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More about 802.11

More about 802.11. 802.11 channels. In 802.11 b/g, there are 11 channels, starting at 2.412GHz at a spacing of 5MHz. Each channel owns a bandwidth of 22MHz. So, only 3 non-overlapping channels, 1,6,11.

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More about 802.11

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  1. More about 802.11

  2. 802.11 channels • In 802.11 b/g, there are 11 channels, starting at 2.412GHz at a spacing of 5MHz. • Each channel owns a bandwidth of 22MHz. • So, only 3 non-overlapping channels, 1,6,11. • 802.11a has more channels and you may check at http://www.moonblinkwifi.com/80211a_frequency_channel_map.cfm

  3. 802.11a/g modulation • From wiki: http://en.wikipedia.org/wiki/IEEE_802.11a-1999

  4. 802.11e • 802.11e has been proposed as an enhancement for 802.11. • There are many things proposed in 802.11, we are going to discuss the Quality of service (QoS). • There are different types of traffic • Data: usually large volume, but no specific requirements for deadline. Better not lose it. • VoIP: small in volume (a VoIP session may need only 16Kbps or even less with some data processing), but very sensitive to delay. If passed the deadline, dropping is fine. • Video: large volume. but very sensitive to delay. If passed the deadline, dropping is fine. • …

  5. QoS • Two kinds of QoS. • Give guaranteed bounds, such as “the delay will be no more than 10ms for the VoIP packets, but the delay will be no more than 1s for the data packets.” • Give prioritized service, such as “when I have both a VoIP packet and a data packet, I will choose to serve the VoIP packet with probability 0.90.”

  6. Priorities • There are 8 priorities defined in 802.11e. • Voice • Voice • Video • Video • Video • Best Effort • Background • Background

  7. Access Category (AC) • Provides 4 Access Categories (AC). • AC_VO • AC_VI • AC_BE • AC_BK • Defined EDCA to provide prioritized service. How?

  8. EDCA • The basic idea is to let higher priority packets have • Shorter IFS • Smaller contention window

  9. EDCA • Each station maintains 4 queues, one for each AC. • Internally, each AC contends for access the transmitter. • The AC that wins will be the next packets to be sent.

  10. EDCA • AC_VO: • AIFS = SIFS + 2 * SLOT (QSTA), AIFS = SIFS + SLOT (QAP), • CWmin = (aCWmin+1)/4 • CWmax = (aCWmin+1)/2 • AC_VI: • AIFS = SIFS + 2 * SLOT (QSTA), AIFS = SIFS + SLOT (QAP), • CWmin = (aCWmin+1)/2 • CWmax = aCWmin • AC_BE: • AIFS = SIFS + 3 * SLOT • CWmin = aCWmin • CWmax = aCWmax • AC_BK: • AIFS = SIFS + 7 * SLOT • CWmin = aCWmin • CWmax = aCWmax

  11. TXOP (Transmission Opportunity) • Another key feature is the Transmission Opportunity. • In 802.11 DCF, if there is one node at 6Mbps and the other at 54Mbps, both loaded with traffic to the AP sent in 1500-byte packets. Assume the AP does not have downlink traffic. What will the average network speed like? Closer to 6Mbps, at 30Mbps, or closer to 54Mbps?

  12. TXOP • Closer to 6Mbps – every time the slow node gets a chance to transmit, it uses about 2ms. Every time the fast node gets a chance to send, it uses about 300us. But they gets equal opportunity to send! • So, a slow node can drag down the speed of the entire network.

  13. TXOP • So, TXOP is something quite reasonable to have. Every node, when gets a chance to send, can send for no more than a TXOP length. • For a fast node, it may use the TXOP to send multiple packets. The packet is followed by SIFS then an ACK then SIFS then another packet – no need to backoff. • For a slow node, it can transmit only for this long. It may have to fragment the packet if the packet cannot be fit in a TXOP length. • So, everyone is competing for an equal share of air time.

  14. TXOP • The length of the TXOP: 1.5ms, 3ms for 802.11g/a; 3ms, 6ms for 802.11b.

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