rmines which pieces of the message are encrypted. Some solutions encrypt the entire message (IP header and data), whereas others encrypt only the data. The four transmission modes used in VPN solutions are In-place transmission mode This is typically a vendor-specific solution where only the data is encrypted. Packet size is not affected, which ensures that downstream transport mechanisms are not affected. Transport mode Only the data is encrypted and the packet size increases in size. This mode provides adequate data privacy for node-to- node VPNs. Encrypted tunnel mode The IP header information and the data are encrypted with a new IP address created and mapped to the VPN endpoints, providing excellent overall data privacy. Nonencrypted tunnel mode Nothing is encrypted, which means that all data transported is clear text. This is not an effective solution for data privacy. Oddly enough, some VPN solutions do not perform any encryption at all. Instead, they rely on data encapsulation techniques such as a tunneling or forwarding protocol for data privacy. Not all of the tunneling and forwarding protocols use a cryptographic system for data privacy. This means that the protocol would transmit all data in the clear, leaving one to wonder how nonencryption-based solutions Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface SGSN-to-GGSN (G) and GGSN-to-PDN (G) Interface can provide any form of data privacy protection-a critical requirement for a VPN. Unfortunately, the industry terminology itself may be contributing to some of this confusion. To clarify this issue, one must look specifically at what transmission mode is being used. As with the qualification between data authentication and user authen- tication, transmission modes should be distinguished between encrypted and nonencrypted. If a VPN sol