1. (30 points) Suppose an IEEE 802.11a Wi-Fi network operates under the DCF mode

Question

1. (30 points) Suppose an IEEE 802.11a Wi-Fi network operates under the DCF mode. Suppose it has the following parameters : aSIFSTime-16s, aSlotTime = 9us, cwmins 15, and CWmax-1023. Suppose station A generates a random value of 12 for its backoff counter at time instance of t1. The carrier sensing mechanism of station A detects the wireless medium as busy or idle as follows: Busy for 50us from t1; Idle for 70us after the previous busy period; Busy for 90us after the previous idle period; Idle for long enough time for A's backoff interval to expire at time instance of t2 What is the time interval between tl and t2? Use a timeline figure to illustrate station A's behavior between t1 and t2 2. (40 points) Consider the same IEEE 802.11a Wi-Fi network as above (which operates under the DCF mode, with aSIFSTime = 16us, aSlotTime 9us, CWmin = 15, and CVVmax 1023) but under a different scenario. Suppose three stations (A, B, and C) are associated with the AP and they also are the only stations in the network; suppose the wireless medium is clean without interferences from any other sources. All three stations have a single frame to transmit, and the arrival time is T (at station A), T+10us (at station B), and T+20us (at station C) a) What is the probability that station A transmits its frame successfully at the first attempt? b) What is the probability that station C transmits its frame successfully at the first attempt? 3. (30 points) Give short answers to the following questions. a) RTS/CTS was proposed to deal with the "Hidden Nodes" problem. The "Exposed Nodes" problem is another interesting phenomenon in wireless networks. Research to find out what the "Exposed Nodes" problem is and discuss whether or how RTS/CTS also may be able to deal with it. b) Research to find out what the "CTS-to-self" approach is and describe an example scenario to explain how it works. c) Research to find out what the "Extended Inter Frame Space (EIFS)" is and describe an example scenario to explain how it is used

Explanation / Answer

answer 3

a)- Exposed terminal problem

In wireless networks, the exposed node problem occurs when a node is prevented from sending packets to other nodes because of a neighboring transmitter.
Consider an example of 4 nodes labeled R1, S1, S2, and R2, where the two receivers (R1, R2) are out of range of each other, yet the two transmitters (S1, S2) in the middle are in range of each other.

Here, if a transmission between S1 and R1 is taking place, node S2 is prevented from transmitting to R2 as it concludes after carrier sense that it will interfere with the transmission by its neighbor S1. However note that R2 could still receive the transmission of S2 without interference because it is out of range of S1.

IEEE 802.11 RTS/CTS mechanism helps to solve this problem only if the nodes are synchronized and packet sizes and data rates are the same for both the
transmitting nodes. When a node hears an RTS from a neighboring node, but not the corresponding CTS, that node can deduce that it is an exposed node and is permitted to transmit to other neighboring nodes. If the nodes are not synchronised (or if the packet sizes are different or the data rates are different) the problem may occur that the sender will not hear the CTS or the ACK during the transmission of data of the second sender.

b) CTS-to-Self

CTS-to-Self is used strictly as a protection mechanism for mixed-mode environments.One of the benefits of using CTS-to-Self over RTS/ CTS as a protection mechanism is that the throughput will be higher because fewer frames are being sent. When a station using CTS-to-Self wants to transmit data, it performs a NAV distribution by sending a CTS frame.

This CTS notifies all other stations that they must wait until the DATA and ACK have been transmitted.
Any station that hears the CTS will set their NAV to the value provided. Since CTS-to-Self is used as a protection mechanism for mixed-mode environments, the ERP (802.11g) station will transmit the CTS by using DSSS technology that all stations can understand. Then the DATA and the ACK will be transmitted at a faster 802.11g speed by using ERP-OFDM data rates.
CTS-to-Self is better suited for use by an AP.
It is important to make sure that all stations hear the CTS to reserve the medium, and this is most likely to occur if it is being sent by an AP.
If a client station were to use CTS-to-Self, there is a chance that another client station on the opposite side of the BSS might be too far away from the
CTS-to-Self and would not realize that the medium is busy.
Even though this is true, from our experience, it appears that most use CTS-to-Self on client stations to reserve the medium instead of RTS/ CTS.
CTS-to-Self is used because of the decreased overhead when compared with RTS/ CTS.
Some vendors allow the user to select whether the client station uses RTS/ CTS or CTS-to-Self when in Protected mode.

c) Extended Interframe space (EIFS)

Extended Interframe space (EIFS) is used in IEEE 802.11 based WLANs. If a previously received frame contains an error then a station has to defer EIFS duration instead of DIFS before transmitting a frame. This is because, although this station was not able to decode the frame, it could be that the intended receiver could decode the frame. It should have the opportunity to return an Acknowledgment frame; the EIFS ensures the transmission of the Ack can proceed without interference from those not able to decode the frame.

This duration is calculated by the below method.

EIFS = Transmission time of Ack frame at lowest phy mandatory rate + SIFS + DIFS

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