditional or non-converged Ethernet network.
- https://en.wikipedia.org/wiki/Small_form-factor_pluggable_transceiver - a compact, hot-pluggable network interface module used for both telecommunication and data communications applications. An SFP interface on networking hardware is a modular slot for a media-specific transceiver in order to connect a fiber-optic cable or sometimes a copper cable. The advantage of using SFPs compared to fixed interfaces (e.g. modular connectors in Ethernet switches) is that individual ports can be equipped with any suitable type of transceiver as needed.
- WiGLE.net - All the networks. Found by Everyone.
- 802.16 - WiMax
- 802.22 - white space
- 802.20 - Mobile-Fi
- http://akbars.net/how-steve-perlmans-revolutionary-wireless-technology-works-and-why-its-a-bigger-deal-than-anyone-realizes.html 
- iw - a new nl80211 based CLI configuration utility for wireless devices. It supports all new drivers that have been added to the kernel recently. The old tool iwconfig, which uses Wireless Extensions interface, is deprecated and it's strongly recommended to switch to iw and nl80211.
- https://github.com/uoaerg/wavemon - wavemon is an ncurses-based monitoring application for wireless network devices
- hostapd - a user space daemon for access point and authentication servers. It implements IEEE 802.11 access point management, IEEE 802.1X/WPA/WPA2/EAP Authenticators, RADIUS client, EAP server, and RADIUS authentication server. The current version supports Linux (Host AP, madwifi, mac80211-based drivers) and FreeBSD (net80211). hostapd is designed to be a "daemon" program that runs in the background and acts as the backend component controlling authentication. hostapd supports separate frontend programs and an example text-based frontend, hostapd_cli, is included with hostapd.
- wifite - An automated wireless attack tool.
- You Are a Rogue Device - A New Apparatus Capable of Spying on You Has Been Installed Throughout Downtown Seattle. Very Few Citizens Know What It Is, and Officials Don’t Want to Talk About It. 
- WiGLE - Wireless Network Mapping
sudo wpa_cli wps_pbc # WPS Push-button mode sudo wpa_cli wps_pin any <the pin> # For WPS Pin mode
- https://github.com/wiire-a/pixiewps - a tool written in C used to bruteforce offline the WPS PIN exploiting the low or non-existing entropy of some software implementations, the so-called "pixie-dust attack" discovered by Dominique Bongard in summer 2014. It is meant for educational purposes only. As opposed to the traditional online brute-force attack, implemented in tools like Reaver or Bully which aim to recover the pin in a few hours, this method can get the PIN in only a matter of seconds or minutes, depending on the target, if vulnerable.
- https://github.com/aanarchyy/bully - a new implementation of the WPS brute force attack, written in C. It is conceptually identical to other programs, in that it exploits the (now well known) design flaw in the WPS specification. It has several advantages over the original reaver code. These include fewer dependencies, improved memory and cpu performance, correct handling of endianness, and a more robust set of options. It runs on Linux, and was specifically developed to run on embedded Linux systems (OpenWrt, etc) regardless of architecture.
- https://en.wikipedia.org/wiki/UUCP - an abbreviation of Unix-to-Unix Copy. The term generally refers to a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers. Specifically, a command named uucp is one of the programs in the suite; it provides a user interface for requesting file copy operations. The UUCP suite also includes uux (user interface for remote command execution), uucico (the communication program that performs the file transfers), uustat (reports statistics on recent activity), uuxqt (execute commands sent from remote machines), and uuname (reports the UUCP name of the local system).
- https://en.wikipedia.org/wiki/Serial_Line_Internet_Protocol - SLIP, an encapsulation of the Internet Protocol designed to work over serial ports and modem connections. It is documented in RFC 1055. On personal computers, SLIP has been largely replaced by the Point-to-Point Protocol (PPP), which is better engineered, has more features and does not require its IP address configuration to be set before it is established. On microcontrollers, however, SLIP is still the preferred way of encapsulating IP packets due to its very small overhead.
- https://en.wikipedia.org/wiki/Parallel_Line_Internet_Protocol - PLIP - direct computer-to-computer communications using the parallel port normally used for connections to a printer.The Parallel Line Internet Protocol provides Link Layer services for the Internet Protocol, the protocol used for forming small local area networks and large computer networks, such as the Internet, enabling computers without standard dedicated networking hardware, such as Ethernet, but with older parallel port devices, to communicate.
- https://en.wikipedia.org/wiki/Point-to-Point_Protocol - PPP - a data link protocol used to establish a direct connection between two nodes. It can provide connection authentication, transmission encryption (using ECP, RFC 1968), and compression. PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. PPP is also used over Internet access connections. Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet, since IP packets cannot be transmitted over a modem line on their own, without some data link protocol. Two derivatives of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line (DSL) Internet service connection with customers.
Audio / ultrasonic
- https://github.com/quiet/quiet - Transmit data with sound. Includes binaries for soundcards and .wav files. 
- https://github.com/Katee/quietnet - Simple chat program that communicates using inaudible sounds
- CUE - Smartphone Light Shows | Data over Sound
- https://github.com/ggerganov/wave-share - A proof-of-concept for WebRTC signaling using sound. Works with all devices that have microphone + speakers. Runs in the browser.Nearby devices negotiate the WebRTC connection by exchanging the necessary Session Description Protocol (SDP) data via a sequence of audio tones. Upon successful negotiation, a local WebRTC connection is established between the browsers allowing data to be exchanged via LAN. 
- https://github.com/ggerganov/ggwave/tree/master/examples/r2t2 - Transmit data with the PC speaker