For our tests, we used a Dell Latitude D620 notebook PC and the Draytek N61 802.11n USB adapter for file transfers, along with two test instruments. First, we used the same notebook and interface with the AirMagnet Laptop Analyzer software. Second, we used an outstanding hand-held wireless device from Berkeley Varitronics called the YellowJacket. Our primary tests measured signal strength, which we explain shortly. We also timed an FTP transfer of a 100-MB file from a PC attached to the Draytek 2930n on the LAN side to our test notebook, at distances of 10, 25, 50 and 100 feet from the router at three of the four compass points relative to that unit.
Signal Strength Measurements
Thanks to the YellowJacket, we compiled two measures of signal strength for the Draytek 2930n: Receive Signal Strength Indicator (RSSI) uses a sensing circuit to measure strength for an incoming signal. This circuit detects RF signals and generates output equivalent to signal strength. The YellowJacket measures RSSI in range from -20 to -90 dBm on the 5.0 GHz band at 1 dBm resolution. Our RSSI measurements ranged from -30 dBm to as high as -70 dBm, all well within tolerance ranges for 802.11n, where higher negative values correlate to slower throughput. Because numbers are negative, higher values are “worse” than lower ones.
Signal to Noise ratio (S/N), which compares relative signals and noise in a specific measurement as a ratio. Here, a higher value is “better” because it indicates a higher ratio of signal (good data) to noise (meaningless, unusable data). Our S/N data measurements ranged from a high of 63 (good) at a 10 foot distance to a low of 11 (low) at a 100 foot distance.
We measured data transfer by the whole number of seconds it took to transfer a 100-MB file from a LAN-attached desktop PC to a wireless notebook PC across the LAN, through the 2930n and onto the notebook’s hard drive. We divided 100 by the number of seconds each transfer required to determine megabytes per second (MBps), then multiplied by eight to calculate megabits per second (Mbps; a standard metric for network throughput). Table 2 provides measurements, after which we discuss implications.
At close range, the results were as good or better than anything we’ve ever measured for 802.11g. At the outer edge of the distance range, however, throughput fell off dramatically, as is usual for broadcast communications. The theoretical maximum distance for 802.11n is around 50m/164ft indoors, and 125m/410ft outdoors. Our measurements show that even at 100 feet, the throughput advantages of 802.11n start to diminish significantly. These results also track our S/N and RSSI measurements. We’d rate the Draytek 2930n as “good, but not great” for signal strength and signal quality at longer distances. That said, count on better reception and throughput at longer distances with 802.11n at 5.0 GHz as compared to 802.11g at 2.4 GHz.