Annex 3 to AVC-743R

H.24X Protocol Stack

1.  Introduction
  Protocol stack for H.24X channel is considered taking the following points into account.
	- applicable to both multiple VC and single VC solution
	- alignment with H.24P protocol specification
	- commonality with T.120 series protocol

2.  SSCOP and SSCF
  AVC-721 suggested X.224 class 0 with SSCF and SSCOP. Considering a single VC solution, where H.24X channel is multiplexed in H.222.1 stream, SSCOP cannot be applied. SSCOP should be allocated directly onto the AAL5 CPCS, because they are tightly coupled. In our current understandings, H.222.1 will be mapped on either AAL1 or AAL5. Therefore, we need another data link control protocol instead of SSCOP in case of single VC.

3.  H.24P and T.120 series protocol
  H.24P uses V.42 (LAPM) protocol as underlying transport mechanism. On the other hand, SG8 has been already using LAPF (Q.922) as a lower layer protocol for T.120 series in N-ISDN environments. The LAPM has very similar functionality and characteristics to LAPF. Therefore, LAPF is considered to be suitable for our purpose.

4.  Necessity of X.224 class 0
  We further discussed the necessity of X.224 class 0 protocol just below H.24X. At first, X.214 service is agreed to be suitable for lower layer interface of H.24X, considering the applicability to various network environment without the change in H.24X specification.
  As pointed out in AVC-721, we have concluded that H.24X protocol stack includes X.224 class 0 to provide X.214 service. The reasons are;
	- that T.120 protocol stack has both LAPF and X.224 class 0 in N-ISDN.
	- that the end-to-end protocol should be necessary, in case of signaling through non-ATM network with and without network gateway.
	- that it is desirable to be extendable to connection multiplexing and an application to other network environments for the future use.

5.  Proposed protocol stack
  Proposed protocol stack for single and multiple VC is shown in Fig. 1. The scope of H.32X type A2 and B2 terminals includes Fig.1 (a) and (b).
  For achieving the alignment between H.24X and H.24P, H.24X needs to have the specification to directly interface with LAPM. However, we recommend LBC Experts group to include X.224 class 0 between H.24P and LAPM in the interest of commonality with T.120 series protocol stack.

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