Introduction to Wireless Networking ECE 401WN Spring 2009

Transcription

1 Introduction to Wireless Networking ECE 401WN Spring 2009 Lecture 3: Wireless LANs and IEEE Part I Now we study a significant new area of wireless communications over the past 10 years Wireless Local Area Networks. I. Overview Chapter 13 Wireless LAN Technology WLANs are an indispensable adjunct to traditional wired LANs. Satisfy requirements for Mobility Relocation Ad hoc networking Coverage of locations difficult to wire. Until recently, however, WLANs were little used. High prices Low data rates Occupational safety concerns. Licensing requirements. Products were produced since the late 1980s. 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 happened to any great extent. - Bit rates were still much lower. - Coverage could still be uneven. WLANs still mainly serve as alternatives or enhancements to networks already present. Lecture 3, Page 1 of 20

2 Application Areas of WLANs LAN Extension WLANs are useful in special environments. What are examples? Buildings with large open spaces. Manufacturing plants. Stock exchange trading floors. Warehouses Small offices Home offices Outdoors (courtyards, parks, etc.) A wired LAN will already likely exist. - But wireless extends the range and mobility. - Hence, the term LAN extension. Lecture 3, Page 2 of 20

3 Sample single-cell WLAN configuration Figure 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 Figure 13.2 Lecture 3, Page 3 of 20

4 - Multiple control modules are connected by a wired LAN. - What challenges are involved in operating with such a multi-cm configuration? Connecting to the best CM, even if several are in range. Handing off to another when moving. Making sure CMs do not interfere. Load balancing avoid one CM overloaded, use bandwidth most wisely. Cross Building Interconnect Connecting LANs in different buildings. Can be accomplished with a point-to-point wireless link between the two buildings. Not a LAN per se, but closely related. Nomadic Access Laptops can move freely. Lecture 3, Page 4 of 20

5 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. - 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. - Example: A set of computers that talk to each other during a meeting. - 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 Throughput Make as efficient use as possible of the wireless medium. Provide data rates fast enough to not hinder users. - Make the network invisible. Lecture 3, Page 5 of 20

6 Number of nodes May need to support hundreds of nodes across multiple cells. Difficult scenario: Many users in a lecture hall. Connection to a backbone LAN Service area Diameter of 100 to 300 meters Battery power consumption Do not require battery-powered workstations to constantly transmit. - No monitoring of control modules. - No frequent handshakes or keepalives. Allow workstations to not use transmission power when not using the network. 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. Collocated network operation Allow two or more WLANs to operate in the same area. License-free operation It is preferable to not need to buy licenses to operate. Handoff/roaming Enable mobile stations to move from one cell to another. Dynamic configuration Permit addition, deletion, and relocation of end systems. In a dynamic or automatic way. Without disruption to end users. Lecture 3, Page 6 of 20

7 The following diagram provides a useful illustration to compare wired, wireless, and mobile data networks. Wireless LAN Physical Layer Technology Three physical layer approaches are discussed in Chapter 13, but we will not investigate them in details, since only one method is commonly used. Infrared LANs Use infrared light that does penetrate walls. - Confined to a single room. High bit rates possible. But sunlight and indoor lighting can cause substantial interference. Narrowband microwave Use licensed frequencies. Guarantees no interference. Spread Spectrum This is the one that is used. 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. Lecture 3, Page 7 of 20

8 II. The IEEE 802 Architecture Chapter 14 Wi-Fi and the IEEE Wireless LAN Standard The Institute of Electrical and Electronics Engineers A technical, professional, and student society. Publishes many journals and magazines. Also has developed a few technical standards. Most notably Local Area Network standards. Ethernet (802.3) and others is the working group for Wireless LAN s Created by the IEEE LAN /MAN Standards Committee (LMSC) Started in 1980 Working Groups (those of most interest to us in bold) Higher Layer LAN Protocols Working Group (active) Logical Link Control (LLC) Working Group (inactive) Ethernet Group (active) standard for wired LAN s Token Bus Working Group (disbanded) Token Ring Working Group (inactive) Metropolitan Area Network (MAN) Working Group (disbanded) BroadBand Technical Adv. Group (BBTAG) (disbanded) Fiber Optic Technical Adv. Group (FOTAG) (disbanded) Integrated Services LAN (ISLAN) Working Group (disbanded) Standard for Interoperable LAN Security (SILS) Working Group (disbanded) ** Wireless LAN (WLAN) Working Group (active) Demand Priority Working Group (disbanded) Cable Modem Working Group Working Group (disbanded) ** 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 ** Broadband Wireless Access (BBWA) Working Group (active) Wireless Metropolitan Area Networks Lecture 3, Page 8 of 20

9 Resilient Packet Ring (RPR) (active) 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 Radio Regulatory Technical Advisory Group (active) Monitoring of, and active participation in, ongoing radio regulatory activities, at both the national and international levels 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 Mobile Wireless Access Working Group (active) 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 Media Independent Handoff Working Group (active) Enable handover and interoperability between heterogeneous network types including both 802 and non 802 networks Wireless Regional Area Networks (active) Develop a standard for a cognitive radio-based PHY/MAC/air_interface for use by license-exempt devices on a non-interfering basis in spectrum that is allocated to the TV Broadcast Service. Architecture Defines layering of protocols that organize basic functions. Open Standards Interconnection Model (OSI) Developed by the International Organization for Standardization (ISO) Lecture 3, Page 9 of 20

10 Lecture 3, Page 10 of 20

11 Another view The IEEE standards focus on the lower layers and subdivide them. Lecture 3, Page 11 of 20

12 Layers 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 - Specifics of infrared, spread spectrum, etc. 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. 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. - See Section 8.4 (starting on page 222) for more details. Performs error control - Detects errors - Through error control bits that indicate a packet has errors. - Corrects errors - Or retransmits frames 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. Lecture 3, Page 12 of 20

13 - 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. Lecture 3, Page 13 of 20

14 MAC Frame Format There are several 802 MAC protocols But all MAC formats follow a format close to the following. MAC Control specific control information for a particular MAC protocol. Destination MAC address Destination physical attachment point on the LAN. Source MAC address Source physical attachment. Data Body of the MAC frame. 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 Optionally keeps track of unsuccessful frames and retransmits them. Not all LAN protocols do this. Lecture 3, Page 14 of 20

15 Special characteristics of the IEEE 802 LLC Must support the multiaccess, shared-medium nature of a link. Takes care of some of the functions of a typical MAC layer. Defines source and destination LLC users. - Called service access points. - See textbook for more details (pp ). III Architecture and Services Working Group Started in 1990 To develop MAC protocol and physical medium specifications. And use existing 802 LLC functions. Initial interest was to use unlicensed frequencies. Called the ISM (Industrial, Scientific, and Medical) bands in U.S. The Working Group has an ever-expanding list of standards. Lecture 3, Page 15 of 20

16 Some in the above table are now finished and others have been added. The latest list of active groups is available from the web site n, Investigating the possibility of improvements to the standard to provide high throughput (>100Mbps) p, Wireless Access in the Vehicular Environment - enhancements to required to support Intelligent Transportation Systems (ITS) applications - communication between vehicles and the roadside and between vehicles while operating at speeds up to a minimum of 200 km/h for communication ranges up to 1000 meters r, Fast Roaming Fast Handoff s, Mesh Networking T, Wireless Recommended Practice for Evaluation of Wireless Performance u, Interworking with External Networks v, Wireless Network Management w, Protected Management Frames y, Contention Based Protocol Application of based systems to the MHz band in the USA (FCC 05-56) z, Direct Link Setup aa, Video Transport Stream Robust audio video streaming, while maintaining co-existence with other types of traffic. Here is a table of terminology. Lecture 3, Page 16 of 20

17 Wi-Fi Alliance The first standard to gain industry acceptance was b. 2.4 GHz, up to 11 Mbps. There was concern whether products would successfully interoperate Linksys Access Point with a Cisco PCMCIA card? Wireless Ethernet Compatibility Alliance (WECA) formed in Renamed the Wi-Fi (Wireless Fidelity) Alliance. Certifies interoperability for b products. Certified products are called Wi-Fi. - Over 100 products certified. Certification also for g products g is at 2.4 GHz, up to 54 Mbps. The Wi-Fi Alliance is concerned with markets for WLANs in enterprises, homes, and public hot spots. The Architecture See Figure 14.4 Lecture 3, Page 17 of 20

18 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. BSSs connect through a Distribution System (DS). Can be a switch, a wired network, or a wireless network. 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. If there is no connection to other BSSs, the BSS is called an. Stations can communicate directly using ad hoc networking approches. No AP is necessary. 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. Lecture 3, Page 18 of 20

19 IEEE Services Nine services are provided to give functionality equivalent to wired LANs. Two ways the services are categorized. 1. Provided by a station or by provided by the distribution system. 2. LAN access and security versus delivery of MAC packets (called MAC Service Data Units MSDUs). MSDU Delivery Service MSDUs are the blocks of data passed down from the MAC user (like the LLC layer) 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. 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). Lecture 3, Page 19 of 20

20 - 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. Services for Access and Privacy WLANs cannot rely on a physical wired connection for security. - Valid wired LAN stations must be physically connected. - 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 by stations and APs. - Privacy - Contents of messages cannot be read by unintended recipients. - Encryption can optionally be used. Next lecture: Details on the , its MAC layer, and its security features. Lecture 3, Page 20 of 20