IEEE 802.11 Standard Why we study this standard: • Showcase issues with WIRELESS and MOBILITY • As a case study to tell you WHY • MAC layer spec. • Wireless channel access • mobility support • HOW to apply ADAPTION principle in one case study?
How popular is 802.11? Growth of 802.11 devices Source : Business Online
802.11 Architecture Infrastructure mode and ad-hoc mode
802.11 Top Goal • Act as wireless Ethernet ! • It is all about WIRELESS DATA, DATA, DATA • Not the same as voice, which is #1 goal in cell phone service • Other goals are also there, but secondary • Voice support, security, …
802.11 Protocol Entities • MAC entity • basic access mechanism • fragmentation & encryption • MAC layer management entity • synchronization • power management • roaming • Physical layer convergence protocol (PLCP) • PHY-specific, common PHY SAP support • provides carrier sense • Physical medium dependent sublayer (PMD) • modulation & coding • PHY layer management • channel tuning & PHY MIB MAC Sublayer MAC layer Management PLCP sublayer PHY layer Management PMD sublayer
PHY spec • Infrared PHY • diffuse infrared • 1 and 2Mbps • Frequency hopping PHY • Direct Sequence PHY • CCA: how to sense a channel is clear: • energy level is above a threshold • can detect a signal • use both
Direct Sequence Spread Spectrum • Spreading factor = code bits/data bit, 10-100 commercial (min 10 by FCC). • Signal bandwidth > 10*data bandwidth • code sequence synchronization • correlation between codes -> interference -: orthogonal • 2.4Ghz band, 1,2Mbps; DBPSK, DQPSK; 11 chip barker sequence
802.11b Frequency Channels • In US, the 2.4ISM band is from 2400MHz to 2483.5MHz • Divided up to 11 “channels” from 2412~2462MHz, spaced 5MHz apart • Each 802.11b channel is 22MHz • Channel 1: centered at 2412MHz, 2400~2423MHz • Channel 2: centered at 2417MHz, • Channel 6: centered at 2437MHz, 2426~2448MHz • Channel 11: centered at 2462MHz, 2451~2473MHz • 3 channels (e.g., Channels 1, 6, 11) are safe to use simultaneously • 3MHz of buffer zone between channels
1st Point for Wireless Networks: The network is the Channel! • Experiences show that the key difference from the wired network is the wireless channel • Wireless channel has very different characteristics from the wired channel!
Wireless Channel Characteristics • Radio propagation • Multipath, fade, attenuation, interference & capture • Received power is inversely proportional to the distance: distance-power gradient • Free space: factor 2 • Inbuilding corridors or large open indoor areas: <2 • Metal buildings: factor 6 • Recommended simulation factors: 2~3 for residential areas, offices and manufacturing floors; 4 for urban radio communications
Wireless Channel Features • Wireless transmission is error prone • Wireless error and contention are location dependent • Wireless channel capacity is also location dependent
Question: How to Design Wireless MAC in 802.11? • Goal: support DATA • Condition: single shared, wireless channel in an ad-hoc setting • Why not use CSMA/CD MAC for wired Ethernet???
Start from a very simple model • Wireless PHY model (simple enough to start): • A single shared physical channel among users • Omni-directional antenna, limited transmission range • Same transmission rate for all users • A node cannot transmit & receive simultaneously • Carrier sensing (no detection of the channel activity)?? • Energy consumption is not the concern • Wireless MAC: how to address channel access in a wireless environment
What you had at that time • Ethernet 802.2 MAC CSMA/CD • Random multiple access • Carrier sensing to detect other senders sharing the same wire; defer until idle channel • Collision resolution: binary exponential backoff Does it work over wireless ?
New Issues/Challenges • Wireless Channel is the Key! • wireless transmission is spatial and local • sender & receiver: different views of the world • relevant contention is at the receiver side • contention may induce collisions • contention/collision/congestion is location dependent • channel access is a collective behavior from the fairness perspective: the notion of “local” is misnomer
Hidden Station!!! • Hidden Stations: within the range of the intended receiver, but out of range of the transmitter • hidden sender C A B C D Problem: A transmits to B, if C transmits (to D), collision at B Solution: hidden sender C needs to defer (Question: who tells C, A or B?) • hidden receiver C A B C D Problem: A transmits to B, if D xmits to C, C cannot reply. D confuses (4 cases) Solution: D needs to be notified that its receiver C is hidden
Exposed Station!!! • Exposed Stations: within the range of the intended sender, but out of range of the receiver • exposed sender B A B C D Problem: C transmits to D, if B transmits (to A), B cannot hear from A Solution: exposed sender B needs to defer • exposed receiver B A B C D Problem: C transmits to D, if A xmits to B, B cannot hear. A confuses (4 cases) Solution: A needs to be notified that its receiver B is exposed (how can B hears A?)
Summary of hidden and exposed station issues • Receiver’s perception of a clean/collided packet is critical • Hidden/exposed senders need to defer their transmissions • Hidden/exposed receivers need to notify their senders about their status
Collision Avoidance • Basic approach: when a station needs to send, • listens to the channel • if it overhears an ongoing transmission, waits until it completes before re-executing the channel access • otherwise, it initiates a control packet handshake • after successful handshake, starts data transmission • RTS-CTS-Data-ACK sequence • draw the basic handshake sequence • explain why they are necessary • deferral: • exposed sender: defers m+2 slots when sees RTS • hidden sender: defers (m+1) slots when sees CTS • solves hidden/exposed sender!!!
Synchronization in 802.11 • All stations maintain a local timer • Timing synchronization function (TSF) • keeps timers from all stations in synch • AP controls timing in infrastructure networks • timing conveyed by periodic beacons • beacons contain timestamp for the entire BSS • timestamp from beacons to calibrate local clocks • not required to hear every beacon to stay in synch • used for power management • beacons sent at well known intervals • all station timers in BSS are synchronized
Roaming Approach • Station decides that link to its current AP is poor • station uses scanning function to find another AP • station sends Reassociation Request to new AP • if Reassociation Response is successful • then station has roamed to the new AP • else station scans for another AP • if AP accepts Reassociation Request • AP indicates Reassociation to the Distribution System • Distribution System information is updated • normally old AP is notified thru distribution system
Scanning • Scanning required for many functions • finding and joining a network • finding a new AP while roaming • initializing an ad hoc network • 802.11 MAC uses a common mechanism • passive or active scanning • Passive scanning • by listening for Beacons • Action Scanning • probe + response
802.11a/g Standard • 802.11a • PHY layer • 12 nonoverlapping channels in 5GHz band • OFDM • Offers rate up to 54Mbps • MAC • Roughly the same as 802.11b • 802.11g • Backward compatible with 802.11b, operating at 2.4Ghz, fall back to 11Mbps with 802.11b AP • OFDM based
802.11 standard • Show case: • How to address wireless issue: hidden/exposed station? • How to address mobility support? • WHY BIG SUCCESS? • Very SIMPLE design • Simple technology is the BEST!