wireless lans and pans - 國立中興大學

Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved

Wireless LANs and PANs

Chapter 15

1


Outline

Introduction Wireless Local Area Networks (WLANs) Enhancement for IEEE 802.11 WLANs Wireless Metropolitan Area Networks (WMANs) using WiMAX and Mesh Networks Mesh Networks Wireless Personal Area Networks (WPANs) ZigBee Summary

2


Data rate

Scope of Various WLAN and WPAN Standards

802.11n*

Power consumption Complexity

802.15.I Bluetooth

802.11a HiperLAN

802.11g*

802.11

WPAN

802.11b

WLAN

* Standard in progress

WMAN

802.16 WiMAX

WMN 802.16*

802.15.4

3


Wireless Local Area Networks (WLANs)

IEEE group published a standard for WLANs named as IEEE 802.11 (now known as IEEE 802.11n) Higher bit rates at 2.4GHz ISM band resulted in high-speed standard called the IEEE 802.11b (popularly known as Wi-Fi) Can be used to have an ad hoc network using peer-to-peer mode, Or, as a client/server wireless configuration (Infrastructure)

Ad hoc Client/server

4


IEEE 802.11

It is the standard for wireless LANs. It specifies MAC procedures and operate in 2.4

GHz range with data rate of 1Mbps or optionally 2Mbps.

User demand for higher bit rates and international availability of 2.4 GHz band has resulted in development of a high speed standard in the same carrier frequency range.

This standard called 802.11b, specifies a PHY layer providing a basic data rate of 11 Mbps and a fall-back rate of 5.5 Mbps.

5


IEEE 802.11

In the ad hoc network mode, as there is no central controller, the wireless access cards use the CSMA/CA protocol to resolve shared access of the channel. In the client/server configuration, many PCs and laptops, physically close to each other (20 to 500 meters), can be linked to a central hub [AP] A larger area can be covered by installing several APs The access points track movement of users and make decisions on whether to allow users to communicate WLAN cards could be operated in continuous aware mode (radio always on) and power saving polling mode (radio in sleep state to extend battery life)

6


Distributed Wireless Network

Station

Access point

Wired network

Access point

Distributed system

Access point

Station

Station Station

7


IEEE 802.11 and variants

IEEE 802.11a With a throughput up to 54Mbps IEEE 802.11a operates on 5GHz It has less interference as compared to IEEE 802.11b/g since 2.4GHz band is heavily used Uses orthogonal frequency-division multiplexing (OFDM) with 52 subcarriers spanning over a 20MHz spectrum

IEEE 802.11b (WiFi) Operates on 2.4GHz band with throughput of up to11Mbps Direct-sequence spread spectrum DSSS on PHY layer

IEEE 802.11g Operates on 2.4G using either DSSS or OFDM Can achieve higher throughput of up to 54Mbps

IEEE 802.11n Multiple-input multiple-output (MIMO) technology Bandwidth can be 40MHz in 2.4GHz and 5GHz

8


Enhancement for IEEE 802.11 WLANs

The keys behind all the above networks are the wireless cards and wireless LAN access points

In an ad hoc network mode, there is no central controller, the wireless access cards use CSMA/CA protocol to resolve shared access

MAC layer access uses one of following methods: distributed coordination function (DCF), point coordination function (PCF), and hybrid coordination function (HCF)

DCF is carrier sense multiple access with collision avoidance (CSMA/CA) and senses the medium before sending frame

9


Enhancement for IEEE 802.11 WLANs

IEEE 802.11e working group has developed enhanced DCF (EDCF) so that the differentiated service could be provided

MSs with shorter AIFS have a higher priority to access channel than stations with longer AIFS

Two EDCF priority schemes: interframe space (IFS) priority scheme and contention window (CW) priority scheme

IFS priority scheme works better when the number of competing stations is large and can improve up to 50% for the real-time packets

10


Issues in MAC Protocol Challenges security related and support of multicast

and location management Many mobile applications require support for group

communication Location-based services include providing listings of

local restaurants or movie theaters, emergency services, and vehicle tracking

Scalability is a major concern to WLANs In client server model, many PC’s or laptops

physically close to each other (20-500m) can be linked to a central hub (access point) which acts as a bridge between the wireless and wired network

A large area can be covered by installing several access points in the building

11


Roofnet and HyperLAN

Roofnet is an experimental multi-hop IEEE 802.11b consists of about 50 nodes in apartments of Cambridge

Few nodes act as gateways to wired Internet Requires no pre-configuration and users can connect on

the fly HiperLAN stands for high-performance LAN Employs 5.15GHz and 17.1GHz frequency bands and

has a data rate of 23.5Mbps with a coverage of 50m and mobility < 10 m/s

Supports 25 audio connections at 32 kbps with a maximum latency of 10 ms, 1 video connection of 2Mbps with 100ms latency, and data rate of 13.4 Mbps

12


Features of Hiper LAN/2

HiperLAN type 2 has been specifically developed to have a wired infrastructure

Type 1 has a distributed MAC with QoS provisions, whereas type 2 has a centralized scheduled MAC

Type 1 is based on Gaussian minimum shift keying (GMSK), whereas type 2 is based on OFDM

HiperLAN/2 automatically performs handoff to the nearest AP which is basically a radio BS that covers an area of about 30 to 150 meters

13


Features of Hiper LAN/2

The goals of HiperLAN are: QoS (to build multiservice networks) Strong security Handoff when moving between local area and wide areas Increased throughput Ease of use, deployment, and maintenance Affordability Scalability

The connection oriented approach makes support for QoS easy It supports automatic frequency allocation, eliminating the need

for manual frequency planning as in cellular networks

14


HyperLAN/2 Features

Fixed network

AP

AP AP

AP

MS MS

A simple HyperLAN/2 system

15


HyperLAN/2 Features MS may at any time request the AP and enter a low-power state for a sleep period Control is centralized at AP Channel spacing is 20MHz allowing high bit rates per channel Selective repeat ARQ is an error control mechanism used Radio link control (RLC) protocol provides following services: Association control with feature negotiation Encryption algorithms and convergence layers, authentication, key negotiation, and convergence layer negotiation Radio resource control to support handoff capability, to perform radio measurements in assisting the APs in selecting an appropriate radio channel, and to run the power-saving algorithm Connection control for the establishment and release of user connections

16


HomeRF

Two kind of networks: HomeRF (for home), Hiper LAN (for business workspace).

43 million US homes now contain more than one PC. A home network typically consists of one high speed

internet access port providing data to multiple networked nodes.

Home networking allows all computers in a home to simultaneously utilize the same high speed ISP (Internet Service Provider) account.

Home networking allows two options: wired solution and wireless solution.

17


HomeRF (cont’d)

Wired Solutions such as Ethernet, phone line offers a fast reliable secure connections, but the cost of wiring and installation is high.

Wireless networks such as PC-Centric Data offer more mobility to the users of the network.

18


Architecture of HomeRF System

Baby monitor

Phone connection

Cell phone

Main PC

Clock

Palmtop

Wireless headset

Satellite dish

Fridge data pad

Television Handheld communicator

Laptop 2nd PC Cable modem

19


Advantages of Wireless HomeRF

Mobility Flexibility: Simultaneous internet access while sharing

a single internet connection with other PCs. Simple: Installation time is small. Economical: Less than $100 for each networked PC. Secure Based on industry Standards: Enables interoperability

between many different manufacturers.

20


HomeRF Technology

In HomeRF all the devices can share the same connections for voice and data

Provides the foundation for a broad range of interoperable consumer devices

A specification for wireless communications in the home called Shared Wireless Access Protocol (SWAP) has been developed

21


Home RF Network

A network consists of Resource providers, which are gateways to different resources like cordless phones, printers, fileservers and TV.

The goal of Home RF is to integrate all of them in to a single Network suitable for all applications and also remove all wires and utilize RF links in the network.

This will support the mobility of devices. With Home RF, cordless phone can connect to

PSTN ordinarily, but can also connect through a PC for enhanced services.

22


Comparison of WLAN Standards Technology Wireless LAN

IEEE 802.11b (WiFi)

HomeRF HiperLAN

Operational spectrum

2.4 GHz 2.4 GHz 5.GHz

Physical layer DSSS FHSS with FSK OFDM with QAM

Channel access CSMA/CA CSMA–CA and TDMA

Central resource control/TDMA/TDD

Nominal data rate

2 Mbps 10 Mbps 32–54Mbps

Coverage 100 m >50m 30–150m

Power level issues <350mA current drain

<300mA peak current

Uses low power states like sleep

Interference Present Present Minimal

Price/complexity Medium (<$100) Medium High (>$100)

Security Low High High

23


Wireless Metropolitan Area Networks (WMANs)

IEEE 802.16 based WiMAX Offers less expensive opportunity Supports point-to-multipoint broadband wireless access Very high bit rates in the range of 3.5 MHz Support a variety of backhaul requirements, including both

ATM and packet-based protocols Convergence sublayers are used to map the transport-layer–

specific traffic to a MAC and offers features such as payload header suppression, packing, and fragmentation

Supports 99.999 percent link availability MAC supports automatic repeat request (ARQ)

24


Protocol Stack

Packet convergence Sublayer (PCS)

ATM Packet SSCS

(security sublayer 16-2004)


Brief Functions Convergence Sublayer (CS)

Mapping external network data into MAC SDU Classifying external network SDU Associating to MAC connection ID Payload header suppression (PHS)

Common Part Sublayer (CPS) Core MAC functionality

System access Bandwidth allocation Connection establishment Connection maintenance Handover and Power amanagement

Security Sublayer Authentication (RSA X.509/EAP) Security key exchange (3DES) Encryption (AES)

PHY Multiple sections


Service Specific Convergence Sublayer (SSCS)

The CS performs the following functions: accepting higher-layer PDUs from the higher layer performing classification of higher-layer PDUs processing (if required) the higher-layer PDUs

based on the classification delivering CS PDUs to the appropriate MAC SAP receiving CS PDUs from the peer entity

Currently, two CS specifications are provided Asyncronous Transfer Mode (ATM) CS Packet CS

Such as IP, PPP, Ethernet, etc., Other CSs may be specified in the future.


Packet Convergence Sublayer (PCS) Packet convergence sublayer (PCS)

The packet CS resides on top of the Common Part Sublayer (CPS)

The PCS performs the following functions, utilizing the services of the MAC sublayer: a) Classification of the higher-layer protocol PDU into the

appropriate connection b) Suppression of payload header information (optional) c) Delivery of the resulting CS PDU to the MAC SAP

associated with the service flow for transport to the peer MAC SAP

d) Receipt of the CS PDU from the peer MAC SAP e) Rebuilding of any suppressed payload header

information (optional)


Wireless Metropolitan Area Networks (WMANs)

Data to the subscriber stations are multiplexed in TDM fashion. The uplink (UL) is shared between SSs in TDMA fashion SS has a standard 48-bit MAC address MAC PDU consists of a fixed-length MAC header, a variable-

length payload, and an optional cyclic redundancy check (CRC) MAC supports various higher-layer protocols such as ATM or IP

CID lsb (8) HCS (8)

LEN lsb (8) CID msb (8)

Type (6) LEN Msb(3)

HT=

0 (1

)

Rsv(

1)

EC(1

)

Rsv(

1)

CI(1

)

EKS (2)

29


IEEE 802.16 MAC MAC supports both TDD and FDD 10–66GHz: line-of-sight (LOS) needed Burst design allows coexistence of both TDD and FDD forms 2–11GHz: three air interfaces are defined

Air Interface Specification WMAN–SC2 A single-carrier modulation is used WMAN–OFDM

License-exempt bands necessarily use this TDMA access interface. OFDM is present with a 256-point transform

WMAN–OFDMA

Each receiver is assigned a set of multiple carriers to enable multiple access. OFDM is present with a 2048-point transform

Three 2–11GHz Air Interface of the IEEE 802.16a Draft 3 Specifications

30


IEEE 802.16 MAC Physical Layer

Channel bandwidths are 20, 25MHz (typical U.S. allocation) or 28MHz (typical European allocation) Frame size can be 0.5, 1, or 2 ms Negotiated burst profile is used to provide synchronization with the Down Link

P MAC PDU which has started in previous TC PDU

First MAC PDU, this TC PDU

Second MAC PDU, this TC PDU

TC sublayer PDU 31


Wireless Mesh Network

Figure 15.9 Illustration of a Wireless Mesh Network (WMN)

Internet

IGW 1

IGW 2

MR1

MR2 MR3

MR4

MR5 MR6

Backbone

Mesh Clients

32


Wireless Mesh Network

Comprise of: Internet Gateways (IGWs) Mesh Routers (MRs) Mesh Clients (MCs) Multi-hop WMN, traffic is predominantly oriented towards IGWs from MRs Traditional routing solutions of MANETs are not adequate for WMNs TCP could result in excessive packet delays Vulnerable to variety of security attacks

33


Ricochet

A mobile data access service that is always on, provides high speed, secure mobile access to the desktop from outside the office.

It allows to link to the Internet or the corporate network without needing phone lines or cable connections.

The Ricochet service is provided by Metricom.

34


Ricochet Mobile Communication Network

Network interconnection facility Microcell radios on street lights, utility poles

Wireless access point

Name server

Router

Gateway

Gateway to Internet, Intranets, LANS, Compuserve, AOL and other on-line services Computer device

Modem radio

35


Ricochet The Ricochet service is a wide area wireless system using

spread spectrum packet switching data. The network operates within 902-928 MHz portion of RF

spectrum. The Ricochet wireless Micro Cellular Data Network

(MCDN), consists of shoebox sized radio receivers, called Micro cell radios.

Micro cells are typically mounted to street poles. Micro cells require a small power from the street lights. Each Micro cell radio employs 162 frequency hopping

channels.

36


The Ricochet Wireless Modem It weighs 13 ounces. Has the general dimensions of a small paperback

book, plugs directly into a desktop. When a Ricochet modem is configured to operate

in bridge mode, it translates signals from other Ricochet modems into signals that a wired modem can receive.

V.34, 28,800 bps access Good Availability Unlimited access Flexible pricing

37


Services Provided by Ricochet

Provides immediate, dependable and secure connections without the cost and complexities of land based phone lines.

Sending E-mails, access to documents in home networks.

Many real estate agents use this to search for property listings while on road.

38


Comparison of WMAN Standards

Technology Wireless MAN

IEEE 802.16 Ricochet

Operational spectrum 10–66GHz, LOS required, 20/25/28MHz channels

900MHz

Physical layer TDMA-based uplink, QPSK, 16-QAM, 64-QAM

FHSS

Channel access TDD and FDD variants CSMA

Minimum data rate possible 120/134.4Mbps for 25/28MHz channel 176 kbps

Coverage Typically a large city As of September, 2002 only Denver, CO

Power level issues Complicated power control algorithms for different burst profiles

Low power modem compatible with laptops and hand-held

Interference Present but limited Present

Price complexity Not available Medium

Security High. Defines an extra privacy sublayer for authentication

High (Patented security system)

39


Wireless Personal Area Network

Bluetooth initially conceived to replace RS232 cables, is the only WPAN technology to be commercially available

Since 2002, its presence has become visible in devices ranging from laptops to wireless mouse to cameras, to headsets, to printers and cell phones

IEEE 802.15.x protocols to address needs of WPANs with varied data rates

Bluetooth has adopted as IEEE 802.15.1 (medium rate) while the IEEE 802.15.3 (high rate) and 802.15.4 (low rate) are also available

40


IEEE 802.15 Task Groups IEEE 802.15 WPAN/Bluetooth TG1 IEEE 802.15 Coexistence TG2: TG2 (the IEEE 802.15.2) is developing recommended practices to facilitate coexistence of WPANs (the IEEE 802.15) and WLANs (the IEEE 802.11). IEEE 802.15 WPAN/High Rate TG3: The TG3 for WPANs is chartered to draft a new standard for high-rate (20Mbps or greater) WPANs IEEE 802.15 WPAN/Low Rate TG4: The goal is to provide a standard for ultra-low complexity, cost, and power for low-data- rate (200 kbps or less) wireless connectivity among inexpensive fixed, portable, and moving devices

41


Bluetooth It is named after the King of Denmark that unified

different factions in Christianity through the country. It is a short range RF communication. Low cost, low power, radio based wireless link

eliminates the need for short cable. Bluetooth radio technology built into both the cellular

telephone and the laptop would replace the cable used today to connect a laptop to cellular phone.

Printers, desktops can all be wireless. It also provides a universal bridge to existing data

networks. 42


Bluetooth Cellular Link

Use of Bluetooth to connect notebook

Base Station

43


Bluetooth connecting printers, PDA’s, desktops, fax machines, keyboards, joysticks and virtually any other digital device

44


Fixed Line

Bluetooth providing a universal bridge to existing data networks

45


Bluetooth: A mechanism to form ad hoc networks of connected devices away from fixed network

infrastructures

Bluetooth

Personal Ad hoc Network

46


Bluetooth

The ultimate goal is to make small products (PC/Laptops) have only one wire attached to power cord.

In case of PDA, the power cord is also eliminated. A simple application of Bluetooth is updating the

phone directory of the PC from a mobile telephone. A typical Bluetooth has a range of 10 m.

47


Features

Fast frequency hopping to reduce interference. Adaptive output power to minimize interference. Short data packets to maximize capacity. Fast acks allowing for low coding overhead for links. Flexible packet types that support a wide application range. CVSD (Continuous Variable Slope Delta Modulation)

voice coding that can withstand high bit error rates. Transmission/reception interface tailored to minimize

power consumption

48


Architecture of Bluetooth System and Scatternet

Piconet 1 Piconet 4

Piconet 3

Piconet 2

M2

M1

M3

M4

S2,1

S2,2

S2,3

S3,1 S3,2

S3,3

S2,4 /S3,4

S1,2 /S2,5

S 1,3 /S 4,4

S1,1

S1,4

S1,5

S4,1

S4,2 S4,3

49


Bluetooth Technological Characteristics Frequency band 2.4 GHz (unlicensed ISM band)

Technology Spread spectrum

Transmission method Hybrid direct sequence and frequency hopping

Transmission power 1 milli-watt (0 dBm)

Range 10 meters (40 feet)

Number of devices 8 per piconet, 10 piconets per coverage area

Data speed Asymmetric link: 721+57.6 kbps

Symmetric link: 432.6 kbps

Maximum voice channels 3 per piconet

Maximum data channels 7 perpiconet

Security Link layer w/s fast frequency hopping (1600 /sec)

Power consumption 30 μA sleep, 60 μA hold, 300 μA standby, 800 μA max transmit

Module size 3 square cm (0.5 square inches)

Price Expected to fall to $5 in the next few years

C/I co-channel 11 dB (0.1% BER)

C/I 1 MHz -8 dB (0.1% BER)

C/I 2 MHz -40 dB (0.1% BER)

Channel switching time 220 μs

50


Architecture

Bluetooth radio typically hops faster and uses shorter packets as compared to other systems operating in the same frequency band.

Use of FEC (Forward Error Correction) limits the impact of random noise.

As the interference increases, the performance decreases.

51


Architecture (cont’d) Bluetooth devices can interact with other Bluetooth devices. One of the devices acts as a master and others as slaves. This network is called “Piconet”. A single channel is shared among all devices in Piconet. There can be up to seven active slaves in the Piconet. Each of the active slaves has an assigned 3 bit Active Member

address. A lot of other slaves can remain synchronized to the Master

through remaining inactive slaves, referred to as parked nodes. A parked device remains synchronized to the master clock and

can become active and start communicating in the Piconet anytime.

52


Architecture (cont’d)

If Piconets are close to each other, they have overlapping areas

The scenario where the nodes of two or more Piconets mingle is called Scatternet

Before any connections in the Piconet are created all devices are in STDBY mode

In this mode an unconnected unit periodically “listens” for message every 1.28 seconds

Each times a device wakes up, it tunes on the set of 32 hop frequencies defined for that unit

53


Upper Layer

Baseband

SDP

LMP Audio L2CAPE

Low Radio Layer

SDP – Service Discovery Protocol

L2CAP – Logical Link Control and Adaptation Layer Protocol

LMP – Link Manager Protocol

Bluetooth Core Protocol

SDP: Provides a mean for applications to discover which services are provided by or available through a Bluetooth device

L2CAP: Supports higher level protocol multiplexing, packet segmentation and reassembly and conveying of QoS information

LMP: Used by Link managers for link set up and control Baseband: Enables the physical RF link between Bluetooth units forming a Piconet

54


625µsec 1-slot packet

3-slot packet

5-slot packet

Core Protocols

Type User Payload (bytes)

FEC

Symmetric (kbps)

Asymmetric (kbps)

DM1 0–17 Yes 108.0 108.8 108.8

DH1 0–27 No 172.8 172.8 172.8

DM3 0–121 Yes 256.0 384.0 54.4

DH3 0–183 No 384.0 576.0 86.4

DM5 0–224 Yes 286.7 477.8 36.3

DH5 0–339 No 432.6 721.0 57.6

HV1 0–10 Yes 64.0

HV2 0–20 Yes 128.0

HV3 0–30 No 192

55


Bea

con Contention

access period (CAP)

Bea

con

Guaranteed time slot (GTS)

Superframe

WPAN parameters

Non-QoS data frames: • Short bursty data • Channel access requests

CAP/GTS boundary dynamically adjustable

Data frames with QoS provisions: • Image Files • MP3 music files (multimedia files) • Standard definition MPEG2, 4.5 Mb/s • High-definition MPEG2, 19.2 Mb/s • MPEG1, 1.5 Mb/s • DVD, up to 9.8 Mb/s • CD audio, !.5 Mb/s • AC3 Dolby digital, 448 Kb/s • MP3 streaming audio, 128 Kb/s

IEEE 802.15.3 MAC and PHY Layer Details

56


IEEE 802.15.4 Some applications that require high data rates such as shared Internet access, distributed home entertainment, and networked gaming However, there is an even bigger market for home automation, security, and energy conservation applications IEEE 802.15.4 defines specification for low-rate, low- power WPANs Application areas include industrial control; agricultural, vehicular, and medical sensors; and actuators

57


Upper layers

Network layer

Data link layer

IEEE 802.15.4 868/915

MHz PHY

IEEE 802.15.4

2400 MHz PHY

IEEE 802.2 LLC, type 1

SSCS

Other LLC

IEEE 802.15.4 MAC

IEEE 802.15.4 Data Link Layer (DLL) Details

58


Bytes:

PHY Layer

General MAC frame format

PHY protocol data unit (PPDU)

MAC protocol data unit (MPDU)

Synchronization header

Physical header PHY service data unit (PSDU)

Payload Frame check sequence

Address Info

Sequence number

Frame control

MAC header (MHR) MAC service data unit (MSDU)

MAC footer (MFR)

2 2 Variable 0-20 1

MAC Layer

59


Preamble

Start of packet

delimiter

PHY header

PHY service data unit (PSDU)

6 bytes ≤ 127 bytes

PHY protocol data unit (PPDU)

PHY packet fields: • Preamble (32 bits) – synchronization • Start of packet delimiter (8 bits) – signify end of preamble • PHY header (8 bits) – specify length of PSDU • PSDU (≤ 127 bytes) – PHY layer payload

802.15.4 PHY layer Packet Structure

60


Comparison of WPAN Systems Technology Bluetooth (802.15.1) 802.15.3 802.15.4

Bluetooth 3.0 HS

Operational spectrum

2.4GHz ISM band 2.402–2.480GHz ISM band

2.4GHz and 868/915MHz

2.4–2.4835GHz or 6–9GHz

Physical layer details

FHSS, 1600 hops per second

Uncoded QPSK trellis coded QPSK or 16/32/64-QAM scheme

DSSS with BPSK or MSK (O–QPSK)

UWB

Channel access

Master slave polling, time division duplex (TDD)

CSMA–CA, and guaranteed time slots (GTS) in a superframe structure

CSMA–CA, and guaranteed time slots (GTS) in a superframe structure

802.11 radio protocol

Maximum data rate

Up to 1Mbps 11–55Mbps 868MHz–20, 915MHz–40, 2.4GHz–250 kbps

480 Mbps

Coverage <10m <10m <20m ?

Power level issues

1mA–60mA

<80mA Very low current drain (20–50 mA)

ultra-low power

Interference Present Present Present Minimum

Price Low (<$10) Medium Very low ?

61


ZigBee ZigBee is pertinent in various sensor applications ZigBee is designed to respond quickly, while Bluetooth takes much longer ZigBee is a control technology on wireless standard Data rate of 250 Kbps in 2.4 GHz ISM band, 20 kbps in the 868 MHz band in Europe, and 40 kbps in 915MHz band used in North America and Australia ZigBee can choose up to 16 different 5 MHz channels within 2.4 GHz band, several do not overlap with 802.11 and WiFi ZigBee has active and sleep modes All devices must have a short 16-bit IEEE addressing Application layer maintains table of binding for matching two or more devices

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wireless lans and pans - 國立中興大學

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