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       ATM Forum Document Number: ATM Forum/95-972

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       Title: Parameter Values for Satellite Links.

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       Abstract:
       We show that Xrm is the key parameter which affects performance
       on satellite links. With current 8-bit XRM field, the maximum
       throughput on satellite links is limited to a few Mbps even on
       155 or 622 Mbps links.  The problem can be fixed by extending the
       field size to 32 bits.
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       Source:

       Raj Jain, Shiv Kalyanaraman, Sonia Fahmy and Fang Lu
       The Ohio State University
       Department of CIS
       Columbus, OH 43210-1277
       Phone: 614-292-3989, Fax: 614-292-2911, Email: Jain@ACM.Org

       The presentation of this contribution at the ATM Forum is sponsored by
       NIST.

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       Date: August 6-11th, 1995

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       Distribution: ATM Forum Technical Working Group Members
       (Traffic Management)

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       Notice: This contribution has been prepared to assist the ATM
       Forum. It is offered to the Forum as a basis for discussion and
       is not a binding proposal on the part of any of the contributing
       organizations. The statements are subject to change in form and
       content after further study. Specifically, the contributors
       reserve the right to add to, amend or modify the statements
       contained herein.

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       The ABR specification is designed for LANs, WANs as well as ATM
       over Satellite networks. The effects of the round trip delay
       become clear in satellite networks.

       We have simulated a bidirectional one-source configuration with a
       satellite link. Of the parameters Xrm, XDF, TOF and RDF, Xrm is
       the most significant parameter and the other parameters, notably
       XDF have little or no effect in such networks.

       Xrm is designed to protect against a broken network. But, at
       small values like 32 or 256 (the maximum value allowed
       currently), the ACR goes down to zero almost vertically and
       exhibits oscillations in the steady state.

       From simulation, Xrm follows the relationship:
                          Xrm = N * PCR * RTTQ / Nrm

       where
       N = number of active sources
       RTTQ = Maximum Round Trip Time including propogation,
       transmission and queueing.


       Hence, Xrm needs to be chosen considering one round trip delay.
       The optimum values for satellite links are not even allowed in
       the current specification which allows only an 8-bit field for
       Xrm.

       We note, that increasing Nrm to compensate for Xrm only reduces
       the frequency of feedback from the network. Hence, with a view to
       accomodating long round trip time networks, we propose a 32 bit
       field for Xrm.

       Simulation results will be presented at the meeting.