For example, the 802.11 MAC layer uses CSMA/CA. Transmitters expect to receive a positive acknowledgement (ACK) for every packet sent. If they do not, collision (loss) is assumed, prompting retransmission. These ACKs, along with collision avoidance techniques like RTS/CTS control frames, make 802.11 a relatively high-overhead protocol. In 802.11a/b/g wireless LANs (WLANs), effective throughput is usually little more than half the available bandwidth. (By comparison, on an uncongested Ethernet LAN, 802.3 overhead consumes roughly a third of available bandwidth.) For both 802.11 and 802.3, higher-layer protocols -- usually TCP -- are used by applications that require reliable delivery. Why? Packets can be lost anywhere in the food chain -- not just when traversing the LAN, but on any hop as each packet makes its way through the network.
However, to compare the reliability and efficiency of different wireless network types, I had to ask my own favorite wireless radio expert, Craig Mathias of Farpoint Group. Craig's response:
"Oh my, that's tough one! There's an inherent degree of statistical variability in any wireless link. Radio is pretty much radio, whether we're talking WLANs or WWANs. The key is keeping the signal to noise ratio ( SNR) up -- the louder the signal, the better the chance the packet makes it through. This is a function of transmit power (which is limited by regulation), receiver gain, the geometric relationship between transmitter and receiver, the types of antennas used, and many, many other factors. All things being equal, though, all commercial systems are about equally reliable from a basic technology perspective. But then there's [also] how good a given carrier network is, or how good the WLAN implementation is. And 'efficiency' and 'reliability' may not correlate."
Craig noted that the future of both WWANs and WLANs is Orthogonal Frequency Division Multiplexing (OFDM), which has good reliability and efficiency at its core. For example, a protocol like WiMAX uses hybrid automatic retransmission in the event of error, which further improves link layer reliability. But ultimately, so long as there is sufficient link margin (the difference between the strength of the actual signal received and the minimal signal required for successful communication), any wireless technology can be made reliable by higher layer protocols like TCP that compensate for lower-layer packet loss.
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