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From: harald.alvestrand@elab-runit.sintef.no
Newsgroups: comp.protocols.iso
Subject: Re: question about transport layer protocol
Keywords: X.21 is NOT Class B
Message-ID: <1991Jan22.214654.5942@ugle.unit.no>
Date: 22 Jan 91 21:46:54 GMT
References: <1991Jan18.220611.14357@bronze.ucs.indiana.edu> <42617@cos.com>
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Reply-To: harald.alvestrand@elab-runit.sintef.no
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This message has been on the net now for at least 1 day, and I have had no
counters..... 

In article <42617@cos.com>, howard@cos.com (Howard C. Berkowitz) writes:

|> 
|> >     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.
|>
 
No, I do not think so.
The definition you quoted, first above, is right.
A Class B network will detect any loss OR CORRUPTION of data. Otherwise
transport classes 1 and 3 would not work, since they contain no
checksums at all.

This means that an ISDN B-channel or X.21 channel is NOT Class B.
You might consider the same channel with HDLC added as Class B.

(BTW: I have encountered at least one bug where the X.25 network failed to
meet the Class B criteria...)


                   Harald Tveit Alvestrand
Harald.Alvestrand@elab-runit.sintef.no
C=no;PRMD=uninett;O=sintef;OU=elab-runit;S=alvestrand;G=harald
+47 7 59 70 94