[Mobile Internet Technology] Wireless LAN

 Wireless LAN

Notes from RWTH Aachen University course 
“Mobile Internet Technology” Summer semester 2020
professor: Drik Thißen

Wireless LAN

IEEE standard

$$802.11.a$$

• 802 standards family (defines service and interface)
• 11 sub-standard (MAC and PHY layer)
• a study group (special focus)

Wireless LAN

• Design goal:
  • Global, seamless (無縫的) operation
  • No special permissions or license
  • low power for battery use
  • Robust transmission technology
  • Simplified spontaneous cooperation at meetings
  • easy to use
  • protection of investment in wired networks
  • security, privacy
  • safety (low radiation)
  • Transparency concerning applications and higher layer protocols

Structure of WLAN

1. Infrastructure network

• Access point (AP): are attached to an existing network
• Each AP manages all communication in its reception range
• Control functionality
• medium access
• mobility management
• authentication

• Station (STA)
• Basic Service Set (BSS)
• Portal
• Distributed system (DS)
• Extended Service Set (ESS) = BSSs with different APs

2. Ad-hoc network

• If no AP is available, station (STA) can build up an own LAN.
• higher complexity
• Station (STA)
• Independent Basic Service Set (IBSS)
• No delegated stations for forwarding data, routing

3. Mesh network

• AP wirelessly connected
  • lower cost
  • simple installation
  • resilient
• Mesh Point (MP) establish AP’s links
• Mesh AP MP+AP
• Mesh Portal MP+portal

802.11 Protocols

• Applications should not be aware of the existence of the wireless network (except lower capacity & higher delays)
• MAC layer
  • Access mechanism
  • fragmentation
  • encryption
  • authentication
  • MAC management:
    • synchronization
    • roaming between APs
    • power management
• PHY layer
  • channel selection
  • modulation
  • error coding

802.11 Physical layer

• DSSS (Directed Sequence Spread Spectrum)
  • Chipping sequence (called Barker-code)
• FHSS (Frequency Hopping Spread Spectrum)
• Infrared

802.11b

• higher speed PHY layer extension in ISM band
• DSSS

Dynamic rate shifting

• Complementary Code Keying (CCK)

  • Barker code: one code
  • CCK: several codes to encode information
    • reduces the robustness of transmission
  • 8-bit CCK
    • chipping sequence length $8\rightarrow 4^8$ (64K) resulting states
    • select 64 (for 11 Mbit/s) resp. 4 (for 5.5 Mbit/s) of the states which have as good cross correction characteristics as possible (i.e. as different as possible)
    • More options to encode data

• Higher data rate $\Rightarrow$ disturbance
  $\therefore$ smaller range (distance) for higher transmission rates

  CCK (802.11b): higher data rate, less distance
  Barker code (802.11): less data rate, more distance

• Problem: close APs using the same frequency band cause interference

  • $\Rightarrow$ Divide the given frequency band into channels
  • Channels overlap

• Solution: Spectrum mask limits the interference

  • Bounds for transmission power
  • Useful for distant competing stations

802.11a

• Extension for higher-frequency ISM band
• PHY layer: OFDM + up to 64-QAM
• Channels overlap
• OFDM:
  • 64 subcarriers
    • 6 + 6 guard space
    • 48 data subchannels
    • 4 used as pilots
      • give phase reference
      • allow for estimating distortion

802.11g

• DSSS/CCK $\Rightarrow$ OFDM

802.11n (faster)

• Decrease guard space between OFDM signals $\Rightarrow$ enlarge bandwidth (52 data subchannels)
• increase FEC coding rate
• MIMO (multiple input multiple output)
  • 4 antannas to send and receive data
  • only used in necessary $\Rightarrow$ a lot of power is consumed
  • (optional) antanna diversity $\Rightarrow$ improve signal strength
• (optional) increase bandwidth
• (optional) Greenfield mode: skip support for 802.11a/b/g

802.11ax (even faster)

• 8 MIMO
• Multi-user MIMO (MU-MIMO)
• 256-QAM

OFDMA: OFDM with APs assigns subbands to different stations

CSMA/CA

• Collision Avoidance
• To mitigate effects of hidden stations
• Asynchronous data service (standard)
  • Best effort transmission of data
  • support of broadcast
  • $\rightarrow$ Random access with distributed control
• Time-bound service (optional)
  • Implementation of some degree of QoS
  • Only for infrastructure networks
  • $\rightarrow$ Polling controlled by Access Point

802.11 MAC layer-Access strategies

• DFWMAC-DCF CSMA/CA (standard)
  • DFW: Distributed Foundation Wireless
  • DCF: Distributed Coordination Function
  • ACKs for acknowledging correct receipt
• DFWMAC-DCF with RTS/CTS (optional)
  • Avoidance of hidden stations
  • MACA variant (medium access with collision avoidance)
• DFMAC-PCF
  • PCF: Point Coordination Function
  • Collision-free
  • Centralized polling strategy

---

• Prioritized time-controlled medium access
• SIFS (Short inter frame spacing)
  • highest priority
  • ACK, CTS
• PIFS (PCF IFS)
  • for time-bounded service using PCF
• DIFS (DCF IFS)
• lowest priority
• for asynchronous data service

DFWMAC-DCF CSMA/CA method

1. sender: carrier sense (CS)
2. If the medium is idle for DIFS, the station may send (MA)
   If the medium is occupied, when it become idle, the station waits for DIFS and the randomly choose a backoff time (CA)
   • If the medium is occupied during backoff time:
     • timer stops
     • In the next try, the the remaining time is used
3. the station continues to listen to the medium (DIFS)

• Unicast transmission: receipt is acknowledged since collisions possibly are not detected by the transmitter

1. DIFS
2. Sender: Data
3. SIFS
4. Receiver: ACK
   • If no ACK: collision might happen
   • wait DIFS
5. (other stations) DIFS
6. (other stations) contention

DFWMAC-DCF RTS/CTS

• Avoid hidden stations

1. DIFS
2. RTS
3. SIFS
   (other stations) NAV (RTS)
4. CTS
   RxBusy: receiver busy
5. SIFS
   (other stations) NAV (CTS)
6. Data
7. SIFS
8. ACK
   NAK: negative acknowledgement
9. (other stations) DIFS
10. (other stations) contention

collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages

• Fragmenting data can decrease the damage caused by transmission errors.
• $$\text{data = frag}_1+\text{frag}_2+...$$

6. $\text{frag}_1$
7. SIFS
8. $\text{ACK}_1$
9. SIFS
10. $\text{frag}_2$
11. $...$

• Special mechanism: adapt size of the fragments to current error rate of the medium

DFWMAC-PCF

• AP cycling queries all stations (polling)
• super-frames = contention-free period + contention period
• contention-free period:
1. coordinator asks all $D_x$ (round-robin)
2. If so, station answers with $U_x$
   (If not, no anser)
3. $Cf_{end}$ means contention-free period ends and contention period starts
   If contention-free period ends earlier $\Rightarrow$ contention period longer

---

• Standard CSMA/CA must be implemented

• RTS/CTS is disabled by default

  • A frame size threshold is defined
  • $>$ threshold: RTS/CTS
  • $<$ threshold: CSMA/CA
  • the only way to avoid collisions

• PCF is usually not implemented

  • not good
  • not possible in ad-hoc

  $\Rightarrow$ $802.11e\rightarrow 802.11n$

802.11e Quality of Service (QoS)

• PCF variant should allow for some QoS

  • allow for delay of information
  • guarantees data rate

• But

  • pulling mode can have different lengths
  • data rate depends on channel conditions

  $\Rightarrow$ No timing guarantees

• Solution: 802.11e

• 802.11e adds on 802.11a/b/g/n/ac (not stand-alone)

• Definition of

Extended Distributed Channel Access (EDCA)

• Refinement of DIFS $\Rightarrow$ AIFS (Arbitrary IFS)
• lower priority $\rightarrow$ more waiting time
  higher priority $\rightarrow$ less waiting time

• 8 clases
  TC (traffic classes) 7 (highest priority) = DIFS
• Each station handle up to 8 queues
• Each class given different
  • backoff
  • AIFS
  • BC (backoff counter)
• virtual collision handler as central transmit queue
• Each class is assigned a TXOP (transmission opportunities) = maximum sending duration
• after getting medium access, for time of TXOP several frames can be sent

Hybrid Coordination function Controlled Channel Access (HCCA)

• As PCF
• Contention-free period
  • stations can place reservations for the polling phase
  • AP polls stations by granting a TXOP oriented at reservation wishes and current traffic load
• Contention period
  • EDCF

Direct Link Protocol (DLP)

• increase data rate by allowing two devices to communicate directly

802.11n

• mainly PHY layer
• RIFS (reduced IFS)
  • shorten waiting time between sending several frames
  • Use frame aggregation:
    • assemble several frames
    • remove redundant header information
  • changes in 802.11ac
    • larger aggregated frame
    • dynamic bandwidth adaption

802.11ax

• OFDMA
  • AP assigns certain bandwidths to certain stations
• AP synchronizes stations and coordinates power control

802.11 Frame

Physical Layer Convergence Protocol (PLCP)

• Allows to use different PHY layers in parallel
• Frames
  • Types:
    • data
    • administrative
    • control
  • Header purpose: signaling
    • medium allocation
    • addressing
    • detecting duplicated frames
    • communicate encoding of data
• Frame Format
  • Frame control
    • protocol
    • types
    • fragmenting
    • encryption information
    • meaning of addresses
  • Duration ID
    • with RTS/CTS, setting the NAV
  • Sequence control
    • recognition of duplicated frames
  • CRC
    • checksum for detecting erros
  • Addresses
    • final/source destination
    • BSS identifier
    • intermediate sender/receiver address
• special frames
  • ACK / CTS

    frame control	duration	receiver address	CRC

  • RTS

    frame control	duration	receiver address	transmitter address	CRC

  • FHSS

    Synchronization	SFD	PLW	PSF	HEC	Payload

    • SFD: start frame delimiter
    • PLW: PLCP-PDU length word
    • PSF: PLCP Signaling Field
    • HEC: Header Error Check
  • DSSS

    Synchronization	SFD	signal	service	length	HEC	Payload

• longer frame format: synchronization bit pattern (128 bits)
  short frame format: inverted bit pattern (56 bits)

802.11 Management

• MAC management
  • synchronization
  • power management
  • association/ re-association
    • scanning
    • roaming
  • security
    • authentication
    • encryption

Beacon

• Synchronization using beacon
  • timestamp
  • administrative information
    • power saving
    • roaming
  1. infrastructure networks
     • AP sends beacon
     • different time between beacons
       $\because$ medium can be occupied
     • 
       
       
       
       
  2. Ad-hoc networks
     • All stations send beacon
     • in fixed time intervals
     • random backoff
     • 
       
       
       
       
       

Power management

• Ideal: switch off is not needed
  1. Time synchronization function
  2. frequent activation of all stations
  3. Transmissions for sleeping stations are buffered
  4. wake up $\Rightarrow$ receive buffered frames

1. Infrastructure networks
   • AP can store all pending frames
   • With each beacon frame, a Traffic Indication Map (TIM) is sent along which indicates, for which stations frames are buffured.
   • Delivery TIM (DTIM): list for broadcast receivers
   • 
     
     
     
2. Ad-hoc networks
   • No AP
   • all stations have to temporarily buffer frames
   • Ad-hoc Traffic Indication Map (ATIM)
   • 
     
     
     

Roaming

1. Scanning
   • listen all channels
   • send probes and wait for response
2. Reassociation request / response
3. AP accepts reassociation request
   • announce new station to the distribution system
   • update its database
   • the old AP is informed by distribution system

• When is roaming possible?
  • All APs have the same SSID
  • Provide a single DHCP server within the distribution system
  • Provide a mobility database within the distribution system
  • (optional) 802.11f Inter-Access Point Protocol (IAPP)
    • direct communication between APs
  • central authentication system within distribution system
    • WISPr (Wireless Internet Service Provider roaming)
      • draft protocol

Security

1. MAC filter
   Registration of allowed MAC address
   $\Rightarrow$ but MAC address can be faked

2. Security by obscurity
   hidding SSID
   $\Rightarrow$ listening into the regular traffic to capture join requests

3. Cryptography

   • WEP (Wireless Equivalent Privacy)
   • WPA/WPA2 (Wi-Fi Protected Access)
   • Authentication with AP or server

   $\Rightarrow$ but only possible if authentication data is presented in a WLAN

4. VPN

   • protection with IPSec