function [fi_tot] = getABCD(ex,start,stop,topology,f,Zterm);
%getABCD - Get the ABCD parameters
%          between start and stop node in the topology
%
% Parameter:    ex         experiment struct
% Parameter:    start      start node
% Parameter:    stop       stop node
% Parameter:    topology   Scenario Topology
% Parameter:    f	   Frequency axis
% Parameter:    Zterm      Termination Impedance
% Returns:      fi         The ABCD parameters
%                          A=fi_tot{1};  
%                          B=fi_tot{2};  
%                          C=fi_tot{3};  
%                          D=fi_tot{4};  
%
% Example(s):
%   segs = size(ex.tt.topology);
%   f = ex.param.frequency.f;
%   fi_tot = getABCD(ex,1,segs(1),ex.tt.topology,f,ex.param.Zterm);
%   Zl = ex.param.Zterm;
%   Zin = (fi_tot{1,1}.*Zl+fi_tot{1,2})./(fi_tot{2,1}.*Zl+fi_tot{2,2});
%
% Reference:
%    FTW's xDSLsimu manual
%% ===========================================================================

%% ===========================================================================
% Copyright (C):                                        
%       1999 by Telia Research AB, Lulea, Sweden;                
%       2000-2003 by Forschungszentrum Telekommunikation Wien, Austria;
%                                                         All rights reserved.
% Project       : FTW's xDSLsimu
% Author(s)     : Tomas Nordstrom (Tomas.Nordstrom@FTW.at)
%               : Daniel Bengtsson (Daniel.J.Bengtsson@Telia.se)
%               : Bo Engstrom (bosse@upzide.com)
%
% CVS:       $Id: getABCD.m,v 3.2 2003/11/03 12:33:22 tono Exp $
%% ===========================================================================
% Change History
%      1999-08-11 (DaB)  Created
%      2000-03-14 (ToNo) Modified how cables are defined (now in a list)
%      2000-04-03 (UvAn) Changed according to new interface of getTwoPortModel
%      2000-07-05 (GS)   Modified for also accepting bridged tap directly at LT
%      2000-07-17 (ToNo) Split out ABCD calcs to its own function
%                        (Makes it easier to do calc things like Zin)    
%      2001-01-04 (InJo) Fixed bug in index start (added +1)
%      2001-02-22 (ToNo) Added support for insertion loss numbers
%                        Useful for splitters and cable models
%                        given as an IL number
%      2001-06-18 (Bosse) Octave port
%      2002-07-01 (PeKa) Octave to Matlab compatibility port
%      2002-07-30 (ToNo) Speed things up by reducing duplicated calculations
%      2003-11-03 (ToNo) Lists now use cell arrays for both Octave and Matlab
%% ===========================================================================

% Start with unity matrix
fi_tot={1,0,0,1};

for index=start+1:stop,
  seg=topology{index};
  cablename=seg{2};
  if ~isempty(cablename),
    % Take care of a cable section
    cablemodel = getList(ex.clist,cablename);
    switch cablemodel.model
      case 'IL'
        d=seg{1}/1000;
	fi={1,getBFromIL(cablemodel,f,Zterm,d),0,1};
      otherwise
	[Z0,gamma] = getTwoPortModel(cablemodel,f);
	d=seg{1}/1000;
        gd=gamma*d; cgd = cosh(gd); sgd = sinh(gd);
        fi={cgd,Z0.*sgd,1./Z0.*sgd,cgd};
    end;
    fi_tot=ABCDprod(fi_tot,fi);
  end

  if length(seg)>5,
    cablename=seg{6};
  else
    cablename='';
  end
  
  if ~isempty(cablename),
    % Take care of a bridge tap (with open circuit termination)
    d=seg{5}/1000;
    cablemodel = getList(ex.clist,cablename);
    [Z0,gamma] = getTwoPortModel(cablemodel,f);
    fi={1,0,1./Z0.*tanh(gamma*d),1};
    fi_tot=ABCDprod(fi_tot,fi);
  end;
end;