e1 signal structure

    the e1 line operates at a nominal rate of 2.048 mbps. the data transferred over the e1 line is organized in frames. each e1 frame includes 256 bits. the 256 bits consist of 32 timeslots of eight bits each, which carry the data payload. the frame repetition rate is 8,000 per second, therefore the data rate supported by each timeslot is 64 kbps. the number of timeslots available for user data is maximum31, because timeslot 0 is always used for frame and multiframe synchronization.

    the frames are organized in larger patterns, called superframes. the number of frames included in one superframe is either 2 or 16. the larger superframe is generally used when timeslot 16 serves for the transmission of end-to-end signaling and supervision information. when timeslot 16 must be reserved for the transmission of system information, the maximum payload rate is reduced from 1984 kbps to 1920 kbps.

    e1 superframe formats

    fcd-2 supports two types of superframes:

    1 the 256n superframe, that consists of two frames. with this superframe, timeslot 16 is generally free for user data.

    2 the 256s superframe, that consists of sixteen frames. with this superframe, timeslot 16 is generally reserved for system use (transfer of e1 signaling and supervision information), and is not available to the user.

    the superframe format, as well as the availability of timeslot 16, is selected by the user, from the front panel, or from the supervision terminal.

    timeslot mapping

    fcd-2 supports two timeslot mapping methods:

    1 sequential (bundled) mapping

    2 free user selection oftimeslots.

    with sequential allocation, the user can specify the starting timeslot. to permit automatic identification of empty ("idle") timeslots, a special code is transmitted in the free timeslots.

    note however that the current fractional-e1 service offerings may impose additional restrictions on timeslot utilization. the restrictions depend on the carrier implementation and therefore vary according to location.

    since timeslot 16 may have to be allocated to system data, it is handled in a special way:

    1 if the superframe format is 256s, timeslot 16 is always allocated to system data.

    2 if the superframe format is 256n, the user can select the use of timeslot 16.

    to prevent errors when the superframe format is changed from 256n to 256s, but timeslot 16 is still designated for user data, fcd-2 generates a configuration error message that prompts the operator to change the allocation of timeslot 16.

    e1 line signal

    the basic e1 line signal is coded using the high-density bipolar 3 (hdb3) coding rules, used by the fcd-2 main link. the hdb3 coding format is an improvement of the alternate mark inversion (ami) code. in the ami format, ones are alternately transmitted as positive and negative pulses, whereas zeros are transmitted as a zero voltage level. the ami format cannot transmit long strings of zeros, because such strings do not carry timing information.

    the hdb3 coding rules restrict the maximum length of a zero string to 3 pulse intervals. longer strings are encoded at the transmit end to introduce non-zero pulses. to allow the receiving end to detect the artificially-introduced pulses and enable their removal to restore the original data string, the encoding introduces intentional bipolar violations in the data sequence. the receiving end detects these violations and when they appear to be part of an encoded zero string it removes them. other bipolar violations are probably due to line errors, and can be counted separately, to obtain information on the quality of the transmission link.