Figure 6: Throughput Example
1. Multilink PPP and Dynamic Bandwidth Allocation
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
The performance of all linked devices must be considered to determine end-to-end throughput. Connection performance is affected by each device in the chain. Therefore, the slowest link in the chain determines the maximum throughput.
Maximizing Throughput
An example of one of the better throughput scenarios is shown in Figure 6.
Figure 6: Throughput Example
a -- COM Port Speed
The COM port speed maximum for a PC or a Macintosh® computer is 115.2 Kbps. Excluding overhead, the maximum throughput possible will be 80% of 115.2 Kbps, or 92.2 Kbps.
Because internal cards are not affected by the limitations of a computer COM port, an internal ISDN product, like the 3ComImpact Internal ISDN ISA Adapter, may provide higher throughput.
b -- 3ComImpact Digital Modem Speed
Using Multilink PPP, the 3ComImpact External Digital Modem will provide transmission speeds of up to 128 Kbps. At this point in the chain, the COM port speed of the PC limits the maximum speed to 92.2 Kbps.
c -- Internet Access Provider Speed
The router providing access to the Internet may also reduce the speed because of network loading. And, the Internet itself may have speed limitations.
d -- Remote Web Site Access Speed
In this example, the remote Web server site provides a 128 Kbps connection.
In the example shown in Figure 6, the maximum attainable transmission speed is 92.2 Kbps because the slowest link in the chain, in this case the PC COM port, determines maximum throughput.
The actual performance may also be lower than 92.2 Kbps because of Web server loading, Internet service provider access equipment overhead and loading, and many other influences on transmission and routing equipment inside the Internet cloud.
Throughput and Cost Implications of Multilink PPP
Although using Multilink PPP provides a 128 Kbps connection for data traffic, the overall end-to-end throughput may not achieve the full rate of 128 Kbps. You should compare your throughput maximum with the additional cost of Multilink PPP. For instance, if your slowest link is to the Web site at 56 Kbps, you know this is your maximum throughput. Each B channel in a Multilink PPP connection will cost from $0.01 to $0.06 per minute of usage. In this example, a single B channel connection would achieve the same performance as a Multilink PPP connection and save you 50% of the usage costs of the Multilink PPP call.
However, many Web and Internet servers as well as computer COM ports continue to be upgraded to improve throughput. Because of these continuing improvements, significant speed limitations will be the exception.
Testing of the 3ComImpact External Digital Modem achieved end-to-end throughput of 85-90 Kbps demonstrating top-performance while justifying the cost of the extra B channel.
6. ISDN Routers and Bridges Versus ISDN Modems
7. Interoperability with Equipment from Other Vendors