This question appears in Mumbai University > Wireless Networks subject

Marks: 10 M

Year: May 2014

1. Backoff algorithm:

i. Backoff algorithm is a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) medium access technique used by IEEE 802.11 WLAN standard.

ii. If the MAC layer of a station wishes to transmit, it senses the medium. If the medium is free for a specified time called ‘Distributed Inter Frame Space (DIFS)’, it transmits. If the medium is busy or becomes busy during DIFS, the station defers using backoff algorithm.

iii. According to this algorithm, if the station finds the medium free for a time equal to DIFS, it waits for additional, randomly selected non-zero number of time slots called as ‘Contention Window (CW)’. (one slot=20microsec for 802.11b)

iv. During this wait, the station continues sensing the medium to check whether it remains free or another transmission begins. At the end of its CW, if the medium is still free the station can send its frame. If during CW, another station begins transmitting data, backoff counter is frozen and counting starts again when the channel becomes free.

Fig8. CSMA/CA in IEEE 802.11b

2. Dimension of CW:

i. If a small CW is used, it is possible that when many stations want to transmit the data at same time, they will have same backoff interval. This will lead to collisions and degrade network performance.

ii. If a large CW is used, many stations wanting to transmit data will have wait for long interval. This too degrades network performance.

iii. The solution is to use exponentially growing CW size. It starts with small value (CW_min=31) and doubles after each collision, until it reaches maximum value (CW_max=1023).

3. Instances when the station use backoff algorithm :

i. When the medium is busy before first transmission. ii. After each retransmission. iii. After a successful transmission.

4.Backoff algorithm is not used when the station is free for more than time equal to DIFS. This will allow single station to occupy a channel for long.

5.Transmission time for data frame = (PCLP+D/R) microsec PCLP: time required to transmit PLCP (Physical Layer Convergence Protocol) D: frame size R: channel bit rate

6.CSMA/CA packet transmission time = (BO+DIFS+2PLCP+D/R+SIFS+A/R) microsec A: ACK frame size BO: backoff time SIFS: Short Inter Frame Space PCLP: time required to transmit PLCP (Physical Layer Convergence Protocol) D: frame size R: channel bit rate DIFS: Distributed Inter Frame Space

7.The loss of performance strongly depends on packet size and data rate, but a 30% loss is more than likely to occur.

8. Some terms :

i. SIFS: It is time required for transmitting station to switch back to receive mode. It thus separates transmission belonging to single dialog (e.g. ; fragment: ACK ) (=10microsec for 802.11)

ii. Point coordinate inter frame space (PIFS): Used by AP (Access Point) to gain access to medium before any station. PIFS = SIFS +one slot (30 microsec for 802.11)

iii.Extended Inter Frame Space (EIFS): It is a longer inter frame space and used by stations which have received a packet but cannot understand it. Such a station waits for EIFS to avoid collision with a future packet of current dialog.

9. Reasons why CSMA/CD is not used in WLAN :

i. Implementing a collision detection mechanism would require implementation of a full duplex radio capable of transmitting and receiving at the same time, an approach that would increase the cost significantly.

ii. In a wireless environment, we cannot assume that all stations hear each other (which is basic assumption of collision detection). Also, if a station that wants to transmit and senses the medium free, does not necessarily mean that medium is free around receiver area.