Tech Stories from the Trenches

Like many of you, I have a home office and consider daily Internet visits essential to download software and pick up email. When I face downloading the latest service pack for Windows NT or Exchange, I agonize over how long the process takes with a modem connection that tops out at 31.2Kbps. To reduce the download time, using my three phone lines, I decided to test pooling modems with the Multilink Point-to-Point Protocol (MPPP), which Remote Access Service (RAS) supports.

Before I started the test, I researched MPPP on the Internet and at Microsoft's Web site. A search for the string MPPP returned only a few links, which surprised me. After digging around, I found that MPPP is more commonly referred to as Multilink Protocol (MP); Microsoft is one of the few vendors that uses the term MPPP. When I finally got the search string right, my research turned up Request for Comments (RFC) 1990, which contains the specifications for MP.

The Multilink Protocol
According to the RFC, the goal of a multilink operation is to coordinate multiple independent links between a fixed pair of systems. This coordination provides a virtual link with greater bandwidth than any of the constituents. The aggregate link, or bundle, can consist of different physical links, as in multiple asynchronous modem lines, but can also be instances of multiplexed links, such as ISDN, X.25, or frame relay. The aggregate link can also be of different kinds, such as pairing dial-up asynchronous links with leased synchronous links.

To establish communications over a Point-to-Point Protocol (PPP) link, each end of the PPP link (RAS client and RAS server) sends Link Control Protocol (LCP) packets to configure the data link. A system signals its peer that it is willing to multilink by sending the multilink option as part of the initial LCP negotiation.

After the systems establish the link, PPP provides an authentication phase in which authentication protocols define an identifier for each system that the link connects. The identifiers ensure that only links between the two original systems are bundled. The RFC also specifies that you can see compression separately on each member link, or run over the bundle, as a logical group link. You can read RFC 1990 at http://sunsite.auc.dk/RFC/rfc/rfc1990.html.

Configuring RAS Multilink Connections
After reading the RFC, I thought that two 28.8Kbps modems should connect at 56Kbps, so I decided to try it. The process to set up a multilink connection is fairly straightforward. You need to perform the following steps. First, install modems on the RAS server and client, reboot each system, and verify that each system recognizes the modems. Second, make sure you configure the modems on the RAS server to receive calls; otherwise, the multilink option is not available. Third, enable the multilink protocol on the RAS server and the RAS client.

To enable multilink on the RAS server, go to Control Panel and double-click the Network icon or right-click Network Neighborhood and select Properties. Select the Services tab, and double-click Remote Access Service. Examine the configuration of each modem to make sure it is set to answer calls. Next, click Network and select the Enable Multilink check box, as shown in Screen 1, page 104. If you have not configured a RAS server before, see Michael D. Reilly, "Remote Access Service," May 1997. Remember that the server settings show up only when you configure RAS ports to receive calls.

On the client side, configuring a multilink dial-out connection is easy. Go to My Computer, and double-click Dial-Up Networking. Here, either create a new entry or edit an existing one. In the Dial using box, select Multiple Lines, and configure each modem with the correct phone number to dial, as shown in Screen 2, page 104. Be aware that the Multiple Lines option is available only when you have more than one modem installed on the client. Highlight the first modem and click Phone numbers to specify one or multiple numbers for that modem to dial. Next, click Configure to enable or disable the modem parameters, hardware flow control, modem error control, modem compression, and the speaker. I used the defaults for all options. Repeat this process for the second modem.