Remote Dial-in Access Applications and Virtual Private Networks

Remote dial-in access (or simply remote access) is the way many users access the Internet. It is also used in the telecommuting service whereby corporate road warriors and those who work at home access corporate IP networks. On the surface, both types of access look the same. In fact, the very procedure (dialing a number and entering a password) and the end-user equipment necessary (a telephone line and a modem) to accesses the network are identical. At least one important piece of the underlying technology pertinent—carrying IP packets over PSTN lines—is also present in both types of services. There are significant differences as well, with underlying issues, between accessing the Internet privately (through an ISP) and accessing corporate networks. We treat remote dial-in access simply as part of VPNs, discussed in a section that follows.

One problem inherent to data access from home via a telephone line is that the line will be busy for the duration of the data access session. The busy line has posed a number of problems for households with only one telephone line. One solution, perhaps most typical for many American households, is to have two lines. Another is to have ISDN installed (which is still rather expensive). Yet another solution is to use a special modem that splits the line into data and voice. Then, of course, there are service solutions (such as the Internet call-waiting service). As it becomes more available, the xDSL technology will eliminate the need to dial at all!

With remote access, in its oldest form (See Figure 1), a user simply established a point-to-point link between a terminal (connected to a modem) and a remote computer (also connected to a modem).

Figure 1: Terminal-to-host dial-in access.

As PCs became household appliances, the terminals virtually disappeared. PCs are presently used both as terminals (with so-called shell accounts, provided by both ISPs and corporations) and IP hosts. Even with the shell accounts, users get access to the Internet (including the Web) through a host. However, with shell accounts, the graphical interface that has made the World Wide Web so popular is not available.

To act as a host, a PC typically dials in the remote access server (RAS) of an IP network (as depicted in Figure 2, where the PSTN path happens to traverse three telephone switches). What actually distinguishes a host equipped with a set of modems from an access server? The answer is simple: A RAS is an IP router equipped with a set of modems or digital signal processors capable of terminating a call. Remote access servers are sometimes called remote access concentrators. Although both terms are used interchangeably, some people use the latter term only when referring to large multiservice modules, which have access to asynchronous transfer mode (ATM) networks, X.25-based public data networks (PDNs), and other non-IP networks (hence the term multiservice).

Figure 2: Host-to-host dial-in access.

Having just defined a RAS, we should note that the data network access may be not so remote. First of all, an enterprise or ISP can place several access servers (see Figure 3) so that users in different geographic areas can call local numbers. Second, remote access can also be outsourced. A remote access outsourcing application uses the existing network infrastructure of a larger service provider to offer remote access termination service to enterprise customers. Then, with the Internet offload application (also called Internet call diversion), the very edge (that is, the central office switch) of the PSTN can recognize the call (based on the dialed number, for example) as a data call, and terminate it at a colocated or even internal access server. From there the call is passed to the appropriate ISP (or enterprise) network (Figure 3).

Figure 3: Internet offload.

The Internet offloading application has been created out of urgency. The ever increasing use of Internet access has manifested a serious problem with the PSTN: The duration (or holding time, in telephony parlance) of such data calls far exceeds what telephone companies expected from voice calls (and consequently engineered their network for). As a result, more and more real (voice) calls became blocked throughout the PSTN, and the problem became so serious that specialized solutions to offload the data traffic from telephone switches to data networks, were urgently requested by telephone companies. The problem warrants special attention here if only because it was among the first instances where data-over-voice applications required significant restructuring of the PSTN. What is especially interesting is that even the old PSTN paradigm—the more calls and the longer they are, the better—has changed! The danger of these long data calls blocking voice traffic became so serious that the telephone companies decided they did not want data calls in the voice network. This conflict has created a classic example of PSTN-Internet integration by necessity.