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In version e is added the field "QoS Control" of lenght 2 bytes.

It carries, depending on the type of frame:

• TID (traffic identifier), of 4 bits. Classifies the packet as belonging to a UP (TID=0-7) or to TSPEC (TID=8-15).

• Some flags used in Automatic Power Save delivery (APSD)

• ACK policy (2 bits): if are used regular ACKs or Block ACKs (see later).

• TXOP limit (8 bit): max duration of TXOP time.

AIFS (Arbitrary IFS)

At each AC is associated a different duration for IFS: AIFS[0], AIFS[1]...

in general the CA with higher priority has AIFS[0] is set equal to DIFS, while other AIFS are > DIFS.

CW: CONTENTION WINDOW

Each AC has also a different CW for the backoff procedure

the CW value is update: (not important)

BACKOFF HP: high priority

MP: medium priority

LP: low priority

The HP has the lowest AIFS and also lowest CW

It may happen that the backoff counters of two or more AC of the same station (virtual stations) reach 0 at same time.

But there is a mechanism inside the STA that avoid the virtual collision, taking also into account the priority of the

involved AC.

EXAMPLE: this is the DCF of standard b. this is the e standard.

We have 4 ACs with different priorities and different set of MAC

parameters.

If there is a collision, the access is given to the AC with highest

priority.

ECDA BURSTING

When a station obtain the access to the channel for TXOP_Limit time, it can send more than one frame, separated by SIFS

to avoid collisions. The possibility of transmitting multiple packets of ECDA is called ECDA bursting.

So the receiver has to acknowledge for each packet. This can be done sending an ACK for each one, or a block ACK (bACK)

at end of TXOP to acknowledge all the packets sent during TXOP.

The block ACK implementation is optional.

The choice between the multiple ACK and block ACK is specified in the MAC packet header in the "ACK policy" field.

The block ACK must be required within TXOP time (with packet bACK req).

Block ACK uses a bitmap to specify the success or failure of the transmitted frames.

Pros of sending multiple packets (bursting) in a TXOP are:

• reduces the network overhead.

• increase the throughputs

• better fairness (equal division in time) among the same priority queues, because a station gets a TXOP every time wins a

contention

Polling is the key in HCCA.

HC can know some information on the station before the polling, because they can send to HC these information using the

Q oS control field in frame headers.

In general stations send to HC information on their queue siz e, desired TXOP and request to initiate a traffic stream

specifying the TSPEC needed.

Traffic scheduling, that is the way stations are polled is not specified in the standard.

HCCA CONCLUSIONS

HCCA (as PCF) has the goal tor provide QoS to real-time traffic. But there are some problems that makes this difficult:

• unpredictable beacon delay: as in PCF, beacon is sent every superframe time, but before to transmit it the channel must be

sensed idle, and if the previous communication is not ended, the beacon can start with a delay.

• Polling list: HC has to poll all the station in the list also if they have nothing to transmit.

Pros:

• In HCCA is solved the problem of transmission duration that there is in PCF since all station can transmit only for TXOP time.

802.11n

It was approved in 2009 .

It operates at 5GHz and 2.4GHz, so it can operate also with the old WiFi cards.

The channel bandwidth has been increased. It can use the 20MHz, to be compatible with the previous standard, or also 40MHz.

It has 13 non-overlapping channels in 5G Hz band.

At physical layer uses OFDM + MIMO + Beamforming.

It can achieve data rate up to 600Mb/s.

MIMO

MIMO (multiple input multiple output) consists in using multiple antennas at Tx and R x, to use parallel spatial data streams. 2x2

(2 antennas at Tx and 2 at R x) are mandatory, but can be implemented also 3 x3 and 4x4.

MIMO is used to increase throughput, because the antennas can be used to transmit more data but also to improve the reliability

of transmission, if the antennas transmit the same data, and if some fails there are the others.

It has almost 70 MCS (Modulation and Coding Sets)

BEAMFORMING

It the process by which the sender of a transmission can direct its energy toward a receiver to increase the SNR.

It uses multiple antennas to direct the propagation in a specific direction, instead of transmit in all directions.