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From: dennis@gpu.utcs.toronto.edu (Dennis Ferguson)
Date: Tue, 3-Jan-89 16:59:08 EST
Message-ID: <1989Jan3.165908.7652@gpu.utcs.toronto.edu>
Organization: Mechanical Engineering, University of Toronto
Newsgroups: comp.dcom.lans
Subject: Re: Token Ring (was: Re: Info on LANs)
Summary: I don't think this is correct
References: <12786@cup.portal.com> <920001@hposdl.HP.COM> <10777@s.ms.uky.edu> <18659@agate.BERKELEY.EDU> <13096@bellcore.bellcore.com> <18672@agate.BERKELEY.EDU>
Reply-To: dennis@gpu.utcs.toronto.edu (Dennis Ferguson)

In article <18672@agate.BERKELEY.EDU> glass@tehran.berkeley.edu (Brett Glass) writes:
> One of the reasons a 4 Mbps Token Ring can outperform a 10 Mbps Ethernet
> (other than the lack of collisions I mentioned earlier) is the built-in
> acknowledgement provided by the trailer at the end of a returning frame.  On
> an Ethernet, one must send a packet to acknowledge receipt of a message --
> with all the delays inherent in setting up a buffer, waiting for the cable
> to clear, etc. This overhead can cut the net throughput of an Ethernet by
> more than 75% under any protocol requiring reliable data transport.

Unless I'm misreading something, I don't think this is true.  The bits
in the trailer don't constitute any sort of useful acknowledgement, other
than indicating there may have been some station out there willing to accept
the packet.  It would be silly for any protocol to rely on this as a reception
acknowledgement.  The protocol wouldn't work across bridges, where the bits
would only indicate that the bridge accepted the frame while telling you
nothing about the final destination.  I also note that those bits, on an
802.5 ring, are in a part of the frame which is not included in the CRC
check, so I wouldn't necessarily want to rely on them for anything important.

I have a feeling what is being confused here is the on board support for
IEEE 802.2 Type 2 LLC circuits the IBM TR adapters have.  Be assured that
this does send packets back to acknowledge receipt of frames (this is
fairly obvious if you think about it, since this works across bridges.  It
will also work equally well (poorly?) over 802.3-style ethernets).  If an
802.2 Type 2 LLC is a performance winner for some PC networking software
it is only because the protocol is implemented on board on IBM adapters, where
it can be intimate with the TR controller chip and with a resulting decrease
in bus traffic to the board (since the network software doesn't have to
receive the acknowledgements itself).  For real computers, with good quality
software and a properly designed interface to the hardware, the on-board
firmware would make a whole lot less difference.

Dennis Ferguson
University of Toronto