- Roaming patterns of wireless clients
- Applications used by wireless clients
WLAN Deployment and Application Impact
WLAN deployments impact application use differently. It is important for you to understand these impacts as you plan your WLAN deployment. The keys are the following:
- Effective per-client throughput
- Streaming versus bursty application types
- Medium contention and application latency
Effective per-client throughput is decreased as each new client joins the base station subsystem (BSS). Although each user is not explicitly guaranteed a specific amount of bandwidth, the distributed coordination function (DCF) medium-access mechanism provides fair access to the wireless medium, suggesting that each client has equal access to (and a portion of) the wireless medium. In a world where switched 10 Mbps and 100 Mbps Ethernet are commonplace, sharing 11 Mbps or even 54 Mbps (802.11b and 802.11a, respectively) among 10 to 25 other clients can be perceived as a step backward.
Given an 11 Mbps data rate for 802.11b networks and a shared, half-duplex medium, it is reasonable to expect no more than 6 Mbps of actual throughput. The total available throughput among 25 clients yields roughly 245 Kbps per client. Extending the same ratio to 802.11a BSSs with a 54 Mbps data rate, a reasonable throughput rate is roughly 22 Mbps, yielding an average throughput rate in the range of 880 Kbps per client. Note that this number is just a reference, assuming all clients are transmitting and receiving equal amounts of data.
WLAN Deployment Planning
There are two general methodologies in deploying WLANs:
- Coverage oriented
- Capacity oriented
Coverage-Oriented WLANs
A coverage-oriented WLAN is designed to provide maximum WLAN coverage with the least amount of APs. (A typical coverage-oriented network provides a user to AP ratio of 25 to 1.) Some characteristics of a coverage-oriented deployment include the following:
- Bursty, low packet rate application types, such as barcode scanning and database queries
- Low bandwidth requirements, allowing data rate scaling down to lower data rates such as 1 or 2 Mbps
- Ease of maintenance because of little or no support staff local to the WLAN
In coverage-oriented deployments, the typical applications have low packet rates and low bandwidth requirements. Because of these meager demands on the WLAN, the users can expect effective throughput in these deployments to be quite high.
This setup allows more users to leverage the WLAN while still maintaining adequate performance.
These types of deployments might be common in warehouse environments or retail environments where WLANs are mission critical for inventory control and just-in-time purchasing and where the IT staff is in a central site, with no local support staff to troubleshoot coverage issues.
Also, it might be common for small or medium branch offices to opt for such a deployment as an alternative to installing wired Ethernet. In these cases, the branch offices experience a significant amount of relocation and Category 5 cabling expenses are minimized. A simple-todeploy WLAN provides basic network connectivity such as file and printer sharing.
Figure 8-2 illustrates the example floor plan from Figure 8-1 with a coverage-oriented WLAN deployment.
Capacity-Oriented WLANs
A capacity-oriented WLAN is designed to provide maximum throughput and packet rate for each client in a BSS. Capacity-oriented cell sizes are smaller than coverage-oriented cells, requiring a higher AP density. Capacity-oriented deployments are required for areas that have the following characteristics:
- High packet rate applications
- Latency-sensitive applications
- Smaller-sized subnet deployments (or multiple subnets per coverage area)
- Dense client population
14 APs).
