It is the transmit power (≈ voltage) with which your device sends its signal to the cable. It effectively means, how strongly your device "shouts" on the cable to the receivers (which is mainly an ADSL router by your ISP).
It is in decibel. Decibel is a
- relative
- logarithmic quantity.
Relative means, that it shows the proportion of the measured quantity, compared to a reference value.
Logarithmic is an uncommon thing. For example, in a linear unit, like meters, 2m means 2 times higher distance as 1m. In the case of decibel, +10 decibel means a 10 times of increase.
Thus, if you increase the Tx power with 10, you will get 10 times higher power. If you increase with 20, you get 100 times higher power.
Note, a 10 dB difference means a 10 times growth in the voltage, and thus a 100 times growth in the power (because the power depends on the voltage in these cases mostly quadratically). This is not important from a network engineering view (but it is from an electronic engineer view).
Tuning the TX parameter can be useful if you have to deal with too long or noisy cables. These cause mainly exponential losses with the cable length, this is why the logarithmic tuning is needed.
It may be also useful to suppress noise sources.
Similar tuning is useful also on other network types, where noise is a significant problem (for example, wifi).
Because it is a logarithmic quantity and the loss grow exponentially with the cable length, typically the required decibel has to be tuned linearly with the cable length (i.e. if you require, for example, 10 dB length grow for a cable length growth of 20m, you need again 10 dB for the next 20m).
Most L0 electronic can tune the TX on need.
In my experience, if you have to tune this value manually, the chances are high that it won't work very well with any setting (this is why the automatical tuning failed).
Twisted pair cable used for the local loop has a velocity factor of about .58 - each km of cable takes ca. 6 µs to travel, adding ca. 12 µs to latency. The rest of the Internet probably uses fiber with a VF of .67, resulting in ca. 10 µs or .01 ms latency per km.
ADSL has a basic encoding latency of around 10 ms. However, your ISP may likely interleave DSL packets to reduce burst error sensitivity, adding 5 to 25 ms to the latency, depending on the interleave depth. Interleaving is more likely used on longer access lines.
These are the line latencies only, each router on the way adds its own latency.
So, the length of your local loop has little impact on overall round-trip time unless interleaving is used because of the length. The ADSL encoding overhead and the general distance to a server and the way the routing is done over the Internet outweigh it by far.
Best Answer
You need to check a few things;
DSL is an aggregation technology which is normally on 'shared' media (you will be sharing with other, both the media to the point of presence and aggregation devices such as DSLAM, LNS so there is always potential for oversubscription.
It will also depend on latency, you need to ask the carrier for latency numbers for the leased line and test this against your DSL links. (Leased line should be better) however remember if latency is high no matter how much Mb (bandwidth) you purchase your download speeds will be limited;
Bandwidth-in-bits-per-second * Round-trip-latency-in-seconds = TCP window size in bits / 8 = TCP window size in bytes example below;
RTT is 20 ms, and connection speed is 10Mbps.
2 x (10Mbps/8 * .020s) = 50Kbytes (using standard window size of 64K)
Maybe you should look at using QoS effectively to limit wasting bandwidth