SUBROUTINE  communication(  a,  ip_xy,  ip_xz,  ip_yz,  voisin,  comm3d)
      USE MPI
      USE numerics
      IMPLICIT  NONE
      REAL(rp),  DIMENSION(sx-1:ex+1,sy-1:ey+1,sz-1:ez+1), INTENT(INOUT)  ::  a
      INTEGER,  INTENT(IN)  ::  ip_xy,  ip_xz,  ip_yz
      INTEGER,  PARAMETER  ::  NB_VOISINS  =  6
      INTEGER,  PARAMETER  ::  N=1,  E=2,  S=3,  W=4,  AV=5,  AR=6
      INTEGER,  DIMENSION(NB_VOISINS),  INTENT(IN)  ::  voisin
      INTEGER,  INTENT(IN)  ::  comm3d
      END  SUBROUTINE  communication