1. Overview of WiFi transmissionWiFi encapsulation in OSI802.11 frame typesManagement frame Control frameData frameMedium access control2. Case studyLab WiFi environmentSTA and AP retransmit so much in low RSSI condition3. WiFi enhancementPhysical layer enhancementMAC layer enhancement
Wi-Fi Transmission and Enhancement Contents Overview of WiFi transmission WiFi encapsulation in OSI 802.11 frame types Management frame Control frame Data frame Medium access control Case study Lab WiFi environment STA and AP retransmit so much in low RSSI condition WiFi enhancement Physical layer enhancement MAC layer enhancement WiFi encapsulation in OSI The IEEE 802.11-2007 standard defines communication mechanisms only at the Physical layer and MAC sublayer of the Data-Link layer of the OSI model By design, the 802.11 standard does not address the upper layers of the OSI model When capturing wireless frames, if wireless encryption is implemented, all of the layer through layer information will be grouped and displayed as the encrypted payload 802.11 frame types There are major type of frame and further subdivided into multiple subtypes: Management frames are used by wireless stations to join and leave the basic service set Control frames assist with the delivery of the data frames 802.11 Data frames carry the actual MSDU data that is passed down from the higher layer protocols Management frame Association request Association response Reassociation request Reassociation response Probe request Probe response Beacon ATIM Disassociation Authentication Deauthentication Control frame subtypes Power Save Poll (PS-Poll) Request to send (RTS) Clear to send (CTS) Acknowledgment (ACK) Contention Free-End (CF-End) CF-End + CF+ACK Block ACK Request (BlockAckReq) Block ACK (BlockAck) Data frame subtypes Data (simple data frame) Null function (no MSDU payload) Data + CF-ACK Data + CF-Poll Data + CF-ACK + CF-Poll CF-ACK (no MSDU payload) CF-Poll (no MSDU payload) CF-ACK + CF-Poll (no MSDU payload) QoS data QoS Null (no MSDU payload) QoS data + CF-ACK QoS data + CF-Poll QoS data + CF-ACK + CF-Poll QoS CF-Poll (no MSDU payload) MAC Sublayer Frame Format 802.11 MAC Protocol Data Unit (MPDU) Management frame Management Frame structure Management frames always have a standard 24-byte-long MAC header with three addresses, followed by a body of variable size When 802.11n is in use, the header is extended byte of the HT Control section Subtype bits Subtype description 0000 Association request 0001 Association response 0010 Reassociation request 0011 Reassociation response 0100 Probe request 0101 Probe response 1000 Beacon 1001 Announcement traffic indication message (ATIM) 1010 Disassociation The SA field is the MAC address of the station transmitting the frame 1011 Authentication The BSSID can be the AP BSSID or a wildcard value 1100 Deauthentication The size and content of the body depend on the management frame subtype 1101 Action 1110 Action no ack Duration/ID field can be used for virtual Carrier Sense – This is the main purpose which used to reset the NAV timer of the other stations The DA field is the destination address of the frame It can be broadcast or unicast depending on the frame subtype Management frame Beacon Frame Connection establishment Beacon frames are used by the access points (and stations in an IBSS) to communicate throughout the serviced area the characteristics of the connection offered to the cell members Beacon frames are sent periodically, at a time called target beacon transmission time (TBTT), this unit is 1,024 microseconds normally All stations in the cell use the AP beacon as a time reference Management frame Beacon Frame example Timestamp Field represent the time on the access point, which is the number of microseconds the AP has been active Capability Information Field contains number of subfields that are used to indicate requested or advertised optional capabilities Short Slot Time Subfield determines whether short slot time is allowed in the cell Supported Rates at least one mandatory rate must be set by AP & any station wanting to join the cell must support all basic rates Control frame Valid Type and Subtype combinations Frame Control fields Data frame Data frames: valid Type and Subtype combinations QoS and Non-QoS Data Frames Transmitting station Receiving station Non-QoS station Non-QoS station Non-QoS frame Non-QoS station QoS station Non-QoS frame QoS station QoS station QoS frame QoS station Non-QoS station Non-QoS frame All Broadcast Non-QoS frame, unless the transmitting station knows that all stations in the BSS are QoS capable, in which case a QoS frame would be used Multicast Non-QoS frame, unless the transmitting station knows that all stations in the BSS that are members of the multicast group are QoS capable, in which case a QoS frame would be used All Data frame subtype used Data-Carrying vs Non-Data-Carrying Frames Medium access control PIFS (PCF Inter Frame Spaces) PIFS are used by STAs during the contention-free period (CFP) in PCF mode Because PCF has not been implemented in 802.11 devices, you will not see PIFS used for this purpose PIFS = SIFS + SlotTime Summarize SIFS,DIFS,PIFS & SlotTime values Medium access control Random backoff The random backoff is a quiet period before a frame transmission, It is a period of time that changes based on a random number chosen by each AP or station APs and stations stay quiet during the random backoff by randomly choosing a number of slot times and then counting down until the number of slot times equals zero Once the number of slot times hits zero, an AP or station is allowed to transmit a frame As soon as one device exhausts its slot times, it will transmit, thus turning the CCA to a busy state in all other devices on the channel The lower limit for the random backoff is always The upper limit for the random backoff is always equal to the contention window (CW) The contention window (CW) parameter takes the initial value CWmin and effectively doubles on each unsuccessful MPDU transmit, for example each time an ACK response is not received for a data frame If the CW reaches CWmax it remains at that value until it is reset The CW is reset to CWmin after every successful MPDU transmit Medium access control Random backoff procedure To begin the random backoff procedure, the station selects a random backoff count in the range [0, CW] All backoff slots occur following a DIFS during which the medium is determined to be idle During each backoff slot the station continues to monitor the medium If the medium goes busy during a backoff slot then the backoff procedure is suspended The backoff count is resumed when the medium goes idle again for a DIFS period When multiple stations are deferring and go into random backoff, then the station selecting the smallest backoff count (STA 3) will win the contention and transmit first The remaining stations suspend their backoff and resume DIFS after the medium goes idle again The station with the next largest backoff count will win next (STA 4) and then eventually the station with the longest backoff count (STA 2) A station that begins a new access (STA again) will select a random backoff from the full contention window and will thus tend to select a larger count than the remaining backoff for stations (such as STA 2) that have already suspended their backoff from a previous access attempt Case study STA and AP restransitted so much in low RSSI condition for which happen with all ONT’s vendor WiFi Lab environment Case study STA and AP restransitted so much in low RSSI condition for which happen with all ONT’s vendor Physical layer enhancement Short Preamble is not allowed in Beacon 1.1 Standard SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = 9μS with short preamble SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = 20μS with long preamble STA and AP need longer time to calculate DIFS and Backoff algorithm 1.3 Next action RnD please build the firmware which support to allow short preamble bit Refer to BMS ID: H660x:5360 VTAC test again at Lab 1.2 Beacon packet capture Physical layer enhancement Reduce Interframe Spacing (RIFS) is prohibitted 1.1 Standard RIFS were introduced with 802.11n to improve efficiency for transmissions to the same receiver in which a SIFS-separated response is not required 802.11n standard use RIFS & Block Acknowledgement (mandatory in 802.11n) RIFS is used only when Block ACK is enabled RIFS = 2μS 1.3 Next action RnD please build the firmware which support to permit RIFS Refer to BMS ID: H660x:5361 VTAC test again at Lab 1.2 Beacon packet capture Physical layer enhancement Include VHT information of 802.11 ac in management frame of 802.11n 1.1 Standard Some packets including Beacon, Probe response, association response increase hearder packets, it make longer time to transmit the packets in WiFi medium 1.3 Next action RnD please build the firmware which support to don’t include info of 802.11ac in some management packets of 802.11n Refer to BMS ID: H660x:5362 VTAC test again at Lab 1.2 Beacon packet capture MAC layer enhancement Modify MCS set parameters 1.1 Standard Non-HT radios that used OFDM technology (802.11a/g) defined data rates of Mbps to 54 Mbps based on the modulation that was used HT radios, however, define data rates based on numerous factors including modulation, the number of spatial streams, channel size, and guard interval The 802.11n amendment defines 77 MCSs that are represented by an MCS index from 0–76 The eight mandatory MCSs for 20 MHz channels are comparable to basic (required) rates MAC layer enhancement Modify MCS set parameters 1.3 Next action If the TX MCS Set Defined subfield is set to 0, it indicates the STA is not specifying a TX MCS set When the TX MCS Set Defined subfield is set to and the TX RX MCS Set Not Equal subfield is set to 0, the STA is indicating it will use the same MCS set defined by the RX MCS Bitmask subfield => Request to set Tx MCS set is defined to be equal to the Rx MCS set 1.2 Beacon packet capture MAC layer enhancement Support A-MSDU and A-MPDU 1.1 Standard 1.2 Action packet capture An 802.11n access point using A-MSDU would receive multiple 802.3 frames, remove the 802.3 headers and trailers, and then wrap the multiple MSDU payloads into a single 802.11 frame for transmission\ The size of an A-MSDU must not exceed the maximum A-MSDU size that a STA is capable of receiving An STA can support one of two maximum lengths: Maximum A-MSDU Length = (3839 Bytes) or = (7935 Bytes) The individual MSDUs must all be of the same 802.11e QoS access category 1.3 Next action => Support A-MSDU with maximum length is 3839 Bytes MAC layer enhancement Support U-APSD (Unschedule Automatic Power Save Delivery) 1.1 Standard Every power management method that is used in the real world works from the same basic power management structure, as illustrated in the following steps and figures: Step 1: Before a station goes into the doze state, it sends a frame, usually a null data frame, to the AP indicating that power management is enabled Step 2: Once the station indicates that it is in Power Save mode, the AP begins to buffer all frames destined to that station Step 3: When the station goes into the awake state (more on that later), it sends a frame to the AP in order to begin the data retrieval process Step 4: When the AP has finished sending all buffered data to the station, the station goes back into the doze state There are three methods of power management that are used today in the 802.11 family: 802.11 power management Unscheduled automatic power save delivery (U-APSD) from the 802.11e amendment Power save multi-poll (PSMP) from the 802.11n amendment 802.11e Unscheduled Automatic Power Save Delivery Third Step: When U-APSD is used, the station typically sends null data frames in order to retrieve buffered unicast frames from the AP Fourth Step: When U-APSD is used, stations must notify the AP that they are going back into Power Save mode by sending a frame MAC layer enhancement Support U-APSD (Unschedule Automatic Power Save Delivery) 1.2 Action packet capture 1.3 Next action => Support 802.11e Unscheduled Automatic Power Save Delivery Reference WiFi web link study: https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/ CWNA document: CWAP document: CWSP document: THANK YOU ... WiFi enhancement Physical layer enhancement MAC layer enhancement WiFi encapsulation in OSI The IEEE 802.11-2007 standard defines communication mechanisms only at the Physical layer and. .. Overview of WiFi transmission WiFi encapsulation in OSI 802.11 frame types Management frame Control frame Data frame Medium access control Case study Lab WiFi environment STA and AP... control Random backoff The random backoff is a quiet period before a frame transmission, It is a period of time that changes based on a random number chosen by each AP or station APs and stations