Chaosnet has no centralized control element; when a network node has a message to transmit, its interface seizes the ether and transmits a packet. The time when it seizes the ether is determined only by state inside that particular interface and by the local state of the cable at the point where that interface’s transceiver is attached.
If two interfaces should decide to seize the ether and transmit at the same time, their transmissions will interfere and no useful information will be transmitted. This is called a collision . Collisions are the principal limitation on the bandwidth of a heavily-loaded ether-type network, and should be avoided. (However, neither PARC’s network nor MIT’s network has yet been operated with a heavy enough load to make collisions really significant.)
Chaosnet uses a novel collision avoidance technique. First of all, an interface will never initiate transmission unless the ether is seen to be not busy, i.e. it has been in the low state for some time. This ensures that collisions can only occur near the beginning of a packet. Once transmission of a packet has gotten well started, the ether is effectively "seized" (all interfaces realize that it is busy) and transmission will continue successfully through to the end of the packet. The amount of ether transmission time wasted by a collided packet is therefore limited to the round-trip cable propagation delay. This technique is called carrier sense .
Secondly, the hardware uses a time-division technique to attempt to prevent two interfaces from initiating transmission at the same time. This technique should prevent essentially all collisions while imposing only a modest delay in the initiation of transmission. It is designed so that it works better as the load on the ether increases; the wasted time between packets and the relative rate of collisions both decrease.
The basic idea is that each interface is assigned a time-slot, or turn , according to its address. It may only initiate transmission during its turn. The turns are spaced far enough apart that if one interface initiates transmission, every other interface will perceive that the ether is busy by the time its own turn arrives, and will not initiate an interfering transmission. Each interface contains a time-slot counter which counts while the ether is not busy, keeping track of whose turn it is. Each packet synchronizes the counters in all of the interfaces by setting them from the source address of that packet; at the time the packet was transmitted, it must have been the turn of the interface that transmitted it.
Another way to think of this is to make an analogy with ring networks. One can imagine a virtual token which passes down the cable until it gets to the end, then jumps to the beginning of the cable and repeats. An interface may only initiate transmission at the instant the token passes by it. When an interface transmits, the token stops moving and remains at that interface until the end of the packet, whereupon it continues down the cable, passing every other interface, giving them each a chance to transmit before letting the first interface transmit a second packet.