ADSL speed

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 at least 13-15% when using ADSL technology. This is a technological constraint that we'll talk about in a further 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, there are a number of other factors that reduce transmission speed, in addition to the technological limitation. 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 speed to the subscriber (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:
Connection speed (10240) - 15% (1500) = 8700 kbps, then divide that number by 8 (to convert kilobits into kilobytes) and we're gonna get 1000 KB/s.

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

Communication equipment (IP ADSL-switches) uses ATM (Asynchronous Transfer Mode) technology as a transport protocol. ATM is a network high-performance switching and multiplexing technology based on data transmission in the form of frames (cells) of a fixed size (53 bytes).
As you know, the Internet uses IP as a communication protocol, and certainly TCP/IP. ADSL technology uses ATM as the transport protocol, so the data is transmitted over your ADSL line using TCP/IP via ATM. I.e. IP-frames are packed (encapsulated) in АТМ-cells and transmitted via DSL-line, and then unpacked by the receiving equipment, and regular IP-frames are obtained.
Large packets will be divided into 48-byte parts. If the packet is not divided by 48 bytes exactly, it is supplemented with a filling to obtain an integer number of 48-byte cells. After the packet is divided into 48-byte cells, a header (5 bytes) is added to each of the cells.
As a result, the actual speed is reduced to 10% of the data rate.

TCP/IP protocol usage reduces the speed by approximately 3% of the declared data transfer rate, as the transmitted useful information is supplemented by service (protocol) information.

The above-mentioned factors are the technological limitations referred to at the beginning of the article. These limitations mean that the actual data rate is always lower than the channel rate by at least 13-15%.

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:

For example, due to packet loss during data transfer. High losses are possible on bad lines or by using the maximum permitted connection speed.
If there is a packet loss during frame transmission, the TCP/IP protocol detects the missing packet in the common data stream, does not accept its reception and then initiates the retransmission of the lost data. The retransmission procedure causes additional delays.
Thus, in addition to the important monitoring and data transfer function, TCP/IP slows down the data transfer rate in the case of high line losses.
You can use the 'ping' utility to check the quality of your Internet connection to the server. In the command line of the operating system perform ping -t <site_name>, for example ping -t www.download.com. Wait 30 seconds and then press Ctrl+C to exit the utility. The statistics will show the % of packet losses. If the packet loss exceeds 5%, the performance of the TCP/IP protocol will be poor for this site.

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 high load on the users 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 tunneling network protocol of the channel layer transmission of PPP frames over Ethernet. 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.

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 channels of standard resolution. 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 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, the accuracy of your Internet channel speed measurement will depend on the selected server and its load, its location, the 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 to 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 of the methods to get more reliable results is to measure the download speed of a large file that has been previously placed in the cloud file service (for example, Dropbox or Google Drive).
Here's an example. A 950 Mbyte 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 Mbytes 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 MByte/s. The Internet channel speed is usually specified in megabits, so let's convert a 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 actual data rate on the ADSL channel. At the same time, the channel speed of the connection in our example was about 15 Mbit/s.
 
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