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Alvarion, Inc. 1890 Rutherford Road, Carlsbad, CA 92008 760.517.3100 www.alvarion.com IEEE 802.11 Technical Tutorial Introduction The purpose of this document is to give technical readers a basic overview of the new 802.11 Standard, enabling them to understand the basic concepts, principle of operations, and some of the reasons behind some of the features and/or components of the Standard. The document does not cover the entire Standard and does not provide enough information for the reader to implement an 802.11 compliant device (for this purpose the reader should refer to the Standard itself). IEEE 802.11 Architecture Architecture Components An 802.11 LAN is based on a cellular architecture where the system is subdivided into cells. Each cell (called Basic Service Set, or BSS, in the 802.11 nomenclature) is controlled by a Base Station (called Access Point or, in short, AP). Although a wireless LAN may be formed by a single cell, with a single Access Point, (and as will be described later, it can also work without an Access Point), most installations will be formed by several cells, where the Access Points are connected through some kind of backbone (called Distribution System or DS). This backbone is typically Ethernet and, in some cases, is wireless itself. The whole interconnected Wireless LAN, including the different cells, their respective Access Points and the Distribution System, is seen as a single 802 network to the upper layers of the OSI model and is known in the Standard as Extended Service Set (ESS). The following diagram shows a typical 802.11 LAN including the components described above:Alvarion, Inc. 1890 Rutherford Road, Carlsbad, CA 92008 760.517.3100 www.alvarion.com The standard also defines the concept of a Portal. A portal is a device that interconnects between an 802.11 and another 802 LAN. This concept is an abstract description of part of the functionality of a “translation bridge”. Even though the standard does not necessarily request it, typical installations will have the AP and the Portal on a single physical entity. This is also the case with BreezeCOM’s AP which provides both functions. IEEE 802.11 Layers Description As any 802.x protocol, the 802.11 protocol covers the MAC and Physical Layer. The Standard currently defines a single MAC which interacts with three PHYs (all of them running at 1 and 2 Mbit/s) as follows: n Frequency Hopping Spread Spectrum in the 2.4 GHz Band n Direct Sequence Spread Spectrum in the 2.4 GHz Band, and n InfraRedAlvarion, Inc. 1890 Rutherford Road, Carlsbad, CA 92008 760.517.3100 www.alvarion.com Beyond the standard functionality usually performed by MAC Layers, the 802.11 MAC performs other functions that are typically related to upper layer protocols, such as Fragmentation, Packet Retransmissions, and Acknowledges. The MAC Layer The MAC Layer defines two different access methods, the Distributed Coordination Function and the Point Coordination Function: The Basic Access Method: CSMA/CA The basic access mechanism, called the Distributed Coordination Function, is basically a Carrier Sense Multiple Access with Collision Avoidance mechanism (usually known as CSMA/CA). CSMA protocols are well-known in the industry, the most popular being the Ethernet, which is a CSMA/CD protocol (CD standing for Collision Detection). A CSMA protocol works as follows: A station desiring to transmit senses the medium. If the medium is busy (i.e. some other station is transmitting) then the station defers its transmission to a later time. If the medium is sensed free then the station is allowed to transmit. These kinds of protocols are very effective when the medium is not heavily loaded since it allows stations to transmit with minimum delay. But there is always a chance of stations simultaneously sensing the medium as being free and transmitting at the same time, causing a collision. These collision situations must be identified so the MAC layer can retransmit the packet by itself and not by upper layers, which would cause significant delay. In the Ethernet case this collision is recognized by the transmitting stations which go into a retransmission phase based on an exponential random backoff algorithm. While these Collision Detection mechanisms are a good idea on a wired LAN, they cannot be used on a Wireless LAN environment for two main reasons: 1. Implementing a Collision Detection Mechanism would require the implementation of a Full Duplex radio capable of transmitting and receiving at once, an approach that would increase the price significantly. 2. In a Wireless environment we cannot assume that all stations hear each other (which is the basic assumption of the Collision Detection scheme), and the fact that a station wants to transmit and senses the medium as free doesn’t necessarily mean that the medium is free around the receiver area. In order to overcome these problems, the 802.11 uses a Collision Avoidance (CA) mechanism together with a Positive Acknowledge scheme, as follows:Alvarion, Inc. 1890 Rutherford Road, Carlsbad, CA 92008 760.517.3100 www.alvarion.com 1. A station wanting to transmit senses the medium. If the medium is busy then it defers. If the medium is free for a specified time (called Distributed Inter Frame Space (DIFS) in the standard), then the station is allowed to transmit. 2. The receiving station checks the CRC of the received packet and sends an acknowledgment packet (ACK). Receipt of the acknowledgment indicates to the transmitter that no collision occurred. If the sender does not receive the acknowledgment then it retransmits the fragment until it receives acknowledgment or is thrown away after a given number of retransmissions. Virtual Carrier Sense In order to reduce the probability of two stations colliding because they cannot hear each other, the standard defines a Virtual Carrier Sense mechanism: A station wanting to transmit a packet first transmits a short control packet called RTS (Request To Send), which includes the source, destination, and the duration of the following transaction (i.e. the packet and the respective ACK), the destination station responds (if the medium is free) with a response control Packet called CTS (Clear to Send), which includes the same duration information. All stations receiving either the RTS and/or the CTS, set their Virtual Carrier Sense indicator


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Berkeley ELENG 228A - IEEE 802.11 Technical Tutorial

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