The following printed references can be found at your local library, if you wish to study further, or are more comfortable using a book instead of the Internet.
> Velte, Toby J, Cisco: A Beginners Guide 2nd Edition. > Kumar, Vineet and Korpi, Markku, IP Telephony with H.323. > Durkin, James F., Voice Enabling the Data Network, Cisco Press. > Smith, Richard E., Internet Cryptography. > Collins, Daniel, Carrier Grade Voice Over IP. > Boney, James, Cisco IOS In a Nutshell. > Hall, Eric A., Internet Core Protocols.