1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+342!.3-)33)/. G0G03).!,).G0G0!.$G0G037)4#().).4%2(b) that 64 kbit/s satellite channels on TDMA systems are not yet operational;(c) that 64 kbit/s channels on SCPC systems are under in
2、vestigation;(d) that for an interim period only 48, 50 or 56 kbit/s channels via a satellite will be available in many cases;(e) that there is a requirement to interface transmission systems using an 8-bit envelope structure with suchsatellite systems,recommendsthat the fundamental parameters for a
3、multiplexing scheme using an 8-bit envelope structure for transmissionvia a 56-kbit/s SCPC satellite channel should be as described in this Recommendation.Note - In some cases, it may be necessary to adapt between 64 kbit/s and 56 kbit/s bearer rates.1 Gross bit rateFor transmission on the internati
4、onal digital satellite link, the multiplexed bit stream shall have a gross bit rateof 56 kbit/s. On the tributary, each transmitted and received tributary data stream has the 8-bit envelope structure asrecommended by Recommendation X.50.The adaptation to the SCPC 56-kbit/s channel is achieved:- by s
5、uppressing the F bits (framing bits):- by using one out of 7 S bits (status bits) for framing.2 Fundamental multiplex2.1 System capacityThe capacity is the same as recommended in Recommendation X.50.Note - Further study is required to accommodate user classes 7 and 11.2.2 Multiplex structureThe mult
6、iplex is based on envelopes of 7 bits.In a 7-bit envelope, bits 1-6 are information bits of the tributary channel and bit 7 is reserved for framing andsignalling purposes (as described in Recommendation X.50) (see Figure 1/X.55).2 Fascicle VIII.3 - Rec. X.552.3 Framing patternOne bit out of 7 S bits
7、 is used for framing purposes. The 72-bit framing pattern and the housekeeping bitsABCDEFGH as defined in Recommendation X.50 shall be used.The resulting framing scheme is shown in Figure 2/X.55.2.4 Framing strategy2.4.1 Loss of frame alignmentThe criterion for loss of frame alignment shall be the r
8、eception of 8 consecutive erroneous F bits.2.4.2 ReframingThe criterion for reframing shall be the detection of 8 consecutive F bits.Fascicle VIII.3 - Rec. X.55 32.4.3 Alarm and consequent actionWhen a loss of alignment is achieved:- the outgoing signals shall be set to all ones;- the state shall be
9、 signalled to the distant end as recommended in Recommendation X.50 via thehousekeeping bit A.3 Adaptation between 56 kbit/s and 64 kbit/s bearers (when used)The 6 + 2 envelope structure of the 64 kbit/s bearer is described in Recommendation X.50.3.1 Insertion and deletion of F bits3.2 Sharing of S
10、bits between framing and signalling mechanisma) At the transmit part:One S bit out of 7 S bits is suppressed to be replaced by an F bit.b) At the received part:The F bit is suppressed and replaced by the last value of the S bit of the tributary channel.Note - The mechanism, described above, is chosen due to the fact that the information on the S bit changesvery slowly. The process will only bring a delay of 6 bits for the signal signalling transition when the corresponding Sbit is affected.For each channel, only one out of 7 S bits is affected.
