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III. FRAMES

The data packets from the network layer, are encapsulated by a level 2 MAC header, forming an MPDU (MAC Protocol Data Unit). The MPDU is then encapsulated in a second frame to level 1 (physical) to allow transmission media. This encapsulation is to add a preamble and a header to the MPDU, this set forms a PLCP-PDU. The preamble and the header following the different physical layer used. We will see different frames of the physical layer (PLCP-PDU), then those of the data link level (MPDU).

A. PHYSICAL LEVEL

The preamble allows the detection of start of frame, the frame synchronization, it allows the Channel to the issuance or CCA (Clear Channel Assesment).
The header contains various information, which varies depending on the physical interface used.

1. FRAME FHSS (802.11 FHSS):

preamble (preamble) into two parts:

-80 synchronization bits (alternating between 0 and 1) to select the best access point and to sync (AP and STA).

-SFD (Start Frame Delimiter) 16-bit (0000 1100 1011 1101): indicates the beginning of the frame.

Header (header) into three parts:

-PLW (PLCP-PDU Length Word) 12-bit: Indicates the length (in bytes) of the frame (PLCP-PDU), this allows the physical layer to determine the end of the frame.

-PSF (PLCP Signaling Field) 4 bits: indicates the rate used over the radio interface. (1 or 2 Mbps) for data transmission (MPDU).

-HEC (Header Error Check) is a 16-bit CRC to detect errors in fields in the header (PLW and PSF).

Note: the preamble and header are always transmitted at 1 Mbps

2. FRAME DSSS (802.11 DSSS, 802.11b):

Preamble: FSSS identical to the frame, if not longer long synchronization longue.et worth 0xF3A0 (1111 0011 1010 0000) for the SFD.

header into four parts:

Signal-to 8-bit: indicates the selected speed for the transmission of data (MPDU):
0x0A for 802.11 mode BPSK (1Mbps)
0x14 for 802.11 mode QPSK (2Mbit / s)
0x37 for 802.11b mode QPSK (5.5 Mbps) for 802.11b
0x6E mode QPSK (11Mbps)

Service 8 bits: reserved for future use
(value 0x00? IEEE802.11)

-Length 16-bit Indicates the length (in bytes) of the frame to follow (MPDU), this allows the physical layer to determine the end of the frame.

-HEC (Header Error Check) is a 16-bit CRC to detect errors in fields in the header (Signal, Service and Length).

Note: the preamble and header are always transmitted at 1 Mbps
addition to the 802.11b standard there is a second type of encapsulation where the preamble is shorter (instead of 72bits 144bits):

3. FRAME OFDM (802.11a, 802.11g):

preamble achieved through a sequence it twelve symbols for the detection of the signal by the receiver and the beginning of the frame.

header in six fields:

-RATE: indicates the transmission rate
-1 Reserved bit always 0
-Length: indicates the number of bytes in the frame. -1
parity bit of the three previous fields
-Tail (in-tail): fields reserved, always 0
- Service: "field, always 0

MPDU:

-Tail:" field, always 0
-Pad: Padding field (filling) of 6-bit minimum to cash a structure in bytes.

B. MAC LEVEL

There are three kinds of frames: data frames, the frames used to control access to the support (RTS, CTS, ACK ...) and the management frames used for the association to an access point or for synchronization and authentication. The representation of the frame is a general representation and its fields change depending on the function thereof. The maximum size of a frame is 2347 bytes.

1. GENERAL FORMAT.

HEADER (7 fields of 30 bytes): This is the most complex because it incorporates many features.



• Control frame (11 sub-fields of 2 bytes):



-protocol version: always 0 for the current version (2 bits)

-Type and sub type: is the 3 kinds of frames and their functions (2 +4 bits)

-To DS and From DS: DS = Distribution Service (access point). To
DS (1 bit) the frame is addressed to the access point that has to do follow. From
DS (1 bit) the frame comes from the access point.

-More Fragments: 1 if the data is fragmented, 0 if they are not fragmented or if last fragment (1 bit).

-Retry: to 1 if a retransmission (1 bit).

-Power Management: 1 if the station is in power saving mode, 0 if it is active (1 bit). From the AP, the frames are still in active mode.

-More Data: This bit is also used for energy management. It is used by the AP to indicate that other frames are stored for this station. The station may then decide to use this information to ask the other frames or to switch to active mode (1 bit).

-WEP: This bit indicates that the frame body is encrypted according to the WEP algorithm.

-Order: if 1 indicates that the frame is sent using a strictly ordered service class. Do not allow the station to send multicast frames.

· Duration / ID (2 bytes):

This field has two meanings, depending on the type of frame:
- for frames polling mode power saving is the station's ID or AID (Association IDentity)
- in other frames, the value of time used for calculating the NAV.

· Fields addresses (4 x 6 bytes):

The MAC address of 48 bits is divided into three parts-
a group of 2 bits at the beginning of address: the first indicates if the address is individual ( bit a1) or group (bit 0), the second indicates whether the address is local (bit 1) or Universal (bit 0). If the address is local, the 46 bits are defined locally.
-a group of 22-bit number constructor defined by the IEEE-
a group of 24-bit serial number defined by the manufacturer

Group addresses:
-broadcast address: defines the set of stations network. (the 48 bits are to 1)
-multicast address: defines a group of stations in finite number.

Address Types:
802.11 The addressing structure is richer than for a wired network. Because if you want to access a station on the same network (BSS) is through the access point thus indicating its MAC address to relay the packet. Similarly to access a station of another network (ESS), middle two addresses can be specified. These address fields are defined in accordance with the directions of the fields To DS and From DS.
We'll see four types of address:

-BSSID (Basic Service Set Identifier):
infrastructure mode -> MAC @ PA
In Ad-Hoc -> @ local BSSID MAC (generated in the creation of the IBSS).

-DA (Destination Address), address, individual or group identifies (s) Recipient (s).

-SA (Source Address): Address individual that transmitted the frame.

-RA (Receveir Address): destination BSSID (access point receiver).

-TA (Transmitter Address): Source BSSID (access point transmitter).


* Wireless Distribution Service (liaison between PA) addressing
Example:

The Ad-Hoc: transmission in an IBSS, sends its data to ST1 ST2.



Step 0: @ 1: ST1, @ 2: ST2, @ 3: BSSID of the IBSS.

Infrastructure mode:
- Case 1: transmission in the same BSS sends ST1 its data to ST2 (via PA1).



Step 1: ST1 sends the frame to PA1 ST2 final destination.
To DS: 1, @ 1: PA1 (dest) @ 2: ST1 (sce), @ 3: ST2 (final dest).

Step 2: send the frame to PA1 ST2. From
DS 1, @ 1: ST2 (dest), @ 2: PA1, @ 3: ST1 (original scene).

- 2em Case: transmission in the same ESS sends ST1 ST3 to its data (via PA1 and PA2).



Unlike the previous example, step 3 follows in step 1.
Step 3: send the frame to PA1 PA2
To DS and From DS are 1, @ 1: PA2 (dest), @ 2: PA1 (sce), @ 3: ST3, @ 4: ST1.
• Control sequence (2 bytes):
-sequence number (12 bits): number assigned to each frame.
-fragment number ('bits): number assigned to each fragment, if the frame is fragmented.

· Body of the frame (data) (0-2312 bytes): The size can be greater than 1500 bytes because of WEP. There are no data for control frames and management.

· FCS (Frame Check Sequence) (4 bytes): 32 bit CRC for integrity checking of the frame.

2. CONTROL FRAME FORMAT:

Control frames can access support and have the function to send commands and supervision information to network elements. In the frame control part, the fields of "ToDS" to "order" are 0.

· Screens main
RTS (Request To Send) is used to claim the right to transmit a data frame



-RA is the address of the recipient receiving the next data frame or management.
-TA is the address of the station transmitting the RTS frame.

CTS (Clear To Send) is the reservation channel to transmit a data frame



- RA is the address of the originating station (field TA) of the RTS frame.

ACK allows acknowledgment of data frames




RA is the address of the originating station, which comes from the address field 2 of the data frame or previous management.

3. FRAME FORMAT MANAGEMENT:

There are four categories of management frames: Frames
· related to the functions of association-disassociation frames
· interrogation radio neighboring
· Frames related to the functions of authentication
Frames · tags used by the access point for disseminating information in the BSS management power saving mode through the TIM and DTIM beacons.


TABLE VALUES (Types and Sub Types)