Head end. The source point. For example in a cable television Interoffice calls. Calls which are made between offices. See also network the head end is where the video, radio and other intraoffice calls. services are formulated before being launched on to the net- Interoffice trunk. A direct line between telephone central offices. work. Interrupt packet. A high priority data packet which interrupts other Header. Information contained at the start of a block of user data, tasks and is passed through the network. which is used for auxiliary functions, such as error checking, Intersymbol interference (i.s.i.). Interference resulting from a synchronisation and timing. symbol in one signalling interval, of a digital transmission Heat pipe. A component for conducting heat away from a source, system, overlapping the sample time of a symbol in another to another region where it can be dissipated more easily. signal interval. Hermetic sealing. Sealing a package to prevent atmospheric con- Interworking. The ability for two or more systems or pieces of taminants from entering. equipment to work together, usually at the protocol level. 62/26 Glossary Intraoffice calls. Calls which are made within the same office. See Limited access network. A network which restricts access to some also interoffice calls. of its outlets. Isarithmic flow control. A method of flow control in which a de- Limit switch. A switch designed to sense the position of an object vice can only transmit when it has the permission to do so. This and to operate a contact when a set position is reached. There permit circulates throughout the network. is mechanical contact between the switch and the object being Isochronous signal. A signal in which the time between any two sensed. See also proximity switch. significant instances is equal to one or more multiples of the Line code. Defines equivalence between sets of digits generated unit interval. In isochronous transmission the characters are in a terminal and the corresponding sequence of symbol ele- sent in