Building-to-building WLANs present some challenges. As the distance between
sites increases, it becomes more difficult to create quality links. Also,
antennas must be deployed depending on the distance between sites. The cost to
install a tower may become the most expensive item in the project.
Aside
from the cost issue, local, state, or federal regulations may provide obstacles
when erecting towers. Even building mount antennas may be against some local
building regulations. Be sure to investigate these issues and obtain permits
before finalizing the design plan. Even one denied permit can seriously
jeopardize a project. It is best to deal with it during the design phase.
When considering building-to-building designs, distance and bandwidth have a
great impact on the overall design. Greater distances are possible using slower
speeds. This is because the signal gets weaker as it extends outward and so
does the noise level. Higher bandwidth requires lower noise because of the
compression and modulation techniques used.
Many corporations would like
to have as much bandwidth between new locations for a variety of applications,
even though the 802.11 standard is limited to 11 Mbps. For WLANs, it is
possible to use fast etherchannel or multilink trunking to bond or aggregate
three bridges together and give the customer a potential of 33 Mbps.

One option
to provide greater bandwidth is to use 802.11a or 802.11g. The Cisco Aironet
1400 series offers an 802.11a bridging solution that will provide up to 54 Mbps
on each link. However, 802.11a cannot reach distances as far as 802.11b. The
802.11g standard will soon provide the same distance as 802.11b but it will
also operate at 54 Mbps.
Finally, WLANs must be integrated properly to
maximize the bandwidth between sites
. This
can be accomplished several ways including filtering on the bridge, Layer 2
filtering using a switch, or Layer 3 filtering using a router. The router
solution is by far the best solution, which allows very granular control of the
traffic. A router can control the following:
- Routing protocols such as Routing Information Protocol (RIP), Open Shortest
Path First (OSPF), and Enhanced Interior Gateway Routing Protocol (EIGRP)
minimize the amount of bandwidth needed for routing protocols. Static routes do
not require bandwidth and are recommended when creating a stub network.
- Routed protocols such as Internet Protocol (IP), Internetwork Packet
Exchange (IPX) and AppleTalk minimize routed protocols across the link. Due to
frequent advertisements, IPX can consume needed bandwidth. If possible, limit
the traffic to pure IP.
- Source and destination minimize the addresses which are allowed across the
link.
- Security maximizes the security across the link using IPSec to create a
virtual private network (VPN).
- LAN broadcast eliminates Layer 2 and Layer 3 broadcast traffic such as ARP,
NetBeui, Cisco Discovery Protocol (CDP), IPX, and IP created by LAN devices
such as workstations, servers, and printers.