Understanding Mobile Networking and Remote Access in Vista

Users often want to connect to their organization's network from an off-site computer. To do so, they need a dial-up, broadband, or virtual private network (VPN) connection. Dial-up networking enables users to connect off-site computers to their organization's network using a modem and a standard telephone line. Broadband enables users to connect off-site computers to their organization's network using high-speed Digital Subscriber Line (DSL) routers or cable modems. VPN uses encryption to provide secure connectivity over an existing connection, which can be a local area, dial-up, or broadband connection. Increasingly, wireless connections are being used as well. With a wireless connection, computers establish connections using a network adapter that has an antenna that enables it to communicate with similar wireless devices.

Understanding Mobile Networking and Remote Access

Although the underlying technologies are fundamentally different, direct dial, broadband, and dial-up connections make it possible for users to access your organization's network remotely. With a typical direct dial network configuration, off-site users utilize their computer's modem and a standard telephone line to connect to a modem pool located at the office. A Microsoft Windows Server managing the modem pool and running Routing And Remote Access authenticates the logon ID and password and authorizes the user to connect to the internal network. The user can then access network resources just as she does when working on-site.

Figure 1 shows direct dial connections using modem pools. Analog modems use dedicated telephone lines to connect users to the internal network at speeds up to 33.6 kilobits per second (Kbps). Digital modems use channels of a T1 line to connect users to the internal network at speeds up to 56 Kbps. In a standard configuration, you might have 8, 12, or 16 modems configured in the pool, each with its own line (or channel). Typically, the modem pool has a lead number that users can call. This number connects to the first modem in the pool. When the lead number is busy, the line rolls over to the next number, which connects to the next modem in the pool, and so on, enabling users to dial a single number to gain access to all modems in the pool.

Figure 1: Use a dial-up connection to access an office network through a modem pool.

Unlike direct dial connections, which can be made directly to the office network, broadband connections are made through an Internet service provider's (ISP's) network. The user's DSL router or cable modem establishes a connection to the ISP, which in turn connects the user to the public Internet. To connect to the office network, broadband users must establish a VPN between the user's computer and the office network. Figure 2 shows how VPN works.

Figure 2: Use broadband and VPN to access an office network.

A VPN is an extension of a private network across the public Internet. Once a user is connected, it appears to him that he is directly connected to the office network and can access network resources just as he does when working on-site. These seamless connections are possible because a virtual tunnel is established between the user's computer and the office network, where the VPN technology takes care of routing information over the public Internet. One of two VPN technologies is typically used: Point-to-Point Tunneling Protocol (PPTP) or Layer 2 Tunneling Protocol (L2TP).

Both L2TP and PPTP offer encryption and protection from attacks, but only L2TP uses IP Security (IPSec) for advanced encryption, making it the more secure of the two technologies. Unfortunately, L2TP is more difficult to configure. When you use L2TP, you'll need to use Microsoft Certificate Services or a third party certificate server to issue individual certificates for each system that will connect to the network using L2TP.

In addition to using VPN with broadband connections, you can also use VPN with dialup connections. In this configuration, as shown in Figure 2, users go through their ISP to establish a connection to the public Internet and later establish a private connection to the office network. When this configuration becomes standard procedure for dial-up users, your organization won't need dedicated private lines like those reserved for a modem pool.