If the witness is an actual party in the case, the Notice of Deposition is compulsory because they are already subject to the Court's authority and can be sanctioned for discovery violations if they don't appear. However, if the witness is not a party to the case, serving them with a deposition notice does NOT mean that they are compelled or required to appear. If you want to compel the witness to appear for the deposition you will need to serve them with a subpoena in addition to the deposition notice.