Wireless Local Area Networks (WLANs) based on IEEE 802.11 Standard aka Wi-Fi 1 Wireless Local Area Networks (WLANs) • Offers clear benefits over wired LANs: – Avoid the inconvenience and cost of running cables – Flexible network connectivity: get connectivity where desired instead of having to connect at locations wired network allows • IEEE 802.11 has become the de facto standard for WLANs – Survived the competition from other proposed WLAN technologies and standards (e.g., HiperLAN) • Now WLANs are synonymous with 802.11 based WLANs (also called Wi-Fi) – Wi-Fi is to wireless LANs as Ethernet is to wired LANs 2 The Success of Wi-Fi Source: ABI Research • Contributing factors: – Operation in license exempt (unlicensed) spectrum bands è no barrier to deployment – Continually evolving standards aimed at higher data rates and enhanced functionality 3 – Low cost commodity hardware from reaching economies of scale (Partial) History of 802.11 WLANs • US Federal Communications Commission (FCC) allowed unlicensed use of ISM bands 1985 • First version of 802.11 standard published 1997 • 802.11b and 802.11a amendments supporting higher data rates up to 54Mbps 1999 • Wi-Fi Alliance formed to certify interoperability between IEEE 802.11 devices from different manufacturers • 802.11g amendment using 802.11a OFDM PHY and supporting up to 54Mbps data rates 2003 • 802.11-2007 (a new release of the standard) that includes amendments a, b, d, e, g, h, i & j 2007 • 802.11n amendment with high throughput improvements via MIMO, channel bonding and frame aggregation 2009 • 802.11-2012 (a new release of the standard) that includes amendments k, n, p, r, s, u, v, w, y and z 2012 • 802.11ad amendment to enable very high throughput operation in frequencies around 60GHz è 802.11ay • 2013: 802.11ac amendment with very high throughput enhancements including multi-user MIMO è 802.11ax • 2014: 802.11af amendment supporting operation in Television White Spaces (TVWS) 2013 - • 2016: 802.11ah for sub 1GHz operation in license-excem4pt bands • 802.11-2016 (a new release of the standard) that includes amendments ac, ad, af, aa and ae Useful 802.11 Links • Get latest 802.11 standards via: – http://ieeexplore.ieee.org/browse/standards/get- program/page/series?id=68 • Official IEEE 802.11 working group project timelines: – http://grouper.ieee.org/groups/802/11/Reports/802.11_ Timelines.htm 5 IEEE 802.11 Standard Overview • Defines multiple physical layers (PHYs) and a common medium access control (MAC) layer for WLANs • Member of IEEE 802 family of local area networking (LAN) and metropolitan area networking (MAN) standards – Inherits the 802 reference model and 48-bit universal addressing scheme 6 802.11 in the TCP/IP Internet Protocol Stack application transport network 802.2 logical link control (LLC) link 802.11 MAC 802.11 PHYs physical 7 A Typical Implementation of 802.11 Network Interface Frequency conversion Amplifier (De)modulation Physical carrier sens8ing Shielding 802.11 Medium Access Control (MAC) Overview • 802.11 adopted the distributed MAC protocol based on carrier sense multiple access (CSMA) from Ethernet (the wired counterpart of 802.11) – listen/sense medium (carrier) and transmit if idle • Ethernet uses a CSMA variant called CSMA with collision detection (CSMA/CD) – Each Ethernet device can receive its own transmission and detect collisions – Upon collision detection: stop transmission è random backoff è retry 9 802.11 Medium Access Control (MAC) Overview • 802.11 uses a different variant called CSMA with collision avoidance (CSMA/CA) – Coz half-duplex wireless interfaces do not allow receiving one’s own transmission – Moreover, collisions occur on receiver side • Idea: be conservative in attempting a transmission – 802.11 devices on finding a busy medium defer by different randomly chosen periods (counting down only when medium is idle) 10