WLANs use Radio Frequencies (RF) instead of cables at the physical layer and MAC sublayer of the data link layer. WLANs share a similar origin with Ethernet LANs. The IEEE has adopted the 802 LAN/MAN portfolio of computer network architecture standards. The two dominant 802 working groups are 802.3 Ethernet, which defined Ethernet for wired LANs, and 802.11 which defined Ethernet for WLANs. There are important differences between the two.
WLANs also differ from wired LANs as follows:
- WLANs connect clients to the network through a wireless access point (AP) or wireless router, instead of an Ethernet switch.
- WLANs connect mobile devices that are often battery-powered, as opposed to plugged-in LAN devices. Wireless NICs tend to reduce the battery life of a mobile device.
- WLANs support hosts that contend for access to the RF media (frequency bands). 802.11 prescribes collision-avoidance (CSMA/CA) instead of collision-detection (CSMA/CD) for media access to proactively avoid collisions within the media.
- WLANs use a different frame format than wired Ethernet LANs. WLANs require additional information in the Layer 2 header of the frame.
- WLANs raise more privacy issues because radio frequencies can reach outside the facility.
The table summarizes the differences between wireless and wired LANs.
|Characteristic||802.11 Wireless LAN||802.3 Wired Ethernet LANs|
|Physical Layer||radio frequency (RF)||physical cables|
|Media Access||collision avoidance||collision detection|
|Availability||anyone with a wireless NIC in range of an access point||physical cable connection required|
|Regulation||different regulations by country||IEEE standard dictates|
802.11 Frame Structure
Recall that all Layer 2 frames consist of a header, payload, and Frame Check Sequence (FCS) section. The 802.11 frame format is similar to the Ethernet frame format, except that it contains more fields, as shown in the figure.
The diagram shows the fields of an 802.11 frame. On the left is the header consisting of the following fields: frame control, duration, address 1, address 2, address 3, sequence control, and address 4. Next is the payload and last is the FCS field.
All 802.11 wireless frames contain the following fields:
- Frame Control – This identifies the type of wireless frame and contains subfields for protocol version, frame type, address type, power management, and security settings.
- Duration – This is typically used to indicate the remaining duration needed to receive the next frame transmission.
- Address1 – This usually contains the MAC address of the receiving wireless device or AP.
- Address2 – This usually contains the MAC address of the transmitting wireless device or AP.
- Address3 – This sometimes contains the MAC address of the destination, such as the router interface (default gateway) to which the AP is attached.
- Sequence Control – This contains information to control sequencing and fragmented frames.
- Address4 – This usually missing because it is used only in ad hoc mode.
- Payload – This contains the data for transmission.
- FCS – This is used for Layer 2 error control.
WLANs are half-duplex, shared media configurations. Half-duplex means that only one client can transmit or receive at any given moment. Shared media means that wireless clients can all transmit and receive on the same radio channel. This creates a problem because a wireless client cannot hear while it is sending, which makes it impossible to detect a collision.
To resolve this problem, WLANs use carrier sense multiple access with collision avoidance (CSMA/CA) as the method to determine how and when to send data on the network. A wireless client does the following:
- Listens to the channel to see if it is idle, which means that is senses no other traffic is currently on the channel. The channel is also called the carrier.
- Sends a ready to send (RTS) message to the AP to request dedicated access to the network.
- Receives a clear to send (CTS) message from the AP granting access to send.
- If the wireless client does not receive a CTS message, it waits a random amount of time before restarting the process.
- After it receives the CTS, it transmits the data.
- All transmissions are acknowledged. If a wireless client does not receive an acknowledgment, it assumes a collision occurred and restarts the process.
Wireless Client and AP Association
For wireless devices to communicate over a network, they must first associate with an AP or wireless router. An important part of the 802.11 processes is discovering a WLAN and subsequently connecting to it. Wireless devices complete the following three-stage process, as shown in the figure:
- Discover a wireless AP
- Authenticate with AP
- Associate with AP
The figure shows the three-stage process used by a wireless client to associate with an AP. A laptop represents a wireless client that is communicating wirelessly with an AP. An arrow flowing from the client to the AP represents stage one in which the client discovers the AP. Below that, a double arrow between the devices represents the authentication stage. Below that, another double arrow between the devices represents the association stage.
In order to have a successful association, a wireless client and an AP must agree on specific parameters. Parameters must then be configured on the AP and subsequently on the client to enable the negotiation of a successful association.
- SSID -The SSID name appears in the list of available wireless networks on a client. In larger organizations that use multiple VLANs to segment traffic, each SSID is mapped to one VLAN. Depending on the network configuration, several APs on a network can share a common SSID.
- Password – This is required from the wireless client to authenticate to the AP.
- Network mode – This refers to the 802.11a/b/g/n/ac/ad WLAN standards. APs and wireless routers can operate in a Mixed-mode meaning that they can simultaneously support clients connecting via multiple standards.
- Security mode – This refers to the security parameter settings, such as WEP, WPA, or WPA2. Always enable the highest security level supported.
- Channel settings – This refers to the frequency bands used to transmit wireless data. Wireless routers and APs can scan the radio frequency channels and automatically select an appropriate channel setting. The channel can also be set manually if there is interference with another AP or wireless device.
Passive and Active Discover Mode
Wireless devices must discover and connect to an AP or wireless router. Wireless clients connect to the AP using a scanning (probing) process. This process can be passive or active.
In passive mode, the AP openly advertises its service by periodically sending broadcast beacon frames containing the SSID, supported standards, and security settings. The primary purpose of the beacon is to allow wireless clients to learn which networks and APs are available in a given area. This allows the wireless clients to choose which network and AP to use.
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