The WaveLAN is a wireless device designed for Ethernet-style, shared-medium in-building access. This device operates in the 902-928MHz ISM band and utilizes direct sequence CDMA spread spectrum technology. Due to the fact that the WaveLAN shares the same frequency band as the Metricom, we decided to test for possible interference between these two devices.
Figure 6: Interaction with the AT&T WaveLAN Device
The first experiment we performed was very simple. We measured the throughput of a transfer over the WaveLAN while simultaneously sending intermittent packets over the Metricom network using the UNIX ping program. Figure 6 shows the results. We see that the interference is quite high and WaveLAN performance is adversely affected. In particular, the throughput drops by 85% with an interfering traffic of only two small ping packets per second across the Ricochet network.
In the same manner, we also measured the reverse case, i.e., the interference of a WaveLAN stream with a Metricom transfer. The interference in this case was not as significant, with a maximum degradation of 10-15% in Ricochet network throughput when both networks were simultaneously transferring data using TCP.
Finally, in order to determine the maximum possible degradation in performance of the Ricochet network in the presence of the WaveLAN, we flodded the WaveLAN link with packets and measured the performance degradation of a Ricochet TCP connection. Our measurements show a throughput degradation of about 50% for 1 KB packet sizes in the experimental Ricochet network.
The reasons for the asymmetry in interference effects are mainly due to the fact that the WaveLAN device is a direct sequence, spread spectrum device that spreads its transmission energy across a wider frequency range in order to obtain diversity. In contrast, the Ricochet radios use frequency hopping to obtain diversity in the ISM band, and at each instant in time, focus a larger amount of energy in a narrower frequency band. The other important reason for this asymmetric interaction arises from the large difference in bandwidths in the two systems. The WaveLAN has a maximum signalling rate of 2 Mb/s, whereas the Ricochet modem has a 100 kb/s maximum signalling rate. Thus, the energy per bit of the WaveLAN is much lower, making it much more susceptible to interference.