def frequency_to_channel(freq):
    """ Convert frequency in GHz to ITU DWDM channel in C-band

    >>> frequency_to_channel(193100)
    0
    >>> frequency_to_channel(192750)
    -7
    >>> frequency_to_channel(193550)
    9
    """
    itu_channel = (freq - 193100) / 50
    assert itu_channel == int(itu_channel)
    return int(itu_channel)
