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PSB 2115
PSF 2115
Functional Description
Semiconductor Group
56
11.97
through to the IOM-2 bus. The TIC bus request remains unaffected (i.e. if access was
granted the TIC address and BAC bit are activated). As soon as the S-bus D-channel
is clear and the S/G bit was set back to “GO” the controller will commence with data
transmission.
The S/G Bit generation in IOM-2 channel 2 is handled automatically by the IPAC
operating in TE mode.
5. After the transmission of an HDLC frame has been completed the D-Channel
controller withdraws from the TIC bus for two IOM-2 frames. This also applies when a
new HDLC frame is to be transmitted in immediate succession. With this mechanism
it is ensured that all connected controllers receive an equally fair chance to access the
TIC bus.
2.3.4.2
The S-bus access procedure specified in ITU I.430 was defined to organize D-channel
access with multiple TEs connected to a single S-bus.
To implement collision detection the D (channel) and E (echo) bits are used. The D-
channel S-bus condition is indicated towards the IOM-2 interface with the S/G bit (see
previous section).
The access to the D-channel is controlled by a priority mechanism which ensures that all
competing TEs are given a fair access chance. This priority mechanism discriminates
among the kind of information exchanged and information exchange history: Layer-2
frames are transmitted in such a way that signalling information is given priority (priority
class 1) over all other types of information exchange (priority class 2). Furthermore, once
a TE having successfully completed the transmission of a frame, it is assigned a lower
level of priority of that class. The TE is given back its normal level within a priority class
when all TEs have had an opportunity to transmit information at the normal level of that
priority class.
The priority mechanism is based on a rather simple method: A TE not transmitting layer-
2 frames sends binary 1s on the D-channel. As layer-2 frames are delimited by flags
consisting of the binary pattern “01111110” and zero bit insertion is used to prevent flag
imitation, the D-channel may be considered idle if more than seven consecutive 1s are
detected on the D-channel. Hence by monitoring the D echo channel, the TE may
determine if the D-channel is currently used by another TE or not.
A TE may start transmission of a layer-2 frame first when a certain number of
consecutive 1s has been received on the echo channel. This number is fixed to 8 in
priority class 1 and to 10 in priority class 2 for the normal level of priority; for the lower
level of priority the number is increased by 1 in each priority class, i.e. 9 for class 1 and
11 for class 2.
A TE, when in the active condition, is monitoring the D echo channel, counting the
number of consecutive binary 1s. If a 0 bit is detected, the TE restarts counting the
number of consecutive binary 1s. If the required number of 1s according to the actual
S-Bus Priority Mechanism for D-Channel