802.11 protocol continued. dcf the basic idea is non-persistent. can do an optimization: for a new...

802.11 protocol continued

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.)

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.

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.

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 + 2*SLOT

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:

– A<-------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 didn’t get the packet header correctly, wait for EIFS.

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 won’t get the packets. So both A and B will retry. And will fail again. If we don’t set a limit to it, this will happen forever.

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.

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

Beacon signal• http://www.wi-fiplanet.com/tutorials/article.php/1492071• 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 = SIFS+SLOT

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

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.

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.

Rate Adapation• The MadWifi

code:• From

“Improving Loss Resilience with Multi-Radio Diversity inWireless Networks” in Mobicom 2005.

802.11 protocol continued. dcf the basic idea is non-persistent. can do an optimization: for a new...

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