Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!uunet!cos!howard
From: howard@cos.com (Howard C. Berkowitz)
Newsgroups: comp.protocols.iso
Subject: Re: question about transport layer protocol
Summary: Class A=X.25(1984) PDN, Class B=X.21 or ISDN CSDN
Message-ID: <42617@cos.com>
Date: 21 Jan 91 17:47:01 GMT
References: <1991Jan18.220611.14357@bronze.ucs.indiana.edu>
Reply-To: howard@cos.UUCP (Howard C. Berkowitz)
Organization: Corporation for Open Systems, McLean, VA
Lines: 53

In article <1991Jan18.220611.14357@bronze.ucs.indiana.edu> lizhen@silver.ucs.indiana.edu (Zhen Li) writes:
>     Class A:  networks that detect, as an error, any loss 
>	       of data.  These networks never duplicate, 
>	       re-order, or corrupt data. Furthermore, a 
>	       class A  network has a relatively small 
>	       probaility of actually losing data. Such a
>	       network provides the CONS.


      A Class A network, in practice, is provided by the typical Public
Data Network which uses X.25 interfaces.

      Note that not all X.25-interfaced networks provide the full CONS.
CONS is fully supported only by the 1984 version of CCITT X.25, which,
for DTE-DCE applications, is identical to ISO 8208 for the
packet level.

      In COS stack specifications for X.25-based services, we currently
expect 1984 version X.25.  Transitional 1980 versions are under 
consideration.

      If the network provides X.25-1980 interfaces, than the Subnetwork
Dependent Convergence Protocol must run on top of X.25 to provide the
CONS.  See ISO 8878 and appropriate implementors' agreements.

      An X.25 interface to a LAN could potentially provide a Class A
service.  I don't remember the ISO standard number for this, but think
it's 8880.

>     Class B:  like class A networks, class B networks detect, 
>	       as an error, any loss of data. However, such losses 
>	       are more common that the transport service would 
>	       prefer. Class B networks aren't necessarily unreliable,
>               it's just that they are less reliable than class A
>	       networks.  The distinction between the two classes is 
>	       decided locally. Class B provide CONS.
>


     A typical Class B service is found on European X.21-interfaced
Circuit Switched Data Networks (CSDN), on which the provision of
error correction (e.g., using LAP-B) is a user responsibility.

     A "raw" B-channel on ISDN would be a Class B service.  I suspect
a D channel would be considered a Class A service when passing packet
data, although I don't have a formal definition of this.

    
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