5.2.4  Clear request and clear indication packets
                Figure 8/X.25 illustrates the format of clear request and clear  indication
          packets, in basic and extended formats.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)         Logical channel group number
             2                                Logical channel number
             3                                 Packet type identifier
                      0          0         0         1         0     


























































          PAGE70  Fascicle VIII.8 - X.25

                                                                            0         1         1
            4                                    Clearing cause
            5                                   Diagnostic code a)
           4                                                    Address block b)
                                                  (see S 5.2.1)
                                                Facility length b)
                                                   Facilites b)
                                                Clear user data b)
          a)      This field is not mandatory in the basic format of clear request packets (see S 
         5.2.4.1).
         b)       Used only in the extended format (see S 5.2.4.2).
         Note - Coded X001 (modulo 8) or X010 (modulo 128).
                                                FIGURE 8/X.25
                              Clear request and clear indication packet format
         5.2.4.1   Basic format
         5.2.4.1.1 Clearing cause field
               Octet 4 is the clearing  cause  field  and  contains  the  reason  for  the
         clearing of the call.
               In the clear request packets, the clearing cause field  should  be  set  by
         the DTE to one of the following values:
                                         bits:  8 7 6 5 4 3 2 1
                                         value: 0 0 0 0 0 0 0 0
                                           or: 1 X X X X X X X
         where each X may be independently set to 0 or 1 by the DTE.













































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                The DCE will prevent values of the clearing cause field  other  than  those
          shown above from reaching the other end of the call by either accepting the clear
          request packet and  forcing  the  clearing  cause  field  to  all  zeros  in  the
          corresponding clear indication packet, or considering the  clear  request  as  an
          error and following the procedure described in Annex C.
                The coding of the clearing cause  field  in  clear  indication  packets  is
          given in Table 20/X.25.
                                                 TABLE 20/X.25
                           Coding of clearing cause field in clear indication packet
                                                                 Bits
                                                8     7     6     5     4     3     2     1
           DTE originated                        0     0     0     0     0     0     0     0
           DTE originated a)                      1     X     X     X     X     X     X     X
           Number busy                           0   























































          PAGE70  Fascicle VIII.8 - X.25

                                                       0     0     0     0     0     0     1
           Out of order                          0     0     0     0     1     0     0     1
           Remote procedure error                0     0     0     1     0     0     0     1
           Reverse charging acceptance not       0     0     0     1     1     0     0   
          subscribed b)                         
































































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                                           1
           Incompatible destination              0     0     1     0     0     0     0     1
           Fast select acceptance not            0     0     1     0     1     0     0     1
          subscribed b)                         
           Ship absent c)                         0     0     1     1     1     0     0     1
           Invalid facility request              0     0     0     0   































































          PAGE70  Fascicle VIII.8 - X.25

                                                                         0     0     1     1
           Acces barred                          0     0     0     0     1     0     1     1
           Local procedure error                 0     0     0     1     0     0     1     1
           Network congestion                    0     0     0     0     0     1     0     1
           Not obtainable                        0   
































































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                      0     0     0     1     1     0     1
          RPOA out of order b)                   0     0     0     1     0     1     0     1
              a) When bit 8 is set to 1, the bits represented by Xs are those included by the 
             remote DTE in the clearing or restarting cause field of the clear or restart 
             request packet respectively.
             b) May be received only if the corresponding optional user facility is used.
             c) Used in the conjunction with mobile maritime service.






























































         PAGE70  Fascicle VIII.8 - X.25

                 5.2.4.1.2 Diagnostic code
                Octet 5 is the diagnostic code and contains additional information  on  the
          reason for the clearing of the call.
                In a clear request packet, the diagnostic code is not mandatory.
                In a clear indication packet, if the clearing cause  field  indicates  "DTE
          originated", the diagnostic code is passed unchanged from the  clearing  DTE.  If
          the clearing DTE has not provided a diagnostic code in its clear request  packet,
          then the bits of the diagnostic code in the  resulting  clear  indication  packet
          will all be zero.
                When a clear indication packet results from a restart request  packet,  the
          value of the diagnostic code will  be  that  specified  in  the  restart  request
          packet, or all zeros in the case where no diagnostic code has been  specified  in
          the restart request packet.
                When the clearing cause field  does  not  indicate  "DTE  originated",  the
          diagnostic code in a clear indication packet is network generated. Annex E  lists
          the codings for network generated diagnostics. The bits of  the  diagnostic  code
          are all set to 0 when no specific additional  information  for  the  clearing  is
          supplied.
                Note - The contents of the diagnostic code field do not alter  the  meaning
          of the cause field. A DTE is not required to undertake any action on the contents
          of the diagnostic code field. Unspecified code  combinations  in  the  diagnostic
          code field shall not cause the DTE to refuse the cause field.
          5.2.4.2   Extended format
                The extended format is used for clear request and clear indication  packets
          only when the DTE or the DCE need to use the called and/or  calling  DTE  address
          fields, the facility field and/or the clear user data field in  conjunction  with
          one or several optional user facilities described in SS 6 and 7. The  called  DTE
          address field is used only when the called  line  address  modified  notification
          facility is used in clearing, in response to an incoming  call  or  call  request
          packet.
                When the extended format is  used,  the  diagnostic  code  field,  the  DTE
          address length fields and the facility length field must be present.  Optionally,
          the clear user data field may also be present.
          5.2.4.2.1 Address block
                The address block is described in S 5.2.1.
          5.2.4.2.2 Facility length field
                The octet following the address block indicates the length of the  facility
          field, in octets. The facility length indicator is binary coded and bit 1 is  the
          low order bit of the indicator.
          5.2.4.2.3 Facility field
                The facility field is present in the clear request or the clear  indication
          packet only in conjunction with one or several optional user facilities requiring
          some indication in this packet.
                The coding of the facility field is defined in SS 6 and 7.
                The facility field contains  an  integral  number  of  octets.  The  actual
          maximum length of this field depends on the facilities which are offered  by  the
          network. However, this maximum does not exceed 109 octets.
                Note - It is for further study whether another  value  should  be  defined,
          relative to the total number of octets in the packet.
          5.2.4.2.4 Clear user data field
                This field may  be  present  only  in  conjunction  with  the  fast  select
          facility (see S 6.16) or the call deflection selection facility (see S 6.25.2.2).
          It has a maximum length of 128 octets in the first case, of 16 or 128  octets  in
          the second case: whether the maximum length is 16 or 128 octets  when  using  the
          call deflection selection facility is specified in S 6.25.2.2.
                Note 1 - Some networks require the clear user  data  field  to  contain  an
          integral number of octets (see the note in S 3).
                Note 2 - The network does not act on  any  part  of  the  clear  user  data
          field. See Recommendation X.244.
          5.2.5  DTE and DCE clear confirmation packets
                Figure 9/X.25 illustrates the format of the DTE and DCE clear  confirmation
          packets, in the basic or extended format.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)         Logical channel group number
             2                                Logical channel number
             3    



                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                               Packet type identifier
                      0          0         0         1         0         1         1         1
            4                                    Address block a) 
                                                   (see S 5.2.1)
                                                 Facility length a)
                                                    Facilities a)
           a)      Used only in the extended format of DCE clear confirmation packets.
          Note - Coded X001 (modulo 8) or X010 (modulo 128).
                                                 FIGURE 9/X.25
                                 DTE and DCE clear confirmation packet format
                The extended format may be used for DCE clear confirmation packets only  in
          conjunction with the charging information facility described in S 6.22. It is not
          used for DTE clear confirmation packet.
          5.2.5.1   Address block
                The address block is described in S 5.2.1.
                The calling and called DTE address length fields are coded with  all  zeros
          and the called and calling DTE address fields are not present.
          5.2.5.2   Facility length field
                The octet following the address block indicates the length of the  facility
          field, in octets. The facility length indicator is binary coded and bit 1 is  the
          low order bit of the indicator.
          5.2.5.3   Facility field
                The coding of the facility field is defined in SS 6 and 7.
                The facility field contains  an  integral  number  of  octets.  The  actual
          maximum length of this field depends on the facilities which are offered  by  the
          network. However, this maximum does not exceed 109 octets.
                Note - It is for further study whether another  value  should  be  defined,
          relative to the total number of octets in the packet.
          5.3    Data and interrupt packets
          5.3.1  DTE and DCE data packets
                Figure 10/X.25 illustrates the format of the DTE and DCE data packets.
                                                   Bits
                      8          7         6         5         4         3         2     




































          PAGE70  Fascicle VIII.8 - X.25

                                                                                                   1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number
                      Q          D          0          1                                       
             2                                Logical channel number
            3                P(R)                  M                  P(S)                  0
                                                     User data
                                                    (Modulo 8)




























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number
                      Q          D          1          0                                       
             2                                Logical channel number






























































          PAGE70  Fascicle VIII.8 - X.25

            3                                    P(S)                                      0
                                                 P(R)                                      M
           4                                       User data
         (When extended to modulo 128)
         D       Delivery confirmation bit
         M       More data bit
         Q       Qualifier bit
                                               FIGURE 10/X.25
                                       DTE and DCE data packet format




























































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.3.1.1   Qualifier (Q) bit
                Bit 8 of octet 1 is the qualifier (Q) bit.
          5.3.1.2   Delivery confirmation (D) bit
                Bit 7 of octet 1 is the delivery confirmation (D) bit.
          5.3.1.3   Packet receive sequence number
                Bits 8, 7 and 6 of octet 3, or bits 8 through 2 of octet 4  when  extended,
          are used for indicating the packet receive sequence number P(R). P(R)  is  binary
          coded and bit 6, or bit 2 when extended, is the low order bit.
          5.3.1.4   More Data bit
                Bit 5 in octet 3, or bit 1 in octet 4 when extended, is used for  the  More
          Data       (M   bit):   0   for   no   more   data   and   1    for
          more data.
          5.3.1.5   Packet send sequence number
                Bits 4, 3 and 2 of octet 3, or bits 8 through 2 of octet 3  when  extended,
          are used for indicating the packet send sequence  number  P(S).  P(S)  is  binary
          coded and bit 2 is the low order bit.
          5.3.1.6   User data field
                Bits following octet 3, or octet 4 when extended, contain user data.
                Note - Some networks require the user data field  to  contain  an  integral
          number of octets (see the note in S 3).
          5.3.2  DTE and DCE interrupt packets
                Figure 11/X.25  illustrates  the  format  of  the  DTE  and  DCE  interrupt
          packets.

                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)         Logical channel group number
             2                                Logical channel number
             3    






































          PAGE70  Fascicle VIII.8 - X.25

                                              Packet type identifier
                     0          0          1          0          0          0          1          1
           4                                  Interrupt user data
          Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                               FIGURE 11/X.25
                                     DTE and DCE interrupt packet format































































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.3.2.1   Interrupt user data field
                Octet 4 and any following octets contain  the  interrupt  user  data.  This
          field may contain from 1 to 32 octets.
                Note - Some networks require the interrupt user data field  to  contain  an
          integral number of octets (see the note in S 3).
          5.3.3  DTE and DCE interrupt confirmation packets
                Figure 12/X.25  illustrates  the  format  of  the  DTE  and  DCE  interrupt
          confirmation packets.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)         Logical channel group number
             2                                Logical channel number
             3                                 Packet type identifier
                      0          0         1      





















































          PAGE70  Fascicle VIII.8 - X.25

                                                      0          0          1          1          1
          Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                               FIGURE 12/X.25
                              DTE and DCE interrupt confirmation packet format

































































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.4    Flow control and reset packets
          5.4.1  DTE and DCE receive ready (RR) packets
                Figure 13/X.25 illustrates the format of the DTE and DCE RR packets.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number
                      0          0          0          1                                       




























































          PAGE70  Fascicle VIII.8 - X.25

             2                                Logical channel number
            3                P(R)                  0          0          0          0          1
                                                    (Modulo 8)
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number





























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                     0          0          1          0                                       
            2                                Logical channel number
           3                                Packet type identifier
                     0          0          0          0          0          0          0          1
           4    P(R)                                                                       0
                 (When extended to modulo 128)                                          
                                               FIGURE 13/X.25
                                        DTE and DCE RR packet format





























































         PAGE70  Fascicle VIII.8 - X.25

                 5.4.1.1   Packet receive sequence number
                Bits 8, 7 and 6 of octet 3, or bits 8 through 2 of octet 4  when  extended,
          are used for indicating the packet receive sequence number P(R). P(R)  is  binary
          coded and bit 6, or bit 2 when extended, is the low order bit.
          5.4.2  DTE and DCE receive not ready (RNR) packets
                Figure 14/X.25 illustrates the format of the DTE and DCE RNR packets.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number
                      0          0          0          1                                       

























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                                               
             2                                Logical channel number
            3                P(R)                  0          0          1          0          1
                                                    (Modulo 8)
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier        




























































          PAGE70  Fascicle VIII.8 - X.25

                                                                Logical channel group number
                     0          0          1          0                                       
            2                                Logical channel number
                                             Packet type identifier
           3        0          0          0          0          0          1          0          1
           4                                    P(R)                                      0
                                           (When extended to modulo 8)
                                               FIGURE 14/X.25
                                        DTE and DCE RNR packet format




























































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.4.2.1   Packet receive sequence number
                Bits 8, 7 and 6 of octet 3, or bits 8 through 2 of octet 4  when  extended,
          are used for indicating the packet receive sequence number P(R). P(R)  is  binary
          coded and bit 6, or bit 2 when extended, is the low order bit.
          5.4.3  Reset request and reset indication packets
                Figure 15/X.25 illustrates the  format  of  the  reset  request  and  reset
          indication packets.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1           General format identifier              Logical channel group number
            2                                Logical channel number
            3                                Packet type identifier
                      0          0          0      






















































          PAGE70  Fascicle VIII.8 - X.25

                                                       1          1          0          1          1
            4                                    Resetting cause
            5                                  Diagnostic code a)
           a)      This field is not mandatory in reset request packets.
          Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                                FIGURE 15/X.25
                               Reset request and reset indication packet format
          5.4.3.1   Resetting cause field
                Octet 4 is the resetting cause  field  and  contains  the  reason  for  the
          reset.
                In reset request packets, the resetting cause field should be  set  by  the
          DTE to one of the following values:
                                         bits:      8 7 6 5 4 3 2 1
                                           value: 0 0 0 0 0 0 0 0
                                             or: 1 X X X X X X X
          where each X may be independently set to 0 or 1 by the DTE.
                The DCE will prevent values of the resetting cause field, other than  those
          shown above, from reaching the other end of the virtual call or permanent virtual
          circuit by either accepting the reset request packet and  forcing  the  resetting
          cause field to all  zeros  in  the  corresponding  reset  indication  packet,  or
          considering the reset request as an error and following the  procedure  described
          in Annex C.
                The coding of the resetting cause field in a  reset  indication  packet  is
          given in Table 21/X.25.
                                                 TABLE 21/X.25
                          Coding of resetting cause field in reset indication packet
                                                            Bits
                                           8     7     6     5     4     3     2     1
           DTE originated                   0     0     0     0   








































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                    0     0     0     0
           DTE originated a)                 1     X     X     X     X     X     X     X
           Out or order b)                   0     0     0     0     0     0     0     1
           Remote procedure error           0     0     0     0     0     0     1     1
           Local procedure error            0   
































































          PAGE70  Fascicle VIII.8 - X.25

                                                  0     0     0     0     1     0     1
           Network congestion               0     0     0     0     0     1     1     1
           Remote DTE operational b)         0     0     0     0     1     0     0     1
           Network operational b)            0     0     0     0     1     1     1   

































































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                                     1
          Incompatible destination         0     0     0     1     0     0     0     1
          Network out of order b)           0     0     0     1     1     1     0     1
                 a)  When bit 8 is set to 1, the bits represented by Xs are those indicated 
                by the remote DTE in the resetting cause field (virtual calls and 
                permanent virtual circuits) or the restarting cause field (permanent 
                virtual circuits only) of the reset or restart request packet, 
                respectively.
                b)  Applicable to permanent virtual circuits only.
         5.4.3.2   Diagnostic code
               Octet 5 is the diagnostic code and contains additional information  on  the
         reason for the reset.
               In a reset request packet the diagnostic code is not mandatory.
               In a reset indication packet, if the resetting cause field  indicates  "DTE
         originated", the diagnostic code has been passed  unchanged  from  the  resetting
         DTE. If the DTE requesting a reset has not provided  a  diagnostic  code  in  its
         reset request packet, then the bits of the diagnostic code in the resulting reset
         indication packet will all be zeros.
               When a reset indication packet results from a restart request  packet,  the
         value of the diagnostic code will  be  that  specified  in  the  restart  request
         packet, or all zeros in the case where no diagnostic code has been  specified  in
         the restart request packet.
               When the resetting cause field does  not  indicate  "DTE  originated",  the
         diagnostic code in a reset indication packet is network generated. Annex E  lists
         the codings for network generated diagnostics. The bits of  the  diagnostic  code
         are all set to 0 when  no  specific  additional  information  for  the  reset  is
         supplied.
               Note - The contents of the diagnostic code field do not alter  the  meaning
         of the cause field. A DTE is not required to undertake any action on the contents
         of the diagnostic code field. Unspecified code  combinations  in  the  diagnostic
         code field shall not cause the DTE to not accept the cause field.






































         PAGE70  Fascicle VIII.8 - X.25

                 5.4.4  DTE and DCE reset confirmation packets
                Figure  16/X.25  illustrates  the  format  of  the  DTE   and   DCE   reset
          confirmation packets.
                                                   Bits
                       8        7         6         5         4         3         2         1
           Octets                                                                             
              1        General format identifier (see           Logical channel group number
                                    Note)                 
              2                                Logical channel number
              3                                Packet type identifier
                        0        0         0          1      


























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                  1          1          1          1
          Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                                FIGURE 16/X.25
                                 DTE and DCE reste confirmation packet format
          5.5    Restart packets
          5.5.1  Restart request and restart indication packets
                Figure 17/X.25 illustrates the format of the restart  request  and  restart
          indication packets.
                                                       Bits
                  
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)       0          0          0      























































          PAGE70  Fascicle VIII.8 - X.25

                                                                                               0
           2        0          0          0          0          0          0          0          0
            3                                 Packet type identifier
                     1          1          1          1          1          0          1          1
            4                                   Restarting cause
            5                                   Diagnostic code a)
          a)      This field is not mandatory in restart request packets.
         Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                               FIGURE 17/X.25
                            Restart request and restart indication packet format



























































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.5.1.1   Restarting cause field
                Octet 4 is the restarting cause field  and  contains  the  reason  for  the
          restart.
                In restart request packets, the restarting cause field  should  be  set  by
          the DTE to one of the following values:
                                           bits:  8 7 6 5 4 3 2 1
                                           value: 0 0 0 0 0 0 0 0
                                             or: 1 X X X X X X X
          where each X may be independently set to 0 or 1 by the DTE.
                The DCE will prevent values of  the  restarting  cause  field,  other  than
          those shown above, from reaching the  other  end  of  the  virtual  calls  and/or
          permanent virtual circuits by either accepting the  restart  request  packet  and
          forcing the clearing or resetting cause field to all zeros in  the  corresponding
          clear and/or reset indication packets, or considering the restart request  as  an
          error and following the procedure described in Annex C.
                The coding of the restarting cause field in the restart indication  packets
          is given in Table 22/X.25.
                                                 TABLE 22/X.25
                         Coding of restarting cause field in restart indication packet
                                                            Bits
                                           8     7     6     5     4     3     2     1
           Local procedure error            0     0     0     0     0     0     0     1
           Network congestion               0     0     0     0     0   














































          PAGE70  Fascicle VIII.8 - X.25

                                                                         0     1     1
          Network operational              0     0     0     0     0     1     1     1
          Registration/cancellation        0     1     1     1     1     1     1     1
         confirmed a)                     
                 a)  May be received only if the optional on-line facility registration 
                facility is used.
         5.5.1.2   Diagnostic code
               Octet 5 is the diagnostic code and contains additional information  on  the
         reason for the restart.
               In a restart request packet, the diagnostic  code  is  not  mandatory.  The
         diagnostic code, if specified,  is  passed  to  the  corresponding  DTEs  as  the
         diagnostic code of a reset indication packet for permanent virtual circuits or  a
         clear indication packet for virtual calls.
               The coding of the diagnostic code field in a restart indication  packet  is
         given in Annex E. The bits of the diagnostic code are all set  to  zero  when  no
         specific additional information for the restart is supplied.
               Note - The contents of the diagnostic code field do not alter  the  meaning
         of the cause field. A DTE is not required to undertake any action on the contents
         of the diagnostic code field. Unspecified code  combinations  in  the  diagnostic
         code field shall not cause the DTE to not accept the cause field.

















































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.5.2  DTE and DCE restart confirmation packets
                Figure  18/X.25  illustrates  the  format  of  the  DTE  and  DCE   restart
          confirmation packets.
                                                    Bits
                      8          7         6         5         4         3         2         1
          Octet                                                                               
            s    
             1      General format identifier (see Note)       0          0          0          0
             2        0         0         0         0         0     




























































          PAGE70  Fascicle VIII.8 - X.25

                                                                             0         0         0
             3                                 Packet type identifier
                      1         1         1         1         1         1         1         1
           Note - Coded 0001 (modulo 8) or 0010 (modulo 128).
                                                FIGURE 18/X.25
                                DTE and DCE restart confirmation packet format
          5.6    Diagnostic packet
                Figure 19/X.25 illustrates the format of the diagnostic packet.
                                                   Bits
                      8          7         6         5         4         3         2         1
          Octet                                                         
            s    

























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                                                               
             1      General format identifier (see Note        0         0         0         0
                                    1)                    
             2        0         0         0         0         0         0         0         0
             3                                 Packet type identifier
                      1         1         1         1         0         0         0         1
             4                                    Diagnostic code
             5                          Diagnostic explanation (see Note 2)





























































          PAGE70  Fascicle VIII.8 - X.25

         Note 1 - Coded 0001 (modulo 8) or 0010 (modulo 128).
         Note 2 - The figure is drawn assuming the diagnostic explanation field is an integral 
         number of octets in length.
                                               FIGURE 19/X.25
                                          Diagnostic packet format
































































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.6.1  Diagnostic code field
                Octet 4 is the diagnostic  code  and  contains  information  on  the  error
          condition which resulted in the transmission of the diagnostic packet. The coding
          of the diagnostic code field is given in Annex E.
          5.6.2  Diagnostic explanation field
                When the diagnostic packet is issued as a result of  the  reception  of  an
          erroneous packet from the DTE (see Tables  C-1/X.25  and  C-2/X.25),  this  field
          contains the first three octets of header  information  from  the  erroneous  DTE
          packet. If the packet contains less than 3 octets, this field  contains  whatever
          bits were received.
                When the diagnostic packet is issued as a result of  a  DCE  time-out  (see
          Table D-1/X.25), the diagnostic explanation field  contains  2  octets  coded  as
          follows:
                 -   bits 8, 7, 6 and 5 of the  first  octet  contain  the  general  format
                     identifier for the interface;
                 -   bits 4 to 1 of the first octet and bits 8 to 1 of the second octet are
                     all 0 for expiration of time-out T10 and give the number of the logical
                     channel on which the time-out occurred for expiration of  time-out  T12
                     or T13.
          5.7    Packets required for optional user facilities
          5.7.1  DTE reject (REJ) packet for the packet retransmission facility
                Figure 20/X.25 illustrates the format  of  the  DTE  REJ  packet,  used  in
          conjunction with the packet retransmission facility described in S 6.4.
                                                    Bits
                         8          7         6         5         4         3         2         1
            Octets                                                                                
              1            General format identifier               Logical channel group number
                         0         0         0     









































          PAGE70  Fascicle VIII.8 - X.25

                                                          1                                      
              2                                 Logical channel number
              3                                                    Packet type identifier
                                  P(R)                  0          1          0          0         1
                     (Modulo                                                                      
                     8)         
                                                    Bits
                         8          7         6     





























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                          5         4         3         2         1
           Octets                                                                                
              1            General format identifier             Logical channel group number
                         0         0         1         0                                      
              2                                 Logical channel number
              3                                  Packet type identifier
                         0         0     






























































          PAGE70  Fascicle VIII.8 - X.25

                                              0         0         1         0         0         1
             4                                      P(R)                                      0
                                             (When extended to modulo 128)
                                               FIGURE 20/X.25
                                            DTE REJ packet format
































































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.7.1.1   Packet receive sequence number
                Bits 8, 7 and 6 of octet 3, or bits 8 through 2 of octet 4  when  extended,
          are used for indicating the packet receive sequence number P(R). P(R)  is  binary
          coded and bit 6, or bit 2 when extended, is the low order bit.
          5.7.2  Registration packets for the on-li e  facility  registration
                 facility
          5.7.2.1   Registration request packet
                Figure 21/X.25 illustrates the format of the registrat e s t 
          packet.
                                                    Bits
                         8          7         6         5         4         3         2         1
           Octets                                                                                
              1     General format identifier (see Note 1)      0         0         0         0
              2         0         0         0     























































          PAGE70  Fascicle VIII.8 - X.25

                                                          0         0         0         0         0
              3                                  Packet type identifier
                         1         1         1         1         0         0         1         1
              4                DTE address length                       DCE address length
                                           DCE and DTE address(es) (see Note 2)
                                                                     0          0         0         0
                         0                              Registration length






























































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                     Registration
          Note 1 - Coded 0001 (modulo 8) or 0010 (modulo 128).
         Note 2 - The figure is drawn assuming the total number of address digits present is odd.
                                               FIGURE 21/X.25
                                     Registration request packet format
         5.7.2.1.1 Address length fields
               Octet 4 consists of the  field  length  indicators  for  the  DTE  and  DCE
         addresses. Bits 4, 3, 2  and  1  indicate  the  length  of  the  DCE  address  in
         semi-octets. Bits 8, 7, 6 and 5  indicate  the  length  of  the  DTE  address  in
         semi-octets. Each address length indicator is binary coded and bit 1 or 5 is  the
         low order bit of the indicator.
               These fields are  coded  with  all  zeros  under  the  procedures  in  this
         Recommendation.
         5.7.2.1.2 Address field
               Octet 5 and the following octets consist of the DCE address, when  present,
         and the DTE address, when present.
               Each digit of an address is coded in a semi-octet in binary  coded  decimal
         with bit 5 or 1 being the low order bit of the digit.



















































         PAGE70  Fascicle VIII.8 - X.25

                Starting from the high order digit, the address is coded  in  octet  5  and
          consecutive octets with two digits per octet. In each  octet,  the  higher  order
          digit is coded in bits 8, 7, 6 and 5.
                The address field shall be rounded up to an integral number  of  octets  by
          inserting zeros in bits 4, 3, 2 and 1  of  the  last  octet  of  the  field  when
          necessary.
                This field is not present under the procedures in this Recommendation.
          5.7.2.1.3 Registration length field
                The  octet  following  the  address  field  indicates  the  length  of  the
          registration field in octets. The registration length indicator is  binary  coded
          and bit 1 is the low order bit of the indicator.
          5.7.2.1.4 Registration field
                The registration field is present only when the DTE wishes to  request  the
          DCE to agree to, or to stop a previous agreement for, an optional user facility.
                The coding of the registration field is defined in S 7.3.
                The registration field contains an integral number of  octets.  The  actual
          maximum length of this field depends on the network. However, this  maximum  does
          not exceed 109 octets.
          5.7.2.2   Registration confirmation packet
                Figure 18/X.25 illustrates the  format  of  the  registration  confirmation
          packet.
                                                    Bits
                         8          7         6         5         4         3         2         1
           Octets                                                                                
              1     General format identifier (see Note 1)      0         0         0         0
              2         0         0         0     











































                                                        Fascicle VIII.2 - Rec. X.25  PAGE93

                                                          0         0         0         0         0
              3                                  Packet type identifier
                         1         1         1         1         0         0         1         1
                                                          Cause
                                                       Diagnostic
              4                DTE address length                       DCE address length
                                           DCE and DTE address(es) (see Note 2)
                                                                     0          0         0     





























































          PAGE70  Fascicle VIII.8 - X.25

                                                                                                     0
                        0                              Registration length
                                                     Registration
          Note 1 - Coded 0001 (modulo 8) or 0010 (modulo 128).
         Note 2 - The figure is drawn assuming the total number of address digits present is odd.
                                               FIGURE 22/X.25
                                   Registration confirmation packet format






























































                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

                 5.7.2.2.1 Cause field
                Octet 4 is the cause field  and  contains  the  cause  of  any  failure  in
          negotiation of facilities or  an  indication  that  the  registration  field  was
          verified by the DCE.
                The coding of the cause field in the registration  confirmation  packet  is
          shown in Table 23/X.25.
                                                 TABLE 23/X.25
                           Coding of cause field in registration confirmation packet
                                                            Bits
                                           8     7     6     5     4     3     2     1
           Registration/cancellation confirmed  0     1     1     1     1     1     1     1
           Invalid facility request         0     0     0     0     0     0     1     1
           Local procedure error          
























































          PAGE70  Fascicle VIII.8 - X.25

                                           0     0     0     1     0     0     1     1
          Network congestion               0     0     0     0     0     1     0     1
               5.7.2.2.2 Diagnostic code
               Octet 5 is the diagnostic code and contains additional information  on  the
         reason for failure of facilities negotiation.
               Annex E lists the coding for diagnostics. The bits of the  diagnostic  code
         are all  set  to  0  when  negotiation  is  successful,  or  when  no  additional
         information is supplied.
         5.7.2.2.3 Address length fields
               Octet 6 consists of the  field  length  indicators  for  the  DTE  and  DCE
         addresses. Bits 4, 3, 2  and  1  indicate  the  length  of  the  DCE  address  in
         semi-octets. Bits 8, 7, 6 and 5  indicate  the  length  of  the  DTE  address  in
         semi-octets. Each address length indicator is binary coded and bit 1 or 5 is  the
         low order bit of the indicator.
               These fields are  coded  with  all  zeros  under  the  procedures  in  this
         Recommendation.
         5.7.2.2.4 Address field
               Octet 7 and the following octets consist of the DCE address, when  present,
         and the DTE address, when present.
               Each digit of an address is coded in a semi-octet in binary  coded  decimal
         with bit 5 or 1 being the low order bit of the digit.
               Starting from the high order digit, the address is coded  in  octet  7  and
         consecutive octets with two digits per octet. In each  octet,  the  higher  order
         digit is coded in bits 8, 7, 6 and 5.
               The address field shall be rounded up to an integral number  of  octets  by
         inserting zeros in bits 4, 3, 2 and 1  of  the  last  octet  of  the  field  when
         necessary.
               This field is not present under the procedures in this Recommendation.
         5.7.2.2.5 Registration length field
               The  octet  following  the  address  field  indicates  the  length  of  the
         registration field, in octets. The registration length indicator is binary  coded
         and bit 1 is the low order bit of the indicator.
         5.7.2.2.6 Registration field
               The registration field is used to indicate which optional  user  facilities
         are available, and which are currently in effect.
               The coding of the registration field is defined in S 7.3.
               The registration field contains an integral number of  octets.  The  actual
         maximum length of this field depends on the network. However, this  maximum  does
         not exceed 109 octets.
         6      Procedures for optional user facilities (packet layer)
         6.1    On-line facility registration
               On-line facility registration is an optional user  facility  agreed  for  a
         period of time. This facility, if subscribed to, permits the DTE at any  time  to
         request registration of facilities, or obtain current  values  of  facilities  as
         understood  by  the  DCE,  by  transferring  across  the  DTE/DCE   interface   a
         registration request packet.
               The DCE will, in response to a  registration  packet,  report  the  current
         value of all facilities applicable to the DTE/DCE interface,  by  transferring  a
         registration  confirmation  packet  across  the   DTE/DCE   interface.   Optional
         facilities which are not offered by the network  will  not  be  reported  in  the
         registration confirmation packet. To avoid requesting  facilities  that  are  not
         available in a particular network, or values that are not allowed,  the  DTE  may
         transfer a registration request packet across the DTE/DCE interface containing no
         optional facilities. It may then modify any negotiable facilities reported in the
         corresponding  registration  confirmation  packet  by   transferring   a   second
         registration request packet across the DTE/DCE interface.
               When the DCE returns the registration confirmation packet,  the  facilities
         values shown are in effect for any subsequent virtual calls. The  values  of  the
         extended packet sequence numbering, packet retransmission, and D bit modification
         facilities and the allocation of logical channel type ranges can be modified only
         when there are no virtual calls (i.e., all  logical  channels  used  for  virtual
         calls are in state p1). When these facilities take effect and when there  is  one
         or more logical channels assigned to  permanent  virtual  circuits,  the  network
         restarts the interface with the cause "Registration/cancellation  confirmed"  and
         the diagnostic "No additional information" in order to change the values  of  the




                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

         permanent virtual circuits at the interface. At the remote end of each  permanent
         virtual circuit, the corresponding reset indication packet is sent with the cause
         "Remote DTE operational" and the diagnostic "No additional information".
               If a requested value of a particular  facility  is  not  allowed,  the  DCE
         shall report in the registration confirmation packet:
               a)  if the facility has a boolean value, the value allowed;
               b)  if the value is  greater  than  the  maximum  allowed  value  of  that
                  facility, the maximum allowed value; or
               c)  if the value is less than the minimum allowed value of that  facility,
                  the minimum allowed value.
               The registration confirmation packet  shall  also  contain  an  appropriate
         cause code. The DTE may choose to accept the value reported  by  the  DCE  or  to
         attempt to negotiate another value for the requested facilities.
               If the DCE cannot make all the modifications requested  in  a  registration
         request packet, it will not alter the values of some facilities. Circumstances in
         which the DCE can not make all of the modifications requested include:
               a)  conflict in facilities settings, and
               b)  when the interface has at least  one  virtual  call  established  when
                  attempting to negotiate those facilities that require all virtual  call
                  logical channels to be in state  p1  (including  the  collision  of  an
                  incoming call packet and a registration request packet).
               The DTE  should  wait  for  the  registration  confirmation  packet  before
         sending a call request packet,  or  sending  a  packet  on  a  permanent  virtual
         circuit.
               For every optional user facility, Annex F indicates:
               -   if the value of the facility may be negotiated;
               -   if the registration confirmation packets indicate whether or  not  the
                  facility is supported by the DCE;
               -   if the value of the facility may be altered by the DTE either only when 
                  every logical channel used for virtual calls is in state p1, or in  any
                  packet layer state.
               Indication in registration confirmation packet of whether the NUI  override
         facility is supported by the network is for further study.
               A fault condition within the network may affect the  facilities  previously
         negotiated by means of registration packets. In this situation, the DCE initiates
         the restart procedure to inform the DTE of the failure.
               A restart procedure initiated by the DTE does  not  affect  the  facilities
         values. When the DCE initiates  the  restart  procedure  with  the  cause  "Local
         procedure error", the facilities values are not affected. When the DCE  initiates
         the  restart  procedure  with  the  cause  "Network   congestion"   or   "Network
         operational", the values of facilities previously  negotiated  may  be  affected.
         When   the   DCE   initiates   the   restart    procedure    with    the    cause
         "Registration/cancellation confirmed", the facilities values are as  set  by  the
         related registration procedure.
         6.2    Extended packet sequence numbering
               Extended packet sequence numbering is an optional user facility agreed  for
         a period of time. It is common to all logical channels at the DTE/DCE interface.
               This user facility,  if  subscribed  to,  provides  sequence  numbering  of
         packets performed modulo 128. In the  absence  of  this  facility,  the  sequence
         numbering of packets is performed modulo 8.
         6.3    D bit modification
               D bit modification is an optional user facility  agreed  for  a  period  of
         time. This facility applies to all virtual calls and permanent  virtual  circuits
         at the DTE/DCE interface. This facility is only intended for use  by  those  DTEs
         implemented prior to the introduction of the D bit procedure which were  designed
         for operation on public data networks that support end-to-end P(R)  significance.
         It allows these DTEs to continue to operate  with  end-to-end  P(R)  significance
         within a national network.
               For  communication  within  the  national  network,  this  facility,   when
         subscribed to:
               a)  will change from 0 to 1 the value of bit 7 of  the  GFI  in  all  call
                  request and call accepted packets and the value of the D bit in all DTE
                  data packets received from the DTE, and
               b)  will set to 0 the value of bit 7 of the GFI in all incoming  call  and
                  call connected packets, and the value of the D  bit  in  all  DCE  data




         PAGE70  Fascicle VIII.8 - X.25

               packets transmitted to the DTE.
               For international operation, conversion b) above applies and conversion  a)
         above does not apply. Other conversion rules for international operation are  for
         bilateral agreement between Administrations.
         6.4    Packet retransmission
               Packet retransmission is an optional user facility agreed for a  period  of
         time. It is common to all logical channels at the DTE/DCE interface.
               This  user  facility,  if  subscribed  to,  allows   a   DTE   to   request
         retransmission of one or several consecutive DCE data packets  from  the  DCE  by
         transferring across the DTE/DCE  interface  a  DTE  reject  packet  specifying  a
         logical channel number and a sequence number P(R). The value of this P(R)  should
         be within the range from the last P(R)  received  by  the  DCE  up  to,  but  not
         including, the P(S) of the next DCE data packet to be transmitted by the DCE.  If
         the P(R) is outside this range, the DCE will initiate the  reset  procedure  with
         the cause "Local procedure error" and diagnostic ## 2.
               When receiving a DTE reject packet, the  DCE  initiates  on  the  specified
         logical channel retransmission of the DCE data packets, the packet send  sequence
         numbers of which are starting from P(R), where  P(R)  is  indicated  in  the  DTE
         reject packet. Until the DCE transfers across the DTE/DCE interface  a  DCE  data
         packet with a packet send sequence number equal to the P(R) indicated in the  DTE
         reject packet, the DCE will consider the receipt of another DTE reject packet  as
         a procedure error and reset the logical channel.
               Additional DCE data packets pending initial  transmission  may  follow  the
         retransmitted packet(s).
               A DTE receive not ready situation indicated by the transmission of  an  RNR
         packet is cleared by the transmission of a DTE reject packet.
               The conditions under  which  the  DCE  ignores  a  DTE  reject  packet,  or
         considers it as a procedure error, are those described for flow  control  packets
         (see Annex C).
         6.5    Incoming calls barred
               Incoming calls barred is an optional user facility agreed for a  period  of
         time. This facility applies to all logical channels used at the DTE/DCE interface
         for virtual calls.
               This user facility, if subscribed to, prevents incoming virtual calls  from
         being presented to the DTE. The DTE may originate outgoing virtual calls.
               Note 1 - Logical channels used for virtual calls retain their  full  duplex
         capability.
               Note 2 - Some Administrations  may  provide  a  capability  that  allows  a
         virtual call to be presented to the DTE  only  in  cases  where  the  called  DTE
         address is the address of the calling DTE.
         6.6    Outgoing calls barred
               Outgoing calls barred is an optional user facility agreed for a  period  of
         time. This facility applies to all logical channels used at the DTE/DCE interface
         for virtual calls.
               This user facility, if subscribed  to,  prevents  the  DCE  from  accepting
         outgoing virtual calls from the DTE. The DTE may receive incoming virtual calls.
               Note - Logical channels used for virtual calls  retain  their  full  duplex
         capability.
         6.7    One-way logical channel outgoing
               One-way logical channel outgoing is an optional user facility agreed for  a
         period of time. This user facility,  if  subscribed  to,  restricts  the  logical
         channel use to originating outgoing virtual calls only.
               Note - A logical channel used for virtual calls  retains  its  full  duplex
         capability.
               The rules according to which logical  channel  group  numbers  and  logical
         channel numbers can be assigned to one-way outgoing logical channels for  virtual
         calls are given in Annex A.
               Note - If all the logical channels for virtual calls are  one-way  outgoing
         at a DTE/DCE interface, the effect is equivalent to  the  incoming  calls  barred
         facility (see S 6.5, particularly Note 2).
         6.8    One-way logical channel incoming
               One-way logical channel incoming is an optional user facility agreed for  a
         period of time. This user facility,  if  subscribed  to,  restricts  the  logical
         channel use to receiving incoming virtual calls only.
               Note - A logical channel used for virtual calls  retains  its  full  duplex




                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

         capability.
               The rules according to which logical  channel  group  numbers  and  logical
         channel numbers can be assigned to one-way incoming logical channels for  virtual
         calls are given in Annex A.
               Note - If all the logical channels for virtual calls are  one-way  incoming
         at a DTE/DCE interface, the effect is equivalent to  the  outgoing  calls  barred
         facility (see S 6.6).
         6.9    Non-standard default packet sizes
               Non-standard default packet sizes is an optional user facility  agreed  for
         a period of time. This facility, if subscribed to, provides for the selection  of
         default  packet  sizes  from  the  list  of  packet  sizes   supported   by   the
         Administration. Some networks may constrain the packet sizes to be the  same  for
         each direction of data transmission across the DTE/DCE interface. In the  absence
         of this facility, the default packet sizes are 128 octets.
               Note - In this section, the term "packet sizes" refers to the maximum  user
         data field lengths ofDCE data and DTE data packets.
               Values other than the default packet sizes may be negotiated for a  virtual
         call by means of the flow control parameter negotiation facility  (see  S  6.12).
         Values other than the default packet sizes may be agreed for a period of time for
         each permanent virtual circuit.
         6.10   Non-standard default window sizes
               Non-standard default window sizes is an optional user facility  agreed  for
         a period of time. This facility, if subscribed to, provides for the selection  of
         default  window  sizes  from  the  list  of  window  sizes   supported   by   the
         Administration. Some networks may constrain the default window sizes  to  be  the
         same for each direction of data transmission across the DTE/DCE interface. In the
         absence of this facility, the default windonw sizes are 2.
               Values other than the default window sizes may be negotiated for a  virtual
         call by means of the flow control parameter  negotiation  facility  (see  S  12).
         Values other than the default window sizes may be agreed for a period of time for
         each permanent virtual circuit.
         6.11   Default throughput classes assignment
               Default throughput classes assignment is an optional user  facility  agreed
         for a period of time. This facility, if subscribed to, provides for the selection
         of default throughput classes from the list of throughput  classes  supported  by
         the Administration. Some networks may constrain the default throughput classes to
         be the same for each direction of data  transmission.  In  the  absence  of  this
         facility, the default throughput classes correspond to the user class of  service
         of the DTE (see S 7.2.2.2)  but  do  not  exceed  the  maximum  throughput  class
         supported by the network.
               The default throughput classes are the  maximum  throughput  classes  which
         may be associated with any virtual call at the DTE/DCE  interface.  Values  other
         than the default throughput classes may be negotiated for a virtual call by means
         of the throughput class negotiation facility (see S 6.13). Values other than  the
         default throughput classes may be agreed for a period of time for each  permanent
         virtual circuit.
               Note - Throughput characteristics and throughput class are described  in  S
         4.4.2.
         6.12   Flow control parameter negotiation
               Flow control parameter negotiation is an optional user facility agreed  for
         a period of time which can be used by a DTE for virtual calls. This facility,  if
         subscribed to, permits negotiation on a  per  call  basis  of  the  flow  control
         parameters. The flow control parameters considered  are  the  packet  and  window
         sizes at the DTE/DCE interface for each direction of data transmission.
               Note - In this section, the term "packet sizes" refers to the maximum  user
         data field lengths of DCE data and DTE data packets.
               In the absence of the flow  control  parameter  negotiation  facility,  the
         flow control parameters to be used at a  particular  DTE/DCE  interface  are  the
         default packet sizes (see S 6.9) and the default window sizes (see S 6.10).
               When  the  calling  DTE  has  subscribed  to  the  flow  control  parameter
         negotiation facility, it may request packet sizes and/or window  sizes  for  both
         direction of data transmission (see SS 7.2.1 and 7.2.2.1). If  particular  window
         sizes are not explicitly requested in acall request packet, the DCE  will  assume
         that the default  window  sizes  were  requested  for  both  directions  of  data
         transmission. If particular packet sizes are not explicitly  requested,  the  DCE




         PAGE70  Fascicle VIII.8 - X.25

         will assume that the default packet sizes were requested for both  directions  of
         data transmission.
               When a called DTE has subscribed to the flow control parameter  negotiation
         facility, each incoming call packet will indicate the  packet  and  window  sizes
         from which DTE negotiation can start. No relationship needs to exist between  the
         packet sizes (P) and window sizes (W) requested in the call  request  packet  and
         those indicated in the incoming call packet. The called DTE  may  request  window
         and packet sizes with facility in  the  call  accepted  packet.  The  only  valid
         facility requests in the call accepted packet, as  a  function  of  the  facility
         indications in the incoming call packet, are  given  in  Table  24/X.25.  If  the
         facility request is not made in the call accepted packet, the DTE is  assumed  to
         have accepted the indicated values (regardless of the default  values)  for  both
         directions of data transmission.
                                                TABLE 24/X.25
          Valid facility requests in call accepted packets in response to facility indications in 
                                            incoming call packets
                     Facility indication                         Valid facility request
                        W(indicated)  2                        W(indicated)  W(requested)  2
                       W(indicated) = 1                           W(requested) = 1 or 2
                       P(indicated)  128                      P(indicated)  P(requested)  128
                      P(indicated) < 128                     128  P(requested)  P(indicated)
               When  the  calling  DTE  has  subscribed  to  the  flow  control  parameter
         negotiation facility, every call connected packet will indicate  the  packet  and
         window sizes to be used at the DTE/DCE interface for the  call.  The  only  valid
         facility indications in the call connected packet, as a function of the  facility
         requests in the call request packet, are given in Table 25/X.25.
                                                TABLE 25/X.25
          Valid facility indications in call connected packets in response to facility requests in 
                                            call request packets
                     Facility indication                         Valid facility request
                        W(requested)  2                        W(requested)  W(indicated)  2
                       W(requested) = 1                           W(indicated) = 1 or 2
                       P(requested)  128                      P(requested)  P(indicated)  128
                      P(requested) < 128                     128  P(indicated)  P(requested)
               The network may have constraints  requiring  the  flow  control  parameters
         used for a call to be modified before indicating them to the DTE in the  incoming
         call packet or call connected  packet;  e.g.,  the  ranges  of  parameter  values
         available on various networks may differ.
               Window and packet sizes need not be the same  at  each  end  of  a  virtual
         call.
               The role of the DCE in negotiating  the  flow  control  parameters  may  be
         network dependent.
         6.13   Throughput class negotiation
               Throughput class negotiation is an optional  user  facility  agreed  for  a
         period of time which can be used by a DTE for virtual calls.  This  facility,  if
         subscribed to, permits negotiation on a per call basis of the throughput classes.
         The throughput classes are considered independently for each  direction  of  data
         transmission.
               Default values are agreed between the DTE and  the  Administration  (see  S
         6.11). The default values correspond to the maximum throughput classes which  may
         be associated with any virtual call at the DTE/DCE interface.
               When the calling DTE has subscribed to  the  throughput  class  negotiation
         facility, it may request the throughput classes of the virtual call in  the  call
         request packet for both  directions  of  data  transmission  (see  SS  7.2.1  and
         7.2.2.2). If particular throughput classes are not explicitly requested, the  DCE
         will assume that the default values were requested for both  directions  of  data
         transmission.
               When a called DTE  has  subscribed  to  the  throughput  class  negotiation
         facility, each incoming call packet will indicate  the  throughput  classes  from
         which DTE negotiation may start. These throughput classes are lower or  equal  to
         the ones selected at the calling DTE/DCE  interface,  either  explicitly,  or  by
         default if the calling DTE has not 







                                                       Fascicle VIII.2 - Rec. X.25  PAGE93

         subscribed to  the  throughput  class  negotiation  facility  or  not  explicitly
         requested throughput class values in the call request  packet.  These  throughput
         classes indicated to the called DTE will also not  be  higher  than  the  default
         throughput classes, respectively for each direction of data transmission, at  the
         calling and the called DTE/DCE interfaces. They may  be  further  constrained  by
         internal limitations of the network.
               The called DTE may request with a facility  in  the  call  accepted  packet
         throughput classes that should finally apply to the virtual call. The only  valid
         throughput classes in the call accepted packet are lower than  or  equal  to  the
         ones (respectively) indicated in the incoming call packet. If the called DTE does
         not make any throughput class facility request in the call accepted  packet,  the
         throughput classes finally  applying  to  the  virtual  call  will  be  the  ones
         indicated in the incoming call packet.
               If the called DTE has not subscribed to the  throughput  class  negotiation
         facility, the throughput classes finally applying to the virtual  call  are  less
         than or equal to the ones selected at the calling  DTE/DCE  interface,  and  less
         than or equal to the default values defined at the called DTE/DCE interface.
               When the calling DTE has subscribed to  the  throughput  class  negotiation
         facility, every call  connected  packet  will  indicate  the  throughput  classes
         finally applying to the virtual call.
               When neither the calling DTE nor the  called  DTE  has  subscribed  to  the
         throughput class negotiation facility, the throughput  classes  applying  to  the
         virtual call will not be higher than the ones agreed as defaults at  the  calling
         and called DTE/DCE interfaces. They may be further constrained to lower values by
         the network, e.g., for international service.
               Note  1  -  Since  both  throughput  class  negotiation  and  flow  control
         parameter negotiation (see S 6.12) facilities can be applied to  a  single  call,
         the achievable throughput will depend on how users manipulate the D bit.
               Note 2 - Users are cautioned that the choice of  too  small  a  window  and
         packet size of a DTE/DCE interface (made by use of  the  flow  control  parameter
         negotiation facility may adversely affect the attainable throughput  class  of  a
         virtual call. This is likewise true of flow control mechanisms adopted by the DTE
         to control data transmission from the DCE.
         6.14   Closed user group related facilities
               A set of closed user group (CUG) optional user facilities enables users  to
         form groups of  DTEs  to  and/or  from  which  access  is  restricted.  Different
         combinations of access restrictions to and/or from DTEs having  one  or  more  of
         these facilities result in various combinations of accessibility.
               A DTE may belong to one or more CUGs. Each DTE belonging to  at  least  one
         CUG has either the closed user group facility (see S 6.14.1) or one  or  both  of
         the closed user group with  outgoing  access  and  the  closed  user  group  with
         incoming access facilities (see SS 6.14.2 and 6.14.3). For each CUG  to  which  a
         DTE belongs, either none of the incoming calls barred within a closed user  group
         or the outgoing calls barred within a closed user group facilities (see SS 6.14.4
         and 6.14.5) may apply for that DTE. Different combinations of CUG facilities  may
         apply for different DTEs belonging to the same CUG.
               When a DTE belonging to one or more CUGs places a  virtual  call,  the  DTE
         may explicitly indicate in the call request packet the CUG selected by using  the
         closed user group selection facility (see S 6.14.6) or the closed user group with
         outgoing access selection  facility  (see  S  6.14.7)  (see  Note).  When  a  DTE
         belonging to one or more CUGs receives a virtual call, the CUG  selected  may  be
         explicitly indicated in the incoming call packet through the use  of  the  closed
         user group selection facility or the  closed  user  group  with  outgoing  access
         selection facility.
               Note - For a given virtual call, only one of the above-mentioned  selection
         facilities can be present.
               The number of CUGs to which a DTE can belong is network dependent.
         6.14.1 Closed user group
               Closed user group is an optional user facility agreed for a period of  time
         for virtual calls. This user facility, if  subscribed  to,  enables  the  DTE  to
         belong to one or more closed user groups. A closed user group  permits  the  DTEs
         belonging to the group to communicate with each other but precludes communication
         with all other DTEs.
               When the DTE belongs to more than one closed  user  group,  a  preferential
         closed user group must be specified.




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