A wireless local area network (LAN) is a flexible data communications system implemented as an extension to, or as an alternative for, a wired LAN. Using radio frequency (RF) technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections. Thus, wireless LANs combine data connectivity with user mobility.
The widespread reliance on networking in business and the meteoric growth of the Internet and online services are strong testimonies to the benefits of shared data and shared resources. With wireless LANs, users can access shared information without looking for a place to plug in, and network managers can set up or augment networks without installing or moving wires. Wireless LANs offer the following productivity, convenience, and cost advantages over traditional wired networks: Mobility: Wireless LAN systems can provide LAN users with access to real-time information anywhere in their organization. This mobility supports productivity and service opportunities not possible with wired networks. Installation Speed and Simplicity: Installing a wireless LAN system can be fast and easy and can eliminate the need to pull cable through walls and ceilings. Installation Flexibility: Wireless technology allows the network to go where wire cannot go. Reduced Cost-of-Ownership: While the initial investment required for wireless LAN hardware can be higher than the cost of wired LAN hardware, overall installation expenses and life-cycle costs can be significantly lower. Long-term cost benefits are greatest in dynamic environments requiring frequent moves and changes. Scalability: Wireless LAN systems can be configured in a variety of topologies to meet the needs of specific applications and installations. Configurations are easily changed and range from peer-to-peer networks suitable for a small number of users to full infrastructure networks of thousands of users that enable roaming over a broad area.
In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure . A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna attached to the access point) is usually mounted high but may be mounted essentially anywhere that is practical as long as the desired radio coverage is obtained.
Wireless LANs can be simple or complex. At its most basic, two PCs equipped with wireless adapter cards can set up an independent network whenever they are within range of one another. This is called a peer-to-peer network. On-demand networks such as in this example require no administration or preconfiguration. In this case each client would only have access to the resources of the other client and not to a central server.
Installing an access point can extend the range of an ad hoc network, effectively doubling the range at which the devices can communicate. Since the access point is connected to the wired network each client would have access to server resources as well as to other clients. Each access point can accommodate many clients; the specific number depends on the number and nature of the transmissions involved. Many real-world applications exist where a single access point services from 15-50 client devices.
Access points have a finite range, on the order of 500 feet indoor and 1000 feet outdoors. In a very large facility such as a warehouse, or on a college campus it will probably be necessary to install more than one access point. Access point positioning is accomplished by means of a site survey. The goal is to blanket the coverage area with overlapping coverage cells so that clients might range throughout the area without ever losing network contact. The ability of clients to move seamlessly among a cluster of access points is called roaming. Access points hand the client off from one to another in a way that is invisible to the client, ensuring unbroken connectivity.
Doctors and nurses in hospitals can use hand-held or notebook computers with wireless LAN to deliver patient information instantly. Wireless campus, just like what does in LYU9902 team.Students can use wireless connectivity to ease access to information, information exchanges, and learning. Wireless Order Entry, Wireless Point-of-Sale terminals, Managing restaurant seating. Warehouse workers use wireless LANs to exchange information with central databases, thereby increasing productivity. Trade show and branch office workers minimize setup requirements by installing pre-configured wireless LANs needing no local MIS support. Quickly expanding branch office networks, Wireless trading. Mobile access in Office and consulting environments, temporary workgroups and projects.
cost savings come from not needing to remove and install and test the cable. 2Mbps bandwidth enough for common data transfer, e.g, database quries, email. Some experiments show that wireless LAN do well with roaming notebook PCs and they do allow the user to move within the office while maintaining a network connection mention before, since no need to construct the wired network, it is flexible to more the wireless network as the users needs. Especially when the building is not allow or difficult to install wired network, wireless networks can be very useful at that time. Also mention before, it is easier than installing wired network.
2Mbps not enough for multimedia access, e.g. video need about 1.5Mbps for mpeg-1. And even not very enough for image transfer. The number of nodes connected to an access point is limited. E.g proxim and waveLAN have range about 150metres to 300 metres. It is limit range. Some experiments shows that Wireless LAN cards can cut the life of a PC battery by 20% or more Line-of-sight: This limitation restricts their usefulness in many situations, such as in so-called hard-walled offices and multi-story buildings.
RED The RED congestion control mechanisms monitor the average queue size for each output queue, and, using randomization, choose connections to notify of that congestion. Transient congestion is accommodated by a temporary increase in the queue. Longer-lived congestion is reflected by an increase in the computed average queue size, and results in randomized feedback to some of the connections to decrease their windows. The probability that a connection is notified of congestion is proportional to that connection share of the throughput through the gateway. SFQ SFQ uses a simple hash function to map from the source-destination address pair to a fixed set of queues. Since the assignment of an address pair to a queue is probabilistic, there is the likelihood of multiple address pairs colliding and mapping to the same queue. This would potentially degrade the additional fairness that is gained with Fairness Queuing. If two or more address pairs collide, they would continue to do so. To deal with the situation when such a collision occurs, SFQ periodically perturbs the hash function so that these address pairs will be unlikely to collide subsequently. CBQ It is an approach of hierarchical link-sharing management. A hierarchical link-sharing structure can be used to specify guidelines for the distribution of “excess” bandwidth. While one could imagine complex requirements in terms of exactly how “extra “ bandwidth is distributed or what fraction of bandwidth each class requires over a range of time intervals. However, CBQ is flexible and easy to manage. Moreover, it gives good result even we just define a simple class structure. In the following section, the advantages of CBQ will be discussed in more details.
Mp3 player implemented by last year project team. We have port it to wireless networks. A user can now use the notebook with wireless LAN to listen to get the songs from server and listen to the songs.
Compare the performance before using QoS, and after using QoS. Mp3 becomes steady after QoS is used. It is the test under waveLAN.
It is the test under Proxim. The result is similar… so, we can see that the performance for waveLAN and proxim is similar. Actually, the graph is also similar to that in the wired networks. Because we have enough bandwidth to do the experiments.
LYU9903 QoS Schemes in Wireless Networks Ho Pun Mo Matchman Ng Maggie Supervised by Prof. Michael Lyu