# 3: 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 interferene
- $\Rightarrow$ 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 varient should allow for some QoS
- allow for delay of information
- guarentees data rate

- But
- pulling mode can have different **lengths**
- data rate depends on **channel conditions**

$\Rightarrow$ No timing guarentees

- **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)
- *virtural 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
- Provida a mobility database within the distribution system
- (optional) **802.11f Inter-Access Point Protocol (IAPP)**
- direct communication between APs
- cnetral **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