What is the difference between using the ProofSpace technology and using a TSA (Time Stamping Authority)?

According to RFC 3161, the term "TSA" refers to any entity that offers time service in compliance with standards rather than a specific technology. In practice, however, the term "TSA" usually is used to mean an outside third party that issues trusted (signed) timestamps. The ProofSpace technology does not need to rely upon external Time Stamping Authorities to generate trusted time stamps, resulting in a solution that is less expensive, easier to manage and more secure than products based on RFC 3161.