Recommendation R.112

       Replace by the following:



          TDM HYBRID SYSTEM FOR ANISOCHRONOUS TELEGRAPH
          AND DATA TRANSMISSION USING BIT INTERLEAVING
                                
     (Malaga-Torremolinos, 1984; amended at Melbourne, 1988)


       The CCITT,

considering

       (a)  that there is a limited requirement on certain routes to provide for 
rates and codes not included in Table 1/R.101 which may be achieved by using time- 
division multiplexing (TDM) techniques;

       (b)  that wherever possible the rates and codes given in Table  1/R.101
should not be expanded in the future;

       (c)  that administrations may  be  asked  to  provide  code  and  speed
independent channels for cryptography, for telemetry, for  rates  outside  the
Recommendation R.101 tolerance of + 1.4%, where the rate and code may be changed 
frequently, and for maintenance purposes;

       (d)  that the aggregate bit rate may be limited to 2400 bit/s  and  TDM
equipment may be required to pass code independent and code dependent traffic;

       (e)  that the bearer may not be suitable for using the backward channel as 
defined by Recommendation V.26,  5 [1], or in the provision of telegraph channels 
above the Recommendation V.26 aggregate by the technique of subdivision of the 
frequency band as given in Recommendation H.34 [2],

unanimously declares the view

       that where bit-interleaved TDM systems are used for code dependent  and
code independent anisochronous telegraph and data transmission with an aggregate 
rate of 2400 bit/s, carried either by analogue telephone-type circuit or by higher 
order TDM system, the equipment should be constructed to comply with the following 
standards:

1.     System capacity

1.1    The TDM system will be capable  of  multiplexing  the  rates  shown  in
Table 1/R.101 for code dependent channels to alternative B.

1.2    Each code and rate independent channel should replace three, six or twelve 
50 baud code dependent channels.

1.3    The characteristics of the code independent channels should follow  the
limits shown in Table 1/R.112.


                                TABLE 1/R.112
                                       
       Characteristics of code independent channels and system capacity
























2.     Channel inputs

2.1    The nominal modulation rate will be 50, 100 or 200 bauds; the theoretical 
maximum modulation rate shall be 51.06, 102.12 or 204.24 bauds.

2.2    The transition coding process of telegraph signals is in accordance with 
Recommendation R.111.

2.3    Each channel provides its individual coding intervals starting within the 
allocated time slots: each coding interval is subdivided into 4 quarters. In that 
coding interval where the skipping bit occurs in the subframe, the forth quarter 
is shortened by one time slot length.

       For a 50 baud channel, transmission of the code character starts with the 
next corresponding time slot. For channels with higher  modulation  rates  the
transmission of the code characters should be delayed by  the  number  of  the
allocated time slots in the subframe minus 3.

2.4    Where applicable, spurious elements with duration of 1.6 ms (= 8%) or less 
shall be rejected and elements longer than 2 ms shall be accepted at the   50 baud 
channel input. The element lengths to be rejected or accepted at higher channel 
modulation rates is for further study.

3.     Channel outputs

3.1    The maximum degree of inherent isochronous distortion due to the sampling 
process shall be 8.3%.

Note - The long-term system distortion on a tandem  connection  of  transition
encoded channels of an independent TDM system approximates in the worst case to 
the arithmetic summation of the individual link distortions.

3.2    After a TDM link failure, actions  described  in    3.5  and  3.6  of
Recommendation R.101, should be taken on the derived channel output.

4.     Multiplexing details

4.1    The multiplexing details are in accordance with  Recommendation  R.101,
alternative B on a bit basis.

4.2    The maximum transfer delay (excluding the modem) for 50, 100 and 200 baud 
code and rate independent channels for back-to-back terminals shall not exceed 
50 ms for the rate 50 baud and 35 ms for rates 100 and 200 baud. The values of 
delay are subject to further study.

5.     Frame structure

       This is as defined in Recommendation R.101, alternative B.

6.     Synchronizing







       This is defined in Recommendation R.101, alternative B.

7.     Aggregate signals and interface, system clock arrangements  and  system
       control and alarms

       These are defined in Recommendation R.101.

8.     Channel numbering scheme of code independent channels

       Channel numbers used in the given Recommendation represent two last digits 
of the four-digit numbering scheme in respect to the Figure 1/R.112, first two 
digits are shown in Recommendation R.114.

                                  REFERENCES

[1]  CCITT Recommendation 2400 bit per second modem standardized  for  use  of
     4-wire leased telephone-type circuits, Recommendation V.26.

[2]  CCITT Recommendation Sub-division of the frequency band of  a  telephone-type
     circuit between telegraph and other services, Recommendation H.34.

[3]  CCITT Recommendation 4800 bit per second modem standardized for use of         4- 
wire leased telephone-type circuits, Recommendation V.27.































































Note - CD - code dependent channel
       CID - code independent channel
       * - service channel

                                 FIGURE  1/R.112
                                         
               Channel allocation for code independent channels of
                              50, 100 and 200 bauds