Wireless and Mobile Protocol

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Fund. Wireless & Mobile Protocol

COSC 4301/5340

Wireless Local Area Networks (WLANs)

Instructor: Dr. Xingya Liu

Department of Computer Science

Lamar University

Office: MAES 86 Phone: 409-880-8677

Email: [email protected]

Fund. Wireless & Mobile Protocol

IEEE 802.11 Protocol

Architecture

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Physical Layer (PHY)

Distributed Coordination Function (DCF)

Point Coordination Function (PCF)

Normal Data Traffic (Asynchronous) Contention Service

Real Time Traffic Contention Free Service

MAC

Fund. Wireless & Mobile Protocol

IEEE 802.11 –

Medium Access Control • Distributed Mode: Distributed Coordination Function (DCF)

– Based on CSMA/CA protocol

– Uses a contention algorithm to provide access to all traffic.

– Ordinary traffic uses DCF directly.

• Coordinated Mode: Point Coordination Function (PCF)

– Supports real-time traffic

– Based on polling which is controlled by a centralized point coordinator.

– Uses a centralized MAC algorithm and provides contention-free service.

– PCF is built on top of DCF and exploits features of DCF to assure access for its users.

• NOTE: Both the DCF and PCF can operate concurrently within the same BSS to provide alternative contention and contention- free periods

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

MAC Layer Overview

• DFWMAC-DCF CSMA (Mandatory)

– Distributed Foundation Wireless Medium Access Control - Distributed Coordination Function CSMA

• DFWMAC-DCF w/ RTS/CTS (Optional)

– Distributed Foundation Wireless MAC

– Avoids Hidden Terminal problem

• DFWMAC- PCF (Optional)

– Access point polls terminals according to a list

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Fund. Wireless & Mobile Protocol

IEEE 802.11 Access Method –

Interframe Spaces (IFS)

• Priorities

– Defined through different interframe spaces (IFS)

– SIFS (Short Inter Frame Space)

• highest priority, for ACK, CTS, polling response

• SIFS: 16 µsec

• SIFS required for turn around of Tx to Rx and vice versa

– PIFS (PCF IFS) - Point Coordination Function Interframe Space

• medium priority, for real-time services using PCF

• SIFS + one slot time (9 µsec) = 25 µsec

– DIFS (DCF IFS) - Distributed Coordination Function Interframe Space

• lowest priority, for asynchronous data services

• SIFS + two slot times (9 µsec) = 34 µsec

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA

• A station with a frame to transmit senses the medium.

• If the medium is idle, it waits to see if the medium remains idle for a time equal to DIFS. If so, the station may transmit immediately.

• Receivers acknowledge at once (after waiting for SIFS) if the

packet was received correctly

• Receiver transmits ACK without sensing the medium

SIFS

DIFS

data

ACK

waiting time

Other

Stations

Receiver

Sender data

DIFS

contention

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA

• If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the DIFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.

• Once the current transmission is over, the station delays another DIFS.

• If the medium remains idle for this period, the station backs off using a binary exponential backoff scheme and again senses the medium.

• Backoff Timer start decreasing after an idle time of DIFS following the medium busyness

• When Backoff Timer is 0, station may transmit immediately.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA • Maintain a value CW (Contention-Window)

• If Busy

– Wait until channel is idle for DIFS

– Then MAC runs a random number generator to choose a random number between 0 and CW to set a BACKOFF CLOCK for every contending station and starts a backoff timer for proportional amount of time

DIFS Contention Window

Slot time

Defer Access

Backoff-Window Next Frame

Select Slot and Decrement Backoff as long as medium is idle.

SIFS

PIFSDIFS

Free access when medium is free longer than DIFS

Busy Medium

IFS: Inter Frame Space

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA – The first station that expires its clock starts transmission

– Other terminals sense the new transmission and freeze their clocks to be reactivated after the completion of the current transmission in the next contention period

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA • If Collisions (Control or Data)

– Binary exponential increase (doubling) of CW

– Length of backoff time is exponentially increased as the station goes through successive retransmissions

• CWmin<=CW<=CWmax

– CWmin = 31slots, CWmax = 1023slots (for 802.11b)

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-DCF CSMA – Up to CWmax, CW = (CWmin+1)* 2n-1, where n = 0, 1, 2,

... is retransmission number

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC w/ RTS/CTS

SIFS

DIFS

ACK

defer access

Other

Stations

Receiver

Sender data

DIFS

contention

RTS

CTS SIFS SIFS

NAV (RTS)

NAV (CTS)

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RTS …

Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC w/ RTS/CTS

• RTS/CTS is used for reserving channel for data transmission so that the

collision can only occur in control message

• Station can send RTS (request to send) with reservation parameter after

waiting for DIFS (reservation determines amount of time the data packet

needs the medium)

• Every node receiving the RTS has to set its Network Allocation Vector

(NAV) in accordance with the duration of the field (NAV specifies the

earliest point at which the station can try to access the medium

• If receiver receives RTS, it sends CTS (Clear to Send) after medium has been

idle for SIFS. CTS again contains duration field and all stations receiving this

packet need to adjust their NAV

• Sender can now send data after SIFS, acknowledgement via ACK by receiver

after SIFS

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Fund. Wireless & Mobile Protocol

Network Allocation Vector

(NAV)

• Both Physical Carrier Sensing and Virtual Carrier Sensing used

in 802.11

• If either function indicates that the medium is busy, 802.11 treats

the channel to be busy

• Virtual Carrier Sensing is provided by the NAV (Network

Allocation Vector)

• Virtual Carrier Sensing:

– Most 802.11 frames carry a duration field which is used to reserve the

medium for a fixed time period

– Tx sets the NAV to the time for which it expects to use the medium

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Fund. Wireless & Mobile Protocol

Network Allocation Vector

(NAV)

– Other stations start counting down from NAV to 0

– When NAV > 0, the medium is busy

– Channel virtually busy a NAV SIGNAL is turned on

– The transmission will be delayed until the NAV signal has

disappeared

– Provides a unique access right for a station without any

contention.

• When the channel is virtually available, then MAC

checks for PHY condition of the channel

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• The access mechanisms presented so far cannot guarantee a maximum access delay

• To provide a time bounded service, the standards specify a Point Coordination Function (PCF) on top of the DCF mechanisms.

• Using PCF requires an access point that can control medium access and poll the single nodes. Ad Hoc networks cannot use this function.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• Point Coordination Function

– PCF is an alternative access method implemented on top of the DCF.

– The operation consists of polling with the centralized polling master (point coordinator).

– The network is configured such that a number of stations with time-sensitive traffic are controlled by the PC while remaining traffic contends for access using CSMA/CA

– The point coordinator resides in the access point (AP). It provides contention free frame transfer.

– The PC issues polls in a round-robin fashion to all stations configured for polling

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• Point Coordination Function (Cont.)

– The PC makes use of PIFS when issuing polls.

– Because PIFS is smaller than DIFS, the point coordinator can

seize the medium and lock out all asynchronous traffic while

it issues polls and receives responses.

– When a poll is issued, the polled station may respond using SIFS.

– If the point coordinator receives a response, it issues another poll to another station using SIFS.

– If no response is received during the expected turnaround time, the coordinator issues another poll.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

PIFS

Stations‘

NAV

Wireless

Stations

Point

Coordinator

SIFS

NAV

SIFS

SuperFrame t0

medium busy

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• At time t0 the contention-free period should start, but

another station is transmitting data

• After the medium has been idle, the PCF has to wait for

PIFS before accessing the medium.

• The point coordinator now sends data D1 to the first

station. The station can answer after SIFS. After

waiting for SIFS, the point coordinator can poll the

second station by sending D2.

• The second station replies with U2

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

Stations‘

NAV

Wireless

Stations

Point

Coordinator

NAV

PIFS

SIFS

CFend

contention

period

contention free period

t2 t3 t4

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• Polling continues with the third node which has nothing to answer.

• After waiting for PIFS, the point coordinator can issue another poll.

• At the end of contention-free period, an end marker (CFend) is issued indicating that the contention period may start again.

• The cycle starts again with the next superframe

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• The figure illustrates the use of the superframe.

NAV NAV

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional) • The AP organizes a periodical contention-free period (CFP) for

the real-time information.

• The AP coordinates real-time data to be transmitted at the beginning of each superframe and during those periods it arranges an NAV signal for all other stations.

• The length of the PCF period is variable because of the variable frame size issued by responding stations, and it only occupies a portion of the superframe.

• The rest of the superframe is released for contention and DCF packets.

• If a DCF packet occupies the channel and does not complete before the start of the next superframe, the starting time of the superframe will defer.

• However, the NAV signal for all other terminals goes to operation at the beginning of the CFP.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

DFWMAC-PCF (Optional)

• At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS.

• If the medium is idle, the point coordinator gains immediate access and a full superframe period follows.

• However, the medium may be busy at the end of a superframe.

• In this case, the point coordinator must wait until the medium is idle to gain access; this results in a foreshortened superframe period for the next cycle.

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Fund. Wireless & Mobile Protocol

IEEE 802.11 –

Power Saving Mode (PS)

• IEEE 802.11 stations can maximize battery life by shutting

down the radio transceiver and sleeping periodically

• During sleeping periods, access points buffer any data for

sleeping stations

• The data is announced by subsequent beacon frames

• To retrieve buffered frames, newly awakened stations use PS-

poll frames

• Access point can choose to respond immediately with data or

promise to delivery it later

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Fund. Wireless & Mobile Protocol

IEEE 802.11 – More Developments

• 802.11aa: Streaming of audio video transport streams – completed (June 2012)

• 802.11ae: Prioritization of management frames – completed (March 2012)

• 802.11ac: Very high throughput improvements over 802.11n

– Better modulation, wider channels, multi user MIMO.

• 802.11ad: Very high throughput at 60GHz

• 802.11af: TV Whitespace

• 802.11ah: Smart metering, 1GHz sensor network

• 802.11ai: Fast initial link setup

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Fund. Wireless & Mobile Protocol

Performance Metrics

• Overall coverage area

– Can be evaluated in terms of received signal strength intensity (RSSI)

• Throughput

– Can be evaluated by measuring TCP connection throughputs since WLANs establish a client-server communication link via TCP connection

• Implementations of handoff and dropping are the responsibility of manufacturers, since they vary according to different equipments

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Fund. Wireless & Mobile Protocol

Interference Issue

• An important issue in all wireless systems because nearby users occupy the same bandwidth and cause co- channel interference.

• For WLANs, in addition to co-channel interference, other types of interference exist mainly due to the use of unlicensed ISM band. Interference to WLANs comes from the following major sources:

– Co-channel interference.

– Interference from non-WLAN devices in the same frequency band

– Interference between different WLANs in the same frequency band.

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Fund. Wireless & Mobile Protocol

Environmental Issue

• Has been proved that WLAN is safe for health

– Radiation used by this technology falls well within the limits

of safety guidelines (both in terms of frequency content and

power level) specified by Radio Frequency Safety Standards

and Recommendations.

– Radiation in this frequency range is non-ionizing (as they do

not have enough energy to break the chemical bonds of

genetic material of body cells).

– Vendors designing their products to operate within the power

limit set by the Safety Standards.

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Fund. Wireless & Mobile Protocol

Challenging Issues

• Interference between different types of WLANs

• Lack of Interoperability between WLANs

• Relatively low data rate

• Lack of support for real-time services.

– IEEE 802.11 products, which are based on the CSMA/CA protocol, are unable to provide QoS guarantees for voice, video, and other real time services. (IEEE 802.11 working

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