WIFI
IEEE 802 Layers The IEEE 802 amily and its relation to the OSI model
Need For Speed Wireless LAN Applications Streaming Media (HDTV, DVD) VoIP Interactive Gaming Data Transer Reuire Hundreds o Mps
IEEE 802. Family IEEE 802. IEEE 802.a IEEE 802. IEEE 802.g IEEE 802.i IEEE 802.e IEEE 802.n IEEE 802.p IEEE 802.s IEEE 802.ac IEEE 802.ad IEEE 802.a
IEEE 802. Family Speciications
Types o Networks Wired Backone peer-peer connections AP Ad-hoc Inrastructure
Wireless Channels 2.4 GHz
Wireless Channels 5 GHz
IEEE 802. Access Mechanisms
Medium Access Mechanisms The IEEE 802. standard speciies three types o access mechanisms. The Distriuted Coordination Function (DCF) o o Mandatory Inrastructure & Ad-hoc modes. The Point Coordination Function (PCF). o o Optional Inrastructure The Hyrid Coordination Function (HCF). o Inrastructure & Ad-hoc modes using IEEE 802.e (QoS)
DCF Access Mechanism Applies a CSMA/CA protocol. The DCF allows multiple independent stations to interact without central control, and thus may e used in either ad-hoc or inrastructure networks. Beore attempting to transmit, each station checks whether the medium is idle. I the medium is not idle, stations deer to each other and employ an orderly exponential acko algorithm to avoid collisions.
DCF Access Mechanism In reining the 802. MAC rules, there is a asic set o rules that are always used, and additional rules may e applied depending on the circumstances. Two asic rules apply to all transmissions. Three optional rules are additional.
DCF Access Mechanism Basic Rules. I the medium has een idle or longer than a speciied period o time (DIFS), transmission can egin immediately. Carrier sensing is perormed using oth a physical medium-dependent method and the virtual (NAV) I the previous rame was received without errors, the medium must e ree or at least the DIFS. I the previous transmission contained errors, the medium must e ree or another speciied amount o time (EIFS).
DCF Access Mechanism Basic Rules 2. I the medium is usy, the station must wait or the channel to ecome idle. 802. reers to the wait as access deerral. I access is deerred, the station waits or the medium to ecome idle or the DIFS and prepares or the exponential acko procedure.
DCF Access Mechanism Additional rules may apply in certain situations. Many o these rules depend on the particular situation and are speciic to the results o previous transmissions.. Error recovery is the responsiility o the station sending a rame. Senders expect acknowledgments or each transmitted rame and are responsile or retrying the transmission until it is successul. Positive acknowledgments are the only indication o success. Atomic exchanges must complete in their entirety to e successul. I an acknowledgment is expected and does not arrive, the sender considers the transmission lost and must retry.
DCF Access Mechanism All unicast data must e acknowledged. Any ailure increments a retry counter, and the transmission is retried. A ailure can e due to a ailure to gain access to the medium or a lack o an acknowledgment. However, there is a longer congestion window when transmissions are retried
DCF Access Mechanism 2. Multirame seuences may update the NAV with each step in the transmission procedure. When a station receives a medium reservation that is longer than the current NAV, it updates the NAV. Setting the NAV is done on a rame-y-rame asis. 3. The ollowing types o rames can e transmitted ater the SIFS and thus receive maximum priority: acknowledgments, the CTS in an RTS/CTS exchange seuence. Once a station has transmitted the irst rame in a seuence, it has gained control o the channel. Any additional rames and their acknowledgments can e sent using the short interrame space, which locks out any other stations. Additional rames in the seuence update the NAV or the expected additional time the medium will e used.
Short Interrame Space (SIFS) The SIFS is used or the highest-priority transmissions, such as positive acknowledgments. High-priority transmissions can egin once the SIFS has elapsed. Once these high-priority transmissions egin, the medium ecomes usy, so rames transmitted ater the SIFS has elapsed have priority over rames that can e transmitted only ater longer intervals.
DCF Interrame Space (DIFS) The DIFS is the minimum medium idle time or contention-ased services. Stations may have immediate access to the medium i it has een ree or a period longer than the DIFS. Data packets are sent ater DIFS.
Extended Interrame Space (EIFS) It is used only when there is an error in rame transmission. it is not a ixed interval. Vendors usually apply DIFS in their product instead o EIFS.
Error Recovery with the DCF Error detection and correction is up to the station that egins an atomic rame exchange. When an error is detected, the station with data must resend the rame. Errors must e detected y the sending station. In some cases, the sender can iner rame loss y the lack o a positive acknowledgment rom the receiver. Retry counters are incremented when rames are retransmitted.
Backo with the DCF Ater rame transmission has completed and the DIFS has elapsed, stations may attempt to transmit congestion-ased data. A period called the contention window or acko window ollows the DIFS. This window is divided into slots. Slot length is medium-dependent; higher-speed physical layers use shorter slot times. Stations pick a random slot and wait or that slot eore attempting to access the medium. All slots are eually likely selections. When several stations are attempting to transmit, the station that picks the irst slot (the station with the lowest random numer) wins.
Backo with the DCF The acko time is selected rom a larger range each time a transmission ails. Contention window sizes are always less than a power o 2 (e.g., 3, 63, 27, 255). Each time the retry counter increases, the contention window moves to the next greatest power o two. the contention window is usually limited to 023 slots. When the contention window reaches its maximum size, it remains there until it can e reset. Allowing long contention windows when several competing stations are attempting to gain access to the medium keeps the MAC algorithms stale even under maximum load. The contention window is reset to its minimum size when rames are transmitted successully, or the associated retry counter is reached, and the rame is discarded.
Backo with the DCF
Short and Long Retry Limits The acko time is selected rom a larger range each time a transmission ails. The limits deine the numer o MAC retries or dierent types o packets. The short retries are incremented whenever a control rame or a short rame smaller than the RTS threshold) is retransmitted. Long retries are incremented when a long packet rame is retransmitted. I either o these retries reach their respective limits (short or long depending on the size o the data rame), the rame is automatically discarded.
IEEE 802. DCF Access Mechanism Prolems
Wireless Transmission Range Wireless domain is accompanied with many impairments that makes it hard to distinguish the transmission range oundaries, sometimes to the point where each node may not e ale to communicate with every other node in the wireless network.
The Hidden Node (Terminal) Prolem In the igure, node 2 can communicate with oth nodes and 3 (within the transmission ranges o oth). Nodes and 3 cannot communicate directly since the radio waves cannot reach the ull distance rom to 3. From the perspective o node, node 3 is a "hidden" node. Also, rom the perspective o node 3, node is a "hidden" node. I node wants to transmit to node 2, node 3 won t hear the transmission. Thus, i node 3 also decided to transmit to node 2 while an on-going transmission occurs rom node to node 2, a collision occurs at node 2 without the knowledge o either node or 3.
The Hidden Node (Terminal) Solution To prevent collisions, 802. allows stations to use Reuest to Send (RTS) and Clear to Send (CTS) signals to clear out an area.
The Hidden Node (Terminal) Solution
The Hidden Node (Terminal) Solution Node has a rame to send; it initiates the process y sending an RTS rame. The RTS rame serves several purposes: in addition to reserving the radio link or transmission, it silences any stations that hear it. I the target station receives an RTS, it responds with a CTS. Like the RTS rame, the CTS rame silences stations in the immediate vicinity. Once the RTS/CTS exchange is complete, node can transmit its rames without worry o intererence rom any hidden nodes. Hidden nodes eyond the range o the sending station are silenced y the CTS rom the receiver. When the RTS/CTS clearing procedure is used, any rames must e positively acknowledged.
The Hidden Node (Terminal) Solution The multirame RTS/CTS transmission procedure consumes a air amount o capacity, especially ecause o the additional latency incurred eore transmission can commence. Usually DCF access mechanism with CTS/RTS rame exchange is used only in high-capacity environments and environments with signiicant contention on transmission. For lower-capacity environments, it is not necessary.
Network Allocation Vector All stations receiving RTS or CTS will set their NAV or the given duration.
IEEE 802. Perormance Analysis Using Markov Chains
The Station Model
Throughput / Station Model 2 ) ( M M g z W z x L S P W M m m M M 2 2 2 2 4 2 2 4 2 W W M m m m m M M 2 2 2 2 2 2 2 L r ) ( LR S P Throughput ) (
The Channel Model
Background IEEE 802. Perormance Anomaly
IEEE 802. Perormance Anomaly All the IEEE 802. amily support more than one transmission speed. 802. provides Mps transmission with a allack to 5.5, 2 and Mps. Stations lower their speeds in the presence o errors. Why is it important to analyze the network when stations transmit at dierent speeds?
IEEE 802. Perormance Anomaly It was showed in the literature that when one station that sends data at a low rate competes with other stations, the throughput o all stations is signiicantly limited. Fast stations see their throughput decreased roughly to the order o magnitude o slow station s throughput.
IEEE 802. Perormance Anomaly The air access to the channel provided y CSMA/CA makes a slow station captures the channel a longer period o time than stations transmitting data at ull rate. 802. -- A slow station transmitting at M/s capture the channel eleven times longer than stations transmitting at M/s. Severe degradation in the actual received throughput. Almost in every wireless network that has several users, there is at least one user transmit data at a slow rate due to channel ading, intererence or
The Channel Model E s p s p ss p N s N ss N O s O O s2 O 2 N s2 N ss2 N 2 O s Ls O L OK s OK N s L2s+DIFS N ss L2s+DIFS N L2+DIFS OK sx OK x
The Station Model (Fast & Slow)
Throughput / Station Model 2 ) ( M M g z W z x L S P M m m M M W 2 2 2 2 4 2 2 4 2 M m m m m M M W W 2 2 2 2 2 2 2 r L ) ( 2 R L S P Throughput ) (
Quality o Service IEEE 802.e
IEEE 802.e Recently, research eorts over wireless LAN are gradually migrated to provision o Quality o Service (QoS) or realtime multimedia services. The ollowing Five key parameters are used or dierentiation Minimum Contention Window (CWmin ) Maximum Contention Window size (CWmax) Aritration Inter-Frame Space (AIFS) Virtual Collision Handler TXOP
IEEE 802.e / Four Access Categories AC AC2 AC3 AC4 Virtual Collision
IEEE 802.e / AIFS[AC] AIFS[AC3] AIFS[AC2] AIFS[AC] AIFS[AC0] BackO[AC3] BackO[AC2] BackO[AC] BackO[AC0]
IEEE 802.p
BA, i, V i -2, BA, i, V i -2,2 BA, i, V i -2,AIFS- BA, i, V i -, BA, i, V i -,2 BA, i, V i -,AIFS- IEEE 802.e / Queue Model / W / W / W / W / W / W / W - - - - - - / W Y 0 Y Y W - 2 Y W - X 0 Y 0 Y Y W-2 Y W- - --h +h - - - g / V 0 / V 0 / V 0 - - - - IDLE Z / V 0 - - - - 0, 0 0, 0, V 0-2 0, V 0 - D z T 0 / V 0 / V 0 / V 0 / V 0 -, 0, 0, 0,, 0, V 0-2, 0, V 0 - - g ( - z) - - - g ( h ) g ( h ) (- ) (-z ) (-z ) --h +h - - - g(-)(-) S g (-) (-) (-z) C 0 C 0 2 C 0 L2 AT 0 AT 0 2 AT 0 r / V / V / V / V I (- )(-) (-) - - - - - - - C 0 C 0 2 C 0 L2 AT 0 AT 0 2 AT 0 r S 2 - - - - -, 0,, V -2, V - C, - T / V / V / V / V -,, 0,,,, V -2 --h - - +h,, V - - S L - i-, 0 - - C - - AK - - - C i- C i- 2 C i- L2 AT i- AT i- 2 AT i- r S - T i- / V i / V i / V / V i i C i- C i- 2 C i- L2 - - - AT i- AT i- 2 AT i- r AK x - i, 0 - - - - i, i, V i -2 i, V i - C i S 2 - T i / V i / V i / V i / V i - -, i, 0, i,, i, V -2, i, V - - --h +h - - S L - M -, 0 AK - - C i - - - - - C M C M 2 C M L2 AT M- AT M- 2 AT M- r T M- - - - / V M / V M / V M / V M - C M C M 2 C M L2 M- M- 2 M- r AT AT AT / V M / V M / V M / V M - - - - - M, 0 M, M, V M -2 M, V M - AK x+aifs - T M, M, 0, M, -, M, V M -2, M, V M - C M C M 2 C M L2 - - - AT M AT M 2 AT M r --h +h - - - - - C M C M 2 C M L2 - - - AT M AT M 2 - - AT M r Station Model Queue Model
BA, i, V i -2,AIFS- BA, i, V i -,AIFS- The Queue Model / W / W / W - - X 0 / W Y 0 Y Y W-2 Y W- --h +h - - - - - - - / V 0 / V 0 / V 0 / V 0 D z T 0 -, 0, 0, 0,, 0, V 0-2, 0, V 0 - g ( h ) g ( h ) (- ) (-z ) (-z ) --h +h - - - I - - - - (- )(-) (-) - - - C 0 C 0 2 C 0 L2 AT 0 AT 0 2 AT 0 r / V / V / V / V - T -,, 0,,,, V -2,, V - --h - - - +h - - - - C - S T i- C i- C i- 2 C i- L2 - - - AT i- AT i- 2 AT i- r S 2 / V i / V i / V i / V i - T i, i, 0 - -, i,, i, V -2, i, V - --h +h - - BA, i, V i -2, - BA, i, V i -, S L BA, i, V i -2,2 BA, i, V i -,2 - - - - AK C i - T M- C C C M M 2 M L2 - - - AT M- AT M- 2 AT M- r / V M / V M / V M / V M AK x+aifs - T M, M, 0, M, -, M, V M -2, M, V M - --h +h - - - - - - - C M C M 2 C M L2 - AT M - AT M 2 - AT M r
IEEE 802.e / Queue Model
IEEE 802.e / Queue Model ) ( ) ( ) ( ) ( ) ( ) ( M M M M z z z h g z z AIFS x L S P ) 2 )( ) ( ( )) )( ( ( ) )( ) ( ( ) ))( )( ( ( 2 2 h h z z h AIFS z z z W AIFS z M M AIFS M M. ) 2 )( )( ( ) 2 2 2 2 2 ( 2 2 ) ( M M M m M M m m m W W W W W L h h AIFS h h AIFS AIFS AIFS r AIFS ) ( ) ) ( 2 ( ) )( 2 ( ) ( 2 ) )( ( 2... DataRate L P Throughput suc 0 n Throughput Total Throughput
High Throughput (HT) IEEE 802.n
IEEE 802.n IEEE 802.n is an enhanced standard to signiicantly improve the perormance o the conventional 802.. Many techniues at the physical (PHY) and medium access control (MAC) layers were introduced to enhance the achievale throughput. Multiple-Input-Multiple-Output (MIMO) technology is the Booming actor in enhancing throughput o IEEE 802.n.
MIMO Multiple antennas at the receiver and the transmitter. The presence o multiple elements at oth ends o the communication link opens up independent channels (streams) or transmission in the presence o multi-path or rich scattering. h Spatial Diversity, dependent data are sent on the dierent streams. Increase Range Spatial Multiplexing, multiple independent data are sent on the dierent streams. Increase Capacity s S s p h 2 h p h 2p h h p
Physical Layer Enhancements Enhancements were introduced on IEEE 802.n PHY to achieve a 600Mps nominal rate. What are the enhancements on the legacy IEEE 802.g to achieve the high throughput o IEEE 802.n? Let s start with the maximum throughput o 802.g which is 54 Mps, and see what techniues 802.n applies to oost it to 600 Mps in the ollowing slides.
Physical Layer Enhancements More sucarriers: 802.g has 48 OFDM data sucarriers. 802.n increases this numer to 52, therey oosting throughput rom 54Mps to 58.5 Mps. FEC: 802.g has a maximum FEC (Forward Error Correction) coding rate o 3/4. 802.n sueezes some redundancy out o this with a 5/6 coding rate, oosting the link rate rom 58.5 Mps to 65 Mps.
Physical Layer Enhancements Guard Interval: 802.a has Guard Interval etween transmissions o 800ns. 802.n has an option to reduce this to 400ns, which oosts the throughput rom 65 Mps to 72.2 Mps. MIMO: y using spatial multiplexing, the throughput o a system goes up linearly with each extra antenna at oth ends. Two antennas at each end doule the throughput, three antennas at each end triple it, and our uadruple it. The maximum numer o antennas in the receive and transmit arrays speciied y 802.n is our. This allows our simultaneous 72.2 Mps streams, yielding a total throughput o 288.9 Mps.
Physical Layer Enhancements 40 MHz channels: all previous versions o 802. have a channel andwidth o 20MHz. 802.n has an optional mode where the channel andwidth is 40 MHz. While the channel andwidth is douled, the numer o data sucarriers is slightly more than douled, going rom 52 to 08. This yields a total channel throughput o 50 Mps. So again comining our channels with MIMO, The throughput goes up to 600 Mps
MAC Enhancements In IEEE 802. amily, the actual received throughput is much less than the nominal rate o the standard. For example, the maximum achievale throughput o IEEE 802.g is around 27Mps which is ar less than the nominal rate 54Mps. This happens ecause o the ollowing actors: The MAC header. The control packets such as ACK, RTS and CTS. The acko algorithm. Enhancements were introduced on IEEE 902.n MAC to reduce the eect o these actors. The two main enhancements are: Frame Aggregation. Block Acknowledgments.
Frame Aggregation There are two standard mechanisms to perorm rame aggregations at the MAC layer in IEEE 802.n (A-MSDU) Aggregation o MAC Service Data Units (MSDUs) at the top o the MAC. (A-MPDU) Aggregation o MAC Protocol Data Units (MPDUs) at the ottom o the MAC
Frame Aggregation / A-MSDU
Frame Aggregation / A-MPDU
Block Acknowledgments Block ACK is a kind o Automatic Repeat Reuest (ARQ) mechanism with selective reusal unctionality. Block ACK cuts the wait time etween rame transmission and allows just the missing rames or rames received in error to e resent y checking a itmap Block ACK can signiicantly improve protocol eiciency and throughput.
IEEE 802.n
Very High Throughput (VHT) IEEE 802.ac
IEEE 802.ac IEEE 802.n is an enhanced standard to signiicantly improve the date rate o the conventional 802.. It is still under standardization. Latest Drat is 5.0 (0-02-203) Shall operate in the 5GHz reuency and. Very High Data rate (around 6GHz)
IEEE 802.ac Enhancements Channel aggregation Mandatory 80 MHz (Maximum) Optional 60 MHz. More MIMO spatial streams Eight spatial streams (Maximum) 256-QAM at rates 3/4 and 5/6. (optional) Beamorming with standardized sounding and eedack or compatiility issues etween vendors.
IEEE 802.ac Enhancements Multi-user MIMO ((MU-MIMO) Multiple STAs transmit or receive independent data streams simultaneously. Downlink MU-MIMO, one transmitting device (usually the AP), multiple receiving devices. (optional)
IEEE 802.ac Enhancements SU-MIMO vs. MU-MIMO
IEEE 802.ac Scenarios
IEEE 802.ac / MCS
Wireless Access in Vehicular Environment IEEE 802.p
IEEE 802.p IEEE 802.p is a standard to add wireless access in vehicular environment (WAVE). Supports Intelligent Transportation Systems (ITS) applications. Supports V2V, V2I communications. Operates in the 5.9 GHz reuency and.
IEEE 802.p Operates using the EDCA access mechanism introduced in IEEE802.e with some modiications. Supports QoS applications. Operates when dotocbactivated o the IEEE802. standard parameter is set to TRUE. Key eatures, Minimizing initial setup time etween communicating devices. Using smaller channel andwidth.
IEEE 802.p / QoS
IEEE 802.p / Channel Bandwidth The OFDM system also provides a hal-clocked operation using 0 MHz channel spacings. Data communications capailities o 3, 4.5, 6, 9, 2, 8, 24, and 27 M/s. The support o transmitting and receiving at data rates o 3, 6, and 2 M/s is mandatory when using 0 MHz channel spacing.
IEEE 802.p & IEEE 609 The IEEE 609 Family o Standards or Wireless Access in Vehicular Environments (WAVE) deine an architecture and a complementary, standardized set o services and interaces that collectively enale secure vehicle-to-vehicle (V2V) and vehicle-to-inrastructure (V2I) wireless communications. Together these standards are designed to provide the oundation or a road range o applications in the transportation environment, including vehicle saety, automated tolling, enhanced navigation, traic management and many others. IEEE 609 is a higher layer standard ased on the IEEE 802.p
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