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Data link layer MAC sublayer
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Point coordination function (PCF)
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Distrilmted coordination function (DC:P)
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One of the two protocols defined by IEEE at the MAC sublayer is called the distributed coordination function (DCF) DCF uses CSMAICA (as defined in 12) as the access method Wireless LANs cannot implement CSMAfCD for three reasons:
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I For collision detection a station must be able to send data and receive collision
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signals at the same time This can mean costly stations and increased bandwidth requirements 2 Collision may not be detected because of the hidden station problem We will discuss this problem later in the chapter 3 The distance between stations can be great Signal fading could prevent a station at one end from hearing a collision at the other end Process Flowchart Figure 144 shows the process flowchart for CSMAICA as used in wireless LANs We will explain the steps shortly Frame Exchange Time Line frames in time Figure 145 shows the exchange of data and control
WIRELESS LANs
CSMAICA flowchart
Wait back-off time
Increment back-off
Time
Time
Time
Time
SECTION 141
IEEE 80211
I Before sending a frame, the source station senses the medium by checking the energy level at the carrier frequency
a The channel uses a persistence strategy with back-off until the channel is idle b After the station is found to be idle, the station waits for a period of time called the distributed interframe space (DIFS); then the station sends a control frame called the request to send (RTS) 2 After receiving the RTS and waiting a period of time called the short interframe space (SIFS), the destination station sends a control frame, called the clear to send (CTS), to the source station This control frame indicates that the destination station is ready to receive data 3 The source station sends data after waiting an amount of time equal to SIFS 4 The destination station, after waiting an amount of time equal to SIFS, sends an acknowledgment to show that the frame has been received Acknowledgment is needed in this protocol because the station does not have any means to check for the successful arrival of its data at the destination On the other hand, the lack of collision in CSMAlCD is a kind of indication to the source that data have arrived Network Allocation Vector How do other stations defer sending their data if one station acquires access In other words, how is the collision avoidance aspect of this protocol accomplished The key is a feature called NAV When a station sends an RTS frame, it includes the duration of time that it needs to occupy the channel The stations that are affected by this transmission create a timer called a network allocation vector (NAV) that shows how much time must pass before these stations are allowed to check the channel for idleness Each time a station accesses the system and sends an RTS frame, other stations start their NAV In other words, each station, before sensing the physical medium to see if it is idle, first checks its NAV to see if it has expired Figure 145 shows the idea of NAV Collision During Handshaking What happens if there is collision during the time when RTS or CTS control frames are in transition, often called the handshaking period Two or more stations may try to send RTS frames at the same time These control frames may collide However, because there is no mechanism for collision detection, the sender assumes there has been a collision if it has not received a CTS frame from the receiver The back-off strategy is employed, and the sender tries again
Point Coordination Function (PCP)
The point coordination function (PCF) is an optional access method that can be implemented in an infrastructure network (not in an ad hoc network) It is implemented on top of the DCF and is used mostly for time-sensitive transmission PCF has a centralized, contention-free polling access method The AP performs polling for stations that are capable of being polled The stations are polled one after another, sending any data they have to the AP To give priority to PCF over DCF, another set of interframe spaces has been defined: PIFS and SIFS The SIFS is the same as that in DCF, but the PIFS (PCF IFS) is shorter than the DIFS This means that if, at the same time, a station wants to use only DCF and an AP wants to use PCF, the AP has priority
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