Wi-Fi, a well known trademark for certified products based on the IEEE 802.11 standards, has become a widely used technology. The recent rise in the use of laptops, PDAs and other devices operating wirelessly has further increased the demand for Wi-Fi connectivity. It is an adequate solution for staying connected outside the office in places such as conference rooms and libraries, but also in hallways, cafeterias and open spaces. IT-CS operates more than 500 wireless base stations around CERN, mainly in public areas. This is progressively being improved.
With the successive improvements to 802.11 technology and its increased popularity, some believe that Wireless Local Area Networks (LANs) can replace classical wired LANs. Is this the case?
Wi-Fi technology is designed to extend the wired network by providing wireless connectivity but it suffers from a certain number of limitations because it is impossible to master the communications medium, i.e. the air. Therefore, one weakness will come from the way in which the radio waves propagate.
Radio-frequency obstacles and interferences
As for mobile telephony (e.g. GSM) or other radio technologies, no radio frequency (RF) coverage can be perfect. As such, during their propagation, radio waves are affected by several phenomena such as reflection, refraction, diffraction or absorption, which will distort the original wave pattern. When dealing with indoor environments, like the CERN buildings, the effects are even worse due to the high number of obstacles such as walls, ceilings, doors and cabinets.
Moreover, this coverage evolves due to changes in the surrounding environment, such as opening of doors and movement of elevators or furniture. When planning RF deployments, we attempt to optimize wireless coverage, but this coverage is not homogeneous around the access points and potentially degraded areas will appear at some places. As a consequence, the Wi-Fi client will decrease its data rate to reduce the bit error ratio and preserve the quality of the connection as far as possible.
Because Wi-Fi uses radio waves on unlicensed bands, the transmission is likely to interact and be disturbed by other sources operating at the same frequency, such as microwave ovens and cordless phones. However, IEEE 802.11 devices may also interfere with each other, e.g. stations connected to the same radio cell facing the hidden node issue. But the most disastrous impact on the RF quality is caused by co-channel interferences coming from neighbouring access points, working on the same channel or overlapping ones (a particular problem with the 802.11b/g where only three channels are usable). Adding more base stations is not a solution because in most cases this will degrade the communication even further.
Wi-Fi performance
The connection data rate indicated on a wireless device corresponds to the nominal bandwidth of the physical layer. This is different from the payload throughput available for transferring data. Before transmitting any payload packet, a complex mechanism takes place to establish and manage the wireless communication. This consumes a lot of time and dramatically increases the transport overhead. Consequently, the available payload throughput is reduced by more than 50%.
Table 1 shows the most common IEEE 802.11 standards and their typical rates. The 802.11a/b/g/n standards are half-duplex, which means that a device can either transmit or receive data, but not at the same time. In the Wi-Fi world, the bandwidth is shared in an unfair way between all of the clients in the same area. Serious performance degradation can be observed due to reduced available throughput and an increase in latency and jitter.
In Wi-Fi communications, many factors reduce the expected performance of wireless connections, e.g. location, interference, station density and limitations in the technology. Wi-Fi does provide mobility and will continue to be deployed at CERN as technology advances are made. However, the CERN wired infrastructure remains better in terms of performance and reliability. Consequently, you are strongly recommended to use wired connections in offices where possible to benefit from network rates up to 1 Gbps. If all of the plugs are in use at a location, it is possible to connect several computers to the same socket using a FANOUT, available from the CERN Stores (SCEM: 80.02.08.030.0).
Useful link
Wi-Fi site: http://cern.ch/wireless