Load a QGIS-Server WMS-Layer to OpenLayers3

HTML

<script src="http://openlayers.org/en/v3.1.1/build/ol.js"></script>
Load a QGIS-Server-WMS-Layer to OpenLayer3
<div id="map"></div>

CSS

#map {
    height: 256px;
    width: 512px;
    border:1px solid black;
}

JavaScript

var map = new ol.Map({
     target: 'map',
     renderer: 'canvas',
     layers: [
     new ol.layer.Tile({
         source: new ol.source.OSM()
     }),

     new ol.layer.Image(
     'roads', "http://ub-server.com/cgi-bin/test/qgis_mapserv.fcgi",
    {
        layers: '5',
        format: 'image/png',
        bgcolor: '#fafafa'
    },
    {
        buffer: 1,
        isBaseLayer: true,
        graphicZIndex: 0,
    }
     )

     ],
     view: new ol.View({
         projection: 'EPSG:3857',
         center: [-8238683.374712203, 4998174.938711717],
         zoom: 2

     })
 });