MAC and Physical.ppt

Bài giảng MAC and Physical.ppt

MAC & Physical Layers

(1 September, 2006)

 Explain how a client joins a network

 Describe the modes of operation wireless

LANs use to communicate

 Explain how wireless LANs avoid collisions

IEEE 802.3 - Ethernet

IEEE 802.3

PROCESS DATA DSAP SSAP CNTRL

FCS LLC-PDU

PREAMBLE DESTINATIONADDRESS SOURCE

ADDRESS

FIELD TYPE LLC-PDU

DATA

ETHERNET

1 1 1-2

0-1500 2

6 6

FCS LLC-PDU

PREAMBLE DA SA LENGTH

IEEE 802.3

CSMA/CD

0-1500 2

2/6 2/6

PAD SFD

 Ethernet was developed by Bob Metcalf, Xerox Corp.

 Standardized in 1980 as IEEE 802.3

 CSMA/CD algorithm is same for both Ethernet and 802.3

 Frame format differs between Ethernet and 802.3

 Frame format differs between Ethernet and 802.11

 THIS FIELD IS NOT IN ETHERNET.

 ETHERNET HAS A TYPE FIELD

 THIS FIELD IS NOT PRESENT IN ETHERNET.

 ETHERNET LENGTH MUST BE >= 64 OCTETS 1

PROCESS DATA

LLC PDU

LLC PDU

IEEE 802.3 FORMAT

 Frame size of 1518 bytes (1500 for payload).

 Jumbo Frames are 9000 bytes

 Fragmented at 1518 bytes by Host or Routers

(IPv4).

IEEE 802.11 – Wireless Ethernet

Generic 802.11 Frame

 Frame Duration Rec Xmit Dest Seq Src Frame FCS

 Control ID Addr Addr Addr Cntl Addr Body

 2 2 6 6 6 2 6 0-2312 4

 Frame Control Specifies control information unique to

wireless transmission

 Duration Generally indices how may microseconds the

medium is expected to stay busy during transmission

 Addresses These are the MAC address of the MS, AP

and Ethernet nodes

 Sequence Field The number of each transmitted frame.

802.11 Frames/Protocols

 Three major 802.11 frame types exist.

 Data frames carry higher level protocol data in

the Frame body.

 Control frames are used to assist in the delivery

of data frames, administer access to the medium and to provide MAC layer reliability.

 Management frames perform supervisory

functions such as joining/ leaving a wireless

network and move associations from AP to AP

802.11 Frame Types

Management Frames

 Association Request frame

 Association Response frame

 Reassociation Request frame

 Probe request Frame

 Probe Response frame

 Power-Save Poll (PS Poll)

 Contention-Free End (CF End)

 CF-End + CF ACK

 The Wireless medium similar to Ethernet is a shared

medium That is, many clients attempt to access (share) the

 In order to prevent collisions from occurring an access

method is required that arbitrates who can access the shared

medium

 For Ethernet this is Carrier Sense Multiple Access with

Collision Detection (CSMA/CD)

 For 802.11 this is Carrier Sense Multiple Access with

Collision Avoidance (CSMA/CA)

 Carrier sensing is used to determine if the medium

is available

functions

 Physical Carrier Sensing

simultaneously.

 Virtual carrier Sensing

 Carrier Sensing

 Stations cannot transmit during that period.

 When the NAV timer reaches zero the Virtual Carrier-Sensing

indicates the medium is idle and the station can transmit.

 For the station to transmit both the physical and virtual carrier

sense must report an idle condition otherwise the station must enter a

deferral condition.

 If the station can transmit it must observe Interframe spacing

(IFS)

 Carrier Sensing

 Interframe Spacing

 Interframe Spacing (IFS) ensures the medium is idle for a

minimum period of time prior to transmission

 IFS Serves two primary functions

 First, IFS ensures that all frames are spaced in time such that they will

be received as distinct frames.

 Secondly, it provides a priority access mechanism whereby certain

types of frames are able to preempt other frames.

 Priority access is provided to frames by allowing them to be

preceded by shorter interframe spacing.

 There are four main lengths of interframe spacing

 Short Interframe Spacing (SIFS)

 Point Coordination Function (PCF) Interframe Spacing (PIFS)

 Extended Interframe Spacing (EIFS)

 Distributed Coordination Function (DCF) Interframe Spacing

(DIFS)

 Interframe Spacing

 There are four different types of interframe spacing.

 Short Interframe Space (SIFS).

 SIFS is used for high priority traffic such as RTS/CTS and

ACK

 Higher priority traffic begins immediately after the expiration

of SIFS

 SIFS is normally used at the following times:

 To send an ACK in response to a data frame.

 To send a CTS in response to an RTS frame.

 To send a data frame following a CTS frame.

 To send all other fragments following the first fragment.

 Interframe Spacing

 There are four different types of interframe spacing.

 PCF Interframe Space (PIFS)

 PIFS is used by PCF during contention-free operation.

 Access points only use PIFS when the network is in PCF mode which must be manually configured by the administrator.

 The PCF mode allows the AP to control which stations may transmit

 No known vendor implements PCF.

 PIFS only works with DCF (BSS, ESS, IBSS) and not Ad-hoc mode

 Extended Interframe Space (EIFS).

 EIFS is used when there is an error in transmission and has

no fixed interval

 Interframe Spacing

 There are four different types of interframe

 DCF Interframe Space (DIFS)

 DIFS is used for contention based services and is the

default interframe space on all 802.11 stations.

 Each station in DCF mode waits until DIFS has expired before contending for the network.

 DCF transmission have lower priority than PCF based transmissions.

 Interframe Spacing

 Interframe Spacing contd

 Interframe Spacing Relationship

 Busy SIFS

 PIFS

 DIFS

 Frame Transmission

Contention Window

 Contention Window

 The interframe spacing time is followed by a contention

window.

 During the contention window all stations desiring to transmit data chooses random backoff time (time to wait)

 Each station uses a random back off algorithm to determine

how long to wait before transmitting.

 A contention period (CP) immediately follows the

(DIFS).

 The station chooses a random number and multiplies it

by the slot time to get a length of time to wait.

 The station performs a Clear Channel Assessment (CCA)

after each time slot to see if the medium is busy.

 Contention Window

 PIFS = SIFS + 1 Slot Time

 DIFS = PIFS + 1 slot Time

 The slot time multiplied by the random number to

obtain the wait time is dependent upon the particular physical layer (DSSS, FHSS, OFDM ,etc)

 Slot Times

 The Contention Window

Initial

1st Retrans

 Fram e

 DIF S

 Fram e

 Fram e

 Contention Window = 31 Slots

2nd Retrans

 DIF S

 DIF S

 Contention Window = 63 Slots

 Contention Window = 127 Slots

 The Contention Window is divided into time slots.

 The length of each slot is medium dependent.

 Stations pick a random slot and wait for that time slot before attempting to access the medium.

802.11

b

Station Backoff with DCF

 Stations contend to transmit after expiration of the DIFS.

 The Contention Window (Backoff Window) follows the DIFS.

 The CW is divided into slots with the slot length depending upon the medium, e.g., DSSS = 20 uS

 The station chooses a random number and multiplies it by the

slot time to get a length of time to wait.

 The station counts down the slot times until its slot arrives

 Each time transmission fails (stations picked the same time

slot) the backoff time is selected from a larger range

 25-1 = 31, 26-1=63, etc., timeslots

DSSS Contention Window Size (802.11b)

 Carrier Sense Multiple Access w/ Collision Avoidance

(CSMA/CA).

 Listen Before Talking (LBT).

 CSMA/CA avoids collisions and uses positive acknowledgements (ACKs) instead of arbitrating the use of the medium such as CSMA/CD

 An ACK is require for each frame sent If no ACK is received

it is assumed that the frame was not received

 Collision avoidance is implemented through two distinct

coordination functions

 Distributed Control Function (DCF) defines how stations

contend for the wireless medium (contention based)

 Point Control function (PCF) defines how the wireless

medium is used during contention-free access

 CSMA/CA

 Distributed Coordination Function (DCF) defines how

stations contend for the wireless medium (contention based

 Point Control function (PCF) defines how the wireless

medium is used during contention-free access.

 PCF refers to the fact that the AP acts as a central point to

 Coordination Function

 802.11e

QoS- 802.11e Background

 802.11 is increasingly being used for multimedia streaming

functions such as voice and video.

 These applications are sensitive to time deviation in

the processing of packet at the receiver

 Time deviations in the arrival of packets resulted in jerky motion or garbled sound

 The Point Coordination Function PCF) mode was

intended to guarantee regular access to the medium and to accommodate VoIP and streaming multimedia

 802.11e

 To address the limitation of 802.11 associated with

streaming multimedia IEEE introduced 802.11e.

 802.11e defines a Quality of Service (QoS) extension to

the 802.11 MAC layer designed to accommodate streaming multimedia

 QoS (Quality of Service) is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance

 QoS provides subscribers better reception for full-motion video, high-fidelity

audio, and Voice over IP through the Internet.

 802.11e modifies the rules associated with DCF and PCF to create,

 EDCAF

 EDCAF defines 8 traffic priority levels with the

higher priority traffic being transmitted first.

 EDCAF does not provide any guaranteed bandwidth but

it does provide an increased probability that stations with high priority traffic will transmit first

 An Arbitration Interframe Space (AIFS) wait period

which corresponds to the traffic priority is transmitted prior

to the data

 The stations with the highest priority traffic which

 HCF

contention-free periods and grants each station a specific start time and maximum duration for transmission.

 During the Contention Free Period (CFP), the AP (Hybrid

Coordinator) controls the access to the medium.

 The HCF defines a number of different Traffic Classes (TC)

 The stations give information about the lengths of their queues for each Traffic Class (TC) to the AP

 The AP uses this information to give priority to one station over another

 In addition, stations can be given a Transmit Opportunity (TXOP) and,

for a given time period selected by the HC, they may send multiple

packets in a row.

 RTS/CTS

 CSMA/CA is based upon two types of carrier sensing mechanisms

 Physical Carrier and

 Virtual Carrier-Sensing

 Virtual Carrier Sensing is provided by Network Allocation

Vector (NAV).

 802.11 frames carry a duration field which reserves the

medium for a fixed time

 The NAV is a timer that indicates the amount of time the

medium is to be reserved

 Stations set their NAV timer upon receipt of a frame containing a duration field

 Stations cannot transmit during that period

 When the NAV timer reaches zero the Virtual

Carrier- Carrier Sensing Review

 For the station to transmit both the physical and virtual

carrier sense must report an idle condition otherwise the station

must enter a deferral condition.

 Each data frame contains a duration field that sets the NAV

timer in all stations

 This value is long enough to (1) transmit an ACK in

response to a data frame and to (2) account for the IFS

 The NAV value is said to protect the ACK

 If an RF coverage area has a high rate of collisions,

CSMA/CA, based upon carrier sensing, will not help the

problem of collission.

 Under these circumstances it might be more efficient to

reserve transmission time.

 This reservation of transmission time is the purpose of

RTS/CTS

 Carrier Sensing Review Contd

 The client station issue a Request to Send (RTS) frame to the

AP This frame contain a duration field value which is issued to set the NAV timer.

 All stations in the BSS will hopefully hear the RTS Some

may not due to the Hidden Node problem.

 The AP responds with a Clear to Send (CTS) frame which

contains a shorter Duration field because all stations may not have

heard the RTS –remember the hidden Node problem.

 All stations in the BSS now set their NAV timer and will not

attempt to transmit unit their NAV timer decreases to zero.

 The client then passes data to the AP which ACKs the data

 RTS/CTS

RTS/CTS Handshaking

The RTS/CTS contains a Duration value which sets the NAV timer

RTS/CTS Process

 A client transmits an RTS frame to the AP.

Station

Access

Point

 RTS/CTS causes significant overheard traffic on the WLAN thereby reducing throughput.

turned OFF by default on a WLAN.

 Most vendor products will allow the Wireless Network Administrator to set the RTS/CTS threshold if required.

 If the network is experiencing a high amount of

collisions this may indicate a Hidden Node

 One solution to a high collision rate may be RTS/CTS

 RTS/CTS Issues

 Fragmentation

Fragmentation and Reassembly

packets, is a techniques used in wireless communication to improve the throughput of the wireless channel as a result of interference caused by microwave ovens, wireless phones, jamming, etc.,

 Interference affects smaller fragments less than larger

fragments

 Fragments all have the same sequence number but ascending

fragment numbers to aid in reassembly.

 Frame control information indicates whether more fragments are coming

 Stations never fragment multicast or broadcast frames.

 There is a tradeoff between the lower frame error rate that

can be achieved by fragmentation and the increase overhead due to fragmentation

 Fragments comprising a frame are normally sent in

fragmentation bursts.

 Fragmentation Burst

Fragments and their ACKs are separated by SIFS so a station retains control of

the channel during the fragmentation burst.

The NAV is used to retain control of the channel

 The RTS/CTS set the NAV from the expected time to the end of the first

fragment.

 The ACK fragments set the NAV thereafter until completion of the

fragmentation burst.

End of Lecture