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A wide area network (WAN) is nothing more than two or more regular networks connected by a low-speed line. You may, for example, have two 10Mb networks—one in San Diego and one in Washington D.C. In order to connect the networks to each other, you would use routers. One router on the network would dial up a router on the other network whenever a network request to access the other network occurred. This router could use a regular modem, an ISDN modem, or some other methodology. The transmission speed could vary from 9,600Kbps up to 45Mbps, but this is still a far cry from the 10Mb to 100Mb networks you are using now. And the costs to connect these networks increase as the transmission speed increases. There are also very few 45Mbps WANS in use commercially, which is why you may want to use the built-in capabilities of Windows NT.
Creating a wide area network with Windows NT Server is extremely easy. You can use the Remote Access Service to connect two physically separate networks, or you can use dedicated hardware (routers). Both of these methods have different strengths and weaknesses, which is the subject of our discussion.
For your first wide area network, I suggest you use the Remote Access Service (RAS) software. This service is included with Windows NT Server and is, therefore, the least expensive alternative for creating a wide area network. It can support up to 256 simultaneous client connections, although to support this, you will require a third-party adapter from a manufacturer such as DigiBoard. If you do plan on supporting 256 simultaneous connections, the server should not perform any other network action. With RAS, you can use an X.25 adapter, a modem, or an ISDN adapter. An X.25 adapter is used to connect to an X.25 source that can provide low speed (9,600Kbps) but world-wide connectivity from a local phone number.
The X.25 Solution
An X.25 RAS connection is a low-speed (about 9,600Kbps) packet-based connection (just like your LAN). The primary advantage you have with an X.25 network is that it is cheaper to implement than the regular modem-to-modem or ISDN-to-ISDN connection when supporting a large number of RAS clients. This is because you can use an X.25 connection from just about anywhere in the world by dialing a local access number.
There are two parts to this process and two ways to implement an X.25 RAS connection. You can use an X.25 network adapter-to-X.25 network adapter connection (using X.25 network adapters such as those manufactured by Eicon). Or, you can use an X.25 network adapter on the server and a modem on the client. The client will dial up a local access number to gain access to the X.25 network. These network packets will then be routed to your network server. Once this has occurred, a two-way communication linkage will be established, and the user will be able to perform any desired network action, such as printing a file or running an application.
This is also a good choice for a network that has to maintain a low-cost and low-usage communication linkage between two remote locations. But, this solution is not for the security minded because the X.25 network can be easily tapped by potential hackers.
The High-Speed Modem Solution
The most common RAS connection is based on high-speed modems. You can use your standard 28.8Kbps modems to provide data transfers in the 57.6Kbps to 152Kbps range. While this is not the fastest data transfer rate, it is an acceptable one for infrequent network usage, and it works extremely well for distributed applications (such as SQL Server). However, as in most software packages, there are issues to be considered.
The first thing you should be aware of is that a reliable modem connection is based on the quality of the phone line. And, the faster the baud rate, the cleaner the line has to be to obtain error-free data transmissions. Reliable 28.8Kbps connections are almost impossible to achieve with local phone lines. Long-distance connections seem to perform better, probably because many of them are based on fiber-optic phone lines.
Next, you have to consider the UART (Universal Asynchronous Receiver Transmitter) your COM port is based on. If you have the standard 16450, the best you can achieve is a 38.4Kbps data transmission. If you have a 16550 UART, the best you can achieve is a 57.6Kbps data transfer rate. To obtain any higher data transfer rate, you must use either an internal modem with a buffered 16550 UART or other buffered UART supported by Windows NT Server. The best solution that I have found is to use a DigiBoard adapter, which can support from 2 to 256 UARTs and includes a dedicated CPU to offload the work from the CPU on your motherboard.
With the newer modems available on the market, such as the 33.6 and 56K modems, you may be able to obtain higher transmission speeds. Unfortunately, both these modems require advanced serial ports that are not included as base products on most of today’s computers. 56K modems also require a digital switch and will not operate at full speed when directly connected to each other. These 56K modems were specifically designed to improve Internet access speeds not modem-to-modem connection transmission rates.
You will also want to test your RAS configuration to determine whether the hardware or software compression performs better. This will depend on the speed of your CPU. For most computers, the software compression will outperform the hardware compression. But, you will need to test this on your hardware platforms to be sure.
And, lest we forget, you must also consider the cost associated with local and long-distance access. This will vary from state to state and according to your phone company. But, you should consider this as well in your budget.
It is possible to use the Internet as a WAN backbone; however, Internet performance varies quite a bit. Security is also an issue with Internet access. The good news here is that RAS supports the PPTP protocol, which you can use to access virtual private networks (VPNS). This means it is possible to use the Internet to connect two or more networks at lower cost. It also opens up potential security risks. So, if you do decide to go this route, be sure to install firewalls or proxy servers (sometimes both) to limit these security risks.
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