Proposed new Recommendation X.220

                  USE OF X.200 SERIES PROTOCOLS
                     IN CCITT MODIFICATIONS


       The CCITT,

considering

       (a)  that administrations in many countries are implementing a variety of 
telecommunications services;

       (b)  that these services may be carried on a variety of networks;

       (c)  that users of these services desire a unifying architecture for the 
applicable protocols;

       (d)  that such an architecture is provided by Recommendation X.200 which 
defines the Reference Model of Open Systems Interconnection for CCITT applications;

       (e)  that a number of protocols conforming to this architecture are defined 
in the X.200 Series of Recommendations and in other Recommendations,

unanimously declares the view

       that for CCITT applications the functional suites  of  protocols,  which
involve the use of protocols in the X.200 Series of Recommendations, are summarized 
in this Recommendation. Details, as well as any conformance  requirements,  are
contained in the relevant Recommendations.

       A growing number of data terminal equipments are being designed to support 
more than one CCITT service and/or are being designed to be  capable  of  being
connected to more than one type of network. In order to facilitate the design of 
such equipments, the various OSI protocol suites involving use of the X.200 Series 
of Recommendations are documented herein.

       These protocol suites are depicted in Figure 1/X.220, which portrays the 
protocols according to the seven layers defined in Recommendation X.200. The CCITT 
applications covered are Message Handling Systems (MHS), Directory, Teletex and 
Document Architecture Transfer and Manipulation. The networks covered are PSPDN, 
CSPDN, PSTN and ISDN. The intent is to give a general view of the set of protocol 
suites in a single figure, while relying on the other Recommendations referenced to 
provide the necessary additional details.

                            Notes to Figure 1/X.220


1. The modem may also be integrated within the terminal and in such cases V.24 need not 
   apply. For telematic terminals, see 3.2.1 of T.70.

2. For automatic calling and/or answering, V.25 or V.25bis may be applicable.

3. For terminals connected to a PSTN, CSPDN or ISDN (circuit switched) and accessing a 
   PSPDN in accordance with X.32 or X.31, the X.25 LAP B procedures are used as  set
   forth in X.32 or X.31.

4. For DTE-to-DTE connections, telematic terminals employ the X.75 LAP B procedures for 
   single link operation (see 3.2.2 and 3.3.2 of T.70 and 2.1.2.2 of T.90). For other 











terminals, the ISO 7776 LAP B procedures may apply for DTE-to-DTE connections.

5. For half duplex operation over the PSTN, the LAP B procedures are extended to include 
   a half duplex transmission module (HDTM) as defined in 5.6 of X.32 and in T.71.

6. Terminals obtaining packet access on the D-channel use the LAP D procedures of Q.921 
   to support both the Q.931 access connection control procedure (if needed) and the 
   X.25 packet layer procedures. Terminals obtaining packet access on the B-channel use 
   the LAP D procedures of Q.921 to support  the  Q.931  access  connection  control
   procedure (if needed) and the X.25 LAP B procedures to support the X.25 packet layer 
   procedures.

7. For terminals connected to a PSTN, CSPDN or ISDN (circuit switched) and accessing a 
   PSPDN in accordance with X.32 or X.31, the network connection is established by two 
   stage selection; the first stage uses the call control procedures of the attached 
   network (as shown in Figure 1/X.220) and the second stage uses the X.25 call control 
   procedures.

8. For terminals connected to a PSTN, CSPDN or ISDN (circuit switched) and accessing a 
   PSPDN in accordance with X.32 or X.31, the X.25 packet layer procedures apply during 
   the data transfer phase of the PSTN, CSPDN or ISDN. However, for telematic terminals 
   connected to a CSPDN and accessing a PSPDN, a minimum network layer functionality is 
   required during the data transfer phase of the CSPDN (see 3.3.3 of T.70).

9. For DTE-to-DTE connections, telematic terminals connected to a CSPDN use the minimum 
   network layer functionality (see 3.3.3 of T.70) during the data transfer phase of the 
   CSPDN and telematic terminals connected to a  PSTN  use  the  X.25  packet  layer
   procedures (see 3.2.3 of T.70); telematic terminals connected to an ISDN (circuit 
   switched) use the X.25 packet layer procedures as  specified  in  ISO  8208  (see
   2.1.2.3.2 of T.90) or, in addition, as a user option, the minimum  network  layer
   functionality (see 2.1.1 of T.90).  For other terminals, the ISO 8208 X.25 packet 
   layer procedures may apply for DTE-to-DTE connections.

10.The Q.931 access connection control procedures are used if needed.

11.For telematic terminals, the transport protocol is in accordance with T.70, section 5 
   plus Annexes A and B; the use of X.224 class 0 plus application rules is optional but 
   needs further consideration to ensure that there are no discrepancies with T.70. For 
   terminals communicating with network based services such as MHS and Directories, the 
   X.224 procedures apply including the mandatory support of class 0.

12.T.62 bis, together with the relevant service and protocol elements of  X.215  and
   X.225, are intended to be equal to T.62.

13.To obtain backward compatibility with X.410 (1984), RTSE uses the "X.410-1984 mode" 
   services of ACSE and the Presentation Layer. The "normal mode" is used in all other 
   cases.

14.Directory uses ROSE but not RTSE.

15.The use of ROSE and RTSE in the T.400-Series is for further study.

16.T.330 describes Group 4 Facsimile and Teletex access  to  the  MHS  Interpersonal
   Messaging System (IPMS) in the T.62bis/X.225 environment.

17.The use of MHS to transfer documents conforming to the T.410-Series is described in 
   T.411.

18.Character repertoire definition of T.61 only.