a timeslot within the frame structure. Each frame consists of 32 time other half of the bandwidth can be simultaneously tested by division multiplexed timeslots each containing 8 bits of data from setting up a similar data call, a data call to another circuit, or a one channel, with the data from each channel being identified by its voice call. position in the frame. The first timeslot of each frame (timeslot 0) is used for frame synchronisation and frame supervisory functions, while timeslot 16 61.5.5.4 Basic rate transmission tests can be used for a variety of signalling methods. In ISDN, Common Channel Signalling (CCS) is used, and timeslot 16 carries supervi- Having achieved a successful call set up as above, the logical path sory signals controlling the status of the calls occupying the remain- established can be tested for transmission quality. For data calls this ing timeslots, by the use of special protocols. Error detection can be normally entails the sending of a pseudo-random binary sequence achieved optionally by the use of a Cyclic Redundancy Check and the checking of a similar received sequence for errors. (CRC4), which groups 8 frames into sub-multiframes and 2 sub- multiframes into multiframes. The results measured can range from a simple count of bit errors and bits received, with a calculated Bit Error Ratio Test (BERT), to Clocks for the generation of frames and data are always locked to a full set of G.821 measurements, including errored and severely the master clock of the network. Terminal equipment therefore uses errored seconds, degraded minutes, and periods of unavailable time. the clock recovered from the received signal to generate the frames and to produce 64kbit/s channel clocks to the tributaries. The duration of the test can range from a few seconds, sufficient Timeslot 0 of each frame is reserved for frame alignment or to ensure that the call has been successfully established and data can synchronisation. Alternate frames contain the Frame Alignment be transferred, to many days for