802.11 WLAN Packet Types
The table below lists the various packet types and subtypes specified in the 802.11 WLAN standard, and describes their usage briefly.
Table E.1 WLAN packet types Packet Types Usage type subtype 00 mgmt 0000 Association Request This packet is sent to an access point (in a BSS or ESS) or to any other peer (in an IBSS or ad hoc network). The sender must already be authenticated in order to gain a successful association. 00 mgmt 0001 Association Response This packet is sent from an access point (in a BSS or ESS) or from any other peer (in an IBSS or ad hoc network) in response to an association request packet. If the request is successful, the response will include the Association ID of the requester. 00 mgmt 0010 Reassociation Request Like an association request, but it includes information about the current association at the same time as it requests a new association (either with the original Station after some lapse of time, or with a new station upon moving from one BSS to another). This packet is sent to an access point (in a BSS or ESS) or to any other peer (in an IBSS or ad hoc network). The sender must already be authenticated in order to gain a successful association. 00 mgmt 0011 Reassociation Response Like an association response, but in response to a reassociation request. This packet is sent from an access point (in a BSS or ESS) or from any other peer (in an IBSS or ad hoc network) in response to a reassociation request packet. If the request is successful, the response will include the Association ID of the requester. 00 mgmt 0100 Probe Request Probe request is used to actively seek any, or a particular, access point or BSS. 00 mgmt 0101 Probe Response Probe response replies with station parameters and supported data rates. 00 mgmt 1000 Beacon Beacon packets are sent by the access point in a BSS (or its equivalent in an IBSS) to announce the beginning of a Contention Free period (CF), during which the right to transmit is conferred by the access point by polling. Beacon management packets carry BSS timestamps to help synchronize member stations with the BSS, and other information to help them locate and choose the BSS with the best signal and availability. 00 mgmt 1001 ATIM Announcement Traffic Indication Message. This packet serves much the same function in an IBSS that the Beacon packet does in an infrastructure (BSS or ESS) topology. The packet sets the synchronization of the group and announces that messages are waiting to be delivered. Stations in Power Save mode wake up periodically to listen for ATIM packets in ad hoc (IBSS) networks, just as they do for Beacon packets in infrastructure (BSS or ESS) networks. 00 mgmt 1010 Disassociation This packet is an announcement breaking an existing association. It is a one-way communication (meaning it does not require or accept a reply), and must be accepted. It can be sent by any associated station or BSS and it takes effect immediately. 00 mgmt 1011 Authentication Authentication packets are sent back and forth between the station requesting authentication and the station to which it is attempting to assert its authentic identity. The number of packets exchanged depends on the authentication method employed. Information relating to the particular scheme is carried in the body of the Authentication packet. 00 mgmt 1100 Deauthentication This packet is an announcement stating that the receiver is no longer authenticated. It is a one-way communication from the authenticating station (a BSS or functional equivalent), and must be accepted. It takes effect immediately. 01 ctrl 1010 PS-Poll Power Save polling packet. Stations in power save mode awaken periodically to listen to selected Beacons. If they hear that data is waiting for them, they will awake more fully and send a PS-Poll packet to the access point (BSS) to request the transmission of this waiting data. In Control packets of the Power Save-Poll type, the Duration/ID field contains the association ID (AID) for the station sending the packet. 01 ctrl 1011 RTS Request To Send. Coordinates access to airwaves. 01 ctrl 1100 CTS Clear To Send. Response to a RTS, coordinates access to airwaves. 01 ctrl 1101 ACK Acknowledges receipt of transmitted data. 01 ctrl 1110 CF End Signals the end of Contention Free period. 01 ctrl 1111 CF End + CF ACK Signals the end of the Contention Free period and Acknowledges the receipt of some packet in a single message. 10 data any any Multiple subtypes exist for Data type packets, but all have the same basic format, as described above. (see Appendix C, "802.11 WLAN Packets and Protocols".) The different Data subtypes essentially just piggyback CF-Poll, CF-ACK, and CF-End messages onto the data message in a single transmission. This allows the BSS to gain higher throughputs possible using PCF (point coordinating function).