] Computer networks - Media access control sublayer - Wireless LAN

] Computer networks - Media access control sublayer - Wireless LAN

802.11 architecture and protocol stack

802.11 network usage patterns:

There architectural pattern (Infrastructure mode)
wireless access point client connected AP, there is called the architectural pattern

Ad-hoc mode (Ad hoc mode)
no AP points, but the client is connected to the client each other, this is called ad hoc mode, but this is the future trend

802.11 protocol stack

802.11 Physical Layer

A plurality of cards compatible with the physical layer, such as 802.11 a / b / g

802.11 MAC sublayer protocol

802.11 is not as using CSMA / CD Ethernet reasons like

1. Almost half-duplex radio. Radio medium like Ethernet collision detection mechanism does not work like that. 802.11 trying to avoid conflict, 802.11 uses CSMA / CA protocol
2. The transmission range of different stations may be different. There will be problems hidden station and exposure station

CSMA (CSMA / CA) with collision avoidance

1. By listening, not determined in a very short time signal; counting down idle time slot and then, when there is the frame counter at the transmission pause; when the counter decrements to 0, the station transmits its own frame
2. If frame transmission is successful, the target station sends a short confirmation immediately
3. If no acknowledgment is received, it may be inferred transmission error has occurred. In this case, the number of time slots doubling back to the sender selected, and then re-attempt to send
4. So again, according to the index as a continuous retreat as Ethernet, the maximum number of times until successfully transmitted or retransmitted reach

Send frame under CSMA / CA mechanism

1. For the first station A, a first transmitting station A towards station D, station D after a reply ack frame to the station A, telling it received, other stations can not be transmitted during this period
after the B station 2. For and station C, they have been transmitted from the start of the listening channel, and finally send to each other when the a and D the end, they are ready to send, and both stations before the countdown of idle time slots when there is a frame transmitted at suspend the counter; when the counter is decremented to 0, the station transmits its own frame transmission began by C, B and begins listening only idle waiting
3. when sending the mutually between C and D, B and then the idle time countdown slot, when there is a frame transmitted on the suspension at the counter; after when the down counter to 0, station B transmits its frame, apparently on a Backoff time B, can be transmitted, it transmits the result, and then return to its D ack frame end .

Summary:
1.CSMA / CA insert escape groove to avoid conflicts
2.MAC use ACKs / retransmission of radio error

802.11 supports two modes of operation

DCF (Distributed Coordination Function)

1. There is no use of any central control means
2. Use the CSMA / CA (with collision avoidance CSMA) this protocol uses both a physical monitor means also uses virtual monitor means . CSMA / CA supports two methods of operation.

• The first mode is, when a station wants to transmit data, it first listens to the channel, if the channel is idle, then now, it begins to transmit, but it does not monitor the channel during the transmission, but directly sends the entire frame, but this may lead to the station recipient received due to interference damaged frames, if the channel happens to be not so idle, then the sender will be postponed until the channel is idle begins transmitting, if a conflict occurs, then the conflict it is re-transmitted after waiting a random time.
• The second mode uses a virtual channel listener methods
  example:
  
  
  Prerequisites:
  1.A A desirable located within a wireless transmission range data toward B, C
  2.D in the B range, but not within the scope of A

Process:
the angle A and B:
1.A decides to send to B a data protocol to work, a first A super B transmits an RTS frame, a transmitted object request permission frame to the B-frame, that is, in other words ask B, I am sending you a thing, I can not be made
after the receipt of the request frame 2.B RTS frame, it may be decided to grant the license, send it to tell you a, B sends a CTS frame to a
3. a B permit after consent, that is, after receiving the RTS, starts to send out the data, and start a timer ACK, ACK timer that is doing it, is to determine whether the data B on time and successfully received a reply ACK frame, if B successfully receives the data and return the ACK frame, the process is terminated protocol exchange, if a timeout so all over again.

For angles C and D:
1. Since A is also within the scope of C, so that it can receive the RTS is transmitted out of the A-frame, but it knows that it is not the destination address, it will not respond to the CTS, but say nothing of it knows tube Zhashui, it was to send the data, in order to consider the overall situation, although it is good to shut up, would not have to send any message, and others until after the transfer is complete, it is how people know Shashi Hou transfer is complete it? This is because it can calculate how long people to be transmitted, how it is to keep it quiet? There is a signal called the NAV (network allocation vector), it is not sent, it is a reminder from the internal RTS frame signal, used to keep quiet for some time.
2.D not listen to the RTS, but it is within the range B, it received the CTS, so after sending the CTS B, it declares NAV signal indicating shut.

3. All must support DCF 802.11

PCF (Point Coordination Function)
1. Use the AP point control of all activities within their own coverage
2.PCF is optional, 802.11 can choose to support, you can choose not to support

The concept summary:

1. In order to reduce the ambiguity to find out which station transmits, 802.11 defines the physical channel sensing comprises sensing and virtual listener
2. The network allocation vector (the NAV), with a virtual listener, each station may be retained in a channel when use of the logical record, which is obtained by tracking the NAV, in each frame carries a NAV field, described a series of data transmission for this frame belongs how long
3. optional RTS / CTS mechanism used to prevent hidden station NAV send (almost worthless design) at the same time. The mechanism is as follows
4. Note: NAV transmission signal is not only used internally by the station, to remind themselves to retain a certain time within a quiet
5. Use the NAV (network allocation vector) and optionally RTS / CTS (usually not used) the virtual channel aware avoid the hidden terminal

MAC some of the mechanisms

Different quality of service support backoff period, a short time interval for the preferred access control e.g., the VoIP
the MAC further includes other mechanisms, e.g., energy

802.11 Structure

1. The frame has a "frame control" specify different types
2. Data frame has three address through ap
defines three different types of frames for communication 3.802.11 standard, a data frame, a control frame, a management frame