Conference 7.286::fddi

    I'll answer my own question having found the answer in the doc by Raj
    Jain( Performance Analysis of FDDI Token Ring Networks)
    
    The timed token access method, requires all stations on the ring to
    agree to a 'target' rotation time and limit their transmissions to meet
    this target as much as possible. If a station calcs the time from
    transmission of token to receipt of token and finds it is greater than
    target, the station can't transmit.
    
    This differs from 802.5. An 802.5 station can transmit for a pre
    defined time on receipt of the token. The effect of this is to
    constrain the number of stations on the ring.
    
      John
    
    

To go a little further, as I understand Raj Jain's explanation, 802.5 operates
at a speed where it is practical for the token to make a complete rotation
before it is replaced with a datagram.  This allows every station to sort of
bid for the token based on priority before to token is taken.  With FDDI, the
speed and distance is such that multiple datagrams need to be present on the
ring to achieve reasonable utilization and this requires deciding whether
you have priority over a down stream stations without being able to get
any direct information from them.

An important consideration in designing a shared communication channel is
utilization and this is determined by the size (number of stations), the
data rate, the message size, and the algorithm used.  Neither 802.3 nor 802.5
are practical at higher data rates unless the size of the network is reduced
or message size increased proportionately.  To meet the requirements fo FDDI,
a new protocol had to be invented, but that protocol is only useful up to
100 megabits per second and 100 km.  To go to a higher data rate, it will be
necessary to invent a new protocol.  Don't expect to see FDDI replaced by
1 gigabit token rings.

And another note, don't be fooled by the 100km figure for FDDI into thinking
that FDDI networks can be larger than Ethernet extended LANs.  The 100 km
figure was necessary in order to match the size of Ethernet LANs.  In order to
connect 1000 stations to a token ring where the stations average 50 meters
from the wiring closets, you need 100km of fiber.  (1000*50*2->100,000 meters)

    Re:.2 Don't be confused by the last paragraph which states FDDI can
    have up to 1000 "stations". What it can really have is 1000 MACs, which
    if you've got 2 MACs in each DAS, brings the number of stations down to
    500. Or think of it as 1000 "connections". 
    
    Debbie
    

From:	BAGELS::SWITZER "GEORGE SWITZER - TAY02-2/N4 - ***DTN 227-3729 - MGR LACS/CSSE  15-Jan-1990 1404" 15-JAN-1990 14:05:00.48
To:	@FDDI_CSTNM
CC:	
Subj:	FDDI Performance Report available

From:	ERLANG::JAIN "Raj Jain, LKG1-2/A19, DTN: 226-7642  12-Jan-1990 2106" 13-JAN-1990 05:42:51.67
To:	@REPORT1,@REPORT2,@REPORT3,@REPORT4
CC:	
Subj:	Technical Report on FDDI performance (DEC-TR-655) Available

The following DEC technical  report  is  now  available  for  internal  and
external distribution.

DEC-TR-655: Performance Analysis of FDDI Token Ring  Networks:   Effect  of
            Parameters and Guidelines for Setting TTRT
            14 pages.

The report is available in PostScript on:
                   FILES::NET$ARCH:[PAPERS]FDDI_PERF.PS

If you would like to receive a hard copy, please reply to this message with
your NAME and MAIL STOP.

If you want a soft copy mailed to you please include the word  PS  in  your
reply.

Any other words or sentences in the reply will only delay the delivery.


A detailed abstract of the report is as follows:


                                DEC-TR-655
             Performance Analysis of FDDI Token Ring Networks:
           Effect of Parameters and Guidelines for Setting TTRT
                                by Raj Jain

Fiber-Distributed Data Interface (FDDI) is a 100-Mbps  Local  Area  Network
(LAN) standard being developed by the American National Standards Institute
(ANSI).  It uses a timed-token access method and allows up to 500  stations
to be connected with a total fiber length of 200 kms.

We analyze the performance of FDDI using a simple analytical  model  and  a
simulation  model.   The  performance metrics of response time, efficiency,
and maximum access delay are considered.  The efficiency is defined as  the
ratio  of  maximum  obtainable  throughput  to the nominal bandwidth of the
network.  The access delay is defined as the time it  takes  to  receive  a
usable token.

The performance of FDDI  depends  upon  several  workload  parameters;  for
example;  the  arrival  pattern,  frame size, and configuration parameters,
such as the number of stations on the ring, extent of the ring, and  number
of  stations that are waiting to transmit.  In addition, the performance is
affected by a parameter called the Target Token Rotation Time ({bf  TTRT}),
which  can  be controlled by the network manager.  We considered the effect
of TTRT on various performance metrics for different  ring  configurations,
and  concluded that a TTRT value of 8 ms provides a good performance over a
wide range of configurations and workloads.