802'11 protocol continued

1
802.11 protocol continued
2
DCF

• The basic idea is non-persistent.
• Can do an optimization For a new packet (Q len
  0), the sender needs only wait for DIFS, and if
  channel is clear, send.
• This is to avoid the unnecessary wait when the
  medium is not congested.
• But, after every successful packet transmission,
  should start a backoff. (imagine the node is
  listening to the medium when it is also
  transmitting. It has another packet to send, but
  the medium is busy, so, it should follow the
  protocol to first wait for DIFS when backoff.)

3
DCF

• Yet another optimization.
• Suppose you have a packet to send, sensed medium
  to be busy, then you generate a random number,
  say, 6, and wait. After medium is free for DIFS
  and 2 time slots, someone else started
  transmitting. Your backoff counter is 4 at this
  time. What should you do?
• First, wait for DIFS. Then, start counting down
  from 4. NOT generate a new random number!
• This is basically to say that people who has been
  waiting for some time likely have higher
  priorities the next time medium is free.

4
DCF

• What should we do if a packet is lost? Meaning
  that we did not get ACK for this packet?
• Currently in 802.11, if a packet is lost, CW CW
  2.
• CWMin 15. CWMax 1023. CW is reset to CWMin
  after every successful transmission. Basically,
  copying Ethernet.

5
DCF

• Slot time
• 802.11b, 20us
• 802.11a, 9us
• 802.11g, 9us (fast slot time)
• SIFS
• 802.11b, 10us.
• 802.11a, 16us.
• 802.11g, 10us.
• DIFS SIFS 2SLOT

6
EIFS

• The EIFS is derived from the SIFS and the DIFS
  and the length of time it takes to transmit an
  ACK frame at 1 Mbit/s by the following
  equation EIFS aSIFSTime (8 x ACKSize)
  aPreambleLength aPLCPHeaderLngth DIFS
• Why having EIFS?
• Consider the case
• Alt-------AP----------------B
• AP sends to A, A got it
• B noticed medium busy, but did not get the packet
  from the AP correctly so it does not know how
  long to wait until the ACK is sent. B cannot hear
  A, so if A sends ACK, B will think that the
  medium is free and may send a packet, so the AP
  cannot get the ACK from A!
• So the solution is, if you didnt get the packet
  header correctly, wait for EIFS.

7
DCF

• After attempted for several times, if a packet
  still cannot get through, 802.11 drops that
  packet.
• In some wireless cards, the retry maximum is only
  1.
• Why abort?
• Consider two hidden stations, A and B, both want
  to send to the AP. They will start to send at the
  same time, and the AP wont get the packets. So
  both A and B will retry. And will fail again. If
  we dont set a limit to it, this will happen
  forever.

8
BSS, DS

• Typically, an 802.11 LAN is organized into a
  cellular structure, where each cell is called a
  Basic Service Set (BSS), controlled by the base
  station called the Access Point (AP).
• The AP is connected to a Distribution System (DS)
  typically by Ethenet.

9
802.11 Frame format

• http//sss-mag.com/pdf/802_11tut.pdf
• All 802.11 frames follow the following formats
• PreamblePLCP header MAC dataCRC
• Preamble
• Alternating 0s and 1s, followed by a bit pattern
  indicating the beginning of a frame

10
802.11 Frame format

• PLCP (physical layer control procedure ) header
• length of the packet
• Bit rate used
• Header CRC
• MAC header
• Check the link provided earlier

11
Beacon signal

• http//www.wi-fiplanet.com/tutorials/article.php/1
  492071
• The AP periodically send beacon signals to
  announce things like
• Beacon interval
• Time stamp
• Service set identifier (SSID)
• Supported rates
• Parameter sets, like channel number
• Capability info
• TIM
• How to make sure that the AP can send this beacon
  signal roughly in time?
• PIFS SIFSSLOT

12
Joining a BSS

• When a station wishes to join a BSS, it can
  either
• wait for the beacon signal broadcast by the AP
• Or actively send a request (Probe request) and
  the AP will reply with probe response
• Then, authenticate the station
• Then, associate with the BSS

13
Power saving

• For laptops, cellphones, battery is important.
• They can enter the power saving mode.
• Basically, the AP can buffer their packets, and
  send in the beacon signal about these packets.
  The stations should periodically wake up to
  receive the beacon signal and if the beacon
  signal says they have packets, they should stay
  awake to receive the packets.

14
Rate Adaptation

• Wireless nodes may have different channels, some
  strong, some weak.
• 802.11a and 802.11g 6, 9, 12,18, 24,36,48,54
  Mbps.
• 802.11b 1, 2, 5.5, 11 Mbps.
• Which rate should you choose? Higher data rate
  requires stronger channels. You should select a
  rate that most suitable for your channel.
• If you select a rate that is above the channel
  can take, the Packet Error Ratio is going to be
  high.

15
Rate Adapation

• The MadWifi code
• From Improving Loss Resilience with Multi-Radio
  Diversity in
• Wireless Networks in Mobicom 2005.