Wireless Media Access Facts

Wireless Media Access Facts

Wireless networks use Carrier Sense, Multiple Access/Collision Avoidance (CSMA/CA) to control media access and avoid (rather than detect) collisions. CSMA/CA uses the following process:

1. The sending device listens to make sure that no other device is transmitting. If another device is transmitting, the device waits a random period of time (called a backoff period) before attempting to send again.
2. If no other device is transmitting, the sending device broadcasts a Request-to-send (RTS) message to the receiver or access point. The RTS includes the source and destination, as well as information on the duration of the requested communication.
3. The receiving device responds with a Clear-to-send (CTS) packet. The CTS also includes the communication duration period. Other devices use the information in the RTS and CTS packets to delay attempting to send until the communication duration period (and subsequent acknowledgement) has passed.
4. The sending device transmits the data. The receiving device responds with an acknowledgement (ACK). If an acknowledgement is not received, the sending device assumes a collision and retransmits the affected packet.
5. After the time interval specified in the RTS and CTS has passed, other devices can start the process again to attempt to transmit.

Note: Using RTS and CTS (steps 2 and 3 above) is optional and depends on the capabilities of the wireless devices. Without RTS/CTS, collisions are more likely to occur.

Wireless communication operates in half-duplex (shared, two-way communication). Devices can both send and receive, but not at the same time. Devices must take turns using the transmission channel. Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting before replying.

Wireless Infrastructure Facts

Wireless Infrastructure Facts

There are two methods of wireless networking:

Method	Description
Ad Hoc	An ad hoc network works in peer-to-peer mode. The wireless NICs in each host communicate directly with one another. An ad hoc network: Uses a physical mesh topology. Is cheap and easy to set up. Cannot handle more than four hosts. Requires special modifications to reach wired networks. You will typically only use an ad hoc network to create a direct, temporary connection between two hosts.
Infrastructure	An infrastructure wireless network employs an access point (AP) that functions like a hub on an Ethernet network. With an infrastructure network: The network uses a physical star topology. You can easily add hosts without increasing administrative efforts (scalable). The access point can be easily connected to a wired network, allowing clients to access both wired and wireless hosts. The placement and configuration of access points require planning to implement effectively. You should implement an infrastructure network for all but the smallest of wireless networks.

The following diagram shows a sample enterprise wireless network operating in infrastructure mode:

The various components of a wireless network are described in the following table.

Component	Description
Station (STA)	An STA is a wireless network card (NIC) in an end device such as a laptop or wireless PDA. STA often refers to the device itself, not just the network card.
Access Point (AP)	An access point (AP), sometimes called a wireless access point, is the device that coordinates all communications between wireless devices as well as the connection to the wired network. It acts as a hub on the wireless side and a bridge on the wired side. It also synchronizes the stations within a network to minimize collisions.
Basic Service Set (BSS)	A BSS, also called a cell, is the smallest unit of a wireless network. All devices in the BSS can communicate with each other. The devices in the BSS depend on the operating mode: In an ad hoc implementation, each BSS contains two devices that communicate directly with each other. In an infrastructure implementation, the BSS consists of one AP and all STAs associated with the AP.
Independent Basic Service Set (IBSS)	An IBSS is a set of STAs configured in ad hoc mode.
Extended Service Set (ESS)	An ESS consists of multiple BSSs with a distribution system (DS). The graphic above is an example of an ESS.
Distribution System (DS)	The distribution system (DS) is the backbone or LAN that connects multiple APs (and BSSs) together. The DS allows wireless clients to communicate with the wired network and with wireless clients in other cells.

Wireless networks use the following for identification:

Identifier	Description
Service Set Identifier (SSID)	The Service Set Identifier (SSID), also called the network name, groups wireless devices together into the same logical network. All devices on the same network (within the BSS and ESS) must have the same SSID. The SSID is a 32-bit value that is inserted into each frame. The SSID is case-sensitive. The SSID is sometimes called the BSS ID (Basic Service Set ID) or the ESS ID (Extended Service Set ID). In practice, each term means the same thing. Note: Using BSS ID to describe the SSID of a BSS is technically incorrect.
Basic Service Set Identifier (BSSID)	The BSSID is a 48-bit value that identifies an AP in an infrastructure network or a STP in an ad hoc network. The BSSID allows devices to find a specific AP within an ESS that has multiple access points, and is used by STAs to keep track of APs when roaming between BSSs. Note: Do not confuse the BSSID with the SSID. They are not the same thing.