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The traditional (and now largely outdated) remote access technique is to use a dial-up connection. With a dial-up connection, a client computer uses a modem to connect to a remote access server over a phone line. Figure 7-8 illustrates how connections are established, with each client requiring a separate physical circuit to the server.
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The architecture of dial-up remote access connections
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Dial-up connections offer the following advantages:
No Internet connection required Dial-up connections use a standard analog phone line
to establish a network connection directly to your internal network. This means you do not need to expose your internal network to authentication requests from the Internet, unlike a VPN. In fact, you do not need to connect your internal network to the Internet at all a common requirement for high-security networks. Minimal privacy risks Although dial-up connections lack encryption, the traffic crosses the public switched telephone network (PSTN), which many security experts consider to offer better privacy than the public Internet. Predictable performance Dial-up connections offer consistent, predictable performance because the connection is dedicated to a single client. However, dial-up connections have the following drawbacks:
When planning to allow employees dial-up access, you need to have as many incoming phone lines and modems available as users who will simultaneously access the dial-up network. To support hundreds or thousands of users, the monthly costs of the telephone circuits can be very expensive, as can be the one-time costs of the modems required. Poor bandwidth Modems for traditional analog phone lines are technically rated for 56 Kbps of bandwidth, but typically, usable bandwidth is between 20 Kbps and 25 Kbps. That bandwidth makes simple tasks such as browsing the Web tedious and makes tasks such as listening to streaming video or audio impossible. Digital phone lines, such as Integrated Services Digital Network (ISDN) circuits, can offer true 128 Kbps bandwidth, but at a much higher cost.
High cost for scalability
Lesson 3: Connecting to Remote Networks
Virtual Private Networks
Whereas dial-up connections use the PSTN to carry traffic to your internal network, VPNs traverse the public Internet. Because your organization probably already has an Internet connection, you might not need to purchase any additional bandwidth (unless you determine that your current bandwidth will not meet the needs of the users simultaneously connected using a VPN). Figure 7-9 illustrates how connections are established, with each client requiring a separate Internet connection but the VPN server requiring only a single connection to the Internet (instead of a separate physical circuit per dial-up client).
VPN client
Internet VPN client VPN server Intranet Destination server
VPN client
Figure 7-9
The architecture of VPN connections
VPNs offer the following advantages:
Higher bandwidth potential Theoretically, VPN bandwidth can be as high as the client
or VPN server s Internet connection (whichever is slower). In practice, because other services will probably use both connections and many other factors can limit bandwidth on the Internet, performance will be somewhat lower than the theoretical maximum. If the client has a broadband connection, however, bandwidth is likely to be much higher than a dial-up connection. Minimal costs Both the VPN server and the client need to be connected to the Internet. However, your organization probably has an existing Internet connection, and many home or traveling users have Internet access. Therefore, there are no connection costs associated with using a VPN, regardless of the number of incoming connections. If the number of incoming VPN connections requires more bandwidth than you have, you might need to purchase additional bandwidth from your ISP. However, this cost is likely
7
Connecting to Networks
to be far less than purchasing a similar number of telephone circuits and modems for dial-up connections. However, VPNs have the following drawbacks:
Internet connection required You must connect the VPN server, and thus your internal network, to the Internet and allow incoming VPN traffic through any firewalls. Additionally, users must have an Internet connection to use a VPN. Organizations typically take one of two approaches:
Work with an ISP to arrange Internet access for all users, using either a dial-up connection or a broadband connection such as a cable modem or DSL.
Require employees to find their own ISPs. Many users currently have an Internet connection at home, and traveling users can often connect to the Internet using public hotspots or wireless broadband services. Poor latency Even if the bandwidth is high, VPN connections often seem slow because of high latency. Latency is the delay that occurs when a packet travels from a client to a server. As Figure 7-9 shows, packets in a VPN have to travel across the Internet to the VPN server, across the intranet to the destination server, and back. The latency on a VPN connection can often be several times greater than the latency on a dial-up connection. Poor efficiency with dial-up connections Although it s possible to dial up to the Internet and then connect to a VPN, the added overhead of the VPN, and the latency added by the Internet, offer even worse performance than using a dial-up connection directly to a remote access server. If users will be using a dial-up connection to access the Internet, they will receive much better performance dialing directly to your intranet.
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