WirelessNets_Lect_03_WLAN_Part1_SP2016(1)

Lecture 3, Page 1 of 20

Introduction to Wireless Networking ECE 477

Spring 2016

Lecture 3: Wireless LANs and IEEE 802.11 – Part I

Now we study a significant new area of wireless communications over the past 10 years – Wireless Local Area Networks.

Chapter 11 – Wireless LAN Technology and the IEEE 802.11 Wireless LAN Standard

I. Overview

♦ WLANs are an indispensable adjunct to traditional wired LANs.

Ø Satisfy requirements for • Mobility • Easy workstation relocation • Ad hoc networking • Coverage of locations difficult to wire.

Ø Until a few years ago, however, WLANs were little used. • High prices • Low data rates • Occupational safety concerns. • Licensing requirements.

Ø Products were produced since the late 1980s, however. • To be substitutes for traditional wired LANs.

- Less costly installation than LAN cabling. - Ease of relocation.

• But architects designed new buildings with extensive LAN wiring already built in.

• Buildings already wired for LANs had little reason to switch to wireless.

• So, use of WLANs to replace wired LANs did not happen to any great extent, until laptops and wireless devices proliferated.

- Bit rates were still much lower. - Coverage could still be uneven.

Ø In office buildings, WLANs still mainly serve as alternatives or enhancements to networks already present.


♦ Application Areas of WLANs

Ø LAN Extension • In many buildings, a wired LAN will already likely exist.

- But wireless extends the range and mobility. - Hence, the term LAN “extension”.

• WLANs are especially useful in special environments. What are examples?

Buildings with large open spaces. Manufacturing plants. Stock exchange trading floors. Warehouses Small offices Conference Rooms Home offices Outdoors (courtyards, parks, etc.)

• WLANs are now also very useful for smartphones instead of using cellular minutes.


• Sample single-cell WLAN configuration

- A Control Module (CM) acts as an interface to the WLAN. - User Modules (UMs) can be used to connect to other wired

facilities. - The CM connects to a larger Ethernet. - And wirelessly connects workstations and smaller wired LANs. - This is a single-cell WLAN

- All systems are within range of a single CM. • Sample multiple-cell WLAN configuration


- Multiple control modules are connected by a wired LAN. - What challenges are involved in getting optimal performance in

such a multi-CM configuration?

Connecting to the best CM, even if several are in range. Handing off to another when moving without interruption. Making sure CMs do not interfere. Load balancing – avoid one CM overloaded, use bandwidth most wisely.

Ø Nomadic Access • Laptops can move freely.


Ø Ad Hoc Networking

• A peer-to-peer network. - No centralized controller.

• In "ad hoc networks" devices talk to whatever other devices they can talk to.

- From a dictionary: “Ad hoc” = Formed for or concerned with one specific purpose (usually also considered temporary).

- Networks of devices that are all peers and talk to whoever is near enough.

- Examples: - A set of computers that talk to each other during a meeting. - Devices that share files, e-mails, calendars, etc. when in range.

- As devices move, they change their connections with other devices.

- May send data through a sequence of neighbors to reach an end destination.

♦ Wireless LAN Requirements

Ø Throughput • Make as efficient use as possible of the wireless medium. • Provide data rates fast enough to not hinder users.

- Make the network “invisible.”


Ø Number of nodes • May need to support hundreds of nodes across multiple cells. • Difficult scenarios: Many users in a lecture halls, several

smartphones watching videos. Ø Connection to a backbone LAN to the Internet Ø Service area

• Diameter of 100 to 300 meters Ø Battery power consumption

• Do not require battery-powered workstations to constantly transmit. - No constant monitoring of CM’s. - No frequent handshakes or keepalives.

• Allow workstations to not use transmission power when not using the network.

Ø Transmission robustness and security • Prevent problems with interference. • Prevent eavesdropping and many other possible security problems. • Since operated by end-users, make configuration easy.

- So that users do not inadvertently leave security features turned off.

Ø Collocated network operation • Allow two or more WLANs to operate in the same area.

Ø License-free operation • Do not need to buy licenses to operate.

Ø Handoff/roaming • Enable mobile stations to move from one cell to another.

Ø Dynamic configuration • Permit addition, deletion, and relocation of end systems. • In a dynamic or automatic way. • Without disruption to end users.


Ø The following diagram provides a useful illustration to compare wired, wireless, and mobile data networks.

♦ Wireless LAN Physical Layer Technology

Ø Spread Spectrum • Method allowed when using unlicensed frequency bands. • Spreads a signal across a wide bandwidth.

- But not very strong at any one frequency. - Allows many uncoordinated sources to use the same bandwidth.

• CDMA and OFDM are used.


II. The IEEE 802 Architecture

♦ The Institute of Electrical and Electronics Engineers

Ø A technical, professional, and student society. Ø Publishes many journals and magazines. Ø Also has developed a few technical standards.

• Most notably Local Area Network standards. • Ethernet (802.3) and others. • 802.11 is the working group for Wireless LAN’s

♦ Created by the IEEE LAN /MAN Standards Committee (LMSC)

Ø Started in 1980 Working Groups (those of most interest to us in bold)1 802.1 Higher Layer LAN Protocols Working Group (active)

802.2 Logical Link Control (LLC) Working Group (inactive) 802.3 Ethernet Group (active) –standard for wired LAN’s

802.4 Token Bus Working Group (disbanded) 802.5 Token Ring Working Group (inactive) 802.6 Metropolitan Area Network (MAN) Working Group (disbanded) 802.7 BroadBand Technical Adv. Group (BBTAG) (disbanded) 802.8 Fiber Optic Technical Adv. Group (FOTAG) (disbanded) 802.9 Integrated Services LAN (ISLAN) Working Group (disbanded) 802.10 Standard for Interoperable LAN Security (SILS) Working Group

(disbanded) ** 802.11 Wireless LAN (WLAN) Working Group (active)

802.12 Demand Priority Working Group (disbanded) 802.14 Cable Modem Working Group Working Group (disbanded)

** 802.15 Wireless Personal Area Network (WPAN) Working Group (active)

Personal Area Networks or short distance wireless networks for devices such as PCs, Personal Digital Assistants (PDAs), peripherals, cell phones, pagers, and consumer electronics

802.16 Broadband Wireless Access (BBWA) Working Group (active)

Wireless Metropolitan Area Networks

1 http://www.ieee802.org/dots.shtml


802.17 Resilient Packet Ring (RPR) (hibernating) Resilient Packet Ring fiber optic networks in Local, Metropolitan, and Wide Area Networks for resilient and efficient transfer of data packets at rates scalable to many gigabits per second.

802.18 Radio Regulatory Technical Advisory Group (active)

Monitoring of, and active participation in, ongoing radio regulatory activities, at both the national and international levels.

802.19 Coexistence Technical Advisory Group (active)

Define the responsibilities of 802 standards developers to address issues of coexistence with existing standards and other standards under development.

802.20 Mobile Broadband Wireless Access Working Group (hibernating)

Efficient packet based air interface that is optimized for the transport of IP based services. Specification of physical and medium access control layers operating in licensed bands below 3.5 GHz, optimized for IP-data transport, with peak data rates per user in excess of 1 Mbps for various vehicular mobility classes up to 250 Km/h in a MAN environment.

802.21 Media Independent Handoff Working Group (active)

Enable handover and interoperability between heterogeneous network types including both 802 and non 802 networks.

802.22 Wireless Regional Area Networks (active)

Develop a standard for a cognitive radio-based PHY/MAC/airinterface for use by license-exempt devices on a non-interfering basis in spectrum that was allocated to the TV Broadcast Service. 802.23 Emergency Services Working Group (disbanded)

Provide consistent access and data that facilitate compliance to applicable civil authority requirements for citizen-to-authority emergency services capabilities (like E-911) for communications systems that include IEEE 802 networks.

802.24 Vertical Applications Technical Advisory Group (active) This group focuses on application categories that use IEEE 802 technology to act as a liaison with industry and regulatory agencies. Examples of current and potential application categories are: Smart Grid, Machine to Machine (M2M), Internet of Things (IoT), Vehicular Networking.


♦ Architecture

Ø Defines layering of protocols that organize basic functions. Ø Open Standards Interconnection Model (OSI)

• Developed by the International Organization for Standardization (ISO)


Ø Another view

Ø The IEEE standards focus on the lower layers and subdivide them.


♦ Layers

Ø Physical Layer • General functions

- Encoding/decoding of data into signals to be sent over a wireless medium through an antenna.

- Preemble generation/removal - Beginning and ending bits added for synchronization

purposes. - Bit transmission/reception

• Physical medium dependent functionality - Specifics of infrared, spread spectrum, etc.

Ø Medium Access Control • Assembles groups of data bits into frames • Also includes addresses, error correction fields, etc. • Upon reception, disassembles the frame, and checks for errors. • Governs how stations get access to the medium.

- Two options - Random access – anyone can transmit at any time, but if

collisions occur they must try again in prescribed ways. - Controlled access – Give stations particular frequencies, time

slots, etc. Ø Logical Link Control

• Provides interfaces to the network layer. • Performs flow control

- Makes sure a transmitting entity does not overwhelm a receiving entity with data.

- Typically allocates data buffers. - So data is not lost while a station is processing other packets.

- And makes sources adjust sending rates. • Performs error control

- Corrects errors - Or has frames retransmitted


Ø Encapsulation • As data is passed down the protocol stack, each layer may add its

own information. - To the header and maybe the trailer of the packet.

- The IP (Internet Protocol) header corresponds to the ________ layer. - IP is far and away the most prominent protocol.

- The TCP (Transmission Control Protocol) corresponds to the

________ Layer. - TCP is used for data, others are used for audio/video.


♦ MAC Frame Format

Ø There are several 802 MAC protocols • But all MAC formats follow a format close to the following.

Ø MAC Control – specific control information for a particular MAC protocol.

Ø Destination MAC address – Destination physical attachment point on the wired or wireless LAN.

Ø Source MAC address – Source physical attachment. Ø Data – Body of the MAC frame. Ø CRC – Cyclic Redundancy Check field.

• Also called a frame check sequence. • Destination does a computation operation on the received bits, if the

result is different than the CRC, an error has occurred. • MAC just detects errors, LLC takes action.


♦ Logical Link Control

Ø Optionally keeps track of unsuccessful frames and retransmits them. • Not all LAN protocols do this.

Ø Supports the �ulti-access, shared-medium nature of a link. • See textbook for more details (Section 11.2).

III. 802.11 Architecture and Services

♦ 802.11 Working Group

Ø Started in 1990 Ø To develop MAC protocol and physical medium specifications.

• And use existing 802 LLC functions. Ø Initial interest was to use unlicensed frequencies.

• Called the ISM (Industrial, Scientific, and Medical) bands in U.S. Ø The 802.11 Working Group has an ever-expanding list of standards.

Table 11.1 IEEE 802.11 Standards

Standard Date Scope

IEEE 802.11 1997

Medium access control (MAC): One common MAC for WLAN applications

Physical layer: Infrared at 1 and 2 Mbps

Physical layer: 2.4-GHz FHSS at 1 and 2 Mbps

Physical layer: 2.4-GHz DSSS at 1 and 2 Mbps

IEEE 802.11a 1999 Physical layer: 5-GHz OFDM at rates from 6 to 54 Mbps

IEEE 802.11b 1999 Physical layer: 2.4-GHz DSSS at 5.5 and 11 Mbps

IEEE 802.11c 2003 Bridge operation at 802.11 MAC layer

IEEE 802.11d 2001 Physical layer: Extend operation of 802.11 WLANs to new regulatory domains (countries)

IEEE 802.11e 2007 MAC: Enhance to improve quality of service and enhance security mechanisms

IEEE 802.11f 2003 Recommended practices for multivendor access point interoperability

IEEE 802.11g 2003 Physical layer: Extend 802.11b to data rates >20 Mbps

IEEE 802.11h 2003 Physical/MAC: Enhance IEEE 802.11a to add indoor and outdoor channel selection and to improve spectrum and transmit power management


IEEE 802.11i 2007 MAC: Enhance security and authentication mechanisms

IEEE 802.11j 2007 Physical: Enhance IEEE 802.11a to conform to Japanese requirements

IEEE 802.11k 2008 Radio Resource Measurement enhancements to provide interface to higher layers for radio and network measurements

IEEE 802.11m Ongoing This group provides maintenance of the IEEE 802.11 standard by rolling published amendments into revisions of the 802.11 standard.

IEEE 802.11n 2009 Physical/MAC: Enhancements to enable higher throughput

IEEE 802.11p 2010 Wireless Access in Vehicular Environments (WAVE)

IEEE 802.11r 2008 Fast Roaming/Fast BSS Transition

IEEE 802.11s 2011 Mesh Networking

IEEE 802.11T Abandoned Recommended Practice for Evaluation of 802.11 Wireless Performance

IEEE 802.11u 2011 Interworking with External Networks

IEEE 802.11v 2011 Wireless Network Management

IEEE 802.11w 2009 Protected Management Frames

IEEE 802.11y 2008 Contention Based Protocol

IEEE 802.11z 2010 Extensions to Direct Link Setup

IEEE 802.11aa 2012 Video Transport Stream

IEEE 802.11ac Ongoing Very High Throughput <6Ghz

IEEE 802.11ad 2012 Very High Throughput in 60 GHz

IEEE 802.11ae 2012 Prioritization of Management Frames

IEEE 802.11af Ongoing Wireless LAN in the TV White Space

IEEE 802.11ah Ongoing Sub 1GHz for applications that benefit from range extension, such as smart meters.

IEEE 802.11ai Ongoing Fast Initial Link Set-up to reduce time to set up an association

IEEE 802.11aj Ongoing China Milli-Meter Wave (CMMW)

IEEE 802.11ak Ongoing Enhancements For Transit Links Within Bridged Networks

IEEE 802.11aq Ongoing Pre-Association Discovery (PAD) to discover services

IEEE 802.11ax Ongoing High Efficiency WLAN (HEW)

IEEE 802.11ay Ongoing Enhanced Throughput for Operation in License-Exempt Bands above 45 GHz

IEEE 802.11az Ongoing Next Generation Positioning


Ø The latest list of active groups is available from the 802.11 web site

http://www.ieee802.org/11/QuickGuide_IEEE_802_WG_and_Activities.htm • Information is listed under Task Groups.

♦ Wi-Fi Alliance

Ø The first 802.11 standard to gain industry acceptance was 802.11b. • 2.4 GHz, up to 11 Mbps.

Ø There was concern whether products would successfully interoperate • Linksys Access Point with a Cisco laptop card?

Ø Wireless Ethernet Compatibility Alliance (WECA) formed in 1990. • Renamed the Wi-Fi (Wireless Fidelity) Alliance in 2003.

(www.wi-fi.org) Ø Certifies interoperability for 802.11 products.

• Certified products are called Wi-Fi. Ø The Wi-Fi Alliance is concerned with markets for WLANs in

enterprises, homes, and public hot spots.

♦ The 802.11 Architecture


Ø The smallest building block of a WLAN is a ________ __________. • Stations executing the same MAC protocol. • Stations competing for access to the same shared wireless medium. • Two BSSs can overlap geographically.

- A single station can participate in more than one BSS. - Using different frequency bands.

Ø BSSs connect through a Distribution System (DS). • Can be a switch, a wired network, or a wireless network.

Ø The __________ is the bridge and relay point. • Stations do not communicate directly with each other.

- But through the AP. • An AP is part of a station.

- STA1 and STA5 above. Ø If there is no connection to other BSSs, the BSS is called an

_______________. • Stations can communicate directly using ad hoc networking

approaches. • No AP is necessary.

Ø An Extended Service Set (ESS) consists of two or more BSSs connected by a distribution system. • The entire ESS appears as a single logical LAN to the LLC.

Ø Here is a table of 802.11 terminology.

Table 11.2 IEEE 802.11 Terminology Access point (AP) Any entity that has station functionality and provides access to the

distribution system via the wireless medium for associated stations

Basic service set (BSS) A set of stations controlled by a single coordination function Coordination function The logical function that determines when a station operating

within a BSS is permitted to transmit and may be able to receive PDUs

Distribution system (DS) A system used to interconnect a set of BSSs and integrated LANs to create an ESS


Extended service set (ESS)

A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to the LLC layer at any station associated with one of these BSSs

MAC protocol data unit (MPDU)

The unit of data exchanged between two peer MAC entities using the services of the physical layer

MAC service data unit (MSDU)

Information that is delivered as a unit between MAC users

Station Any device that contains an IEEE 802.11 conformant MAC and physical layer

♦ IEEE 802.11 Services

Ø Nine services are provided to give functionality equivalent to wired LANs.

Table 11.3 IEEE 802.11 Services Service Provider Used to support

Association Distribution system MSDU delivery

Authentication Station LAN access and security

Deauthentication Station LAN access and security

Disassociation Distribution system MSDU delivery

Distribution Distribution system MSDU delivery

Integration Distribution system MSDU delivery

MSDU delivery Station MSDU delivery

Privacy Station LAN access and security

Reassocation Distribution system MSDU delivery

Ø Two ways the services are categorized. 1. Provided by every 802.11 station (including APs) or by provided

by the distribution system. 2. LAN access and security versus delivery of MAC packets (called

MAC Service Data Units – MSDUs). Ø MSDU Delivery Service

• MSDUs are the blocks of data passed down to the MAC layer. • This service executes the delivery. • And if the MSDU is too large, it may be broken into smaller frames.

- This process is called ____________.


Ø Services for Association • To transfer MSDUs, stations must be known to the WLAN.

- To know where a destination station is located. • A station must be associated.

- Before it can deliver or accept data. • Types of mobility to be supported.

- No transition – only movement within the range of a BSS. - BSS transition – to another BSS in the same ESS.

- Addressing capabilities must recognize the new location. - Hopefully with a fast, seamless transition (no disruption of

service from users’ viewpoint, on the order of 10’s of msec). - ESS transition – to another ESS.

- Likely will cause a service disruption in this case. • Services

- Association - Associate with an AP. - APs share information with other APs.

- Reassociation - Transfer an association to another AP.

- Disassociation - Hopefully tell AP when leaving. - MAC management facility also protects itself against stations

that disappear without disassociating. Ø Services for Access and Privacy

• WLANs cannot rely on a physical wired connection for security. - WLANs are open to anyone within radio range.

• Services - Authentication/deauthentication

- Establishes the identity of stations to each other. - Several authentication schemes are supported. - And also allows for expansion of the functionality. - The standard does not mandate any authentication scheme. - Whatever is used must be agreed upon by stations and APs.

- Privacy - Contents of messages should not be read by unintended

recipients. - Encryption can optionally be used for this.

Next lecture: Details on 802.11, its MAC layer wireless sharing approach, and its security features.

WirelessNets_Lect_03_WLAN_Part1_SP2016(1)

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