The document discusses the components and characteristics of wireless local area networks (WLANs). It describes the basic components of a WLAN including access points, WLAN adapters, and software. It discusses characteristics such as typical ranges of access points, the number of users supported, and how multiple access points can be connected. It also covers topics such as roaming between access points, infrastructure versus ad-hoc network architectures, and standards like IEEE 802.11.

1. Wireless LAN Characteristics IEEE 802.11 PHY MAC Roaming .11a, b, g, h, i … HIPERLAN Bluetooth / IEEE 802.15.x IEEE 802.16/.20/.21/.22 RFID Comparison

2. Wireless LAN Components The WLAN has the following configuration: Access Point : Connects to the wired network from a fixed location via an Ethernet cable Receives, transmits information from mobile devices such as laptops, PDAs etc and the wired infrastructure network. A single access point can function anywhere in the range of 30 metres to several hundred feet. WLAN Adapters: The mobile devices communicate with the operating system via the WLAN Adapters(radio Network Interface Cards NIC), ISA or PCA adapters for desk top computers.

3. Software and HW Access Point HW Access Point

4. SW Access Point - Advantages does not limit the type or number of network interfaces you use. allows considerable flexibility in providing access to different network types, such as different types of Ethernet, Wireless and Token Ring networks.

5. Range of Access Point Typical indoor ranges are 150-300 feet, but can be shorter if the building construction interferes with radio transmissions. Longer ranges are possible, but performance will degrade with distance. Outdoor ranges are quoted up to 1000 feet, but again this depends upon the environment. There are ways to extend the basic operating range of Wireless communications, by using more than a single access point or using a wireless relay /extension point

6. No. of users on an Access Point This depends upon the manufacturer. Some hardware access points have a recommended limit of 10, with other more expensive access points supporting up to 100 wireless connections. Using more computers than recommended will cause performance and reliability to suffer. Software access points may also impose user limitations, but this depends upon the specific software, and the host computer's ability to process the required information.

7. Multiple Access Points multiple access points can be connected to a wired LAN, or sometimes even to a second wireless LAN if the access point supports this. In most cases, separate access points are interconnected via a wired LAN, providing wireless connectivity in specific areas such as offices or classrooms, but connected to a main wired LAN for access to network resources, such as file servers.

8. Extension Point

9. Roaming

10. Roaming A wireless computer can "roam" from one access point to another, with the software and hardware maintaining a steady network connection by monitoring the signal strength from in-range access points and locking on to the one with the best quality. Usually this is completely transparent to the user; they are not aware that a different access point is being used from area to area. Some access point configurations require security authentication when swapping access points, usually in the form of a password dialog box. Access points are required to have overlapping wireless areas to achieve this *** NOT ALL ACCESS POINTS SUPPORT ROAMING

11. LAN to LAN Wireless Communication Each Access Point acts as a Router or Bridge to connect its own LAN to the wireless network

12. Mobile Communication Technology according to IEEE Local wireless networks WLAN 802.11 802.11a 802.11b 802.11i/e/…/w 802.11g WiFi 802.11h 802.15.4 802.15.1 802.15.2 Bluetooth 802.15.4a/b ZigBee 802.15.3 802.20 (Mobile Broadband Wireless Access) + Mobility WiMAX 802.15.3a/b 802.15.5

13. Characteristics of wireless LANs Advantages very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... Disadvantages typically very low bandwidth compared to wired networks (1-10 Mbit/s) due to shared medium many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11) products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000

14. Design goals for wireless LANs global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) transparency concerning applications and higher layer protocols, but also location awareness if necessary

15. Comparison: infrared vs. radio transmission Infrared uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.). Photo diodes act as receivers. Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface available everywhere

16. Paper : An Adhoc Network system based on Infra Red Communication The network should solve the following problems: Route maintenance How to maintain routes between the mobile hosts. Host enumeration How to count up (and identify) the partici- pants(mobile hosts) of the network.

17. Comparison: infrared vs. radio transmission Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices Example Many different products

18. Comparison: infrastructure vs. ad-hoc networks Infrastructure network ad-hoc network AP AP AP wired network AP: Access Point

19. Comparison: infrastructure vs. ad-hoc networks A very good coordination is required between the medium access of wireless nodes and access points. Else, collisions can occur (infrastructure networks) If the access points control the medium access of individual terminals, collisions can be largely minimized.

20. Complexity with Adhoc Networks Each node has to implement : Medium Access Mechanisms to handle hidden and exposed problems Priority Mechanisms Greatest Advantage : Flexibility in installation and configuration

21. 802.11 - Architecture of an infrastructure network Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.x LAN Access Point 802.11 LAN BSS 2 802.11 LAN BSS 1 Access Point STA 1 STA 2 STA 3 ESS Distribution System Portal

22. 802.11 - Architecture of an infrastructure network ESS has its own ESSID ESSID distinguishes different networks If a node wants to participate in the network, it has to know the ESSID for WLAN communication. The access points get connected to the network via a portal. IEEE 802.11f specifies the inter-Access Point communication protocols.

23. 802.11 - Architecture of an ad-hoc network Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency 802.11 LAN IBSS 2 802.11 LAN IBSS 1 STA 1 STA 4 STA 5 STA 2 STA 3

24. IEEE standard 802.11 mobile terminal access point fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC

25. 802.11 - Layers and functions PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions PMD PLCP MAC LLC MAC Management PHY Management MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PHY DLC Station Management MIB : Management Information Base

26. 802.11 – Physical Layer Physical Layer PLCP : Physical Layer Convergence Protocol Provides carrier sense signal Provides a common physical service access point(PSAP) independent of transmission technology PMD (Physical Medium Dependent Sublayer) Modulation Encoding-Decoding

27. 802.11 Management Layer MAC Management Provides: Association and re-associaltion of a station to an access point. Roaming (handover) between access points. Authentication Encryption Synchronization Power Management

28. 802.11 - Physical layer (classical) 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mW (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchronization

29. FHSS PHY packet format synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header 80 16 12 4 16 variable bits Synchronization synch with 010101... pattern SFD (Start Frame Delimiter) 0000110010111101 start pattern PLW (PLCP_PDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) data rates of payload (1 or 2 Mbit/s) 0000 : 1 Mbps. 0010 : 1.5 Mbps (500 kbps granularity) HEC (Header Error Check) CRC with x 16 +x 12 +x 5 +1

30. DSSS PHY packet format Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) 1111001110100000 Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: 802.11 compliant  length of the payload HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x 5 +1 synchronization SFD signal service HEC payload PLCP preamble PLCP header 128 16 8 8 16 variable bits length 16

31. PHY : DSSS Sync SFD signal service length HEC Payload 128 16 8 8 16 16 Variable Uses 11-chip Barker Code : +1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1 Start Frame Delimiter(SFD) : 1111001110100000 Signal : Data Rate of Payload 0x0A : 1Mbps; 0x14 : 2Mbps Service : Future use; 0x00 : indicates IEEE 802.11 compliant Length(16 bits) : Length of payload HEC; Header error check : CRC 16 polynomial for signal, service and length fields Payload

32. 802.11 - MAC layer I – DFWMAC (Distributed Foundation Wireless Medium Access) Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory){Distributed Coordination Function} collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (Point Coordination Function-optional) access point polls terminals according to a list

33. Medium Access Medium Busy Contention next frame Time DIFS Direct Access if “Medium is Free” >= DIFS DIFS PIFS SIFS Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses PCF Inter-frame spacing (PIFS): medium priority, time bounded service DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority

34. Medium Access Priorities defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service t medium busy SIFS PIFS DIFS DIFS next frame contention direct access if medium is free  DIFS

35. Basic DFWMAC-DFC using CSMA/CA Medium Busy Next Frame DIFS Slot time Contention window A mobile device waits for DIFS and if the medium is free after DIFS, it accesses the medium. So, the medium is busy. Once the above device releases the resources in the medium, it waits for DIFS. The contention period starts. A few devices start their random back off timer and the countdown of the timers start. whichever device that completes the timer back off time first will get access to the medium. As soon as the device senses that the medium is busy, it loses the chance for this cycle and has to try after DIFS duration. Now, the backoff time is initialized for the rest of the devices and they start all over again after DIFS. DIFS

36. Basic DFWMAC-DFC using CSMA/CA ISSUE WITH THE ABOVE SCHEME: A node will not have a priority once it has lost the chance. Irrespective of the amount of wait in the last cycle, it has to start all over again.

37. Medium Access Priorities Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses PCF Inter-frame spacing (PIFS): medium priority, time bounded service DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority

38. 802.11 - competing stations - simple version(for broadcast) t busy bo e station 1 station 2 station 3 station 4 station 5 packet arrival at MAC DIFS bo e bo e bo e busy elapsed backoff time bo r residual backoff time busy medium not idle (frame, ack etc.) bo r bo r DIFS bo e bo e bo e bo r DIFS busy busy DIFS bo e busy bo e bo e bo r bo r

39. 802.11 - competing stations - simple version St-3 has the first request and sends the packet. St-3 senses the medium, waits for DIFS and accesses the medium. Stns 1,2 & 5 have to wait for at least DIFS after Stn-3 stops sending the data. All three stations now start off a back off timer and start counting down their back off timers. Back off time = Elapsed back off time + residual back off time. @@ It is to be noted that if the residual time of device-1 is more than that of device-2, it means that device-1 had waited for a lesser time as compared to device-2 and so, device-2 gets a priority to access the medium.

40. 802.11 - competing stations - simple version Stn-2 gets an access since its backoff time is the least. The back off timers for Stns 1 & 5 stop and stns store residual backoff times. Now Stn 4 wants to access the medium. In all, three stans are trying to acess the medium. Since 4 & 5 have the same backoff time, they result in collision. Transmitted Frames are destroyed Stn-1 finally gets access to the medium. Stns 4 & 5 still have to contend in the next cycle.

41. 802.11 - competing stations - simple version Problem with this scheme: If the contention window is small, too many stns will contend and so, collisions will be substantial. If the contention window is larger, there will be noticeable delays.

42. 802.11 - CSMA/CA access method II (for unicast) t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors (No ACK is sent) But sender has to wait for the medium access. No special privileges for retransmitted data. No. of retransmissions are limited.

43. 802.11 – DFWMAC Hidden Terminal Avoidance using RTS & CTS) t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS (RTS specifies receiver’s Id, amount of time needed for transmission of data and also time for ACK ) acknowledgement via CTS after SIFS by receiver (if ready to receive) (all stns receive this) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS. Other stns have to set ‘Net allocation Vector(NAV)’ (contained in CTS) that specifies how long they need to wait before trying again for transmission.

44. Fragmentation If frames of larger sizes are transported, any bit error will corrupt the entire frame and so, frame errors increase. Hence, it is advantageous to consider shorter frame lengths so as to minimize frame errors.

45. Fragmentation t SIFS DIFS data ACK 1 other stations receiver sender frag 1 DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) SIFS ACK 2 frag 2 SIFS

46. Fragmentation While transmitting frag1, one more duration is also transmitted corresponding to the duration of the following fragment and the acknowledgement. Thus the medium is reserved for the following fragment(frag-2) Other nodes which receive this will adjust their NAV If there is no network change (static network), the set of nodes receiving this duration is the same as that indicated in the original RTS control packet. Because of mobility, this is not the case in most situations. The receiver will receive frag1 and send an ACK1 that contains duration of the net fragment transmission. The other set of nodes will adjust their NAV Thus, the current fragments would contain info about the following ones.

47. DFWMAC-PCF I PIFS stations‘ NAV wireless stations point coordinator D 1 U 1 SIFS NAV SIFS D 2 U 2 SIFS SIFS SuperFrame t 0 medium busy t 1

48. DFWMAC-PCF I (Point Coordination Function -Polling) To achieve a time bounded service, the PCF is used on top of the DCF. It requires an access point. The access point splits time into super frame periods. Super Frame : contention-free period + contention period. In the scheme above, contention-free period should ideally start at to. But the medium is busy till t1. Actually, PCF has to wait for PIFS before accessing the medium. As PIFS is less than DIFS, no other stn can send the data earlier than PCF. PCF sends data to stn-1(polling starts) Stn-1 responds after SIFS. After waiting for SIFS, the PCF sends data to Stn-2. Stn-2 answers with U2. Polling continues for D3 but D3 has no data. Finally, after polling all stns, the PCF can send a ‘End Marker (CFend) indicating that the contention period can start all over. Using PCF automatically sets NAV and prevents other stns from sending data.

49. DFWMAC-PCF I (Point Coordination Function -Polling) The Process of polling with PCF is exactly like TDMA where all users get a fair and equal chance to send data. If a node does not send data, it is an overhead.

50. DFWMAC-PCF II t stations‘ NAV wireless stations point coordinator D 3 NAV PIFS D 4 U 4 SIFS SIFS CF end contention period contention free period t 2 t 3 t 4

51. 802.11 - Frame format Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 6 2 4 0-2312 bytes Protocol version Type Subtype To DS More Frag Retry Power Mgmt More Data WEP 2 2 4 1 From DS 1 Order bits 1 1 1 1 1 1 Types control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous sending time, checksum, frame control, data

52. 802.11 - Frame format Frame control : 2 bytes Protocol ver : 2 bits, starts from zero Type : 2 bits (00: Management, 01: control, 10: Data, 11: Reserved) Sub types: Ex., management functions 0000 : Association request 1000 : Beacon 1011 : RTS 1100 : CTS Duration ID : 2 Bytes period for which the medium is occupied(used for setting NAV) Addresses 1 to 4:contain MAC addresses (to address later)

53. 802.11 - Frame format Sequence Control : 2 Bytes Used to filter duplicates Data : 0 to 2312 bytes Checksum(CRC) : 32 bits To protect from frame errors To DS/From DS Shows How MAC frames are being transmitted. - DA Transmitter Address(TA) Receiver Address(RA) 1 1 - DA SA BSSID 0 1 - SA BSSID DA 1 0 - Basic Service Set Id (BSSID) Source Address (SA) Distribution address (DA) 0 0 Address 4 Address 3 Address 2 Address1 From DS To DS

54. 802.11 - Frame format Address 1: Physical receiver of the frame Address 2: Physical transmitter of a frame Addresses 3 & 4 : Logical assignment of frames. More Fragments : Set to 1 if more fragments are to follow. RETRY If the current frame is a duplicate because of retransmissions, this field is set to 1.

55. 802.11 - Frame format Power Management 0 : Stn is in stand-by mode 1 : stn stays active More Data : To indicate to the receiver that more data is to follow. To indicate to the stns that are in sleep mode that more data is to follow. To indicate to an access point that more polling is required. Wired Equivalent Privacy (WEP) : To indicate that standard 802.11 security algorithm is used. Order : If set to 1, frames have to be taken strictly in the same order that they are received.

56. MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

57. Special Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send Frame Control Duration Receiver Address Transmitter Address CRC 2 2 6 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes ACK RTS CTS

58. 802.11 - MAC management Synchronization try to find a LAN, try to stay within a LAN timer etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base All parameters concerning the present state of the wireless stn and access point are stored in MIB. These can be accessed via a protocol line SNMP.

59. Synchronization using a Beacon (infrastructure) beacon interval t medium access point busy B busy busy busy B B B value of the timestamp B beacon frame

60. Synchronization using a Beacon (infrastructure) 802.11 specifies a Timing and Synchronization Function (TSF). It is needed for PCF and for power management. Needed for Synchronization of hopping sequence for all nodes etc. Within a BSS, timing is conveyed by (quasi) periodic transmission of timing frame called “Beacon (contains time stamp and other management functions”. Nodes need to hear the beacons and adjust the timing. But nodes need not adjust to every beacon. If the medium is busy, access point may not be able to send the beacons on certain occasions. Beacon intervals are not shifted if a beacon is missed.

61. Synchronization using a Beacon (ad-hoc) t medium station 1 busy B 1 beacon interval busy busy busy B 1 value of the timestamp B beacon frame station 2 B 2 B 2 random delay

62. Synchronization using a Beacon (ad-hoc) No Access Point Each node maintains a sync timer and starts sending to the rest of the nodes. It uses a standard back-off algorithm and and only one beacon wins. All other stns adjust their internal clock as per the received beacon. They suppress their beacons for this cycle. If there is a collision, the beacon is lost. In this situation, beacon intervals can be slightly shifted and the following cycles synchronizes all stns.

63. Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) For sender, it is not an issue as the transmitter knows when it is ready for sending frames. Transmitter has to buffer the frame to make sure that it will transmit when the receiver is ready to receive. stations wake up at the same time periodically and listen to the transmitter. Waking up at the right time needs the TSF. Along with beacon, a Traffic Indication Map(TIM- containing the list of stns for which buffering has been done in the AP.) is also sent. Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Ad-hoc Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?)

64. Power saving with wake-up patterns (infrastructure) .........Only One Station Shown........... TIM interval t medium access point busy D busy busy busy T T D DTIM interval B B station p d d T TIM D DTIM B broadcast/multicast awake p PS poll d data transmission to/from the station

65. Power saving with wake-up patterns (infrastructure) All stations wake up prior to TIM/DTIM. CASE WITH TIM : Access Point buffers frames when the receiver is in the sleep mode. With every beacon, a Traffic Information Map (TIM-containing the list of stations for which uni-cast buffers are stored in AP) is sent. AP sends a broadcast frame and the receiver stays awake to receive it. Receiver then sleeps and wakes up just before the next TIM. TIM is delayed since the medium is busy. So, the receiver stays awake. AP has nothing to send and so, the receiver goes to sleep. In the next TIM interval, the AP indicates that the stn is the destination for a buffered frame. Stn answers with a PS poll and stays awake to receive data. In the next DTIM interval, the AP has more broadcast data to send. This is deferred since medium is busy.

66. Power saving with wake-up patterns (ad-hoc) awake D transmit data t station 1 B 1 B 1 B beacon frame station 2 B 2 B 2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data A transmit ATIM

67. Power saving with wake-up patterns (ad-hoc) Each participating station has to buffer the data since access points do not exist. In the period that all stations are awake, all the participating station send a list of buffered frames and the stations that are targeted to receive these. These are sent through “Adhoc Traffic Information Map(ATIM)” All stations stay awake during this ATIM period and listen to the ATIM. In the example, ATIM of station-1 contains the address of station-2. Stn-2 acknowledges the ATIM, waits for the data and later acknowledges the data. With more stations wanting to send their frames, collisions can be substantial. Access delay is not easy to predict and so, QoS can’t be guaranteed.