Introduction

Bit errors are a rare phenomenon in most wired local area networks. Wired Ethernet² rarely operates close to the noise threshold -- link distances are limited by restrictions on delay, not by signal power. See Figure 1. As such, the cause of misbehavior in wired Ethernet is typically congestion, not noise, and the type of error usually manifested is the loss of an entire packet, not a flipped bit.

Figure 1: An example 802.3 waveform at 10 Mbit/sec (the sequence number ``212'' in an ICMP ping packet). Up-transitions in the middle of bit intervals indicate ones, down-transitions zeros. Note the almost complete lack of noise.

Wireless networks are different. The range of a 32 mW 802.11 base station indoors is only a few tens of feet before bit errors begin to appear. Outdoors, there is much interest in stretching 802.11 links as far as possible, including MIT's own Roofnet project³ to build a citywide 802.11 mesh network in Cambridge, Massachusetts.

Figure 2: An example 802.11 packet at 1 Mbit/sec, received over a 0.55-mile outdoor line-of-sight link. This packet has 90 octets received incorrectly (indicated with *) out of 1,392.

The causes of misbehavior on an 802.11 link thus include, in addition to congestion, low signal-to-noise ratio and interference. These latter two produce incorrectly received bits, causing a failed CRC, a lack of acknowledgement, and the sender's retransmitting the entire packet. (Low signal-to-noise and interference can also cause a receiver not to hear the sender's preamble and miss the packet entirely.)

Figure 2 shows an example 802.11 payload after transmission 2,900 feet along an outdoor line-of-sight link. The payload was 1,392 octets, of which 1,301 were received correctly and 90 incorrectly (indicated with *). At this error rate, most 802.11 senders will give up on retransmissions before the receiver is able to receive an entire packet correctly.

With some fraction of each packet devoted to forward error correction, the receiver can recover from bit errors. By adding redundancy to each packet in software -- essentially, increasing the noise margin of the transmission path by reducing throughput -- we were able to extend the outdoor range of 802.11 by up to 70 percent, from 0.5 miles to 0.85 miles.