WiFi Technology Blog: Frames and more Frames

Frames

One of the main topics that we should learn is the concept of 802.11 frames to take successfully the CWAP exam. In this chapter, we will introduce the evolution of general frame since creation until current 802.11ac frame and explain briefly each field related to general frame.

802.11 General Frame Format from 802.11-1997 and 1999

In 802.11-2007 were incorporated some changes. The Frame Body field is now from 0-2304 and because of 802.11e QoS, was aggregated QoS Control field to pass information QoS and define queue operations in the STAs.

802.11 General Frame Format from 802.11-2007

With 802.11n ratification, in 2009 was incorporated 802.11-2012. This permits a longer Frame Body with A-MSDU, also we can see that was introduced the HT Control field that contains information related to Transmit Beamforming and Antenna Selection (ASEL) and another more.

802.11 General Frame Format from 802.11-2012

The 802.11ac format shows the Frame Body field as variable for length with a minimum of 0 octets and maximum length based in MMPDU, MSDU, A-MSDU and MPDU that are received for the PPDU format. Finally, fields like QoS Control, Address 4, HT Control, TKIP, CCMP GCMP and MIC can impact the length of Frame Body.

802.11 General Frame Format from 802.11ac-2013

Now we will do a brief description of each field related to a general frame 802.11:

Frame Control

Frame Control Field 802.11-2012

The Protocol Version bits are always set a 00.
The Type and Subtype fields (6-bits) define the frame type (management, control o data) and the subtype. Next table lists the important valid values for these bits.

The subfields To DS and From DS bits determine whether a frame is transmitted from a STA to the AP, from one STA to another in a IBSSS or using the four-address MAC header format (mesh BSS). Next table shows the To DS and From DS values appropriate as defined in the 802.11 standard.

The More Fragments subfield (1-bit) determines whether current frame is part of fragmented frame or not.
The Retry Field (1-bit) is used when a frame is retransmitted that include the Retry field set to 1.
The Power Management field (1-bit) is a bit indicating whether power management is used by the STA.
The Protected Frame field (1-bit) indicates whether the MSDU is encrypted in the frame if it is set to 1.
The Order field is set to1 in a non-QoS data frame to indicate that it contains an MSDU. It is set to 1 in a QoS data or management frame to indicate that the frame contains an HT control field.

Duration ID

The Duration/ID field is used for two purposes. First, it may contain the duration of the frame. Second, it may contain the association identifier (AID) of the STA that transmitted the frame.

Address 1, 2, 3 and 4

The 802.11 general frame format specifies four address fields. The next table provides an overview of the use of these fields.

Sequence Control

It is used with fragmentation and for the removal of duplicate frames should they occur. It is divided into a 4-bit fragment number and a 12-bit sequence number (16-bit total). When an MSDU is fragmented, all fragments have the same sequence number and the fragment number is incremented by 1 for each frame until all fragments are delivered. The sequence number start at 0 until it reaches 4095, then simply resets to 0 and begins again.

QoS Control

It is a 16-bit field that identifies the category to which the frame belongs for queuing purposes. The most important factor in this field is the user priority (UP). EDCA is implemented in QoS WLANs based on WMM certification the bits 0-3 in the QoS Control field are mapped with possible values from 0 to 7. The next table lists the mapping of WMM access categories (ACs) to 802.1d tags.