Presentation on theme: "Communication Technology Laboratory Wireless Communication Group IEEE 802.11 - Wireless Local Area Networks."— Presentation transcript:
Communication Technology Laboratory Wireless Communication Group IEEE Wireless Local Area Networks
2 Communication Technology Laboratory Wireless Communication Group Updated Schedule: 8:15-9:009:15-10:0010:15-11:0011:15-12:00 L01: Fundamentals of wireless communications. 1 L02: Introduction – First Exercise Fundamentals of wireless communications. 2 L03: Fundamentals of wireless communications. 3 L04: Presentation of Ex 1/ 1Presentation of Ex 1/2WLAN - 1 L05: optional: wrap up of simulation basics revised solutions of Ex 1/1 and EX 1/2 (rest of WLAN - 1)WLAN - 2 L06: Introduction- Second Exercise Presentation of Ex 1 - Combination stepVehicular Networks L07: UWB 1 L08: UWB 2 L09: Presentation of Ex 2/1Presentation of Ex 2/2WBAN L10: Introduction – Third Exercise Presentation of Ex 2 - Combination stepWPAN L11: RFID 1 L12: Presentation of Ex 3/1Presentation of Ex 3/2RFID 2 L13: Presentation of Ex 3 - Combination stepRFID 3 *
3 Communication Technology Laboratory Wireless Communication Group Wireless Communication Technology according to IEEE Local wireless networks WLAN a b i/e/…/w g WiFi h Personal wireless nw WPAN Bluetooth a/b ZigBee Wireless distribution networks/ Wireless metropolitan area nw WMAN (Broadband Wireless Access) (Mobile Broadband Wireless Access), e (WiMAX mobile) + Mobility WiMAX a/b Wireless Networks n
4 Communication Technology Laboratory Wireless Communication Group Wireless Access Technologies: Wireless Local Area Networks (WLAN) Wireless Networks, Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management
5 Prologue (1)
6 Communication Technology Laboratory Wireless Communication Group Prologue (2) “Faster Wi-Fi Will Grow Rapidly.” [In-Stat, 2011] “The emerging ac standard, which is aimed at gigabit-speed wireless LANs, will be quickly adopted over the next four years…“ “In-Stat estimates that nearly 350 million routers, client devices and attached modems with 11ac will ship annually by 2015 …” “… 1.5 billion products equipped with 11n will be sold that year , more than double the estimated 700 million in 2011.” “800 mln WiFi households globally by 2016” [IT Facts, 2012] “Already used in some 439 mln households worldwide, equivalent to 25% of all households, Wi-Fi home network penetration will expand to 42%, reaching nearly 800 mln by 2016.” “Hotspot Usage to Reach 120 Billion Connects by 2015” [In-Stat, 2012] “Worldwide hotspot venues will increase to over 1 million in 2013.”
7 Communication Technology Laboratory Wireless Communication Group Prologue (3) WLAN: fast growing market Expanding year after year (along with the rapid spread of broadband infrastructure) Hot Spots: WLANs at airports, railway stations, universities, cafes, etc. Already in 2006: “82% of US hotels offer wireless Internet” Wifi chipsets in PCs, notebooks, smartphones, tablets Dominant standards: IEEE g with 54 2,4 GHz (successor of b with11 2,4 GHz, the former dominant standard) The high throughput n MIMO standard WiFi Alliance (1999/ 2000), WiFi Certification Topics of this lecture: Standardization OSI reference model PHY MAC
Introduction to IEEE 802 IEEE : WLAN – standard (’97/ ‘99) Purpose: provide wireless connectivity to automatic machinery, equipment, or stations, which may be portable or hand-held, or which may be mounted on moving vehicles within a local area Differences: wired LAN / WLAN destination location dest. Address channel: wired wireless portable mobile CSMA/CD <> CSMA/CA security issues, power saving... Other IEEE standards: 802.2: Logical Link Control (LLC) 802.3: CSMA / CD, Ethernet 802.4: Token Bus, Token Ring IEEE802.11(3,4,5) specifies MAC and PHY layer Physical Layer (PHY): Layer 1 of OSI basic reference model Medium Access Control (MAC): lower half of layer 2 in OSI reference model (layer 2a) Logical Link Control (LLC): upper half of layer 2 (layer 2b) can provide connection-oriented and connectionless services flow control, sliding window, error checking, confirmation of received data LLC standardized for all 802 MACs 8 Communication Technology Laboratory Wireless Communication Group Wireless Networks,
9 Communication Technology Laboratory Wireless Communication Group IEEE – Seamless Integration CSMA/CDEt hernet Token Bus Token Ring Wireless Networks,
11 Communication Technology Laboratory Wireless Communication Group Quelle: MR Ethernet LLC Network structure (2) Thanks to Maximilian Riegel, Siemens Mobile; some pictures of this lecture are taken from his presentations (formerly at Wireless Networks,
12 Characteristics of wireless LANs Advantages very flexible within the reception area mobile communications Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings) more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... Disadvantages typically very low data rate per user compared to wired networks due to shared medium products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions low coverage range Wireless Networks,
13 Design goals for wireless LANs Low power for battery use No special permissions or licenses needed (ISM band) Robust transmission technology (for the wireless channel) Simplified spontaneous cooperation at meetings (ad hoc) Easy to use for everyone, simple management Security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (e.g. low radiation) Transparency concerning applications and higher layer protocols, but also location awareness if necessary Using existing LAN infrastructure (global, seamless operation) Wireless Networks,
14 Communication Technology Laboratory Wireless Communication Group F: Inter Access Point Protocol Wireless Networks, WLAN standardization DFS: dynamic frequency selection TPC: transmit power control
WLAN IEEE Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management 15 Communication Technology Laboratory Wireless Communication Group Wireless Networks,
16 Communication Technology Laboratory Wireless Communication Group Network architectures Quelle: MR Wireless Networks, Station (STA): Any device that contains an IEEE conformant MAC and PHY interface to the wireless medium (WM). Basic Service Set (BSS): Set of stations controlled by a single coordination function (CF). CF: logical function, determines when a STA operating within a BSS is permitted to transmit and may be able to receive protocol data units (PDUs) via the WM. Station Services (SS): set of services that support transport of MAC service data units (MSDUs) between STAs within a BSS. Independent Basic Service Set (IBSS): BSS that forms a self-contained network, and in which no access to a distribution system (DS) is available (=> Ad Hoc network)
Ad Hoc Mode: IBSS Network composed solely of stations within mutual communication range of each other via the wireless medium (WM); typically created in a spontaneous manner. Principal distinguishing characteristic: limited temporal and spatial extent. 17 Communication Technology Laboratory Wireless Communication Group Quelle: MR Wireless Networks,
18 Communication Technology Laboratory Wireless Communication Group Quelle: MR transport layer network layer LLC (layer 2b) Infrastructure Mode (1) Wireless Networks, Access Point (AP): Any entity that has STA functionality and provides access to the DS via the wireless medium (WM) for associated stations => AP implements both the MAC and the DS MAC protocols.
19 Communication Technology Laboratory Wireless Communication Group Infrastructure Mode (2) Quelle: MR Access Point (AP) A AP B Wireless Networks, STA 1 STA 2 Several connected BSSs may form (together with integrated LANs) an Extended Service Set (ESS) The architectural component used to interconnect BSSs is the distribution system (DS). : Distribution System Services (DSS) are specified (not the DS itself) The medium used by the DS is called Distribution System Medium (DSM), and is not specified. Examples are a Wireless Medium, a cable, or a fibre- optic cable,...
Infrastructure Mode (3) Portal: The logical point at which MAC service data units (MSDUs) from a non-IEEE local area network (LAN) enter the DS of an ESS. 20 Communication Technology Laboratory Wireless Communication Group Wireless Networks,
21 Communication Technology Laboratory Wireless Communication Group Wireless Networks, WLAN IEEE Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management
OSI basic reference model: specifies the layers 1 and 2a Coexistence with other 802 LANs (Bridge on LLC layer) several WLANs Compatibility to other (802) LANs: Mobility of STAs handled in the MAC layer; so, for upper protocol layers, shows no differences to other 802 networks One MAC for all PHYs Enhancements: e, n, … 22 Communication Technology Laboratory Wireless Communication Group Wireless Networks,
IEEE Communication Technology Laboratory Wireless Communication Group Reference model: Protocol Entities MAC Sublayer PLCP Sublayer PMD Sublayer MAC Sublayer Management PHY Layer Manage- ment Station Management LLC (Logical Link Control) DATA Link PHY Wireless Networks,
24 Communication Technology Laboratory Wireless Communication Group Wireless Networks, WLAN IEEE Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management
5 different Physical layer technologies FHSS (Frequency Hopping Spread Spectrum) 2.4 GHz band: 1 and 2 Mbit/s, 2GFSK, 4GFSK frequency hopping: 79 frequencies DSSS (Direct Sequence Spread Spectrum) 2.4 GHz band: 1, 2, 5.5 and 11 Mbit/s DBPSK, DQPSK, 11-chip Barker Sequence, CCK OFDM (Orthogonal Frequency Division Multiplexing) a: 5 GHz band, 6, 9, 12, 18, 24, 36, 48 and 54 Mbit/s BPSK, QPSK, 16-QAM and 64-QAM (each with 2 different coding rates) 52 sub-carriers g: OFDM in 2.4 GHz with 54 Mbit/s Baseband IR 1 and 2 Mbit/s, 4-PPM and 16-PPM MIMO (Multiple Input – Multiple Output) – OFDM n: upto 600 Mbit/s (details see n chapter) 25 Communication Technology Laboratory Wireless Communication Group Quelle: MR
Physical Layer – OFDM 26 Communication Technology Laboratory Wireless Communication Group IEEE a: (in CH) 5.15 – 5.35 GHz (Indoor) 5.47 – GHz (Indoor&Outdoor) g: 2,4 - 2,4835 GHz Wireless Networks, , g and n and g
Physical Layer – a. OFDM 27 Communication Technology Laboratory Wireless Communication Group Transmitter and receiver block diagram for the OFDM PHY
802.11a PHY Data Format 28
Physical Layer – OFDM (802.11a) 29 Subcarrier frequency allocation Bandwidth: 20 MHz FFT block length : 64 Number of used carriers: 52 (including 4 pilots) Guard interval: 0.8 µs Wireless Networks,
34 Communication Technology Laboratory Wireless Communication Group Comparison of mostly used PHYs Data rates a: OFDM, up to 54 Mbit/s b: DSSS, up to 11 Mbit/s g: DSSS (downwardly compatible to b) / OFDM (up to 54 Mbit/s) Channels a: 8 (non-overlapping) 5.15 – 5.35 GHz (20 MHz each) => up to 8 APs in the same area b/ g: only 3 non-overlapping (out of 13) channels (25 MHz each) => only up to 3 APs in the same area
35 Communication Technology Laboratory Wireless Communication Group Channels: b European channel selection—non-overlapping European channel selection—overlapping Wireless Networks,
36 Communication Technology Laboratory Wireless Communication Group Channels: a OFDM PHY frequency channel plan for the United States
37 Communication Technology Laboratory Wireless Communication Group Wireless Networks, WLAN IEEE Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management
38 Communication Technology Laboratory Wireless Communication Group Overview: MAC Sublayer Wireless Networks, Two multiple access schemes -> two Coordination Functions: (1)Distributed Coordination Function (DCF): CSMA / CA (contention based) or optional (2)Point Coordination Function (PCF): Polling (central allocation) Different frame formats Fragmentation / defragmentation Encryption
Multiple access schemes 39 Wireless Networks, Point Coordination Function (optional): Only in Infrastructure Mode (ESS) Only for systems using an AP as central point of BSS AP gives transmit right to the STAs; STAs are polled one after another (Polling) Higher priority than DCF (see Interframe Spacing) Distributed Coordination Function: For IBSS and Infrastructure mode (ESS) Based on Carrier Sense Function in PHY, called Clear Channel Assessment (CCA) CSMA / CA for broadcast frames CSMA / CA + ACK otherwise Optional: (parameterised) RTS / CTS – handshake for Virtual Carrier Sense (protection against „Hidden Nodes“)
40 Communication Technology Laboratory Wireless Communication Group MAC Sublayer: DCF (1) DIFS Contention Window Slot time Defer access Backoff-Window Next Frame Decrement Backoff Timer as long as medium idle SIFS PIFS DIFS Medium busy transmit, if medium is free >= DIFS CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance) : Wireless Networks, Channel access: If WM seems to be free for a time >= DIFS, STA transmits immediately If WM busy, STA waits until the end of the ongoing transmission and starts Backoff Procedure After this the status of the channel is checked again
41 Communication Technology Laboratory Wireless Communication Group MAC Sublayer: DCF (2) Wireless Networks, Backoff Procedure (for a STA willing to transmit): STA sets its Backoff Timer to a random backoff time. In this time STA waits and uses carrier-sense mechanism. Only if the WM seems to be idle, the STA decrements the Backoff Timer. Backoff procedure starts also, if a collision is detected. Backoff procedure reduces the probability of collisions without such a procedure this probability would be high after a successful transmission, because then all the STAs prepared to transmit would start their transmissions at the same time.
42 MAC Sublayer: DCF (3)
43 Communication Technology Laboratory Wireless Communication Group MAC Sublayer: DCF (4) - CSMA / CA + ACK – Protocol Ack Data Next MPDU Src Dest Other Contention Window Defer transmission Backoff Procedure DIFS SIFS DIFS Wireless Networks, The (physical) Carrier-Sense mechanism is provided by the PHY (CCA). After unsuccessful transmissions the max. backoff time increases exponentially up to a limit. In a direct transmission between 2 STAs successful transmissions (CRC correct) are acknowledged immediately (positive acknowledgement) using ACK Frames. If no ACK is received the frame is repeated.
MAC Sublayer: DCF (5) - RTS-CTS Handshake (optional) Control frames are exchanged: “Ready To Send“: RTS STA wants to start transmission “Clear To Send”: CTS receiver is ready for transmission These frames contain a field indicating the length of the upcoming transmission. All STAs receiving at least one of the two control frames are now informed about the length of the upcoming transmission (Virtual Carrier Sense) STAs store this information in their Net Allocation Vector (NAV) Carrier-Sense mechanism: CCA + NAV 44 Communication Technology Laboratory Wireless Communication Group RTS CTS Ack Data NAV Next MPDU Src Dest Other CW Defer transmission Backoff Procedure NAV (RTS) (CTS) DIFS
MAC Sublayer: DCF (6) - Hidden Nodes The „Hidden Nodes“ problem can be eased by the RTS / CTS mechanism. STAs can be configured, to use the RTS / CTS mechanism always, never or from a given threshold upwards (for short frames the overhead may be too high). 45 Communication Technology Laboratory Wireless Communication Group AP STA CTS Range RTS Range STA AP RTS CTS Data Ack STAs can’t hear each other but the AP. STA
Exposed Nodes STA S1/S2 does not generate interference at STA R2/R1 S1 and S2 on the other hand are in communication range of each other They are „exposed nodes“ Thus S1 and S2 could transmit simultaneously CSMA/CA prevents this, as e.g. S2 senses the channel busy if S1 transmits RTS/CTS can help here: if e.g. S2 detects the RTS message of S1 but does not receive the CTS answer from R1 it can conclude, that it is an exposed node and transmit concurrently with S1 46
47 Communication Technology Laboratory Wireless Communication Group MAC Sublayer: Fragmentation Partitioning MSDUs (MAC Service Data Units) or MMPDUs (MAC Management Protocol Data Units) into smaller MAC level frames Purpose: increase reliability, by increasing the probability of successful transmission Fragmentation: MSDU or MMPDU are sent as independent transmissions, each of which is separately acknowledged (not for Broadcast frames) Backoff procedure and retransmission if no ACK received Information about duration of transm. included in fragments and in ACKs => NAV is set Defragmentation at receiver Can be combined with RTS - CTS Src Dest SIFS RTS NAV (RTS) NAV (CTS) Other PIFS DIFS Backoff NAV (Fragment 0) NAV (ACK 0) SIFS CTS ACK 0 ACK 1 Fragment 0Fragment 1
48 Communication Technology Laboratory Wireless Communication Group MAC Sublayer: PCF (1) CFP CP CFP repetition interval Variable length PCF waits when medium is busy: delay CF burst (PCF) WM busy PCF DCF Async traffic waits "Reset NAV" NAV CFP repetition interval AP of a BSS can become Point Controller (PC) -> Polling Master PCF gains control of the WM by using Beacon Management Frames to set the NAV in STAs Contention Period (CP): DCF has control in this period (CSMA/CA: contention based multiple access) Contention Free Period (CFP): PCF has control (Polling: central allocation, no contention) CP and CFP alternate under PC control Length and repetition interval of the CFPs are controlled by the PC
MAC Sublayer: PCF (2) STA immediately responses to a CF-Poll_Frame Responses have variable length “Reset NAV“ by last frame of AP No RTS / CTS under PCF (Polling) PCF better suited for time critical services 49 Communication Technology Laboratory Wireless Communication Group Wireless Networks, However, legacy PCF was rarely (or even never) used; => But, it is the basis for a CF in e
MAC Sublayer: Interframe Spacing SIFS (Short Interframe Space): time between ACK frames, CTS frames, fragmented data frames and PCF polls. E.g.: Frequency Hopping PHY - 28ms (802.11a: 16µs) PIFS (PCF Interframe Space): PCF has higher priority than DCF => PIFS < DIFS; PIFS = SIFS + Slot Time E.g.: Frequency Hopping PHY - 78ms (802.11a: 25µs) DIFS (DCF Interframe Space): = SIFS + 2 Slot Time E.g.: Frequency Hopping PHY - 128ms (802.11a: 34µs) EIFS (Extended Interframe Space): used if in previous transmission an error occurred IFS independent of the STA bit rate 50 Communication Technology Laboratory Wireless Communication Group
51 Communication Technology Laboratory Wireless Communication Group Wireless Networks, WLAN IEEE Structure: 1.Introduction 2.Network architecture 3.Reference model 4.Physical layer 5.MAC sublayer 6.MAC sublayer management
Overview: MAC Sublayer Management Synchronization needed for Detection of WLANs Staying in a WLAN Synchronization functions: TSF (Time Synchronisation Function) Timer, Beacon generation Power management Sleep mode (without missing a message) Power management functions Periodic sleep, frame buffering, Traffic Indication Map Association and Reassociation Connection to a network Roaming Scanning Management Information Base 52 Communication Technology Laboratory Wireless Communication Group Wireless Networks,
MAC Sublayer Management: Synchronisation All STAs in a BSS are synchronized to a common clock important for Hop Timing in FH PHY for Point Coordination Timing for Power Management … Timing Synchronization Function (TSF) keeps timers for all STAs in a BSS synchronized Each STA has a local TSF Timer Beacons contain timestamps Beacons contain also further management information (e.g. for power management, roaming) The timestamps calibrate the local clocks of the STAs In an IBSS (ad hoc) network all STAs transmit Beacons In an ESS the AP controls the timing Beacons are generated periodically, but they can be delayed due to CSMA/CA 53 Communication Technology Laboratory Wireless Communication Group Time axis Beacon Interval X X X X "Actual time" in Beacon Beacon Medium busy Wireless Networks,
54 Communication Technology Laboratory Wireless Communication Group MAC Sublayer Manag.: Power Management Wireless Networks, Power Save (PS) / Sleep Mode for STAs, to save energy ESS (infrastructure mode): AP buffers frames for STAs in PS mode and transmits them at “known times” STAs that currently have buffered MSDUs within the AP are identified in a Traffic Indication Map (TIM) TIM is included in all Beacons of the AP STAs in PS mode wake up in periodical intervals to receive Beacons (also because of the TSF) => Contention Period, under control of DCF If there is a MSDU for a STA buffered in the AP, the STA transmits PS-Poll to the AP, which shall respond with the corresponding buffered MSDU immediately => CFP, PCF No PS-Poll, STA remains active until MSDU is received or CFP ends Broadcast, multicast frames: also buffered by AP, delivered after Delivery TIM (DTIM) IBSS (ad hoc mode): If destination is in PS mode, source notifies using ATIM (Announcement Traffic Indication Message, or ad hoc TIM)
MAC Sublayer Management: Scanning Scanning necessary for: Finding and joining a new network Setting up an IBSS (ad hoc network) Finding a new AP while performing handover or roaming MAC used by different PHYs, most of them using more than one channel Scanning: Active: STA sends Probe on each channel waits for an answer (Probe Response) Passive: listening for Beacons Beacon as well as Probe Response contain all information needed to join the network Seamless handover not defined in Communication Technology Laboratory Wireless Communication Group Wireless Networks,
56 Communication Technology Laboratory Wireless Communication Group Successful Association or Reassociation MAC Sublayer Management: Association Authentication: “Shared Key” (using WEP/WPA encryption) or “Open System” Goal: provide access control equal to a wired LAN Authentication service: provides a mechanism for one STA to identify another STA Association establishing a logical connection between STA and AP each STA must become associated with an AP before it is allowed to send data through the AP onto the DS connection is necessary for the DS to know where and how to deliver data to the STA Wireless Networks,
57 Communication Technology Laboratory Wireless Communication Group Active Scanning, Association, Handover Quelle: MR Handover: Scanning, (Re) association request Wireless Networks,
58 Communication Technology Laboratory Wireless Communication Group Outline IEEE e –MAC Quality of Service Enhancements IEEE n: –Enhancements for higher throughput e
59 WLAN IEEE e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation e
Introduction e IEEE802.11e: MAC Quality of Service Enhancements nQSTA: HCF not present QSTA: both DCF and HCF are present. PCF is optional in all STAs. Finalized in Nov Defines MAC procedures to support LAN applications with QoS requirements Transport of voice, audio, and Video over WLANs. QoS enhancements available to QoS stations (QSTAs) associated with a QoS access point (QAP) in a QoS basic service set (QBSS).
61 Communication Technology Laboratory Wireless Communication Group Main QoS Problems with legacy TBTT: Target Beacon Transmission Time e Unknown transmission durations of the polled stations No prioritization Unpredictable beacon delays Unpredictable throughput per STA
62 WLAN IEEE e e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation
63 HCF e The HCF uses both methods contention-based: enhanced distributed channel access (EDCA), and contention-free: HCF controlled channel access (HCCA) New feature of e MAC: Transmission Opportunity (TXOP) interval of time when a STA has the right to initiate transmissions defined by a starting time and a maximum duration allocated via contention (EDCA-TXOP) or granted through HCF (polled-TXOP) duration of an EDCA-TXOP is limited by a QBSS-wide TXOP limit distributed in beacon frames duration of a polled TXOP specified by duration field inside the poll frame Although the poll frame is a new frame as part of e, also the legacy stations set their NAVs upon receiving this frame.
64 Traffic Specification (TSPEC) 1/2 TSPEC: Quality of service (QoS) characteristics of a data flow to and from a non- access point (non-AP) QSTA Contains the set of parameters that define the characteristics and QoS expectations of a traffic flow (like data rate, burst size, and service interval). Parameterized QoS: Treatment of MAC protocol data units (MPDUs) depends on the parameters associated with the MPDU Primarily provided through HCCA mechanism Also provided by EDCA when used with a traffic specification (TSPEC) for admission control.
Traffic Specification (TSPEC) 2/2 65 Traffic stream (TS): Set of MAC service data units (MSDUs) to be delivered subject to the QoS parameter values provided to the MAC in a particular TSPEC. TSs are meaningful only to MAC entities of QSTAs. These MAC entities determine the TSPEC applicable for delivery of MSDUs belonging to a particular TS using the TS identifier (TSID)
66 Overview of MAC services (1) By default, asynchronous MSDU transport is performed on a best-effort connectionless basis. However, the QoS facility uses a traffic identifier (TID) to specify differentiated services on a per-MSDU basis. The QoS facility also permits more synchronous behavior to be supported on a connection-oriented basis using TSPECs. No guarantees that the submitted MSDU will be delivered successfully. Asynchronous data service provided by the MAC: Broadcast and multicast transport available.
67 Overview of MAC services (2) QSTAs in a QBSS differentiate their MSDU delivery according to the designated traffic category or traffic stream (TS) of individual MSDUs QSTA: MAC uses a set of rules that tends to cause higher UP (user priority) MSDUs in a BSS to be sent before lower UP MSDUs. MAC sublayer entities determine the UPs for MSDUs based on the TID values provided with MSDUs. a TSPEC has been provided for a TS => MAC attempts to deliver MSDUs belonging to that TS in accordance with the QoS parameter values contained in the TSPEC.
68 WLAN IEEE e e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation
69 Communication Technology Laboratory Wireless Communication Group EDCA (Enhanced Distributed Channel Access) e Up to 8 UPs (or TCs) EDCA transmission attempt e DCF access to the medium depending on Traffic Categories (TCs) 8 different priorities (UP: User Priority) 4 different buffers with different priority of access to the medium: Access Categories (ACs); Average time to wait depends on priority Per-queue channel access: internal collision resolution (EDCAF)
70 Communication Technology Laboratory Wireless Communication Group EDCA(2) UP-to-AC mappings BK: Background BE: Best effort VI: Video VO: Voice For each AC: an enhanced distributed channel access function (EDCAF) contends for TXOPs using a set of EDCA parameters EDCAF: enhanced variant of the DCF
71 Communication Technology Laboratory Wireless Communication Group EDCA (3) e EDCA delivers traffic according to different ACs by varying the following quantities: Amount of time a QSTA senses the channel to be idle before backoff or transmission (AIFS), or length of the contention window to be used for the backoff, or duration a STA may transmit after it acquires the channel (length of TXOP).
72 Communication Technology Laboratory Wireless Communication Group EDCA (4) e with a: Slot: 9 μs SIFS: 6 μs PIFS: 25 μs DIFS: 34 μs AIFS: ≥34 μs EDCA: priority according to AC by varying: AIFS[AC] length of the contention window to be used for the backoff length of TXOP AIFS: Arbitration Inter frame Space - shall be used by QSTAs to transmit: all Data type frames (MPDUs), all Management type frames (MMPDUs), the following control frames: PS-Poll, RTS
73 Communication Technology Laboratory Wireless Communication Group EDCA parameters (lengths of CW, IFS, TXOP) e AIFS number- AIFSN: number of slots after a SIFS duration a QSTA should defer before either invoking a backoff or starting a transmission (minimum value for a QSTA is 2, for a QAP it is 1). BK: Background BE: Best effort VI: Video VO: Voice 0 indicates that a single MSDU or MMPDU AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime a: aCWmin = 15, aCWmax 1023 OFDMDSS
74 Communication Technology Laboratory Wireless Communication Group Wireless Station Backoff AIFS Access Point Backoff AIFS Frame 1 Ack SIFS Frame 2 Ack SIFS Busy Beacon t t t < EDCA TXOP limit TXOP Bursting – Multiple Frame Tx (Reduces Backoff Overhead) TXOP: Transmit Opportunity given to the TC with highest priority of the colliding TCs, all Management type frames (MMPDUs), and the following control frames: PS-Poll, RTS, CTS (when not transmitted as a response to the RTS), BlockAckReq and BlockAck EDCA TXOP e
75 WLAN IEEE e e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation
76 HCCA (1) e The HCCA mechanism uses a QoS-aware centralized coordinator, called a hybrid coordinator (HC). HC: collocated with the QAP higher medium access priority than non-AP STAs allocates TXOPs to itself and other QSTAs in order to provide limited- duration controlled access phase (CAP) for contention-free transfer of QoS data. HCCA TXOP allocation may be scheduled during the CP and CFP.
77 HCCA (2) HCF Controlled Channel Access (HCCA): Like PCF mechanism, HCCA controls channel access through AP-directed polling. AP's Hybrid Controller takes QoS into consideration when scheduling STA transmission times and durations, giving some traffic a bigger share of the channel. STAs using HCCA submit reservation requests AP then assigns transmit opportunities based on 8 possible Traffic Stream Identifiers (TSIDs). -TSIDs are themselves based upon Transmission Specification (TSPEC) parameters like data rate, burst size, and service interval. This parameterized QoS mechanism is arguably more complex than prioritized QoS. -E.g., HCCA requires STAs to know what they'll want to send in advance. -However, in WLANs used primarily for voice or video streams, HCCA with well-tuned QoS parameters can enable more efficient channel utilization by eliminating "wasted" backoff time.
78 Controlled Access Phase (CAP) Generation e (controlled access phase) HC shall sense the WM. If WM is determined idle for one PIFS period, HC transmits first frame of any permitted frame exchange sequence. Duration value is set to cover the CFP or the TXOP. First permitted frame in a CFP after a TBTT is the Beacon frame.
HCCA TXOP 79 Access Point Data + Ack Ack Busy Beacon t t PIFS SIFS Polled TXOP limit Data Downlink TXOP limit Poll + Data SIFS Data SIFS UPLINK TXOPDOWNLINK TXOP Ack SIFS PIFS Ack SIFS Piggybacking (Reduces Poll and Ack Overheads) Wireless Station e
80 WLAN IEEE e e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation
81 Admission Control Important for the provision of guaranteed QoS parameters Goal: Limit amount of traffic admitted into a service class so that QoS of existing flows will not degrade, while the medium resources can be maximally utilized An IEEE network may use admission control to administer policy or regulate the available bandwidth resources. Admission control is also required when a QSTA desires guarantee on the amount of time that it can access the channel. HC administers admission control in the network. Admission control, in general, depends on vendors’ implementation of the scheduler, available channel capacity, link conditions, retransmission limits, and the scheduling requirements of a given stream e
82 Block Acknowledgment The Block Ack mechanism improves channel efficiency by aggregating several acknowledgments into one frame. There are two types of Block Ack mechanisms: immediate and delayed. The Block Ack mechanism is initialized by an exchange of ADDBA (add Block Acknowledgment) Request/Response frames. The number of frames in the block is limited, and the amount of state that is to be kept by the recipient is bounded. Acknowledgments of frames belonging to the same TID (Traffic Identifier assigned by higher layers), but transmitted during multiple TXOPs, may be combined into a single BlockAck frame e
83 No Acknowledgment ACK does not need to be used in a QBSS in case of time-critical services when a retransmission is not reasonable. There is no MAC-level recovery, and the reliability of this traffic is reduced, due to the increased probability of lost frames from interference, collisions, or time-varying channel parameters. A protective mechanism (such as transmitting using HCCA, RTS/CTS, should be used to reduce the probability of other STAs transmitting during the TXOP e
84 DLS (direct-link setup) Operation (1) DCF: In general, STAs are not allowed to transmit frames directly to other STAs in a BSS (exception: IBSS, i.e. Ad-hoc network) They should always rely on the AP for the delivery of the frames However, STAs with QoS facility (i.e., QSTAs) may transmit frames directly to another QSTA by setting up such data transfer using DLS. Need for this protocol: motivated by the fact that the intended recipient may be in PS mode, in which case it can be awakened only by the QAP. Second feature of DLS: exchange of rate set and other information between the sender and the receiver DLS prohibits STAs going into PS mode for the duration of the direct stream as long as there is an active DLS between the two STAs. DLS does not apply in a QIBSS, where frames are always sent directly from one STA to another e
85 DLS (direct-link setup) Operation (2) The handshake involves four steps QSTA-1 intending to exchange frames directly with another non-AP STA (QSTA- 2) sends a DLS Request frame to the QAP (step 1a); request contains the rate set, capabilities of QSTA-1, and the MAC addresses of QSTA-1 and QSTA-2. If QSTA-2 is associated in the BSS, and direct streams are allowed in the policy of the BSS, and QSTA-2 is indeed a QSTA => QAP forwards the DLS Request frame to QSTA-2 (step 1b). If QSTA-2 accepts the direct stream, it sends a DLS Response frame to the QAP (step 2a) containing rate set, capabilities of QSTA-2, and the MAC addresses of QSTA-1 and QSTA-2. QAP forwards the DLS Response frame to QSTA-1 (step 2b) => direct link becomes active and frames can be sent from QSTA-1 to QSTA-2 and from QSTA-2 to QSTA e
86 WLAN IEEE e e Introduction MAC Architecture DCF HCF: EDCA, HCCA Main Differences from legacy IEEE Traffic Classification TXOP Polling during CP Admission Control Block Acknowledgment No Acknowledgment DLS operation
87 Communication Technology Laboratory Wireless Communication Group Outline IEEE e –MAC Quality of Service Enhancements IEEE n: –Enhancements for higher throughput e
88 Communication Technology Laboratory Wireless Communication Group WLAN n: High Throughput WLAN n MIMO- OFDM WLAN standard Goals: Using MIMO PHY for higher data rates higher spectral efficiency higher diversity gains (i.e. increased link reliability) extended communication range Several MAC enhancements (e.g. MSDU aggregation to reduce overhead, RIFS – Reduced IFS, Block ACK, …)
89 Communication Technology Laboratory Wireless Communication Group IEEE WLAN n: MIMO-OFDM Wireless Networks, IEEE n standard specifies MAC and PHY for a high throughput WLAN PHY is based on MIMO OFDM in the 2.4 GHz and 5 GHz band -operating in 20 MHz bandwidth -operation in 40 MHz bandwidth is optional. Mandatory in n for an AP: support of 1 and 2 spatial streams for 20 MHz bandwidth (MIMO) for an n STA: one spatial stream Optional features include transmit beamforming space-time block codes (STBC) or hybrid STBC/ Spatial Multiplexing (SM) support of 3–4 spatial streams in 20 MHz mode and of 1–4 spatial streams in 40 MHz mode is optional.
90 Communication Technology Laboratory Wireless Communication Group WLAN n: PHY n OFDM in 2.4 and 5 GHz (ISM) band Mandatory payload communication capabilities of up to 135 Mbit/s Optional modes capable of supporting data rates up to 540 Mbit/s (600 Mbit/s for reduced OFDM Guard Interval (GI)) 1 TX, 2 TX, 3 TX, and 4 TX (transmit antenna modes) MIMO Spatial Multiplexing (SpaMuX) gain: 1, 2, 3, or 4 (spatial sub- channels) 540 Mbit/s mode: 64-QAM, code rate = 5/6, # spatial sub-channels = 4 (600 Mbit/s: optional 400 ns GI instead of 800ns) NTX-STBC (Space Time Block Coding) Modes: more TX antennas than # of used spatial sub-channels Low Density Parity Check (LDPC) codes: optional error correction codes Extended communication range (Tx beamforming, STBC, LDPC)
Mandatory and Optional PHY Features Eldad Perahia: “IEEE n Development: History, Process, and Technology,” IEEE Communications Magazine, July n Up to 4 spatial streams: 4x4 MIMO (up to factor 4 in data rate) 40 MHz bandwidth (factor 2) a/g encoder rate ¾: for.11n increased to 5/6 (11% increase in data rate) Four extra OFDM subcarriers squeezed into the spectral mask (48 -> 52: 8% increase) Optional OFDM guard interval of 400 ns (11% increase) Mixed format: backward compatible to.11a/g OFDM but additional training fields in preamble for MIMO training (20μs in.11a to 48 μs in.11n with 4 streams) 91
Summary of n MAC Enhancements n Frame aggregation as key method to increase efficiency on MAC Increases the length of the data portion of the frame relative to PHY and MAC overhead Block Ack: -> e Enhanced by reduced interframe spaces (RIFS) Reverse direction protocol: e.g. FTP over TCP, TCP Ack in the same TXOP TxBF: PHY, but MAC for exchange of beamforming weights, CSI, channel sounding, … 92
WLAN n: Code Rates (1) IEEE n, October n 93
WLAN n: Code Rates (2) IEEE n, October n 94
WLAN n: Code Rates (3) IEEE n, October n 95
WLAN n: Code Rates (4) n 96
WLAN n: Transmitter IEEE n, October 2009 CSD: Cyclic Shift Diversity n 97
A Glimpse on ac Expected Throughput: per single link at least 500 Mbit/s, for 2 users simultaneously (i.e. 1 Gbit/s sum throughput) Up to 1.75 Gbit/s expected per single link (as a first step) 5 GHz band mandatory (2.4 GHz band still supported => backward compatible to n) Bandwidth: 80 and GHz MIMO: 8 antennas, multi-user MIMO Symbol alphabet: 256-QAM Beamforming MAC modifications Approval of.11ac standard: expected not before early Communication Technology Laboratory Wireless Communication Group
Multi-User MIMO Uplink: Multiple Access Channel (MIMO-MAC) Downlink: Broadcast Channel (MIMO-BC) 99 Diversity, MIMO AP STA 1 STA 2
MU-MIMO - Downlink MIMO transmit processing at AP (to separate both STAs) No joint decoding at STAs on receive side The Tx MIMO signals for different STAs interfere with each other AP to use Tx signal processing to cancel (or reduce) this interference Rates to the STAs are bounded by the achievable rate region (capacity region) of the MIMO broadcast channel See, for instance: D. Tse, P. Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, [Tse, 2005] 100 LTE Advanced
MU-MIMO - Uplink MIMO receive processing at AP STAs transmit simultaneously to AP Extension to legacy MAC necessary (!) No joint Tx processing at STAs However, each STA may use CSIT for beamforming Rates of STAs are bounded by achievable rate region (capacity region) given by the MIMO multiple access channel See, for instance: D. Tse, P. Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, [Tse, 2005] 101 LTE Advanced
A Glimpse on ad [Wikipedia, ] Wireless Gigabit Alliance (WiGig): Organization promoting the adoption of multi-gigabit speed wireless communications technology operating over the unlicensed 60 GHz frequency band. Creation of WiGig (IEEE ad) was announced on May 7, 2009 The completed version 1.0 WiGig specification was announced in December In May 2010, WiGig announced the publication of its specification, the opening of its Adopter Program, and the liaison agreement with the Wi-Fi Alliance to cooperate on the expansion of Wi-Fi technologies. In June 2011, WiGig announced the release of its certification-ready version 1.1 specification. WiGig specification will allow devices to communicate at multi-gigabit speeds. Enables high performance wireless data, display and audio applications that supplement the capabilities of today’s wireless LAN devices. 102 Communication Technology Laboratory Wireless Communication Group
A Glimpse on ad [Wikipedia, ] WiGig tri-band enabled devices, which operate in the 2.4, 5 and 60 GHz bands, will deliver data transfer rates up to 7 Gbit/s, about as fast as an 8 antenna ac transmission, and nearly 50 times faster than the highest n rate, while maintaining compatibility with existing Wi-Fi devices. However, the promised 7 Gbit/s rate makes use of the 60 GHz band which cannot go through walls; it is a line-of-sight technology. When roaming away from the main room the protocol will switch to make use of the other lower bands at a much lower rate, but which propagate through walls. 103 Communication Technology Laboratory Wireless Communication Group
The Steps to an Amendment of the Standard in the Working Group Discussion of new ideas in the Wireless Next Generation Standing Committee Development of the purpose and scope of the amendment in a Study Group Drafting of the amendment in a Task Group Letter ballot votes in Working Group for iterative improvement of draft Approval of the draft by the Working Group Review by a sponsor ballot pool Approval and ratification by the IEEE Standards Association Board
Overview of Task Groups and Study Groups (as of October 2010) source source