A comparison of wi-fi and wimax with case studies - Florida State ...

user. The awakened MS will send a PS-Poll frame to the base station to get the buffered frames.
The base station can choose either immediate response or deferred response to the MS. [17][55]
The immediate response means the BS (Base Station) will send the buffered frames to MS
after SIFS interval. If the BS chooses a deferred response, it will send an ACK frame back to MS
first, and then transmit data frames later. After sending the PS-Poll frame, the MS must stays
awake until the whole process is over. The BS notifies the MS for buffered frames by sending a
beacon frame. The buffered frames may be fragmented for transmission. [17][55]
3.3.2.7. Security
This is a very important part for the whole MAC layer design. So far the most widely used
security procedure for wireless communication is WEP (Wired Equivalent Privacy). WEP uses
RC4 cipher to encrypt data. The RC4 cipher is a kind of symmetric stream cipher; it generates a
keystream and then uses the XOR algorithm to mix with data to produce the ciphertext stream.
The receiver will use the same XOR algorithm to recover original data. To encrypt the data, the
secret key has to be chosen first and then extended to the same size of data by PRNG
(Pseudorandom Number Generator). This extended secret key is called keystream. For
recovering data, both transmitters and receivers must have the same secret key and PRNG, and
how to distribute the secret key is an important issue; sometimes it may be preloaded by system
designers or manufacturers. The other issue is “key to keystream expansion” of RC4 stream
cipher, because the safety is dependent upon on how random it is. [17][55]
The communication security has three major properties: confidentiality, integrity and
authentication. Confidentiality is needed to protect data from being stolen by unauthorized
people. Integrity is used to make sure the data has not been changed during transmission. This
part is dependent on CRC code. Finally, authentication is the foundation of all security
procedures. For transmitting data, the users must be trusted and the source must be reliable.
Otherwise, authorization and access control will not be allowed. [17][55]
RC4 shared secret key is composed by 40-bit shared secret and 24-bit IV (Initialization
Vector) usually called 64-bit WEP and the other one is 128-bit WEP. When framing a frame,
WEP will generate an ICV (Integrity Check Value) which is a hash value to combine with
payload as an original data. This ICV can use to protect data from unauthorized changes. After
entering the original payload, secret key and IV, WEP can generate an encrypted frame for
transmission in either secure or not secure network. Figure 3-20[17] shows the frame structure
and operation of WEP. Figure 3-21[55] shows the encryption process. [17][55]
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