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EEE521. Computer and Data Communications Semester 2 2011-2012 Mohd Nazri Mahmud Session 4a-12 March 2012. Lesson plan. Reflection on Lab session 1: Local Area Networks Discussion on CSMA/CA and Wireless LAN Preparatory readings:

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Computer and Data Communications

Semester 2 2011-2012

MohdNazri Mahmud

Session 4a-12 March 2012

Lesson plan
Lesson plan

  • Reflection on Lab session 1: Local Area Networks

  • Discussion on CSMA/CA and Wireless LAN

  • Preparatory readings:

    • Chapter 12.1 CSMA/CA part and Chapter14 Wireless LAN (14.1 only). It is recommended for students to complement their study with other supplementary reading resources.

    • Preparatory questions for CSMA/CA and WLAN – from teaching webpage

Wireless network
Wireless network




A’s Transmission range

B’s Transmission range

C’s Transmission range

Collision avoidance
Collision avoidance

  • In a wireless network, much of the sent signal energy is lost in the transmission due to fading (multipath and shadowing). Additional energy that results from overlapping signals becomes very weak for an effective collision detection by the source stations.

  • Therefore, for the wireless network collision detection mechanism needs to be replaced with collision avoidance

The hidden node problem

  • C senses the medium up to the required time but could not notice the transmission by A because C is out of A’s transmission range. C mistakenly thinks that the medium is free and start its transmission to B. Consequently, both A’s and C’s data collide at B and get corrupted.

  • The problem arises because both A and C are “hidden” from each other. This is called the HIDDEN NODE problem.


  • Use handshaking mechanism

  • Following the detection of an idle channel, A sends Request-to-Send (RTS) signal to B. The RTS must specify the duration required for the data transfer (the Network Allocation Vector(NAV))

  • Upon receiving the RTS signal from A, B shall wait for a Short Inter-Frame Spacing (SIFS) period, this is to ensure that the channel is idle for B to send out a response.

  • If the channel is idle throughout the SIFS duration, B responds to A with a Clear-To-Send (CTS). The CTS must also contains the duration for the data transmission.

  • Meanwhile, C should increase its sensing duration by a DIFS (Distributed Inter-Frame Spacing) duration. The CTS send by B can reach C during the DIFS period because B use the shorter SIFS and also because C is within B’s range. C knows that B is ready to communicate with a hidden node A. Therefore, C starts to backoff. C starts its Network Allocation Vector (NAV) timer (NAV-CTS) for the duration specified by the CTS and refrains from accessing the link throughout the NAV-CTS duration.





A’s Transmission range

B’s Transmission range

C’s Transmission range

D’s Transmission range

The exposed node problem

  • B sends RTS to A.

  • Since C is within B’s transmission range, C knows that B will be busy communication with another station. Therefore C back-offs for the NAV-RTS duration specified in the RTS.

  • BUT….

  • Even if C sends RTS to D, the RTS will not interfere with the CTS sent by A to B because A and C are out of each other’s range. Their data will not collide at either A or D.

  • Thus, C has unnecessarily waited for the total duration of NAV-RTS.

  • C is said to be exposed to B.

  • This is called the EXPOSED NODE problem.


  • After overhearing the RTS sent by B to A, C should wait for the CTS

  • If C can hear a CTS coming from B, then C knows that itself is within B’s transmission range.

  • But in this particular case, C is outside B’s range and the CTS from B will never reach C

  • Following the absence of the CTS that corresponds to the overheard RTS, C can safely assume that the transfer of data between C and D will not interfere with data transfer between A and B and proceed without waiting anymore

  • Nevertheless, C has to wait for an SIFS + CTS (plus some transmission and propagation delay) duration because a CTS from a station that is within C’s transmission range would have reached C by this time.

3 strategies for access regulation
3 strategies for access regulation

  • Interframe Space (IFS)

    • collisions are avoided by deferring transmission even if the channel is found idle.

    • Stations wait for a period of time called the interframe space or IFS.

    • Even though the channel may appear idle when it is sensed, a distant station may have already started transmitting.

    • The distant station's signal has not yet reached this station.

    • The IFS time allows the front of the transmitted signal by the distant station to reach this station.

    • If after the IFS time the channel is still idle, the station can send, but it still needs to wait a time equal to the contention time (described next).

    • The IFS variable can also be used to prioritize stations or frame types.

    • For example, a station that is assigned a shorter IFS(SIFS) has a higher priority than stations assigned with longer IFS (eg. DIFS)

3 strategies for access regulation1
3 strategies for access regulation

  • Short Interframe Space (SIFS), is the small time interval between the data frame and its acknowledgment.

  • used for the highest priority transmissions enabling stations with this type of information to access the radio link first.

  • This value is fixed per PHY and is calculated in such a way that transmitting station will be able to switch back to receive mode and be capable of decoding the incoming packets.

  • Examples of information which will be transmitted after the SIFS has expired include the acknowledgement(ACK) and the Clear To Send (CTS) messages.

  • The SIFS in IEEE 802.11 are defined to be the smallest of all interframe spaces (IFS).

  • A SIFS duration is a constant value and it depends on the amendments.

  • Standard SIFS (μs)[1]

    • IEEE 802.11b = 10

    • IEEE 802.11a = 16

    • IEEE 802.11g = 10

3 strategies for access regulation2
3 strategies for access regulation

  • DIFS

  • In the DCF protocol, a station has to sense the status of the wireless medium before transmitting.

  • If the medium is continuously idle for DCF Interframe Space (DIFS) duration, only then it is supposed to transmit a frame.

  • If the channel is found busy during the DIFS interval, the station should defer its transmission.

  • DIFS duration can be calculated by the following method.

    • DIFS = SIFS + (2 * Slot time)

  • Standard DIFS (µs)

    • IEEE 802.11b = 50

    • IEEE 802.11a = 34

    • IEEE 802.11g = 28 or 50

3 strategies for access regulation3
3 strategies for access regulation

2. The contention window

  • an amount of time divided into slots.

  • Standard slot time (in microseconds)

    • IEEE 802.11b = 20

    • IEEE 802.11a = 9

    • IEEE 802.11g = 9 or 20

  • A station that is ready to send chooses a random number of slots as its wait time.

  • The number of slots in the window changes according to the binary exponential back-off strategy.

  • This means that it is set to one slot the first time and then doubles each time the station cannot detect an idle channel after the IFS time.

  • This is very similar to the p-persistent method except that a random outcome defines the number of slots taken by the waiting station.

  • One interesting point about the contention window is that the station needs to sense the channel after each time slot.

  • However, if the station finds the channel busy, it does not restart the process; it just stops the timer and restarts it when the channel is sensed as idle.

  • This gives priority to the station with the longest waiting time.

3 strategies for access regulation4
3 strategies for access regulation

3. Acknowledgment

  • With all these precautions, there still may be a collision resulting in destroyed data.

  • In addition, the data may be corrupted during the transmission.

  • The positive acknowledgment and the time-out timer can help guarantee that the receiver has received the frame.