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channels In 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, a has more channels and you may check at

802.11a/g modulation From wiki:

802.11e e has been proposed as an enhancement for There are many things proposed in , 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. – …

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.”

Priorities There are 8 priorities defined in e. – Voice – Video – Best Effort – Background

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

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

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.

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

TXOP (Transmission Opportunity) Another key feature is the Transmission Opportunity. In 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?

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.

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.

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