
MT9071
Preliminary Information
40
checked. If the D4 secondary yellow alarm is enabled with control register bit D4SECY (see Table 82 - T1
Transmit Alarm Control - R/W Address Y02), then the Fs bit of frame 12 is not verified for the loss of frame
circuit.
In T1DM mode, the synchronization error criteria for the receiver will be the same as D4.
No reframing is forced by the device when in transparent mode, selected by setting control register bit TRANSP
(see Table 78 - T1 Framing Mode Control - R/W Address Y00).
The user may initiate a software reframe at any time by setting control register bit REFR (see Table 78 - T1
Framing Mode Control - R/W Address Y00). Once the circuit has commenced reframing, the signaling bits are
frozen until multiframe synchronization has been achieved.
4.4
T1 Multiframing
In T1 mode, DS1 trunks contain 24 bytes of serial voice/data channels bundled with an overhead bit. The frame
overhead bit contains a fixed repeating pattern used to enable DS1 receivers to delineate frame boundaries.
Overhead bits are inserted once per frame at the beginning of the transmit frame boundary. The DS1 frames
are further grouped in bundles of frames, generally 12 (for D4 applications) or 24 frames (for ESF - extended
superframe applications) deep.
The protocol (D4 or ESF) appropriate for the application is selected with control register bit ESF (see Table 78
- T1 Framing Mode Control - R/W Address Y00). In T1 mode, the MT9071 is capable of generating the
overhead bit framing pattern and for ESF links, the CRC remainder for transmission onto the DS1 trunk. The
beginning of the transmit multiframe may be determined by any of the following criteria:
(i) It may free - run with the internal multiframe counters;
(ii) The multiframe counters may be reset with the external hardware pin TxMF. If this signal is not synchronous
with the current transmit frame count it may cause the far end to go temporarily out of sync.
(iii) Under software control by setting control register bit TXSYNC (see Table 78 - T1 Framing Mode Control - R/
W Address Y00). The transmit multiframe counters will be synchronized to the framing pattern present in the
overhead bits multiplexed into channel 31 bit 0 of the incoming 2.048 Mb/s digital stream DSTi. Note that the
overhead bits extracted from the receive signal are multiplexed into outgoing DSTo channel 31 bit 0.
4.4.1
For D4 links, the frame structure contains an alternating 101010... pattern inserted into every second overhead
bit position. These bits are intended for determination of frame boundaries, and they are referred to as Ft bits.
A separate fixed pattern, repeating every superframe, is interleaved with the Ft bits. This fixed pattern
(001110), is used to delineate the 12 frame superframe. These bits are referred to as the Fs bits. In D4 frames
# 6 and #12, the LSB of each channel byte may be replaced with A bit (frame #6) and B bit (frame #12)
signaling information.
T1 D4 Multiframing