High-Density and De-Densified Smart Campus Communications. Daniel Minoli

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СКАЧАТЬ of bits that described the “color” of the BSS. The color of a BSS corresponds to an identifier (ID) of the BSS that is shorter than the Basic Service Set Identifier (BSSID) defined by 802.11. The BSS color may be contained in the PHY Signal (SIG) field in a PHY header of a PPDU, whereas the BSSID is typically included in a MAC portion of PPDUs. A device (e.g. an AP or client) in a BSS can determine whether a PPDU is from the BSS to which the device belongs (the “same‐BSS”) or some other BSS (e.g. an overlapping BSS [OBSS]), by decoding the SIG field and interpreting BSS color bits included therein [6].

      Machine To Machine (M2M) communication technology has been discussed as a next‐generation communication technology. The technological standard for supporting M2M communication in an IEEE 802.11 WLAN system has been developed as IEEE 802.11ah (other standards or recommendations have been advanced by ETSI). Regarding M2M communication, a scenario of occasionally communicating a small amount of data at low speed in an environment in which numerous devices are present is considered. Communication in a WLAN system is performed in a medium shared by all devices. When the number of devices is increased like M2M communication, there is a need to enhance a channel access mechanism more effectively to reduce unnecessary power consumption and interference [15].

      2.3.2 MAC Layer Operation

      IEEE 802.11 defines a data frame exchange process that enables the stations and APs, to negotiate the timing of the exchange of data between devices over the various shared channels in the 2.4 and 5 GHz bands. In WLAN systems using the IEEE 802.11 standards, frames exchanged between stations (including APs) are classified into management frames, control frames, and data frames. The management frame is a frame used for exchanging management information that is not forwarded to higher layers of a communication protocol stack. The control frame is a frame used for controlling access to the transmission medium. The data frame is a frame used for transmitting data that will be forwarded to higher layers of the communication protocol stack [2]. Each frame's type and subtype are identified using a type field and a subtype field included in a control field of the frame, as described in the applicable standard.

Traditionally, at the frame level, Wi‐Fi WNs use Request to Send/Clear to Send (RTS/CTS) to take advantage of the shared medium. The AP only issues a CTS packet to one WN at a time. When the WN receives the CTS, it sends its entire frame to the AP; the WN then waits for an acknowledgment (ACK) frame from the AP indicating that it received the packet correctly; if the WN does not get the ACK in a specified amount of time, it postulates that the packet in fact collided with some other WN transmission – at that point the WN transitions into a period of binary exponential backoff; it will try to access the medium and retransmit its packet after the backoff time expires.

      Clearly, data are transmitted using MAC framing and channel management mechanisms along with PHY resources. As alluded to earlier, at the MAC layer, the following frames are utilized:

       A data frame is used for the transmission of data forwarded to a higher protocol layer. The WLAN device transmits the data frame after performing backoff if a Distributed Coordination Function (DCF) Inter‐Frame Space (IFS) (known as DIFS) interval has elapsed, during which such DIFS interval, the medium has been idle.

       A management frame is used for exchanging management information that is not forwarded to a higher protocol layer. Subtype frames of the management frame include a beacon frame, an association request/response frame, a probe request/response frame, and an authentication request/response frame.

       A control frame is used for controlling access to the medium. Subtype frames of the control frame include a RTS frame, a CTS frame, and an ACK frame.

      IFSs are waiting periods between transmission of frames operating in the MAC sublayer. These waiting periods are used to prevent collisions as defined in IEEE 802.11‐based WLAN standards; they represent the time period between completion of the transmission of the last frame and starting transmission of the next frame, apart from the variable backoff period. These are techniques used to prevent collisions as defined in IEEE 802.11‐based WLAN standard. Specifically, IFS is the time period between the completion of the transmission of the last frame and the start of transmission of the next frame, apart from the variable backoff period [16]. The list that follows enumerates the different types of IFSs starting from the shortest duration (highest priority) to the longest duration (lowest priority):

       Reduced Inter‐frame Space (RIFS)

       Short Inter‐frame Space (SIFS)

       Point Coordination Function (PCF) Inter‐frame Space (PIFS)

       Distributed Coordination Function (DCF) Inter‐frame Space (DIFS)

       Arbitration Inter‐frame Space (AIFS)

       Extended Inter‐frame Space (EIFS)

The DCF is a required technique utilized to prevent collisions in 802.11‐based WLANs. When using the DCF, a station is required to sense the status of the wireless channel before it can place its request to transmit a frame. DIFS is the time interval that a station must wait before it sends its request frame. SIFS is the time interval required by a WLAN device between receiving a frame and responding to the frame. See Table 2.3.

Figure 2.5 illustrates IFS relationships; the figure illustrates a SIFS, a PIFS, a DIFS, and an AIFS corresponding to an Access Category (AC) “i” (AIFS[i]) [2].

Before making a transmission, a WLAN device assesses the availability of the wireless medium using a CCA procedure. If the medium is occupied, CCA establishes that it is busy, while if the medium is available, CCA determines that it is idle. A WLAN device performs a backoff procedure when the WLAN device that is ready to transfer a frame finds the medium busy. In addition, a WLAN device operating according to the IEEE 802.11n and 802.11ac standards performs the backoff procedure when the WLAN device infers that a transmission of a frame by the WLAN device has failed. The backoff procedure includes determining a random backoff time composed of N backoff slots, each backoff slot having a duration equal to a slot time and N being an integer number greater than or equal to zero. The backoff time may be determined according to a length of a Contention Window (CW). All backoff slots occur following a DIFS or EIFS period during which the medium is determined to be idle for the duration of the period. When the WLAN device detects no medium activity for the duration of a particular backoff slot, the backoff procedure decrements the backoff time by the slot time. When the WLAN determines that the medium is busy during a backoff slot, the backoff procedure is suspended until the medium is again determined to be idle for the duration of a DIFS or EIFS period. The WLAN device may perform transmission or retransmission of the frame when the backoff timer reaches zero. The backoff procedure operates so that when multiple WLAN devices are deferring and execute the backoff procedure, each WLAN device may select a backoff time using a random function, and the WLAN device selecting the shortest backoff time may win the contention, reducing the probability of a collision [2]. When the control frame is not a response frame of another frame, the WLAN device transmits the control frame after performing backoff if a DIFS has elapsed, during which DIFS, the medium has been idle. When the control frame is the response frame of another frame, the WLAN device transmits the control frame after a SIFS has elapsed without performing backoff or checking whether the medium is idle.

TABLE 2.3 Inter‐frame Space Types

Inter‐frame Space Type	Description
Inter‐frame Space (IFS)	The time period between completion of СКАЧАТЬ В начало < 21 22 23 24 25 26 27 28 29 30 > В конец