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MicroCell Deployments: Making a Bad Problem Worse
Conventional Wireless LAN Wisdom
Wireless LANs are widely used in enterprises, homes and hot spots to provide connectivity to the Internet or network resources without wires. They are on their way to becoming ubiquitous, with almost everyone with a mobile laptop having the ability to connect to a Wi-Fi access point at home, on the road or at the office. Wireless LANs provide up to 54 Mbps of over-the-air bandwidth with 802.11g, reaching effective throughputs of ~24 Mbps in ideal conditions. 802.11g is the most popular wireless LAN standard currently being deployed, as it offers backwards compatibility with the previous generation of 11 Mbps 802.11b devices.
For larger hot spot or enterprise deployments, multiple access points are used to create pervasive, contiguous coverage. This allows users to move from one area to another without losing connectivity to the network.
As the client moves from one area to another, it recognizes that the signal it is receiving from the access point it is connected to is getting weaker. When the signal reaches a low enough threshold, the client will disconnect and search for a stronger signal from a new access point. This is termed handoff and is typically not noticeable by users of data applications.
The scenario described above sounds simplistic. Place access points such that you have contiguous coverage and mobile users will be able to roam and remain connected without any problem. Unfortunately, the situation is more complicated than that, and pervasive Wi-Fi deployments require enormous planning and effort in order to operate at a reasonable performance level.
The Challenges
Four basic facts about how Wi-Fi operates cause pervasive wireless LAN deployments to be difficult.
1) Reducing data rate vs. cell size
With a Wi-Fi access point, the data rate reduces as you move farther away from it. Close to the access point, you may experience speeds of up to 54 Mbps (for 802.11g), but as you move farther away, speeds will drop to 48, 36, 24, 12, 9, 6, 2 and eventually 1 Mbps.
Access point range can extend up to 300 feet indoors at the 1 Mbps rate. So, to achieve higher overall throughput, access points must be placed closer together.
2) Limited non-overlapping channels
The 802.11b and 802.11g wireless LAN standards operate in the 2.4 GHz band. This band only offers three non-overlapping channels. Each channel is a separate "pipe" of bandwidth. All clients attached to the access point share that same bandwidth. Therefore, if you have 6 clients all attached to an 802.11g access point that achieves 24 Mbps effective throughput, at most they each have 4 Mbps. If you now place two more access points on different channels with contiguous coverage areas to the 1st one, and distribute the clients evenly among them, each 2 clients will share 24 Mbps of bandwidth, or have 12 Mbps each.
3) Collisions and interference
As wireless clients are not physically connected together, they are unaware of when others are trying to transmit. As a result, it is likely that multiple clients in the same area will transmit at the same time. When this occurs, the clients are programmed to 'back off.' That is, to wait some period of time before trying to transmit again. The waiting period increases with each collision; the idea being that the longer the interval, the higher the probability that the other client won't try to transmit at the same time. Back off causes the performance of the client to be reduced. If the client can normally transmit at 54 Mbps, but has to wait due to other collisions, then the effective transmission rate is reduced because of the time spent waiting.
4) Handoff between access points
When a client roams from one access point to another, the time between disconnecting from the 1st access point and reconnecting to the 2nd access point is non-zero. This process involves the client searching for the next strongest signal by scanning the band. For some clients, this process can take up to several seconds.
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