Remote Access Dial-up Protocols
Recall that up until this point, weve spoken of TCP/IP as a set of protocols that run over Ethernet, using 10xBaseTX cabling. Phone lines dont look or work much like permanent Ethernet cables.
A significant difference between dial-up remote access and permanently wired network connections is that once a computer is connected to the LAN with 10xBaseTX cable, it can start participating in the network immediately. Most networks require additional user ID and password information before users can access shared files and printers, but commands like ping will show you that even if you dont provide this information, you do have limited network access. Unless the wire is physically changed, the computer will always be participating in that particular segment of the network.
With dial-up lines, its necessary for the remote user to place a phone call to their desired network access point, and provide a valid user ID and password for network access. Until providing this authentication information, you have no network access not file sharing, not web browsing, not even ping. Its necessary for the user to verify that theyre authorized to use the network before they can send and receive any packets. Also, the network access point (and phone number) they dial into can change from day to day, if the person is traveling around the country and dials in every day to their ISPs local access number in their current city. Even the speed of the connection might vary, depending on the modem the customer is using to connect, and what theyre connecting into not all modems can speak with each other at their highest possible speeds.
To accept incoming dial-up remote access users on a general purpose computer such as a Windows NT Server, it is necessary to run a special service designed for that task. The service waits for dial-up networking connections and then communicates with the remote user to verify that they are authorized, and then establish the connection. The server software supplied with Windows NT and Windows 2000 for this function is called RAS (Remote Access Service).
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