IEEE 802.11 Wireless Local Area Network (WLAN) Lecture 3 (Part 1) IEEE 802.11 Wireless Local Area Network (WLAN)

Outline 802.11 basics 802.11 Protocol stack 802.11 Network architecture 802.11 MAC protocol: DCF 802.11 MAC protocol: PCF

802.11 Basics

IEEE 802.11: basics Basic standard first ratified in 1997 Primary (original) function: Wireless replacement for Ethernet Frequency band: Unlicensed band 2.4 GHz: 11 channels, 3 of which are non-overlapping 5 GHz: 12 non-overlapping channels Basic data rate 1, 2, 5.5, 11, . . ., 54Mbps Range Indoor: 20 ~ 25 meters Outdoor: 50~100 meters Specify protocols for MAC and PHY layers only 802.11 has defined many new amendments PHY: 802.11b, 802.11a, 802.11g, … MAC: 802.11e, 802.11i, … Application Enterprise and campus networking Ad hoc networking Public access in “hot spots” Home networking

IEEE 802.11: basics 802.11 802.11b 802.11a 802.11g 802.11n Standard Freq Supported Rate Number of Channels Published Market Introduction 802.11 2.4GHz 1, 2 11 1997 N/A 802.11b 1, 2, 5.5, 11 1999 802.11a 5GHz 6, 9, 12, 18, 24, 36, 48, 54 12 2002 802.11g 1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54 2003 802.11n 2.4/5GHz 100Mbps above MAC SAP Expected 2007 Pre-N 2005 802.11 802.11b 802.11a 802.11g 802.11n

802.11 Protocol Stack

Protocol Stacks IEEE 802.11

Protocol Stacks: More Details Data Plane Physical Layer (PHY) Physical Medium Dependent (PMD) Modulation Coding Physical Layer Convergence Protocol (PLCP) Clear channel assessment signal (carrier sense) (CCA) Medium Access Control (MAC) Access mechanisms Fragmentation Encryption clear channel assessment (CCA) function: That logical function in the physical layer (PHY) that determines the current state of use of the wireless medium (WM). Either “busy” or “idle”.

Protocol Stacks: More Details Management Plane PHY Management Channel selection, PHY-MIB MAC Management Power management Determine the characteristics of the available BSS Synchronization Authentication Association

802.11 Network Architecture

Network Architecture Two configurations Infrastructure basic service set Independent basic service set: ad hoc mode

Service Set Basic Service Set (BSS) Extended Service Set (ESS) Basic building block of an IEEE 802.11 WLAN A set of stations controlled by a single coordination function Extended Service Set (ESS) Wireless network of arbitrary size and complexity, consists of a set of BSSs interconnected by DS. All stations of the ESS appear to be in a single MAC layer APs communicate with each other to forward traffic through wired or wireless network Station mobility within an ESS is invisible to the higher layers

Distributed Coordination Function (DCF) 802.11 MAC Protocol Distributed Coordination Function (DCF)

MAC Protocol: Overview DCF: Distributed Coordination Function Mandatory Widely deployed Contention-based random access Using CSMA/CA to access the channel Collision avoidance via binary exponential back-off mechanism ACK packet for acknowledgements For best-effort traffic PCF: Point Coordination Function Optional Almost never used, if ever implemented at all. Contention-free Access point polls stations according to a list For delay-sensitive application DCF PCF MAC Physical Layer (PHY)

MAC Protocol: Overview Superframe structure Consists of both contention period and contention-free period These two periods are of variable length. Their duration depends on the traffic load at the AP and at the mobile hosts. DCF is used in contention period PCF is used in contention-free period PCF has higher access priority than DCF Each superframe is started by a Beacon Each contention-free period is terminated by a CF-End message transmitted by the access point (AP). PCF Superframe Contention Free Period (CFP) Contention Period (CP) CP CFP repetition interval Beacon CFP

Contention Period Protocol DCF: CSMA/CA Algorithm Station which is ready to send starts sensing the channel If the channel is free for the duration of a DCF Inter-Frame Space (DIFS), station transmits immediately. If the channel is busy, the station has to wait for a free DIFS, then the station must additionally wait for a random backoff time (collision avoidance, multiple of slot-time) before transmission The receiving station has to acknowledge the reception of the packet after waiting for a SIFS period, if the packet is received correctly (CRC). If no ACK is received after a timeout period at transmitting station, it reschedules a new transmission time for the packet, with a round of backoff When the number of retransmissions exceeds RetryLimit (e.g. 6), the packet is dropped at the transmitting station. Assume the upper layer retransmission mechanism will take care of this dropped packet. Carrier Sense based on Clear Channel Assessment (CCA)

Inter-frame spaces IEEE 802.11 has defined different inter frame spaces SIFS (Short Inter Frame Spacing) Highest priority, for ACK, CTS, Polling response PIFS (PCF IFS) PIFS = SIFS + slot time Medium priority, for time-bounded service using PCF DIFS (DCF IFS) DIFS = SIFS + 2 x slot time Lowest priority, for asynchronous data service EIFS (Extended IFS) SIFS + ACK_Transmission_Time + DIFS Used after an erroneous frame reception SIFS < PIFS < DIFS < EIFS Values of IFS and slot time are PHY dependent. 802.11b (DSSS) Slot time = 20us, SIFS = 10us 802.11a Slot time = 9us, SIFS = 16us

Basic DCF: An Illustration Random backoff T0 DIFS Resume counting down Channel Busy SIFS Successful Transmission STA 1 Slot Time PIFS DIFS Random backoff T0 Contention window from [1, 2mCWMin[AC]] DIFS Successful Transmission SIFS Channel Busy STA 2 Slot Time PIFS Contention Window DIFS Randomly choose a backoff window size and decrement backoff counter as long as the medium stays idle Defer Access

Basic DCF: An Illustration Collision/Erroneous reception Successful Transmission Transmitting Station Transmitting Station SIFS SIFS Transmission DIFS Transmission DIFS ACK Timeout Receiving Station Receiving Station ACK

Exponential Backoff: More Detail To begin the backoff procedure: Choose a random number in [0, CWMin-1] from a uniform distribution Listen to determine if the channel is busy for each time slot Decrement backoff time by one slot if channel is idle Suspend backoff procedure if channel is busy in a time slot Resume backoff when the channel becomes idle again. When the backoff counter value becomes 0, STA starts transmission. When experiencing collision Choose a new random number according to uniform distribution within interval [0, min{ 2ixCWMin-1, CWMax-1}] Repeat the steps described above for backoff. When you hit the ceiling: retransmission time = RetryLimit (e.g. 6) Drop the packets Most of the time, upper layer protocol will take care of the packet drop and initiate retransmission.

Backoff: An Illustration Evolution of contention window (CW) Increases after each failure: [0, 31], [0, 63], [0, 127], [0, 255], [0, 511], [0, 1023], then gives up Resets to 31 after each successful transmission Channel idle Resume backoff Generate a random number from [0, CWMin-1] and begin backoff Channel busy, suspend backoff counter Channel busy, suspend backoff counter DIFS DIFS DIFS DIFS DIFS STA A Success Success defer Channel idle Resume backoff defer STA B Collision defer Channel idle Resume backoff Generate a random number from [0, 2CWMin-1] and begin backoff defer defer STA C Success defer defer defer STA D Channel busy, suspend backoff counter

Hidden Terminal Problem A and C are two STAs far away from each other. A sends to B, C cannot hear A C wants to send to B If use CSMA/CA: C senses a “free” medium, thus C sends to A Collision at B, but A cannot detect collision Therefore, A is “hidden” for C B A C

Exposed Terminal Problem B sends to A, C wants to send to D If use CSMA/CA C senses an “in-use” medium, thus C waits But A is outside the radio range of C, therefore waiting is not necessary Therefore, C is “exposed” to B B A C D

Solution: DCF with RTS/CTS Request To Send/Clear To Send (RTS/CTS) Address the hidden terminal problem But can’t tackle exposed terminal problem Detailed algorithm When a station wants to send a packet, it first sends a RTS. The receiving station responds with a CTS. Stations that can hear the RTS or the CTS then mark that the medium will be busy for the duration of the request (indicated by Duration ID in the RTS and CTS). Stations will adjust their Network Allocation Vector (NAV) Time that must elapse before a station can sense channel for idle status This is called virtual carrier sensing. RTS/CTS are smaller than long packets that can collide

RTS/CTS: Illustration Usually RTS/CTS mechanism is disabled in the product. When should it be enabled? When data frame is long enough to justify the overhead caused by the exchange of RTS/CTS messages. RTS threshold is adjustable in the WLAN card. NAV RTS STA 1 New random backoff Random backoff DIFS SIFS STA 3 DATA STA 2 STA 4 CTS ACK Defer Access Backoff after defer

Broadcast/Multicast If a packet should be received by all the neighbors, it is a broadcast packet. If a packet should be received by some of the neighbors, it is a multicast packet. For broadcast/multicast, we can not use RTS/CTS mechanism. Why? Broadcast Multicast

MAC Layer Frame Format MAC Header 2 2 6 6 6 2 6 0 ~ 2312 4 Bits 2 2 4 Control Duration/ID Address 1 2 3 Sequence 4 Body FCS 2 2 6 6 6 2 6 0 ~ 2312 4 Bytes Protocol Version Type Sub-type ToDS FromDS More Frag Retry Power Mngmt More Data WEP Order Bits 2 2 4 1 1 1 1 1 1 1 1

MAC Layer Frame Format “Address 1” field always holds the receiver address of the intended receiver. “Address 2” field always holds the address of the station that is transmitting the frame. Scenario To DS From DS Addr 1 Addr 2 Addr 3 Addr 4 IBSS (Ad Hoc network) DA SA BSSID N/A Infrastructure network AP to STA 1 STA to AP AP to AP RA TA

Homework Why do we need 4 MAC address fields, instead of only 2?

MAC Layer Frame Format Protocol version: 0x00 for current version Type + Subtype: identify the function of the frame. Data Control Management More Frag: if “0x01”, the station has more fragment(s) to send Retry: “0x01” implies that this is a retransmission Power Management: “0x01” Power saving mode “0x00” Active mode More data: “0x01” indicates to a station in power saving mode that more MSDUs are buffered at the AP for that station. WEP: “0x01” implies that WEP is used in the frame body

Data Frame Format Data frame format ACK frame format The “receiver address” field in the ACK is directly copied from the “Address 2” field of the corresponding received data frame Frame Control Duration/ID Address 1 2 3 Sequence 4 Body FCS Frame Control Duration/ID Receiver Address (RA) FCS

RTS/CTS Frame Format RTS CTS RA: Address of the STA which is the intended immediate recipient of the frame. TA: Address of the STA which is transmitting the frame. Duration: The time value, in microseconds, required to transmit the pending frame, plus 1 CTS frame, plus 1 ACK frame, plus 3 SIFS intervals. CTS Identical to the frame format of ACK RA: copied from TA field of corresponding RTS Duration: The duration field in corresponding RTS, minus 1 CTS time, minus 1 SIF interval. Frame Control Duration/ID Receiver Address (RA) FCS Transmitter Address (TA) Frame Control Duration/ID Receiver Address (RA) FCS

Performance Metrics Throughput Delay Receiver Transmitter Number of payload bits correctly received at the intended recipient within a unit time. Excluding the PHY and MAC header. Delay Queueing delay. Channel access delay. Transmitter from LLC MSDU T0 Queueing Delay (T1 – T0) Buffer to LLC T3 T1 Media Access Delay (T2 – T1) Defer, Backoff, Transmit, Retransmit Buffer T2 Wireless Channel . . . PPDU PPDU Throughput [1] MSDU: MAC Service Data Unit [2] PPDU: PHY Protocol Data Unit

Fragmentation Motivation Penalty Wireless channel is lossy Longer frame size yields higher packet error rate Penalty For each frame, physical and MAC layer headers much be appended to each frame. This causes additional overhead. So, it is a tradeoff between packet error rate and the overhead.

Fragmentation The length of a fragment MSDU shall always be an even number of octets, except the last fragment. The length of a fragment shall never exceed aFragmentationThreshold unless WEP is invoked. aFragmentationThreshold is a MIB variable MSDU Fragment 1 Fragment 2 Fragment 3 MAC Hdr Frame Body CRC MAC Hdr Frame Body CRC MAC Hdr Frame Body CRC

Fragmentation: More Detail Sequence Number Each MSDU transmitted by a STA is assigned a Sequence Number from a single modulo 4096 counter, starting from 0 and incrementing by 1 for each MSDU Each fragment of an MSDU (and its retransmission) contains the same assigned Sequence Number Sequence number remains the same for all retransmissions of a data unit Fragment Number Indicate the number of each fragment of MSDU Retransmission has the same fragment number 4 bits 12 bits Frame Control Duration/ID Address 1 2 3 Sequence 4 Body FCS Fragment Number Sequence Number

Fragmentation Transmission Fragment burst for basic access scheme DIFS Source Backoff window SIFS SIFS SIFS SIFS SIFS Frag 0 Frag 1 Frag 2 ACK0 ACK1 ACK2 Destination DIFS Fragment burst for RTS/CTS scheme NAV(RTS) NAV(Frag 0) NAV(Frag 1) NAV(CTS) NAV(ACK 0) NAV(ACK 1) Source SIFS SIFS SIFS SIFS SIFS SIFS SIFS Frag 0 Frag 1 Frag 2 RTS Destination ACK0 ACK1 ACK2 CTS

Point Coordination Function (PCF) 802.11 MAC Protocol Point Coordination Function (PCF)

PCF: Illustration Delay (due to a busy medium) CFP repetition interval Contention Free Period (CFP) repetition interval: Superframe NAV NAV Contention Free Period Contention Period (CP) Foreshortened CF Period CP DCF DCF B PCF B PCF Channel Busy STA 1 (AP) PIFS SIFS SIFS SIFS B Data + CF-Poll Data + CF-Poll CF-END Data + CF-ACK CF-ACK CFP: Polling only SIFS SIFS CP CP: Listen before talk

References 1. Raphael Rom, Moshe Sidi, “Multiple Access Protocols – Performance and Analysis”, Springer-Verlag 1989 2. Anurag Kumar, D. Manjunath, Joy Kuri, “Communication Networking – An analytical approach”, Elsevier 2004 3. Giuseppe Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, JSAC, March 2000 4. Z. Hadzi-Velkov, B. Spasenovski, “Saturation Throughput – Delay analysis of IEEE 802.11 DCF in Fading Channel”, ICC 2003