Mumbai University > Computer Engineering > Sem 6 > Mobile Communication & Computing

Marks: 10 Marks

Year: Dec 2016

The Data Link layer header will include at least the following three items:

• MAC sub-layer Destination Address (MAC-DA)
• MAC sub-layer Source Address (MAC-SA)
• Information to identify the higher-layer protocol payload (carried in the LLC sub-layer, by either the LLC sub-layer protocol or the LLC sub-layer protocol in conjunction with the Sub-Network Access Protocol (SNAP)

Figure below shows the decomposition of the LLC and MAC sub-layers into their constituent protocols (of course, the MAC sub-layer has only one protocol). The most common form of the LLC sub-layer protocol occupies three octets, and the SNAP protocol, if present, consumes a further five octets.

The IEEE 802.11 MAC sub-layer protocol structure follows the IEEE LMSC model, and thus has no higher-layer protocol demultiplexing features of its own. Therefore, the IEEE 802.11 MAC sub-layer protocol relies on the LLC sub- layer headers to perform the higher layer protocol de-multiplexing function.

The LLC sub-layer consists of the LLC sub-layer protocol, and optionally the SNAP sub-layer protocol.

The IEEE 802.11 MAC sub-layer protocol uses CSMA/CA in a manner similar to the way that Ethernet uses its Carrier Sense Multiple Access with Collision Detection (CSMA/CD), in that CSMA/CD's rules were defined to govern medium access in half-duplex shared (i.e., non-switched) Ethernet networks.

All IEEE 802.11 frames begin with the two-octet Frame Control (FC) field, the structure of which is depicted in Figure below:

Summary of the Frame Control Field

1. Bits 0 and 1 (which comprise the Protocol Version field) have been defined by IEEE 802.11-1999 as "0x00".
2. The Type (bits 2 and 3) and Subtype (bits 4 through 7) fields define the format of the subsequent frame.
3. Bit 8 (the "ToDS"), and bit 9 (the "FromDS" bit), these two bits control the interpretation of the "Address" fields in the MPDU and MMPDU headers.
4. Bit 10, the "More Frag." (i.e., More Fragments) bit, indicates (if it is set to "1") that this MPDU or MMPDU is a fragment of a larger MSDU or MMPDU, and that this is not the last fragment. If the bit is clear, then this frame is the last fragment, or the frame was never fragmented.
5. Bit 11, the "Retry" bit, is set when this MPDU or MMPDU is a retransmission of an earlier MPDU or MMPDU. The Retry bit allows the receiving STA to detect and eliminate any duplicate MPDUs or MMPDUs that it happens to receive. If a STA receives a MPDU or MMPDU and acknowledges it, but the ACK control frame does not reach the sending STA, then the sender will retransmit the frame with the Retry bit set.
6. Bit 12, the "Pwr. Mgt." (i.e., Power Management) bit, is used to indicate what power management state the sending station will enter upon successfully completing the transmission of the current MPDU or MMPDU.
7. Bit 13, the "More Data" bit, is used when talking to a STA that is in power-save mode, to let it know that after this frame, there are still more frames buffered for it. Upon receiving a frame with this bit set, the receiving STA could choose to stay awake a little longer to collect the frames that are queued for delivery to it.
8. Bit 14, the "Prot. Frm." (i.e., Protected Frame) bit,is used to indicate that a frame is protected by one of the cipher suites supported by IEEE 802.11 This bit was formerly known as the "WEP" bit (Wired-Equivalent Privacy) in the original IEEE 802.11-1999 standard, but the IEEE 802.11i TG is changing its name (as indicated in the text).
9. Bit 15, the "Ord." Bit, is set to indicate that a given MSDU is being sent using the "strictly ordered" service class provided by the 802.11 MAC. The other service class type is "re-orderable multicast." If an MSDU is being sent with the Order bit set, then no other MSDUs may be sent to a STA until this MDSU has been completely transmitted (e.g., in the case where an AP has multiple frames buffered for that STA, any frames with the Order bit set must be transmitted in order).