def get_channel_descriptions(model, channel_name=None):
    """
    Returns a dictionary of channel descriptions for the given model
    :param model: the model
    :param channel_name: the channel name
    :return: dictionary of results, or None
    """
    if not channel_name:
        return model.get_channel_descriptions()
    else:
        return model.get_channel_description(channel_name)