II. ARCHITECTURE
A. HARDWARE ARCHITECTURE
1. Two modes:
Infrastructure: Infrastructure mode
based on a special station called Access Point (AP). This mode allows wireless stations to connect to a network (typically Ethernet) via an access point. It allows a wireless station to connect to another station via their WLAN PA common. A station associated to another wireless AP can also interconnect. All stations within radio range of the PA form a BSS (Basic Service Set). Each BBS is identified by a BSSID (BSS Identifier) of 6 bytes that corresponds to the MAC address of the AP.
Ad-Hoc:
The operation of this mode is fully distributed, there is no hierarchically structuring element or cell for transmitting frames from one station to another. This mode allows Communication between two machines without using an infrastructure. Stations located in radio range form an IBSS (Independent Basic Service Set).
2. Interconnection:
can dial a network with multiple BSS. These are interconnected by a distribution system (DS) connected to their access points. This DS is typically an Ethernet network which connects the PA but it may correspond to the token ring, FDDI or other network 802.11. These different BSS interconnected via a DS form an ESS (Extended Service Set). An ESS is identified by an ESSID (abbreviated as SSID), which consists of a word 32 characters representing the name of the network.
can associate an IBSS in an ESS.
B. ARCHITECTURAL SERVICES
1. Presentation:
The protocol provides 802.11 MAC model as in the IEEE 802. These services are transport frame in offline mode (best effort), the algorithm security WEP (Wired Equivalent Privacy). In addition, the manner, services are provided.
Ad-Hoc mode: This mode uses the basic services:
- authentication of the resort (optional)
- data transport,
- security.
Infrastructure mode: in addition to the above basic services, other services are available:
- association-disassociation: a station that wants to connect to the network must work with a PA,
- Distribution: this service helps convey a frame to its final destination via the AP.
- Integration: Allows you to connect two PA through a DS (this service is generally provided by the local network).
2. Fragmentation reassembly:
Because of higher error rate in radio transmissions, it is essential to use small frames. Now works with the Ethernet protocol frames of 1518 bytes maximum, it was therefore agreed to use a system of cutting and reassembling the data. This helped to increase overall network performance, in addition, can use a wireless frequency hopping where the carrier is periodically interrupted for this frequency change (in our case, every 20 ms), so most package , the smaller the chance of having a low transmission is interrupted.
· FRAGMENTATION: The data frames or MSDU (MAC Service Data Unit) and the control frames and management or MMPDU (MAC Management Protocol Data Unit) are cut to a preset size (Fragmentation threshold). Then all fragments or MPDU (MAC Protocol Data Unit) are transmitted sequentially, the carrier is released once all the fragments are successfully transmitted or the source station fails to receive payment of a transmis.Car fragment in case of non receipt of an acknowledgment, the originating station resumes transmission from the last fragment unpaid. But the retransmission can not last indefinitely and is defined by a variable called MaxTransmitMSDULifeTime This fragmentation mechanism is reduced to a simple algorithm for sending and waiting for results, where the transmitting station is not allowed to transmit a new fragment as one of the following two events not occurred:
1. Receiving an ACK for that fragment.
2. Ruling that the fragment was retransmitted too often, and abandonment of the frame transmission.
· REASSEMBLING: The receiving station reassembles the fragments with two variables, the sequence number (identical to each fragment of the same frame) and the fragment number (incremented by one for each fragment) located in the field " Sequence Control ". Furthermore the field "More Fragment" found in the header of the fragment allows the station whether other fragments follow (a bit) or if it's the last (bit zero). As before, the resort destination MaxReceiveLifeTime sets a value upon receiving the first fragment. If all fragments of that frame could be sent before expiry of this value, these fragments are lost, as well as fragmentation.
It should be noted that fragmentation is only applicable to unicast data transfer, when multicast transmission or broadcast, the frames are not fragmented. Moreover, if we use the mechanism of WEP (Wired Equpement Privacy) is used, the fragments may be larger than Fragmentation Threshold the result of adding two additional fields (ICV and IV: we shall see below).
3. Dynamic flow management:
In radio transmission, conditions can change in a signal degradation caused by interference or removal of the station because the concept of flow is always related to physical distances. In order to access all stations in the network, the standard Wi-Fi uses a feature called Variable Rate Shifting. This function varies the flow of the station by the radio signal quality, generally there are four levels of speed: 11Mbit / s, 5.5Mbit / s 2Mbit / s and 1Mbit / s (these thresholds are not standardized and may differ according to manufacturers). This implies that in a BSS, a station with a low flow rate will bring down significantly the cell performance because the other stations with high flow rates should wait until the transmission of the remote station. This mechanism provides a better connectivity of a cell with a longer range at the expense of reduced network performance.
4. Association Management:
When a station is within the range of one or more access points (She joins a BSS or ESS), it chooses one of the PA based on signal strength, error rate or charge of réseau.Le association process involves several steps:
· Listen OF SUPPORT (in order to discover access points):
-Active listening: when the station enters an ESS or BSS, it sends a request frame (Probe Request Frame), containing its configuration (SSID it belongs, flow ...) on each channel and records the characteristics of access points (with the same SSID) who meet and chooses the access point offering the best compromise of speed and load. If no response is received it goes into passive listening. "Listen
passive station scans all channels and expects to receive a beacon frame (beacon frame) access point.
· AUTHENTICATION: Open System Authentication-
: default mode, there is no real authentication, since any station logging is authenticated.
-Shared Key Authentication: user authentication based on shared secret key between the station and the access point if the station uses a different key of the AP, there is a release by the latter. This mechanism can be activated with the security protocol WEP (discussed later).
· Association: The station sends an association request to AP (Associate Request frame), which responds with a response frame. In this response, the AP generates an identifier association or AID (Association ID), it is more commonly called SSID (Service Set ID) is actually the name of the network. Once accepted, the station sets its channel to the AP. Station periodically scans the channels to determine if another AP is not superior in performance.
· REASSOCIATION: the mechanism of reassociation is similar to the previous mechanism. The reassociation occur when the distance from the base station or when too much traffic on a point (depending on load balancing).
5. Mobility management (roaming):
Unlike mobile telephones, there is no change management cell (handover or handoff) for Wi-Fi devices during transmission. If a station moves, it looks best access point to associate with him, but all communication will be interrupted and not resumed by the new access point. Some manufacturers, such as Lucent, have mitigates this problem by developing a proprietary protocol called IAPP (Inter-Access Point Protocol), providing wireless mobility. IAPP is a protocol level 4 running on UDP (User Datagram Protocol), it allows you to connect access points between them through the distribution system (DS). IAPP should be implemented in the firmware of access points. These PAs establish a dialogue among themselves and exchange their configurations.
order to secure the handover, IAPP defines the use of RADIUS protocol. Before any handover, authentication is required. The station is an association to the new AP, the AP relays the authentication station to a RADIUS server, which verifies the information and authenticates the station with the new AP. Once authenticated, the new AP enters stage with former handover PA.
This protocol was adopted by the IEEE802.11 group to be standardized under the name 802.11f.
C. EXAMPLES OF ARCHITECTURE:
· The extension of the BSS (same SSID) form an ESS. The station may move from access point A to point C. Access
Boiling point / repeater mode access: allows to extend the coverage area of the BSS, share the total bandwidth throughout the area.
· Share Load: three channels covering the same area and thus increase the flow. The station determines the best access point along the signal and the load of the PA.
· Interconnection remote private networks: here the Wi-Fi standard used to interconnect two buildings.
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