Elements of a Wireless Network

• O modo intra-estrutura (ou modo infra-estrutura, mais corretamente em português) é um modo de operação das redes Wi-Fi (IEEE 802.11) em que a comunicação entre os dispositivos passa por um ponto de acesso (AP – Access Point).

Wireless Hosts:

• may be stationary (non-mobile) or mobile
• wireless does not always mean mobility

Base Station

• Typically connected to wired network
• Relay - responsible for sending packets between wired network and wireless host(s) in its “area” (cell towers, 802.11 access points)
• Access Point (AP) - a specific device type in Wi-Fi networks that performs the relay function.
• Base Station (in Wi-Fi) usually refers to an Access Point (AP):
• The central device that STA devices connect to.
• Think of it as "the wireless hub or relay".

STA (Station):

• Any device with a wireless interface that connects to a Wi-Fi network.
• Can be a client device like:
  • Laptop
  • Smartphone
  • Tablet
  • IoT device
• Think of it as "the user device".

Wireless Link:

• typically used to connect mobile(s) to base station, also used as backbone link
• multiple access protocol coordinates link access
• various transmission rates and distances, frequency bands

Infrastructure mode (BSS)

• base station connects mobiles into wired network
• handoff: mobile changes base station providing connection into wired network

Ad hoc mode (IBSS)

• No base stations
• Nodes can only transmit to other nodes within link coverage
• Nodes organize themselves into a network: route among themselves

Wireless link characteristics

Fading (Attenuation): -> Wireless radio signal attenuates (loses power) as it propagates (free space “path loss”)

Multipath: -> radio signal reflects off objects ground, built environment, arriving at destination at slightly different times -> Coherence time:
- amount of time bit is present in channel to be received - influences maximum possible transmission rate, since coherence times cannot overlap - inversely proportional to frequency and receiver velocity

Noise: -> interference from other sources -> larger SNR(signal-to-noise-radio) – easier to extract signal from noise -> SNR might change with mobility

Hidden Terminal Problem (Nó Escondido)

• The coverage area of A does not detect C, and the coverage area of C does not detect A
• If A sends data anywhere C does not detect that data being sent
• Both A and C detect B, because is in their coverage area
• So B can transmit to both, but A and B can not transmit to each other

Problem: A and B decide to send a frame to B at the same time

• A collision occurs at B, because A and C are unaware of each other (and are unaware of this collision because is wireless)
• A anc C are hidden in respect to each other

Solution: -> RTS and CTS

Exposed Terminal Problem (Nó Exposto)

• B and C are aware of each other
• A and C are in the coverage area of B
• B and D are in the coverage area of C

Problem: B sends to A and C is aware of this communication so it does not send anything because is preventing collisions. Though C could send to D.

Solution: RTS and CTS

802.11 Wireless LAN

Architeture

• wireless host communicates with base station denominated access point (AP)

Basic Service Set/cell (BSS): -> wireless hosts -> access point (AP): base station -> ad hoc mode: hosts only

Extended Service Set: -> include one or more BSS

Channels

• spectrum divided in two at diffent frequencies
• Admin of the AP decides frequency
• Interference may occur for neighborhoods with same frequency

Association

• Arriving host: must associate with an AP
• scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address
• selects AP to associate with
• then may perform authentication before association
• then typically run DHCP to get IP address in AP’s subnet

Avoiding Collisions

• idea: sender “reserves” channel use for data frames using small reservation packets ->sender first transmits small request-to-send (RTS) packet to BS using CSMA
  • RTSs may still collide with each other (but they’re short) -> BS broadcasts clear-to-send CTS in response to RTS -> CTS heard by all nodes
  • sender transmits data frame
  • other stations defer transmissions

NOTE: ->ACK - pacote TCP que confirma a recepção de uma mensagem ou de um conjunto de pacotes -> CTS (Clear to Send) - é um quadro de controlo utilizado no protocolo MAC (Media Access Control) para garantir a transmissão de dados de forma ordenada, evitando colisões e interrupções.

Frame Addressing:

• duration - Reserved transmission time (RTS/CTS)
• address 1 - MAC of AP that receives the frame
• address 2 - MAC of AP transmitting the frame
• address 3 - MAC of Router which AP is attached
• address 4 - Used only in ad hoc mode
• seq control - Frame sequence # (for reliable data transfer)

Frame Types

Management Frames: -> Perform supervisory functions (joining, leaving wireless networks; moving AP to AP)

Control Frames: -> Perform control operations in conjunction with data frames -> Allow to deliver data reliably from STA to STA.

Data Frames: -> Send data from STA to STA.

Mobility Within Same Subnet

• H1 remains in same IP subnet: IP address can remain same
• Switch: as it is self-learn, will see frame from H1 and “remember” which switch port can be used to reach H1

Advanced Capibilities

Mobility in cellular data networks:

• Direct routing:
  → Refers to when the communicating node (e.g., a mobile device) sends data directly to the destination without passing through a fixed intermediate point. → although it reduces some hops, can be more complex and less scalable because the end nodes need to maintain dynamic and updated information about the location of mobile devices. → can suffer from overhead in maintaining dynamic routes, especially in large and mobile networks

• Indirect routing:
  → Data passes through an intermediate node, such as a Home Agent in Mobile IP, which acts as a fixed point in the network and forwards packets to the mobile device -> (using fixed nodes like Home Agents) helps manage mobility and location of devices, making routing easier even when devices change networks

• SSID(Service Set Identifier) - Name of wifi netwrok
• Um SSID identifica um Extended Service Set (ESS), que pode ser composto por um ou mais pontos de acesso (APs) que juntos formam a rede Wi-Fi.