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 • Define the Request-to-Send / Clear-to-Send transmission protocol • Explain the effects of fragmentation on a network Coypright 2005 All Rights Reserved

Communication Modes • Wireless LANs vs. Ethernet • Joining a network • Passive scanning • Active scanning • Distributed Coordination Function (DCF) • Point Coordination Function (PCF) Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

2 2 6 6 6 2 6 0-2312 4 • Frame Duration Rec Xmit Dest Seq Src Frame FCS • Control ID Addr Addr 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. Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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) Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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) Coypright 2005 All Rights Reserved

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) Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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 • FHSS 50 uS • DSSS 20 us • Infrared 8 uS • PIFS = SIFS + 1 Slot Time • DIFS = PIFS + 1 slot Time Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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 Coypright 2005 All Rights Reserved

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). Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

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. Coypright 2005 All Rights Reserved

MAC & Physical Layers (1 September, 2006)