Abstract: A data transmission network comprises a plurality of interconnected nodes and input/output units. In response to receipt of a data message for onward transmission through the network, each node accesses information which it stores relating to the then current topology of the network and transmits the message through the appropriate connecting links. Each node operates according to a predetermined algorithm to up-date its stored topology information. Each node at any given time assumes a particular state, state n. When in this state, it interrogates immediately each neighboring node with the question "Who are the neighbors of node of distance (n-1) from you?" where "distance" is defined in terms of the number of links. The addressed nodes respond either by returning the requested information in the form of a positive reply or by responding "Don't yet know".

Abstract: A communication system has a first station (SO) and a plurality of further stations (S1-S4), two communication channels (A, B), for transmission in opposite directions, serially connecting the stations. The first station includes routing means operable in a double looped mode to route signals received on each channel onto the respective other channel and the further stations each including routing means operable in a through mode to forward, in the same channel, signals received on each channel, thereby forming a single folded transmision path. Preferably the further stations each are responsive to the cessation of signalling energy (e.g.

Abstract: To enable binary data to be differentially encoded into symbols (waves) for transmission the data is applied in n-bit data-words to produce symbols which, when displayed in a phasor-diagram, are in three groups in three 120.degree.-phase-sectors respectively, the number and dispositions of the symbols in the three sectors being the same. The encoding is such that, depending upon the data-word to be transmitted the sector in which a symbol is produced is rotated by 0.degree., 120.degree. or 240.degree. relative to the next preceding sector containing a symbol. In some embodiments one of the 2.sup.n possible different data-words is applied to produce an absolute symbol of zero amplitude. At a receiver the received symbols are decoded having regard to the 0.degree., 120.degree. and 240.degree. differential sector-rotations.