So the c. 1985 Macintosh supported inexpensive networking built-in (novel at the time), called AppleTalk. AppleTalk is designed to work over many link layer protocols, the one built-in to the mac works over its RS-422 ports.

LocalTalk uses round DIN-8 connections, which on the Apple IIGS are labeled printer and modem. The protocol which it speaks is AppleTalk, which is designed to work on other link level protocols such as Ethernet, where it is called EtherTalk, or Token Ring (TokenTalk).

AppleTalk on LocalTalk uses the LocalTalk Link Access Protocol (LLAP), which uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as discovered by Robert Metcalfe at Xerox PARC as part of his research into networking Altos, what became Ethernet

LLAP solves node IDs in an interesting way. Instead of using a unique ID built-in to the hardware the way Ethernet’s 48-bit MAC address is (which involves vendor coordination), it simply guesses an 8-bit number and asks if it’s being used—speak now or forever hold your peace

The idea behind two node ID groups is that clients will turn on and off more frequently, and avoids a conflict if a server is very busy and can’t respond to the enquiry. E.g. the print spooler or file server’s node ID being taken would be more impactful than your coworker’s

Another interesting thing about AppleTalk is that it has a type of transaction protocol (ATP)—one request followed by one response—instead of a stream connection (it has a session protocol (ASP) built atop transactions and a separate data steam protocol (ADSP))

The idea behind ATP is that programmers make a single call (I think RPC or HTTP RESTful API) atop a stream connection, resulting in waste. ATP is supposed to make this more efficient.

This book also claims that “plug-and-play” network capability was pioneered in AppleTalk

Another constraint of LocalTalk I think is cool is that no router/switch is required. You can connect two macs together and share a printer and files. Add a LaserWriter or ImageWriter II with a LocalTalk card directly to the network. This is the recommended setup for LANs

The Printer Access Protocol (PAP) and Apple Filing Protocol (AFP) seem ahead of their time, and LocalTalk made it affordable to network just a few computers at home (with Farallon PhoneNET, over phone line) as well as a school or office

LocalTalk on the wire is bit oriented, and has start and stop bit sequences written by the hardware. What if the stop sequence (01111110) is in your data? It won’t be, because the hardware stuffs an extra 0 after five consecutive 1s, and strips it at the receiving end