Introduction to ARCnet

ARCnet is a network type which works in a way similar to popular Ethernet networks but which is also different in some very important ways.

First of all, you can get ARCnet cards in at least two speeds: 2.5 Mbps (slower than Ethernet) and 100 Mbps (faster than normal Ethernet). In fact, there are others as well, but these are less common. The different hardware types, as far as I'm aware, are not compatible and so you cannot wire a 100 Mbps card to a 2.5 Mbps card, and so on. From what I hear, my driver does work with 100 Mbps cards, but I haven't been able to verify this myself, since I only have the 2.5 Mbps variety. It is probably not going to saturate your 100 Mbps card. Stop complaining. :)

You also cannot connect an ARCnet card to any kind of Ethernet card and expect it to work.

There are two "types" of ARCnet - STAR topology and BUS topology. This refers to how the cards are meant to be wired together. According to most available documentation, you can only connect STAR cards to STAR cards and BUS cards to BUS cards. That makes sense, right? Well, it's not quite true; see below under "Cabling."

Once you get past these little stumbling blocks, ARCnet is actually quite a well-designed standard. It uses something called "modified token passing" which makes it completely incompatible with so-called "Token Ring" cards, but which makes transfers much more reliable than Ethernet does. In fact, ARCnet will guarantee that a packet arrives safely at the destination, and even if it can't possibly be delivered properly (ie. because of a cable break, or because the destination computer does not exist) it will at least tell the sender about it.

Because of the carefully defined action of the "token", it will always make a pass around the "ring" within a maximum length of time. This makes it useful for realtime networks.

In addition, all known ARCnet cards have an (almost) identical programming interface. This means that with one ARCnet driver you can support any card, whereas with Ethernet each manufacturer uses what is sometimes a completely different programming interface, leading to a lot of different, sometimes very similar, Ethernet drivers. Of course, always using the same programming interface also means that when high-performance hardware facilities like PCI bus mastering DMA appear, it's hard to take advantage of them. Let's not go into that.

One thing that makes ARCnet cards difficult to program for, however, is the limit on their packet sizes; standard ARCnet can only send packets that are up to 508 bytes in length. This is smaller than the Internet "bare minimum" of 576 bytes, let alone the Ethernet MTU of 1500. To compensate, an extra level of encapsulation is defined by RFC1201, which I call "packet splitting," that allows "virtual packets" to grow as large as 64K each, although they are generally kept down to the Ethernet-style 1500 bytes.

For more information on the advantages and disadvantages (mostly the advantages) of ARCnet networks, you might try the "ARCnet Trade Association" WWW page: http://www.arcnet.com