Friday, November 12, 2010

Understanding IEEE 802.11 MAC Standard

IEEE 802.11 MAC Dictionary

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Undesrstanding Carrier Sense & Back Off:

Consider the following two questions about IEEE 802.11 Standard:
1. Carrier Sense is provided by the physical layer, thus, when MAC layer get a packet to transmit. It will initiate a carrier sense mechanism, and it took >= DIFS time to determine that the channel is free, thus, at least we have a delay as 128 us, right?

2. Assuming 4 nodes A,B,C,D working in  Ad-hoc mode without RTS/CTS . First, A has 10 packets (10 MPDU belongs to 1 MSDU) to transmit to B,  Thus, A access the channel, and get it. Thus, A begin a Packet-SIFS-ACK-SIFS-Packet-SIFS........ sequence. During that,  C got one packet to transmit to D, unfortunately, C sense the channel is busy, thus, C begin a backoff procedure, which he set his CWmin = 7, and Backoff time = Random() * a slottime = 3. a slot means 50 us.
Because, Each MPDU is transmitted with SIFS interval, the Backoff number cannot be decremented unless at least a DIFS is idle and the next slot is also idle. Thus C has no chance to transmit until A finished his own transmission. and After DIFS+ 3 timeslot, C will begin to transmit. Thus, we conclude that, C has no chance to challenge an existing transmission.

Then, what happened if there is a transmission failure during the 10 MPDU, it means a back-off procedure will be initiated after an EIFS, thus, at this time, A and C has the same probability to win the channel as long as the CWmin is the same.

Another interesting thing is that if node A get another MSDU (~ 5 MPDUs) to transmit to C during the period it sends the 10 MSDU. So,
" node A still has advantage to access channel, because A does not generate any back-off procedure if no transmission failure happens, and A will transmit immediately after an DIFS slot when last MSDU is finished. So, it seems that as long as a station get the access, it is always has priority to send than other stations." 

This is Wrong!!!
Citing from 802.11-1999:

"9.2.3.3:
A STA using the DCF shall be allowed to transmit if its
carrier-sense mechanism (see 9.2.1) determines that the medium is
idle at the TxDIFS slot boundary as defined in 9.2.10 after a
correctly received frame, and its backoff time has expired."

The last part (after "and") means: *Whenever* you have to wait DIFS,
you have to backoff, too, thus allowing other stations to content for
the medium. So, everyone has equal probability to access the channel.
The only exception is that channel is free when you access it and after waiting longer than DIFS, it is still free. Then, no backoff, send directly.
To prevent consecutive access as in above example, after a DATA packet transmission, the sending node automatically backoff after DIFS.
More specific in:
"9.2.5.2.
A backoff procedure shall be performed immediately after the end of every transmission with the More Fragments bit set to 0 of an MPDU of type Data, Management, or Control with subtype PS-Poll, even if no additional transmissions are currently queued. In the case of successful acknowledged transmissions, this backoff procedure shall begin at the end of the received ACK frame. In the case of unsuccessful transmissions requiring acknowledgment, this backoff procedure shall begin at the end of the ACK timeout interval. If the transmission is successful, the CW value reverts to aCWmin before the random backoff interval is chosen, and the STA short retry count and/or STA long retry count are updated as described in 9.2.4. This assures that transmitted frames from a STA are always separated by at least one backoff interval."

Fragmentation

As the largest MSDU is 2304 bytes limited, the MAC format for a MPDU has the data field as 0-2312, because 8 bytes used for WEP. Thus, if no fragmentation is used, the MPDU has the same length as MPDU. However, we prefer short packets to transmit to reduce the  packet corruption chance. Thus, Fragmentation is optional.More about MPDU, MSDU & PSDU and Overhaed:
MSUD is the packet from upper layer, MPDU = PSDU including everything in MAC, for example: A Packet would like:

PPDU = PSDU +PLCP header + PLCP preamble.
There are only 3 addresses in the MAC header, but the standard says 4. That's because the address is usually omitted unless a Wireless Distribution System is used and packet is for AP-AP communication.

RSSI



  • RSSI (Receive Signal Strength Indicator) Indicates how well the local station receives the remote unit's transmission in dBm. from the measurement of one packet.




    • NAV (Virtual carrier sense)

    It is interesting that the RTS set its NAV covering  to the end of whole RTS-CTS-DATA-ACK process. How about a RTS has not been ACKed with a proper CTS. Then all the nodes around the sending node will wait for such a long idle period, and just wasting the bandwidth?! The standard provides a solution, that the nodes could reset NAV if it does not hears DATA packet at some time. However, it is still seems not perfect because anyway, the NAV settings broadcasted by RTS has to be re-confirmed by a following DATA frame, the first setting in RTS seems always flexible and non-compulsive. Thus, schemes could just say, we set the NAV first to "2 x aSIFSTime) + (CTS_Time) + (2 x aSlotTime", then announce the final decision in DATA frame. This is very useful, in case that the sending node cannot be sure how long it takes to transmit without a CTS reply in some scheme
    Excerpts from 9.2.5.4
    STAs receiving a valid frame shall update their NAV with the information received in the Duration/ID field,
    but only when the new NAV value is greater than the current NAV value and only when the frame is not
    addressed to the receiving STA. Various additional conditions may set or reset the NAV, as described in
    9.3.2.2. When the NAV is reset, a PHY-CCARESET.request shall be issued.
    Figure 53 indicates the NAV for STAs that may receive the RTS frame, while other STAs may only receive
    the CTS frame, resulting in the lower NAV bar as shown (with the exception of the STA to which the RTS
    was addressed).
    A STA that used information from an RTS frame as the most recent basis to update its NAV setting is permitted
    to reset its NAV if no PHY-RXSTART.indication is detected from the PHY during a period with a duration
    of (2 x aSIFSTime) + (CTS_Time) + (2 x aSlotTime) starting at the PHY-RXEND.indication
    corresponding to the detection of the RTS frame. The ¡°CTS_Time¡± shall be calculated using the length of
    the CTS frame and the data rate at which the RTS frame used for the most recent NAV update was received.
    Figure 53¡ªRTS/CTS/data/ACK and NAV setting

    SIFS & DIFS & Slottime

    In the 1Mbps or 11M DSSS standard (1999), SIFS are 10us, and aSlottime is 20us. So, the basic reason for those values are CCA has to use 15us and a round-trip time is less than 5us, so the maximum round-trip of propagation delay is 5*300/2= 750m. However, some wireless LAN for outdoor claims coverage as several miles. In that case, it has to change this Slottime value or there are only 2 nodes in the network.
    Excerpts from standard:
    aSIFSTime and aSlotTime are fixed per PHY.
    aSIFSTime is: aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime.
    aSlotTime is: aCCATime + aRxTxTurnaroundTime + aAirPropagationTime
    + aMACProcessingDelay.
    The PIFS and DIFS are derived by the following equations, as illustrated in Figure 58.
    PIFS = aSIFSTime + aSlotTime
    DIFS = aSIFSTime + 2xaSlotTime
    The EIFS is derived from the SIFS and the DIFS and the length of time it takes to transmit an ACK Control
    frame at 1 Mbit/s by the following equation:
    EIFS = aSIFSTime + (8xACKSize) + aPreambleLength + aPLCPHeaderLngth+ DIFS

    Beacon:

    It is not strange that in Infrastructure mode, a beacon is necessary to let AP inform the following STAs. But in ad-hoc mode , beacon is also necessary to maintain synchronization and exchange ESSID information between those ad-hoc nodes. In "ad-hoc" nodes are going to form an IBSS ( Independent Basic Service Set). It does not mean those nodes listening a same channel are automatically being in the same ad-hoc network. Actually, nodes need to join a service set identified with SSID ( Service Set IDentifier). Thus, nodes staying in the same channel still cannot communication if they belong to different "SSID". Beacons are very important in 802.11 MAC. Unfortunately, ns-2 MAC implementation of 802.11 ignores the function and overhead of "beacon".

    Broadcast:

    Broadcasting packets is also regarded as an MPDU, so it also has to obey the carrier sense & backoff. Excerpts from the standard:

    In the absence of a PCF, when broadcast or multicast MPDUs are transferred from a STA with the ToDS bit clear, only the basic access procedure shall be used. Regardless of the length of the frame, no RTS/CTS exchange shall be used. In addition, no ACK shall be transmitted by any of the recipients of the frame. Any broadcast or multicast MPDUs transferred from a STA with a ToDS bit set shall, in addition to conforming to the basic access procedure of CSMA/CA, obey the rules for RTS/CTS exchange, because the MPDU is directed to the AP. The broadcast/multicast message shall be distributed into the BSS. The STA originating the message shall receive the message as a broadcast/multicast message. Therefore, all STAs shall filter out broadcast/multicast messages that contain their address as the source address. Broadcast and multicast MSDUs shall be propagated throughout the ESS.
    There is no MAC-level recovery on broadcast or multicast frames, except for those frames sent with the ToDS bit set. As a result, the reliability of this traffic is reduced, relative to the reliability of directed traffic, due to the increased probability of lost frames from interference, collisions, or time-varying channel properties.

    Power & Energy Efficeincy

    802.11 radio consumes as much power when it is idle as when it receives transmissions. Briefly, these reasons have to do with  By contrast, with a TDMA-style MAC, it is possible to put the radio in standby mode during such intervals. That's a big difference regarding energy consumed by the radio during idle intervals.

    Weakness:

    IEEE 802.11 MAC standard is weak in solving following issues: (link layer reliability, QoS support?, power conservation)
    • Hidden Terminal .vs. Expose Terminal . This is a problem basically attributed to the difficulties to distribute "global" information about the network topology and channel state to a "local" terminal. RTS/CTS mechanism partly overcomes this problem.
    • Multi-hop .vs. Clustering. This problem is prevailing when large ad-hoc network  has difficulty  in efficient routing and a long MAC address is also increasing the overhead.
    • Link layer availability and fair utilizing. The major representative of this is the "Head of Line" problem. the first packet of the FIFO queue is re-transmitted endlessly, thereby affecting the performance ("throughout") of the network. As this also occurs in wired network, the adverse wireless environment worsen the problem.

    Further Research Issues about MAC layer of Ad-hoc Network

    1. To revamp the  well-known MACA protocol used by 802.11 from a single-cell MAC in direction that allows  neighboring cells to operate simultaneously whenever possible, thereby increasing the overall system throughput.
    2. The second challenge we discuss is the notion of a  "wireless router" or a forwarding node, whose primary function is to receive  packets from one neighbor and transmit them to a second neighbor using the  same wireless interface. This requires combining channel access functionality with that of next-hop address lookup within the network interface card without host participation.
    3. The third set of challenges arise from the effects of physical/MAC layer characteristics on  network connectivity (i.e. whether two nodes are neighbors depends on the rate used),  transport layer performance (i.e  contention for the physical channel among neighboring hops lead to packets of the same flow contending with each other) and use of  MAC contention mechanisms as a means for supporting transport-layer congestion control.

    Spectrum

    A sample spectrum "image" for IEEE 802.11 wireless LAN
     
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