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Frame Header Compression

Date: March, 2012. Frame Header Compression. Authors:. Date: 2012-03-11. MAC Frame Header. The typical frame payload of Smart Grid is tens/hundreds bytes. 100 bytes payload frame with 3 MAC addresses has about 30/100=30% MAC header overhead!

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Frame Header Compression

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  1. Date: March, 2012 Frame Header Compression Authors: Date: 2012-03-11 Liwen Chu Etc.

  2. MAC Frame Header The typical frame payload of Smart Grid is tens/hundreds bytes. 100 bytes payload frame with 3 MAC addresses has about 30/100=30% MAC header overhead! 300 bytes payload frame with 3 MAC addresses has about 30/300=10% MAC header overhead! The following parts of the MAC header can be compressed: Some MAC address fields, Do we need 2 bytes Sequence Control in a low rate Smart Grid BSS? Do we need Duration/ID since 2-bit ACK Indication is added to PHY SIG? Do we need 2 bytes QoS Control field? Do we need 4 byte FCS? Date: March, 2012 2 6 6 6 2 2 4 Variable 4 Octets: 2 6 Frame Header

  3. Frame Header Compression 1 MPDU type (Normal or compressed) should be identified by either the PHY or MAC header. One bit in PHY PLCP SIG. Other methods. When both “From DS” is 1 and “To DS” is 1, compressed MPDU header is not used. TDLS is not allowed, because there is almost no intra-BSS traffic in smart grid BSS. Duration field can be removed since 2-bit ACK Indication is added to PHY SIG. BSSID are used to identify the transmitter or the receiver of the frames. BSSID field is used to identify whether the frame is in the same BSS. When “From DS” is 1 and “To DS” is 0, BSSID is the transmitter identifier. When “From DS” is 0 and “To DS” is 1, BSSID is the receiver identifier. Date: March, 2012 6 1 1 2 Variable 2 Octets: 2

  4. Frame Header Compression 1 AID/DASA Identifier field are used for indicating AID and DA/SA. Bit15~Bit13 is the DA Identifier (when “From DS” is 0 and “To DS” is 1) and SA (when “From DS” is 1 and “To DS” is 0). DASA Identification Request/Response action frames are used to match DASA identifier to DASA MAC address. Bit12 ~ Bit0 are used to identify AID When “From DS” is 1 and “To DS” is 0, AID is the receiver identifier. When “From DS” is 0 and “To DS” is 1, AID is the transmitter identifier. Date: March, 2012 Bit15 Bit12 Bit13 Bit0 AID/DA RA Identifier • One octet Sequence Control is used. • One octet QoS control is used: 4 bits in QoS Control field are used to indicate TID of the frame. 2 bits are used to indicate ACK policy. Other bits are reserved. • CRC-16-CCITT can detect 1, 2, 3 bit-errors when the frame size is smaller than 128 bytes. So 2-byte FCS is ok to frames with frame header compression. • Single Address frames are not allowed to use compressed frame format.

  5. Frame Header Compression 2 MPDU type (Normal or compressed) should be identified by either the PHY or MAC header. One bit in PHY PLCP SIG. Other methods. When both “From DS” is 1 and “To DS” is 1, compressed MPDU header is not used. TDLS is not allowed, because there is almost no intra-BSS traffic in smart grid BSS. Duration field can be removed since 2-bit ACK Indication is added to PHY SIG. RA is the MAC address of the transmitter. Date: March, 2012 6 2 1 1 Variable 2 Octets: 2

  6. Frame Header Compression 2 AID/DASA Identifier field are used for indicating AID and DA/SA. Bit15~Bit13 is the DA Identifier (when “From DS” is 0 and “To DS” is 1) and SA (when “From DS” is 1 and “To DS” is 0). DASA Identification Request/Response action frames are used to match DASA identifier to DASA MAC address. Bit12 ~ Bit0 are used to identify the TA’s AID. Date: March, 2012 Bit12 Bit13 Bit0 Bit15 AID/DA RA Identifier • One octet Sequence Control is used. • One octet QoS control is used: 4 bits in QoS Control field are used to indicate TID of the frame. 2 bits are used to indicate ACK policy. Other bits are reserved. • Single Address frames are not allowed to use compressed frame format. • CRC-16-CCITT can detect 1, 2, 3 bit-errors when the frame size is smaller than 128 bytes. So 2-byte FCS is ok to frames with frame header compression. • Another possibility is that the receiver is identified by its AID and the transmitter is identified by its MAC address.

  7. Conclusion Frame header compression can decrease short frame overhead through: AID replaces RA/TA MAC address in MAC header, SA/DA identifier indentifies SA/DA MAC address, Single octet QoS Control field, Single octet Sequence Control field, Duration field is removed. Totally 14 bytes is removed from the MAC header of 3-address frame. With 2-byte FCS, 16 bytes MAC overhead can be removed. With further frame header compression, 20 bytes can be removed from MAC header and FCS of 3-address frame. Date: March, 2012

  8. Backup Slides Date: March, 2012

  9. DASA Identification Management A non-AP STA sends DASA Identification Request frame to the AP to indicate the mapping between DASA Identifier and DASA MAC Address. The AP sends DASA Identification Response frame to acknowledge the mapping between DASA Identifier and DASA MAC Address. Date: March, 2012 1 1 1 6 1 6 Octets: 1 At most 16 DASA Identifier and DASA MAC Address pairs DASA Identification Request frame Bit6 Bit7 Bit2 Bit3 Bit0 DASA Identifier field 1 1 2 Octets: 1 DASA Identification Response frame

  10. Further Frame Header Compression 1 Each AP selects a 2-octet Compressed BSSID (BSSID[47--32], randomly, by IT manager etc.) to itself and notifies the Compressed BSSID to its associated STAs. Compressed BSSID is used to identify the RA or TA When “From DS” is 1 and “To DS” is 0, Compressed BSSID is the transmitter identifier. When “From DS” is 0 and “To DS” is 1, Compressed BSSID is the receiver identifier. Compressed BSSID conflict detection and avoidance: An AP selects BSSID[47--32] as its Compressed BSSID if BSSID[47--35] does not conflict with its OBSS. If an STA receives an Beacon with the same Compressed BSSID with its associated AP from another AP, it notifies its associated AP. If an AP receives a Beacon with the same Compressed BSSID with it or receives a Compressed BSSID conflict from its associated STAs, it shall randomly select another Compressed BSSID and notify the new Compressed BSSID to its associated STAs. 20 Bytes MAC overhead can be removed. Date: March, 2012 2 1 1 2 Variable 2 Octets: 2

  11. Further Frame Header Compression 2 Each AP selects a 13-bit Compressed BSSID (BSSID[47--35], randomly, by IT manager etc.) to itself and notifies the Compressed BSSID to its associated STAs. Compressed BSSID conflict detection and avoidance: An AP selects BSSID[47--35] as its Compressed BSSID if BSSID[47--35] does not conflict with its OBSS. If an STA receives an Beacon with the same Compressed BSSID with its associated AP from another AP, it notifies its associated AP. If an AP receives a Beacon with the same Compressed BSSID with it or receives a Compressed BSSID conflict from its associated STAs, it shall randomly select another Compressed BSSID and notify the new Compressed BSSID to its associated STAs. 20 Bytes MAC overhead can be removed. Date: March, 2012 2 2 1 1 Variable 2 Octets: 2

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