Figure 1: Multilink PPP
The 3ComImpact(TM) External Digital Modem utilizes digital service available from the telephone company to transmit information. Generally, access to the Integrated Services Digital Network (ISDN) from the home is provided by a Basic Rate Interface (BRI). A BRI consists of two bearer (B channels) and one data (D channel). A B channel carries voice and data at a rate of 56 Kbps or 64 Kbps. The D channel carries network-related messages such as status and control information. By using Multilink PPP, the B channels can be combined to provide a maximum transmission speed of 128 Kbps, which is four times as fast as V.34 analog modems.
Multilink PPP is a protocol standard which provides higher access speeds and allows interoperability with Multilink PPP-compliant equipment from other vendors. Multilink PPP allows the two B channels to be used as a single transmission pipe. By combining the two B channels, a maximum transmission rate of 112 Kbps or 128 Kbps can be attained. With Multilink PPP, the two B channels are virtually linked, as shown in Figure 1. By default, Multilink PPP is enabled.
Figure 1: Multilink PPP
Dynamic Bandwidth Allocation
A feature of the 3ComImpact External Digital Modem, dynamic bandwidth allocation (DBA) automatically provides bandwidth when needed to place an outgoing voice call. (DBA does not apply to incoming voice calls.) When Multilink PPP is enabled, both B channels are dedicated to the data call. However, while Multilink PPP is active, you may want to place a voice call. With Dynamic Bandwidth Allocation (DBA), you can place a voice call while a Multilink PPP call is active.
Suppose, while connected to the Internet, you want to place a voice call using an analog phone attached to the 3ComImpact External Digital ModemOs phone port as shown in Figure 2. However, both B channels are being used by Multilink PPP for a data call.
Figure 2: DBA Example
With DBA, once you lift the telephone handset, one of the B channels used for Multilink PPP is temporarily and automatically removed and used to place the voice call. The temporary removal of a Multilink PPP link for a voice call reduces the speed of the data call in progress to 64 Kbps. The reliability of the data call is not affected. When the voice call ends (that is, when you replace the handset) the channel used for the voice call is once again reinstated for Multilink PPP.
2. Setting up the 3ComImpact Digital Modem for Multilink PPP
3. Setting up the 3ComImpact External Digital Modem for DBA
4. Access Solutions with the 3ComImpact External Digital Modem
5. Determining Throughput
6. ISDN Routers and Bridges Versus ISDN Modems
7. Interoperability with Equipment from Other Vendors