User's Manual
Design Principles of a M/LC Repeater Network
February 2004 7
2.2.3 Overall System Throughput Design Considerations
Even though intra-repeater (or intra-access point) traffic is at 11.0 Mbps, the “system” bandwidth or
throughput is reduced in direct proportion to the number of wireless access points (or “hops”) that a
message must be routed though in order to get to a bridge (an access point [AP] that is hardwired to the
LAN). Therefore, it is desirable to have as many “zero-hop” or “hardwired” APs as possible. Therefore,
always take advantage of all hardwired LAN locations as possible in order to install an AP. Good
examples of this are crushers, truck shops, and dispatch buildings with are often hardwired to the LAN
via fiber optics or even microwave links.
If a mine site is very large, and the hardwired Ethernet is only available at one location (which may not
even be a good site for an access point), then a point-to-point link should be used in order to construct
another “zero-hop” or “wired” equivalent access point. This is important to minimize bottlenecks and
avoid excessive “message hopping” though a repeater network. Although one of the 802.11b channels
may be used for this, often it is best to use a completely different frequency band in order to minimize 2.4
GHz traffic. The Cisco 1400 Aironet Bridges are highly recommended for this.
2.2.4 Channel Selection
802.11b LAN networks can use 11 channels of operation. These range from 1 (2412 MHz) to 11 (2462
MHz) and are separated by 5 MHz increments. However, because the occupied spectrum of each channel
is approximately 20 MHz, only three of these channels should ever be used at one time. Due to the RF
characteristics of the radio hardware, channel 1 almost always contains the most output power (and
slightly better performance). Assuming 0.3 dB per channel reduction, typically, channel 11 has 1.5 dB
less output power than channel 1. If good coverage and bandwidth can be achieved using only one
channel it should be channel 1. If two channels are decided on then it should be channel 1 and channel 11
since frequency separation is far more important than output power in this case. If three channels are to be
used, then channels 1, 6, and 11 would be used.