Figure 5.5 Frame Format
❑ Frame control (FC).
The FC field is 2 bytes long and specifies the frame type as well as some control information. Table 2 describes the subfields
In all types of frames, except one, this field determines the duration of the transmission
which is used to determine the value of NAV. While in a control frame, this field specifies the frame ID.
There are four address fields, each 6 bytes long. The meaning of each of the fields depends on the value of Right-DS and From-DS.
. Sequence control.
This field specifies the frame sequence number to be used in the flow control
F Frame body.
This field, which can be between 0 and 2312 bytes, contains information based on the type and subtype defined in the FC field.
The FCS field is 4 bytes long and contains a CRC-32 error control sequence
Table 2 Subfields in the FC field
5.5 Frame Types
A wireless LAN defined by IEEE 802.11 has three categories of frames: management frame, control frame, and data frame.
Management frameworks are used for initial communication between stations and Access Points.
Control diagrams to access the channel and recognize the frame.
For control frames the Type field value is 01; the values of the subtype fields for the above frames are presented in Table 5.6.
Data frames are used to transfer data and control information.
Figure 5.6 shows the format
5.6 Address Mechanism
The IEEE 802.11 addressing mechanism specifies the treatment of four cases, defined by the value of the two flags in the FC field, For in – DS and From – DS. Each flag can be 0 or 1, resulting in four different situations. The interpretation of the four addresses (from address 1 to 4) in the MAC frame depends on the value of these flags, as shown in Table 3.
Table 3 Address
Hidden and exposed problems of Stations where we refer to hidden and exposed problems to stations in the previous section. It is time now to discuss these problems and their effects.
Figure 5.7 shows an example of the hidden station problem. Station B has a transmission range indicated by the left ellipse); any station in this range can hear any signal transmitted by station B. Station C has a transmission range indicated by the right ellipse); any station located in this range can hear any signal transmitted by C. Station C is outside the B transmission gamma, also, station B is outside the transmission interval of C. Station A, however, is in the area covered by both B and C, so he can hear any signal transmitted by B or C.
Figure 5.7 Hidden station problem
Assume that station B is sending data to station A. In the middle of this transmission, station C also has data to send to station A. We know that station C is out of range B of the transmission and vice versa. Therefore C thinks the transmission channel is free. Station C sends its data to A, which causes a collision as this station is receiving data from B and C simultaneously. In this case, we say that stations B and C are hidden from each other in relation to station A. Hidden stations may reduce the capacity of the network due to the possibility of collision. The solution to the hidden station problem is to use the hand shake frames (RTS and CTS) that we discussed earlier.
Figure 5.8 shows that the RTS message from B reaches A, but does not reach C. However, because both B and C are within the radius of A, the CTS message, which contains the duration of data transfer from B in A reaches C. Station C realizes that a hidden station is using the transmission channel and refrains from transmission until it is released.
The CTS hand shake frame in CSMA / CA can prevent collision from a hidden station.
Figure 5.8 Using handshaking to prevent the hidden station problem
5.6.1 The problem of the exposed station.
We consider a situation that is the opposite of the previous one. In this problem a station is restrained from using a communication channel even though it is free, available. In Figure 5.9, station A is transmitting to station B. Station C has some data to send to station D, which can be sent without interfering with transmission from A to B. However, station C is exposed to transmission from A to ; he hears that A is transmitting and thus refrains from sending. In other words, C is very attractive and damages the capacity of the channel.
Figure 5.9 the problem of the exposed station
Handshaking RTS and CTS messages cannot help in this case. Figure 5.10 shows the situation. Station C listens to RTS from A, but does not listen to CTS from B. Station C, after
listening to RTS from A, can wait for a while so that CTS from B reaches A; he then sends an RTS to D to indicate that he needs to communicate with D.
Both stations A and B can hear this RTS but station A is in the sending and not receiving situation. While station B responds with a CTS. The problem lies here: If station A starts sending data, station C cannot hear CTS from station D due to the collision, so it cannot start sending data TO ITS RIGHT. He remains unemployed until A completes the submission of data.