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MAC & Physical Layers (1 September, 2006). Objectives. Upon completion of this chapter you will be able to:. Explain how a client joins a network Describe the modes of operation wireless LANs use to communicate Explain how wireless LANs avoid collisions on the network

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MAC & Physical Layers (1 September, 2006)

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MAC & Physical Layers(1 September, 2006)

Coypright 2005 All Rights Reserved


Objectives

Upon completion of this chapter you will be able to:

  • Explain how a client joins a network

  • Describe the modes of operation wireless LANs use to communicate

  • Explain how wireless LANs avoid collisions on the network

  • Define the Request-to-Send / Clear-to-Send transmission protocol

  • Explain the effects of fragmentation on a network

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Communication Modes

  • Wireless LANs vs. Ethernet

  • Joining a network

    • Passive scanning

    • Active scanning

  • Distributed Coordination Function (DCF)

  • Point Coordination Function (PCF)

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IEEE 802.3 - Ethernet

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PROCESS DATA

DATA

IEEE 802.3

DSAP

CNTRL

SSAP

LLC-PDU

PROCESS DATA

1

1

1-2

FIELD

TYPE

DESTINATION

ADDRESS

SOURCE

ADDRESS

LLC PDU

LLC-PDU

PREAMBLE

ETHERNET

FCS

0-1500

2

4

8

6

6

IEEE 802.3

CSMA/CD

LLC PDU

DA

LENGTH

SA

LLC-PDU

SFD

PREAMBLE

PAD

FCS

0-1500

2

4

1

7

2/6

2/6

  • THIS FIELD IS NOT PRESENT IN ETHERNET.

  • ETHERNET LENGTH MUST BE >= 64 OCTETS

  • THIS FIELD IS NOT IN ETHERNET.

  • ETHERNET HAS A TYPE FIELD

  • 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

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  • 802.3 Ethernet

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

    • Jumbo Frames are 9000 bytes

    • Fragmented at 1518 bytes by Host or Routers (IPv4).

  • 802.11 Wireless Ethernet

    • Frame size 2346 (3212 for payload)

    • Fragmented by Access Point to 1518 for traversing wired system.

IEEE 802.3 FORMAT

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IEEE 802.11 – Wireless Ethernet

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  • 2 2 6 6 6 2 6 0-2312 4

  • Frame DurationRecXmitDest Seq Src Frame FCS

  • Control ID AddrAddr Addr Cntl Addr Body

Generic 802.11 Frame

  • 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.

  • Frame Body. The higher layer payload transmitted from station to station.

  • Frame Check Sequence (FCS). Used to validate the integrity of the transmitted data.

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  • 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 Frames/Protocols

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  • Management Frames

    • Association Request frame

    • Association Response frame

    • Reassociation Request frame

    • Probe request Frame

    • Probe Response frame

    • Beacon frame

    • ATIM frame

    • Disassociation frame

    • Authentication frame

    • Deauthentication frame

  • Control Frames

    • Request to Send (RTS)

    • Clear to Send (CTS)

    • Acknowledgement (ACK)

    • Power-Save Poll (PS Poll)

    • Contention-Free End (CF End)

    • CF-End + CF ACK

802.11 Frame Types

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  • CSMA/CA

  • The Wireless medium similar to Ethernet is a shared medium. That is, many clients attempt to access (share) the same medium.

    • When many clients share the same medium it is inevitable that two or more will want to transmit at the same time.

    • When this occurs a transmission collision occurs resulting a an error condition

    • 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)

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  • Carrier Sensing

  • Carrier sensing is used to determine if the medium is available.

  • 802.11 employs two types of carrier-sensing functions.

    • Physical Carrier Sensing.

      • Transceiver must receive and transmit simultaneously.

    • Virtual carrier Sensing

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  • Carrier Sensing

  • 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-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)

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    • Interframe Spacing

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    • 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)

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    • 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.

        • All frames exchanged during the PCF mode.

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    • 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.

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    • Interframe Spacing

    • There are four different types of interframe spacing.

    • 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.

      • A contention period immediately follows the DIFS

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    • Interframe Spacing contd

    • IFSDSSSFHSSinfrared

    • SIFS10 uS28 uS7 uS

    • PIFS30 us78 uS15 uS

    • DIFS50 uS128 uS23 uS

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    • Interframe Spacing Relationship

    Contention Window

    • DIFS

    • PIFS

    • Frame Transmission

    • SIFS

    • Busy

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    • Contention Window

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    • 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.

      • The station can transmit provided:

        • (1) the medium is clear and

        • (2) the NAV is zero

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    • Slot Times

    • 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

    • FHSS50 uS

    • DSSS20 us

    • Infrared8 uS

    • PIFS = SIFS + 1 Slot Time

    • DIFS = PIFS + 1 slot Time

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    • The Contention Window

    • Contention Window = 31 Slots

    • 802.11b

    • Initial

    • Frame

    • DIFS

    • Contention Window = 63 Slots

    • 1st Retrans

    • Frame

    • DIFS

    • Contention Window = 127 Slots

    • 2nd Retrans

    • Frame

    • DIFS

    • 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.

      • The station with the lowest random number (slot) accesses first.

      • The number of slots will always be 1 less than the power of 2.

        • 25-1, 26-1, 27-1, etc

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    • 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

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    • DSSS Contention Window Size (802.11b)

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    • CSMA/CA

    • 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.

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    • Coordination Function

    • Distributed CoordinationFunction (DCF) defines how stations contend for the wireless medium (contention based access).

      • DCF is the implementation of CSMA/CA and encompasses such things as IFS, CCA, Contention window, etc.

      • DCF refers to the fact that the transmission coordination of each station is distributed among all stations.

    • 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 control (manage) when each station will transmit.

      • The AP does this by polling each station.

      • No AP is known to implement this function

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    • 802.11e

    • - QoS-

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    • 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.

      • Few if any vendors implemented PCF.

      • In addition, PCF was not designed to give priority to different application coming from the same MAC address.

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    • 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, respectively:

      • an Enhanced Distributed Channel Access Function (EDCAF) for DCF or DCF plus QoS

      • And a Hybrid Coordination Function (HCF) or PCF plus QoS

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    • 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 corresponds to the smallest AIFS wait period, transmits data.

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    • HCF

    • The Hybrid Coordination Function (HCF) polls stations during 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.

    • Since PCF has not been widely used, this second enhancement has received lower interest levels than Enhanced DCF, although the two can work together.

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    • RTS/CTS

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    • Carrier Sensing Review

    • 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-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.

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    • Carrier Sensing Review Contd

    • 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

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    • RTS/CTS

    • 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 transmission.

    • After this exchange the wireless medium may be used by any station after the Distributed Interframe Space (DIFS).

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    RTS/CTS Handshaking

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

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    RTS/CTS Process

    Station

    Access Point

    • SIFS - Short Interface Frame Space

    • DIFS - Distributed Interface Frame

    • A client transmits an RTS frame to the AP.

    • The receiving AP respond with a CTS frame containing a shorter duration field. This value is used to set the NAV timer by the other stations.

    • After this exchange all clients in the BSS then contend based upon the contention window after the DIFS.

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    • RTS/CTS Issues

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

    • Because it decreases throughput RTS/CTS is normally 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.

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    • Fragmentation

    • and

    • Reassembly

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    • Fragmentation and Reassembly

    • Fragmentation, breaking larger packets into smaller size 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.

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    Fragmentation

    • 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.

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    • 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.

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    End of Lecture

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