WT Week 6

Wireless LAN II

tldr - MAC methods for IEEE 802.11 networks, MAC frame formats & MAC layer operations

MAC Methods for Ethernet Network (Recap)

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

A data link layer protocol defining the access control method for IEEE 802.3 (Ethernet) networks

How it works

1. Before transmission, a station listens
2. During transmission, the station should continue to listen
3. When a collision is detected, each station should stop transmission, send a “jammed” signal and re-transmit after a random interval

Problem with CSMA/CD in Wireless Networks

• Cannot be used for WLAN as collisions are hard to detect
• CSMA/CD requires stations to be able to transmit and receive at the same time
• However, in radio systems, the transmitted signal has more power than the received signal
• The signal from a transmitting station will overpower its own ability to receive a transmission
• This means a transmitting station “drowns out” its own ability to detect a collision
• ∴ CSMA/CD cannot be used for IEEE 802.11 networks

MAC Methods for IEEE 802.11 Networks

Distributed Coordination Function (DCF)

a.k.a. CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

How it works

• After a station has successfully transmitted a frame, all stations wait a random time for medium to clear before starting the next transmission
• For IEEE 802.11b, each time slot is 20 microseconds
• So, if a client’s random back-off period is 3 time slots, it has to wait for 60 ms before attempting to transmit
• ∴ reducing chances of a collision occurring
• With radio-based LANs, a station cannot listen for collision while transmitting
• ∴ the receiving station has to send an acknowledgement (ACK) to the sender if it detects no errors in the received frame
• If the sending station does not receive an ACK after a specified period of time, the sending station assumes a collision occured and retransmits the frame

Drawback

• High overheads, as every data required acknowledgement, and lower throughput

Request To Send (RTS) and Clear To Send (CTS)

DCF incorporates the optional use of RTS / CTS protocol to reduce collisions further

How it works

1. Sending client sends RTS frame to AP
2. AP broadcasts RTS frame to receiving client, requesting for a certain amount of time to deliver data to it
   • RTS frame contains source, destination address and duration of transmission
3. Receiving client responds with CTS frame when ready
4. AP sends CTS frame to sending client
5. AP alerts all client that the medium is now reserved and they should suspend transmissions
6. Upon receiving CTS from AP, sending client begins transmission

Point Coordination Function (PCF)

• Optional
• Priority based
• Suitable for time-sensitive data such as audio & video

How it works

• A point coordinator in AP controls the transmission of frames from all clients
• Each client is asked in sequence if it wants to transmit
• If client has nothing to transmit, it returns a null data frame

Drawback

• High overheads due to polling frames

802.11 MAC Frame Formats

IEEE 802.11 MAC frames are not the same as 802.3 Ethernet frames. They have different formats and functions.

Management Frames

Sets up initial communication between client and AP (e.g. association request & response)

Control Frames

Assists in delivering frames that contains data (e.g. RTS, CTS, ACK)

Data Frames

Carries info to be transmitted to client

MAC layer operations

MAC layer has 3 main roles

Discovering the WLAN

• AP must transmit info about itself so clients can connect to it
• Discovery process is done by passive or active scanning

Passive Scanning

• The AP sends a beacon frame to announce its presence at regular interval
• Client listens for beacon frame containing AP’s SSID

Active Scanning

• Client sends out probe request frame and wait for probe response frame from AP(s)
• Probe request frame may contain SSID of desired AP

Joining the WLAN

• Client initiates the authentication and association steps of joining the network via the AP

Authentication

The process in which the AP accepts a client

AP requires client to be authenticated prior to joining the network

Open System Authentication

Basic and default method

1. Client sends authentication request frame to AP
2. AP replies with authentication response frame (contains acceptance or rejection notice)

Shared Key Authentication

AP and client given the same key in advance (Pre-Shared)

1. Client sends an authentication frame to AP
2. AP responds with an authentication frame containing challenge text
3. Client encrypts text with pre-shared key and responds
4. AP verifies the encryption key used

Association

Enables the client access to the WLAN

1. Client begins to negotiate a WLAN connection with AP
2. To join WLAN, client sends association request frame to AP
   • Frame includes client’s own capabilities and supported data rates
3. If AP accepts the request, it will reply with an association response frame
   • Frame includes status code and client ID number (used while client is connected)
4. Client has joined WLAN and can begin communicating

Re-Association (ROAMING)

Allows a device in an area covered by an AP to move to another area covered by another AP to remain connected to the network (seamless roaming)

1. When client determines the link to its current AP is poor, it begins scanning for another AP
2. It sends re-association request frame to new AP
3. If new AP accepts request, it will reply with a re-association response frame
4. New AP also sends disassociation frame to old AP
5. The disassociation frame terminates the old AP’s association with the client

Note: Re-Association only works in ESS wireless networks

Transmitting on a WLAN

CSMA/CA (As mentioned above)

fin