WLAN Communications Protocols and Routing in Internet (IN5030) - - PowerPoint PPT Presentation

WLAN Communications

Protocols and Routing in Internet (IN5030) Andrew Adrians (27.03.2020)

Wireless Communication

Definition: Transfer of information among

two or more points that are not connected by wires or any kind of electric conductor.

Applications: Broadcast information (e.g.

news, weather reports, road conditions), satellite communications (e.g. GPS), WLAN (PCs, printers, wireless keyboards/mouse/headsets/USB and Bluetooth devices), Mobile phones communication and medical technologies such as MBAN (Mobile Body Area Network) to transfer data such as heartbeat or blood pressure to nursing centers, etc.

Wireless Communication

How does it work?

Information is transmitted via electromagnetic (EM) waves. Some of the examples

• f EM waves are: Light (the visible part of EM), Radio waves, X-rays, etc.

EM waves are modulated to transmit the information. There are two types of modulation techniques: AM or FM AM (Amplitude Modulation):

AM is a type of modulation where the height of the carrier signal changes in accordance with the height of the message signal. In AM, only the amplitude of the carrier wave changes and not its frequency or phase.

FM (Frequency Modulation):

FM is a type of modulation where the information (message signal) is transmitted over a carrier wave by varying its frequency in accordance with the amplitude of the message signal. In FM, the frequency of the carrier signal is varied whereas the amplitude of the carrier signal remains constant.

AM or FM, which one is better?

FM:

• High frequency represents 0
• Low frequency represents 1, or vice versa.
• Simpler and decoded faster
• The receiving device has a table of numbers for each frequency and

can match them quickly. Conclusion: FM is better for digital transfer.

Nano: few micrometers distance NFC (Near-Field Communication): within 4 CM Of distance, e.g. Keycards, contact payment, etc. BAN (Body Area Network): E.g. wearable computing devices to monitor heart rate or blood pressure etc. PAN (Personal Area Network): within a radius of one

• r a couple or rooms.

LAN (Local Area Netwrok): covering the area of one Building. CAN (Campus/Corporate Area Network): unites LAN within a limited geographical area such as a university. MAN (Metropolitan Area Network): a bigger network to cover certain areas of a city using microwave transmission technology. WAN (Wide Area Network): unites LANs and MANs.

Local Area Network (LAN)

• A group of devices that are interconnected within a limited area such

as a school, a hospital, a university, a residential building, etc.

WLAN (Wireless LAN)

It is a LAN where devices communicate wirelessly. The most common WLAN technology is Wi-Fi.

WLAN vs Ethernet LAN

• The difference between WLAN and Ethernet (cabled) LAN is in the layer 1

(Physical Layer) and layer 2 (Data Link Layer) of the OSI model. Layer 1 or Physical layer is about medium, topology, and signaling format:

• Medium: copper wire, fiber, air, etc.
• Topoplogy: How devices are set physically

and logically. (e.g. Ring, star, mesh, bus, etc.)

• Signaling Format: electric current, light, radio waves

Layer 2 or Data Link Layer is about media access method, MAC address and data frame.

WLAN vs Ethernet LAN

• CSMA/CD: Carrier Sense Multiple Access/Collision Detection
• CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance

(A collision happens when two transmitters transmit at the same time using the same medium.)

CSMA/CD va CSMA/CA

• CSMA is a MAC (Media Access Control) protocol that is used to make

sure data packets are not lost and data integrity is maintained.

• CD: Collision Detection is used in wired communication (e.g. In

Ethernet LANs). Once the collision is detected, CSMA CD immediately stops the transmission so that the transmitter does not have to waste a lot of time in continuing it. The last information is retransmitted.

• CA: Collision Avoidance is a mechanism used in wirelss communication

where the transmitter first checks if the medium is free before sending data packets. This reduces the possibility of collison and saves time.

Data Frame in Ethernet LAN vs in WLAN

Data frame is a data unit at the Data Link Layer of OSI

• model. Wi-Fi frame has a much larger MAC header.

This means more overhead compared to Ethernet frame and less efficiency compared to Ethernet traffic.

Data Frame Terminology

• Preamble: Tells the receiver that a frame is coming and where it starts.
• Type: Tells the recipient the basic type of data, such as IPv4 or IPv6.
• Data: actuall data, that must be delivered, is in this section.
• FCS (Frame Check Sequence): It’s for error checking to make sure the

frame is intact.

• Frame control: Holds info about protocols, type of frame and type of

security in use.

• Duration: Tells how long the field’s transmission will last, so that other

devices will know when the channel will be available again.

Data Frame Terminology

• Address 1: Source (or sender) MAC address
• Address 2: Transmitter’s MAC address, which

is the AP’s MAC address

• Address 3: Receiving AP’s MAC address
• Address 4: Destination’s MAC address
• Sequence Control: Indicated how a large packet

is fragmented. Are all 4 address fields always used?

• No. Only Address 1 is mandatory and all the others depend on the frame.

Why WLAN? What are the applications of WLAN?

• Makes mobility easier. It’s more convinient

and saves the cost of cable installations.

• Application: home, office, schools, university

campuses, hotels, restaurants, manufacturing plants, hospitals, etc.

What do we need to set up a WLAN?

Wi-Fi Router vs Wireless AP

Wi-Fi Router can connect directly to internet via modem but AP can’t. Wi-Fi Router can connect to both wireless devices and wired ones but AP can only connect to wireless devices. AP is connected to Wi-Fi Router via an Ethernet port.

Wi-Fi Router vs Wireless AP

Wi-Fi Router has Firewall but AP doesn’t.

Wi-Fi Router vs Wireless AP

Wi-Fi Router can automatically assign IP addresses to devices, but …

Wi-Fi Router vs Wireless AP

AP can’t assign any IP address and it simply passes the IP addresses that are assigned by Wi-Fi Router to the devices.

Using Wi-Fi Router & Wireless AP

AP is used to boost the signal strength. It is connected via Ethernet cable to Wi-Fi Router and the Wi-Fi Router is connected to Modem which is connected to Internet.

Using Wi-Fi Router & Wireless AP

A typical setting using Wi-Fi Router

Using Wi-Fi Router & Wireless AP

A typical setting using Wi-Fi Router and using AP to boost the signals.

Using Wi-Fi Router Only

Using only Wi-Fi Router makes it harder to manage because every change in network must be made in each wi-fi router separately.

WLAN Terminology

• WLAN: The wireless extension of the LAN using wi-fi bands
• Station: The users or subscribers having WLAN or wi-fi capabilities
• Wi-Fi Router: The device that connects the WLAN to the Internet.
• AP (Access Point): A terminal which provides wi-fi connectivity to
• stations. On one end it is connected to internet (via wi-fi router) and its
• ther end gives wireless internet connection to stations.
• BSS (Basic Service Set): A set of stations which are controlled using a

common coordination function. Coordination function determines whether stations within BSS is permitted to transmit and receive or not.

WLAN Terminology

• IBSS: Independent BSS, stations connect to each other without AP
• ESS: Extended Service Set, a set of one or more BSSs and LANs. This

appears as single BSS to LLC layer.

• DS: Distribution System, the system which connects BSSs and

integrated LANs to make ESS.

• MPDU: MAC Protocol Data Unit, unit of data which is exchanged

between two peer MAC entities using PHY layer.

• MSDU: MAC Service Data Unit, Information exchanged as unit

between two MAC users.

• WDS: Wireless Distribution System, enables wireless interconnections
• f APs

Types of WLAN

• 1) Infrastructure: When a base station (AP) acts as wireless hub and
• ther stations communicate through this hub. Aps can provide services

within a limited range. There can be many Aps in a WLAN and the client’s software tries to find the one with the strongest signal.

• 2) Peer-to-Peer (ad-hoc): P2P allows network

devices to communicate with each other

• directly. Wireless devices within range of each
• ther can discover and communicate direclty

without involving central AP.

Data Transmission Mechanism in WLAN

RTS = Request To Send. This packet is sent to AP to ask for trasmission permit. CTS = Clear To Send. It means AP is free to receive my laptop’s request. *RTS and CTS is part of CSMA/CA technology used in IEEE 802.11 based WLAN. (If activity is detected, my laptop waits for a specific amount of time)

Data Transmission Technique in WLAN

• Distributed Coordination Funtion (DCF) is the fundamental medium

(channel) access technique used in IEEE 802.11 standard. It uses CSMA/CA technology to minimize collision. In DCF user stations first “listen” to the channel before trying to transmit any packet. DCF has 4 main components:

1. Interframe Space 2. Duration Field (Frame Transmission) 3. CSMA/CA (using RTS / CTS) 4. Random Backoff Timer

Data Transmission Technique in WLAN

• Interframe Space (IFS): IFS is a period of idle time that exists between

transmissions of wireless frames. The main purpose of IFS:

• 1. To provide a time gap to avoid collision
• 2. To prioritize frame transmission

There are 6 types of Interframe Space which are used according to the importance of the packet being transmitted:

Data Transmission Technique in WLAN

The 6 types of IFS. The shortest time period means the highest priority.

1. Reduced Interframe Space (RIFS, is the shortest which means highest priority) 2. Short Interframe Space (SIFS, the second highest priority) 3. Point Coordination Function Interframe Space (PIFS, the third highest priority. Used in AP) 4. Distributed Coordination Funtion Interframe Space (DIFS), used by user stations 5. Arbitration Interframe Space (AIFS), used by QoS stations 6. Extended Interframe Space (EIFS), used after receipt of corrupted frames

Data Transmission Technique in WLAN

*RIFS is not being used anymore.

Data Transmission Technique in WLAN

• NAV = Network Allocation Vector

It is a virtual carrier-sensing mechanism in IEEE 802.11 standard. NAV equals the duration value of the frame being transmitted. The duration value = Duration of the sequences of frames and interframe spaces AFTER the current frame is transmitted. NAV is not zero => medium is busy NAV is zero => medium is free and can be used.

Data Transmission Technique in WLAN

Data Transmission Technique in WLAN

• Contention Window: If participants determine that the channel is

free, they wait a random amount of time before they start sending. This duration corresponds to the contention window. This time window doubles with each collision and corresponds to the binary exponential backoff (BEB) that is familiar from CSMA/CD.

• Why randomization & growing exponentially?

It is the best try to avoid second collision and the CW grows more aggressively in exponential growth. This means wider range of option for stations and lower possibility to choose the same number.

Data Transmission Technique in WLAN

Data Transmission Technique in WLAN

Data Transmission Technique in WLAN

Data Transmission Technique in WLAN

Data Transmission Technique in WLAN

WLAN Communications

• References:
• Designing and Deploying 802.11 Wireless Networks: A Practical Guide to Implementing 802.11n and

802.11ac Wireless Networks For Enterprise-Based Applications (Publisher: Cisco Press; 2 edition April 29, 2015)

• CWNA Guide to Wireless LANs (Publisher: Cengage Learning; 003 edition (June 19, 2012)
• Crow, B.P.; Widjaja, I.; Kim, L.G.; Sakai, P.T.; "IEEE 802.11 Wireless Local Area Networks,

"Communications Magazine, IEEE , Volume: 35 , Issue: 9 , Sept. 1997.

• M. Kuran and T. Tugcu, "A Survey on Emerging Broadband Wireless Access Technologies." Computer

Networks, 2007. Tilgjengelig online via www.sciencedirect.com.