2014年2月23日星期日

Introduce type of frame 1

Question: What is the frame?
Answer: Frame is a kind of data packet which is on the 2nd layer of the OSI model (as introducing in "description of this project"). It is defined as the transmission unit in the link layer protocol. A frame includes a link-layer header with a following packet (consist of payload and control information).

Question: How many types of frame? And what are they?

Answer: Three types. They are: data frame, control frame and management frame.

Question: What is the control frame?

AnswerControl frames can assist the data frames delivery, administer the access to wireless medium. In addition, they provide functions of MAC-layer reliability.
Frame Control field in control frames


Protocol version: The protocol version is 0(It is the only version in current)

Type: In definition, control frames are assigned the type identifier 01. 

Subtype: the subtype of the control frame

ToDS and FromDS bits: Both of them are 0 because of no sending and receiving to control frames by the distribution system

More Fragments bit: Control frames are not fragmented, 0.

Retry bit: Control frames are not queued for retransmission like management or data frames, so this bit is always 0.

Power Management bit: To indicate the power management state of  sender.

More Data bit: Only being in management and data frames, so it is set to 0.

WEP bit: Control frames might not be encrypted by WEP and the WEP bit is always 0.

Order bit: This bit is set to 0.

Question: How many subtypes of control frame are included? And what are they?
Answer: Four subtypes and they are Request to Send, Clear to Send, Acknowledgment and Power-Save Poll.

Request to Send (RST)
RTS frame is used to enhance control of the medium for large frames transmission.

Clear to Send (CST)
The CTS frame answers the RTS frame.


Acknowledgment (ACK)
ACK frame is used with any data transmission and is used for sending positive acknowledgments required by the MAC.

Power-Save Poll (PS-Poll)

When a mobile station wakes from power-saving mode, a PS-Poll frame will be transmitted to the AP for retrieving any frames buffered since in power-saving mode.

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