Actually, that's a good question! It may indeed by a cassette. If there is a "bulge" in the hub body, it's a cassette. Also, if it's an actual 105 hub, that probably makes it a cassette as well. In that case, you would just need to replace the freehub body, which is a very easy job...although it does involve removing the axle and repacking the hub bearings. And, you'll need a 10mm allen with a rather long extension...and you'll need a new freehub body.