3Com 3C888 User Guide - Page 145

What is a network? 145 connected ISP. By bypassing the analog-to-digital conversion in the downstream path, 56K technology can use nearly all of the available 64K network bandwidth. (Upstream data, typically less sensitive, travels at the standard V.34 rate of 33.6K.) Based on "encoding" rather than "modulation", the result is download speeds once thought not possible. Older, V.34 modems treat the PSTN as if it were entirely analog; they therefore cannot take advantage of the bandwidth made available when one end of the connection--that of your ISP, for instance--is completely digital. Possible Limitations Note that several factors may affect your Dual 56K LAN Modem's performance. Current FCC rules limit download speeds to 53 Kbps. Poor line conditions or heavy traffic may impact performance, just as with older V.34 modems. In real applications, speeds typically range from the 40s to the low 50s (Kbps), with the average in the mid-to upper 40s.There are cases where performance is in the 30s, and others where true V.90 performance may not be possible. Note that both your phone line as well as your service provider must be V.90 capable; this means that your service provider must conform to the V.90 standard. Lastly, V.90 requires that there be no more than one analog-to-digital conversion in the downstream path; PBX devices, such as those found in corporate telephone systems or some hotels, may introduce additional conversions. For assistance with locating an 56K service provider in your area, or for a more detailed explanation of 56K technology, visit 3Com's 56K Web page at http://www.3com.com/56k. What is a VPN tunnel? A VPN tunnel is a private, secure means by which data that uses any routing protocol, and which may be content-sensitive, can be transported over the public, IP-routable-only Internet. A data packet using any protocol (such as IPX or NetBEUI) can be made secure and IP-routable through use of the Point-to-Point Tunneling Protocol (PPTP). Specifically, each data packet is encrypted and encapsulated [behind] an outer, PPTP header. This encryption and encapsulation allows the data packet to be transported over the Internet regardless of protocol and without its inner contents' being publicly visible-hence, the concept of a private tunnel through the public Internet. Once a tunnel server on the private network receives the encapsulated packet on the other side of the WAN, it discards the outer shell and routes the inner packet on to its destination, according to the routing protocols used inside. The LAN Modem allows for the transparent passage of VPN tunnel data from a computer on its LAN side to its WAN side through its support of the PPTP protocol. The LAN Modem cannot, however, initiate or terminate a tunnel.