SUBROUTINE ERLTRU
C
C     This module of the ERLANG program calculates the number
C   of servers (trunks) required to the blocking to be no more
C   than a given value for a given amount of traffic.
C
C     B. Z. Lederman     I.T.T. World Communications
C
      BYTE ANS
C
  300 WRITE (5, 305)
  305 FORMAT ( / '$ Enter Erlangs of traffic, % Blocking: ')
      READ (5, 310, END=260) A, BLOCK
  310 FORMAT(2F10.2)
C
C    Check for impossible data.
C
      IF (BLOCK .GT. 100.) GO TO 810
      IF (BLOCK .LE. 0.0 ) GO TO 810
C
C     Start with one trunk and see if the blocking is acceptable:
C  If not, increase the number of trunks by one and keep trying
C  until blocking is acceptable.
C
      BLOCK = 0.01 * BLOCK
      N = 0
  320 N = N + 1
      BPER = ERLB(N,A)
      IF (BPER .LT. BLOCK) GO TO 325
C
C   Stop if the number of trunks gets out of hand.
C
      IF (N .GE. 2000) GO TO 325
      GO TO 320
C
  325 BPER = 100. * BPER
      WRITE (5, 230) A, N, BPER
  230 FORMAT ('0 Erlangs = ', F7.2, ',  Trunks = ', I4,
     1',  Blocking = ', F6.2, '%' / 1X /
     2'$ Another Trunks Problem [Y/N] ?: ')
      READ (5, 250, END=260) ANS
  250 FORMAT (A1)
      IF ((ANS .EQ. 'Y') .OR. (ANS .EQ. 'y')) GO TO 300
C
  260 RETURN
C
C    Tell the user the data is no good.
C
  810 WRITE (5, 820)
  820 FORMAT ('  Impossible situation, please re-enter.')
      GO TO 300
C
      END