This post covers WIRELESS COMMUNICATIONS AND NETWORKS by William Stallings.

Basic Ideas

• MAC Frame- Fundamental System Design

  

  Figure shows the 802.11 frame format when no security features are used. This general format is used for all data and control frames, but not all fields are used in all contexts. The fields are as follows:

  • Frame Control: Indicates the type of frame and provides control information, as explained presently.

  • Duration/Connection ID: If used as a duration field, indicates the time (in microseconds) the channel will be allocated for successful transmission of a MAC frame. In some control frames, this field contains an association, or connection, identifier.

  • Addresses: The number and meaning of the 48-bit address fields depend on context. The transmitter address and receiver address are the MAC addresses of stations joined to the BSS that are transmitting and receiving frames over the wireless LAN.

    • The service set ID (SSID) identifies the wireless LAN over which a frame is transmitted.
    • For an IBSS, the SSID is a random number generated at the time the network is formed.
    • For a wireless LAN that is part of a larger configuration the SSID identifies the BSS over which the frame is transmitted; specifically, the SSID is the MAC-level address of the AP for this BSS (See figure).
    • Finally the source address and destination address are the MAC addresses of stations, wireless or otherwise, that are the ultimate source and destination of this frame.
    • The source address may be identical to the transmitter address and the destination address may be identical to the receiver address.

  • Sequence Control: Contains a 4-bit fragment number subfield used for fragmentation and reassembly, and a 12-bit sequence number used to number frames sent between a given transmitter and receiver.

  • Frame Body: Contains an MSDU or a fragment of an MSDUThe MSDU is a LLC protocol data unit or MAC control information.

  • Frame Check Sequence: A 32-bit cyclic redundancy check. The frame control field, shown in Figure 14.8b, consists of the following fields:

  • Protocol Version: 802.11 version, currently version O.

  • Type: Identifies the frame as control, management, or data.

  • Subtype: Further identifies the function of frame. Table 14.4 defines the valid combinations of type and subtype.

  • To DS: The MAC coordination sets this bit to 1 in a frame destined to the distribution system
  • From DS: The MAC coordination sets this bit to 1 in a frame leaving the distribution system.
  • More Fragments: Set to 1 if more fragments follow this one.

  • Retry: Set to 1 if this is a retransmission of a previous frame.

  • Power Management: Set to 1 if the transmitting station is in a sleep mode.

  • More Data: Indicates that a station has additional data to send. Each block of data may be sent as one frame or a group of fragments in multiple frames.

  • WEP: Set to 1 if the optional wired equivalent protocol is implemented. WEP is used in the exchange of encryption keys for secure data exchange. This bit also is set if the newer WPA security mechanism is employed.

  • Order: Set to 1 in any data frame sent using the Strictly Ordered service ,

    which tells the receiving station that frames must be processed in order.

• Control Frames : Control frames assist in the reliable delivery of data frames. There are six control frame subtypes:
  • Power Save-Poll (PS-Poll): This frame is sent by any station to the station that includes the AP (access point). Its purpose is to request that the AP transmit a frame that has been buffered for this station while the station was in powersaving mode.
  • Request to Send (RTS): This is the first frame in the four-way frame exchange discussed under the subsection on reliable data delivery at the beginning of Section 14.3. The station sending this message is alerting a potential destination, and all other stations within reception range, that it intends to send a dat frame to that destination
  • Clear to Send (CTS): This is the second frame in the four-way exchange. It is sent by the destination station to the source station to grant permission to send a data frame
  • Acknowledgment: Provides an acknowledgment from the destination to the source that the immediately preceding data, management, or PS-Poll frame was received correctly.
  • Contention-Free (CF)-End: Announces the end of a contention-free period that is part of the point coordination function.
  • CF-End + CF-Ack: Acknowledges the CF-end.This frame ends the contention free period and releases stations from the restrictions associated with that period.
• Data Frames There are eight data frame subtypes, organized into two groups. The first four subtypes define frames that carry upper-level data from the source station to the destination station. The four data-carrying frames are as follows:
  • Data: This is the simplest data frame. It may be used in both a contention period and a contention-free period.
  • Data + CF-Ack: May only be sent during a contention-free period. In addition to carrying data, this frame acknowledges previously received data.
  • Data + CF-Poll: Used by a point coordinator to deliver data to a mobile station and also to request that the mobile station send a data frame that it may have buffered.
  • Data + CF-Ack + CF-Poll: Combines the functions of the Data + CF-Ack and Data + CF-Poll into a single frame.
  • The remaining four subtypes of data frames do not in fact carry any user data. The Null Function data frame carries no data, polls, or acknowledgments. It is used only to carry the power management bit in the frame control field to the AP, to indicate that the station is changing to a low-power operating state.
  • The remaining three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) have the same functionality as the corresponding data frame subtypes in the preceding list (Data + CF-Ack, Data + CF-Poll, Data + CF-Ack + CF-Poll) but without the data.
• Managetnent Frames:Management frames are used to manage communications between stations and APs. The following subtypes are included
  • Association Request: Sent by a station to an AP to request an association with this BSS. This frame includes capability information, such as whether encryption is to be used and whether this station is pollable.
  • Association Response: Returned by the AP to the station to indicate whether it is accepting this association request
  • Reassociation Request: Sent by a station when it moves from one BSS to another and needs to make an association with the AP in the new BSS. The station uses reassociation rather than simply association so that the new AP knows to negotiate with the old AP for the forwarding of data frames.
  • Reassociation Response: Returned by the AP to the station to indicate whether it is accepting this reassociation request.
  • Probe Request: Used by a station to obtain information from another station or AP. This frame is used to locate an IEEE 802.11 BSS.
  • Probe Response: Response to a probe request.
  • Beacon: Transmitted periodically to allow mobile stations to locate and identify a BSS.
  • Announcement Traffic Indication Message: Sent by a mobile station to alert other mobile stations that may have been in low power mode that this station has frames buffered and waiting to be delivered to the station addressed in this frame.
  • Dissociation: Used by a station to terminate an association.
  • Authentication: Multiple authentication frames are used in an exchange to authenticate one station to another.
  • De authentication: Sent by a station to another station or AP to indicate that it is terminating secure communications.