<P> The primary purpose of QoS is to protect high priority data from low priority data . There are also scenarios in which the data needs to be protected from other data of the same class . Admission Control in EDCA address these type of problems . The AP publishes the available bandwidth in beacons . Clients can check the available bandwidth before adding more traffic . </P> <P> Wi - Fi Multimedia (WMM) is the Wi - Fi Alliance specification which is a subset of IEEE 802.11 e . Certified APs must be enabled for EDCA and TXOP . All other enhancements of 802.11 e are optional . </P> <P> The HCF (hybrid coordination function) controlled channel access (HCCA) works a lot like PCF . However, in contrast to PCF, in which the interval between two beacon frames is divided into two periods of CFP and CP, the HCCA allows for CFPs being initiated at almost anytime during a CP . This kind of CFP is called a Controlled Access Phase (CAP) in 802.11 e . A CAP is initiated by the AP whenever it wants to send a frame to a station or receive a frame from a station in a contention - free manner . In fact, the CFP is a CAP too . During a CAP, the Hybrid Coordinator (HC)--which is also the AP--controls the access to the medium . During the CP, all stations function in EDCA . The other difference with the PCF is that Traffic Class (TC) and Traffic Streams (TS) are defined . This means that the HC is not limited to per - station queuing and can provide a kind of per - session service . Also, the HC can coordinate these streams or sessions in any fashion it chooses (not just round - robin). Moreover, the stations give info about the lengths of their queues for each Traffic Class (TC). The HC can use this info to give priority to one station over another, or better adjust its scheduling mechanism . Another difference is that stations are given a TXOP: they may send multiple packets in a row, for a given time period selected by the HC . During the CFP, the HC allows stations to send data by sending CF - Poll frames . </P> <P> HCCA is generally considered the most advanced (and complex) coordination function . With the HCCA, QoS can be configured with great precision . QoS - enabled stations have the ability to request specific transmission parameters (data rate, jitter, etc .) which should allow advanced applications like VoIP and video streaming to work more effectively on a Wi - Fi network . </P>

Which two are considered as characteristics of the original 802.11 protocol