Building-to-building Design
Building-to-building overview

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.