User Manual

Why is the actual data rate always lower than the channel rate when using ADSL technology? Why is the ADSL modem connected at 12 Mbit/s, and the speed measured at speedtest.net does not exceed 8 Mbit/s?

The actual data rate is always less than the channel rate of at least 13-15% when using ADSL technology. This is a technological constraint that we'll talk about in a different section. It does not depend on the provider or the modem used.

Ideally, with a declared connection rate of 12 Mbps, you can expect a maximum actual data rate of up to 10 Mbps.

In fact, several other factors reduce transmission speed, in addition to technological limitations. We will discuss these factors below.

ADSL technology (Asymmetric Digital Subscriber Line) — is a data transmission technology in which the available bandwidth of the channel is distributed between incoming (download) and outgoing (upload) traffic asymmetrically. Thus, when connecting the ADSL modem, the subscriber's speed (download) and the speed from the subscriber (upload) are used.

In ADSL networks, the connection speed is measured in megabits per second (Mbps) or kilobits per second (kbps).

For example, numbers 10240/768 show that the maximum input connection speed to the subscriber will be 10240 kbps (speed of data coming to your ADSL modem), and the maximum output connection speed from the subscriber will be 768 kbps (the speed of data transfer from your ADSL modem to the remote ISP server).

The actual maximum download speed is about 1000 Kilobytes per second (KB/s). This number was obtained using the following formula:

Let's take a closer look at the factors that affect the actual connection speed:

But there are other factors that reduce the data transfer rate.

Theoretically, when loading a file, you should see the transfer rate calculated by the formula 'Connection speed - 15% (costs of using TCP/IP and ATM) / 8 (to convert kilobits into kilobytes)', but the reality is that the speed is lower, and there are reasons for that:

Another factor may be server and gateway overload on the ISP. This depends on the structure of the provider's network (e.g. many gateways) or the low bandwidth of the outgoing provider's channel. The problem is observed at a high load on the user's side. Too many requests to the server may exceed its maximum use during peak hours and cause slowdowns in operation.

Routing problems can also cause speed degradation. When routing problems are detected, packets can be rerouted along alternative routes, causing delays in data transmission.

Using PPPoE may also lead to a speed reduction. PPPoE is a tunnelling network protocol of the channel layer transmission of PPP frames over Ethernet. They are primarily used by ADSL services. PPPoE is a resource-intensive protocol, and when transmitting network data, the requirements for the processor increase. Depending on the implementation and use of PPPoE, a maximum speed reduction of 5-25% can be seen.

Insufficient performance of the BRAS provider's server (Broadband Remote Access Server). A broadband remote access server routes traffic to/from the ADSL switch in the ISP's networks. BRAS is located in the ISP core network and aggregates user connections from the access layer network. The server performs logical termination of point-to-point tunnels (PPP). These can be encapsulated PPP tunnels over Ethernet (PPPoE) or PPP over ATM (PPPoA). BRAS is also an interface to authentication, authorization and traffic accounting systems.

A speed limit on the tariff plan on the BRAS server is possible. Typical cases where the speed of the physical connection is high and the data receiving speed is limited by the paid tariff plan.

When an additional service, such as IPTV, is used, the stream of received television also takes up a certain bandwidth, for example, about 4 Mbit/s for standard resolution channels. The maximum data rate for IPTV service can be calculated using the following formula: Connection speed - 15% - IPTV stream speed.

For example, the connection speed (10240) - 15% (1500) - IPTV stream speed (4000) = 4700 kbit/s (587 KB/s).

Also, personal computer and software settings may cause an additional delay when sending network packets. For example, the problem may be low computer performance, lack of virtual memory or RAM, incorrect network adapter operation, lack of free space on the hard disk, data buffering in a web browser or download manager, etc.

Many users often use popular Internet services to check the Internet channel speed (e.g. speedtest.net). Please note that speed checks with Internet services do not guarantee a reliable measurement. In this case, your Internet channel speed measurement accuracy will depend on the selected server and its load, location, Internet channel load, and other factors. For more accurate results, it is recommended to disable applications that use Internet traffic (downloads, online videos, games, etc.) for the test period. We suggest you repeat the test several times and compare the data received. By default, the service automatically selects the optimal server for speed testing. But it is important to consider the location of the server. There have been cases when the service has selected the server for testing incorrectly. The service provides an option to specify the server manually. If you notice that the service has selected a server from another city or region, specify the correct server to test manually.

One way to get more reliable results is to measure the download speed of a large file previously placed in the cloud file service (for example, Dropbox or Google Drive).

Here's an example.

A 950 MB file was uploaded to Dropbox. Then we download this file from the cloud to your computer, but it will be necessary to track the time of downloading using a clock or stopwatch. In our case, the file of 950 MB size was downloaded in 10 minutes and 50 seconds (650 sec). Now the file size in megabytes should be divided into download time in seconds: 950/650 =1.5 MB. The Internet channel speed is usually specified in megabits, so let's convert megabytes to megabits by multiplying the previously calculated value to 8. We'll get 1.5*8 = 12 Mbit/s. In our example, the value of 12 Mbit/s will be the ADSL channel's actual data rate. At the same time, the channel speed of the connection in our example was about 15 Mbit/s.