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Lecturer: Anjum Naveed

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Architectures Stations

BSS (One AP and Stations connected to it)

IBSS (Ad Hoc, without AP) ESS (Set of connected BSS)

DS (Connects APs of ESS)

Wired

Wireless

Mesh

WDS

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DLL Services

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Framing

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Framing

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FC, Duration, Address 1 and FCS are mandatory

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Important Frame types Management

Beacon

Authentication

Association (Req/Resp) Reassociation (Req/Resp)

Control

RTS

CTS ACK

CF-End

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Addressing 48 bit MAC addresses are used

Locally administered

Globally administered

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Error Control CRC based error detection used at MAC layer

Re-transmissions used for correction

Keep in mind the physical layer FEC

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Flow Control One packet sent at a time

Data rate adjusted based on BER through adaptivemodulation and encoding (AMC)

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The Channel Allocation Problem How to allocate a single broadcast channel among

competing users?

Two possibilities Static channel allocation

Dynamic channel allocation

Our Concern

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Dynamic Channel Allocation Protocols or

Medium Access Control Protocols ALOHA

Pure ALOHA

Slotted ALOHA

CSMA 1-Persistent CSMA

Non-Persistent CSMA

p-Persistent CSMA

CSMA/CD

CSMA/CA

Collision free protocols Token ring (we will not study these protocols)

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Pure ALOHA Uncoordinated stations are contesting for the use of a

single shared channel

Algorithm Stations transmit whenever they have something

to send

Colliding frames will be destroyed

If destroyed (detected via self sensing), the

sender waits a random amount of time and sends

frame again

SLOTTED ALOHA is synchronous variant where framescan only be transmitted at start of a centrally controlledslot size of slot is equal to frame size

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CSMA Protocols There are several types of CSMA protocols

1-Persistent CSMA

Non-Persistent CSMA p-Persistent CSMA

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1-persistent CSMA When a station has to send data

1. It listens to the channel

1. If idle, transmits a frame

2. If not, monitors continuously and waits until it becomes idleand transmits at this stage

2. If collision occurs, waits a random amount of time andstarts all over again

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Non-persistent CSMA When a station has to send data

1. It listens to the channel

1. If idle, transmits a frame

2. If not, waits for a random amount of time and repeats thealgorithm

2. If collision occurs, waits a random amount of time andstarts all over again

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p-Persistent CSMAApplies to time-slotted channels

When a station has to send data

It listens to the channel If channel is idle, it transmits with a probability ofp

With probabilityq = 1 - p it defers until the next slot

If that slot is also idle, it either transmits or defers again

with probabilities p and q

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CSMA with Collision Detection In CSMA protocols

If two stations begin transmitting at the same time, eachwill transmit its complete packet, thus wasting the

channel for an entire packet time In CSMA/CD

The transmission is terminated immediately upon thedetection of a collision (saves time & bandwidth)

Try again later after waiting a random amount of time

This protocol is widely used on LANs (e.g., Ethernet uses1-persistant CSMA/CD)

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CSMA with Collusion Avoidance Before transmitting data:

Transmitter transmits a short frame RTS, informing itsneighbours and receiver of the transmission

Receiver replies with CTS if its not involved in transmission,informing its own neighbours and transmitter allowingtransmission.

Data is transmitted only after receiving CTS.

SELF ASSIGNMENT: What problems are solved by this protocol? What problems are not solved? Why is this protocol required when we have CSMA/CD?

To be discussed at the end of class (time permitting)

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802.11 MAC Sublayer 802.11 supports four modes of operation

DCF (Distributed Coordination Function) which doesnot use any kind of central control

PCF (Point Coordination Function) which uses the basestation to control all activity in its cell

HCF (Hybrid Coordination Function)

MCF (Mesh Coordination Function)

All implementations of 802.11 must support DCF butPCF is optional

QoS enabled must support HCF and Mesh enabledmust support MCF

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DCFWhen DCF is employed, 802.11 uses P-persistent

CSMA/CA Station intending transmission senses the channel

If channel is idle, starts transmission with probability p No sensing during transmission (Unlike CSMA/CD)

If the channel is busy, defers until channel goes idle andthen continue sensing

If a collision occurs, the colliding stations wait arandom time, using the binary exponential backoffalgorithm, and then try again later

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Transmission?? Station A -> Station B Extended mode

A transmits RTS frame

B receives this request, it may decide to grantpermission, in which case it sends a CTS frame back A Receives CTS, A now sends its frame and starts an

ACK timer Upon correct receipt of the data frame, B responds with

an ACK frame, terminating the exchange If As ACK timer expires before the ACK gets back to it,

the whole protocol is run again

Basic Mode RTS & CTS are optional for short frames

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The use of virtual channel sensing using CSMA/CA

Virtual Channel Sensing

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PCF AP broadcasts a beacon frame periodically (10 - 100 times per second)

Between the sending of beacon frames, PCF splits the time into acontention-free period and a contention period During contention-free period, AP sequentially polls the registered

stations

A poll can contain data for the station

Response to poll can be data to AP/other station or empty frame

Contention period uses DCF Why is contention period required?

Once a station signs up for polling service at a certain rate, it iseffectively guaranteed a certain fraction of the bandwidth

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PCF & NAV Power Management

Power is a big issue in mobile wireless devices

Using PCF, AP can direct a mobile device to go to sleepstate

This is specifically used for contention free period

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Inter-frame spacing in 802.11 PCF and DCF can coexist within one cell

SIFS (Short Inter-Frame Spacing)

PIFS (PCF Inter-Frame Spacing)

DIFS (DCF Inter-Frame Spacing) EIFS (Extended Inter-Frame Spacing)

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Fixed values

per PHY

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Reading Giuseppe Bianchi. Performanceanalysis of the ieee

802.11 distributed coordination function. IEEE Journalon Selected Areas in Communications, 18(3), March

2000.

Piyush Gupta and P. R. Kumar. The capacity of wireless

networks. IEEE Transections on Information Theory,46(2), March 2000.

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