Prentice Hall kindly made their new book on voice-over IP available for review. Voice over IP by Uyless Black (ISBN 0-13-022463-4) is an excellent overview of the subject. The key technologies are sufficiently described, and a wealth of references are provided. This book manages a fine balance—a broad-brush overview, with enough technical details to satisfy the curious engineer. While I wish the author had devoted more space to MGCP and especially SIP, the sections on H.323 and SS7/IP integration are excellent. The appendices on PSTN signaling alone are worth the price of the book. It's well worth adding to your technical library, and I consider it an essential reference if you are getting into this field and need to learn a great deal in a short time.
