There are quite a few possibilities here. The below is list of the most common reasons there are discrepancies between the advertised rate and the realized rate:
The internet connection isn’t properly isolated. To accurately benchmark the connection you need to be sure that only 1 computer is on the network. Otherwise, the aggregate utilization among all devices will make your test benchmark slower. Even when you aren’t consciously downloading content, updates, synchronizations, etc. are occurring at all times on most modern devices.
Even if you only have 1 device, if that 1 device is doing something in the background that isn’t part of your benchmark test, less total throughput will be available to the test and the results will be inaccurate as a result. You also need to be sure all devices/equipment within your network that is part of the testing path can handle the maximum throughput of the connection for cable internet Plan
You’re using wireless or hardwired connection that is slower than your internet connection. Networks are only as fast as the slowest link in the communication path. Thus, if your wireless access point, Ethernet NIC, Switch, etc. are slower than the maximum throughput of the connection, you’ll be bottlenecked by how your testing platform connects.
There’s a signal quality issue between you and your ISP so the maximum throughput is less than the subscribed rate due to signal integrity issues. This is common in cable/coaxial connections when they’re experiencing ingress (interference). This is somewhat less common with technologies such as DSL as the copper pairs are generally dedicated to a specific account through to the node. Cable internet is similar to having one line that split as many times as needed to reach all the customers.
You may be benchmarking to a server that has a slow link somewhere between your ISP and the testing server. Ideally, you should use a testing server that is hosted by your ISP so the test stays entirely within their network.
Realize that advertised speeds are the maximum rate between your premise and the ISP ONLY. Once traffic leaves your ISP’s network, all bets are off. To put it another way, you may have a 100 mbps connection between you and your ISP, with perfect signal integrity and properly isolated, but there’s a good chance that most resources on the internet aren’t going to let you download from them at 100 mbps.
Your advertised rate is generally not guaranteed by most broadband providers. They sneak this into the contract and disclaimers using verbiage like “up to…” and “in times of network congestion.” Most internet providers oversubscribe their connections. What this means is that not every customer in the area is able to get their maximum speeds if they all attempt to do so at the same time. As a result, you slow down during peak usage periods.
Essentially, if the advertised rate and the realized rate don’t match, you’ve either got flawed testing methodology, a technical issue or your ISP doesn’t actually guarantee the advertised speed.
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