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1 유무선인프라기술 유비쿼터스컴퓨팅전반및 HW 고려점 서울인터콘티넨탈호텔 김재현 정보통신대학

2 목차 1 2 통신망의미래상 통신망인프라기술 1 2 유선통신기술 무선통신기술 3 4 전자정부통신망인프라기술 미래전자정부인프라구축시고려사항 2 김재현

3 통신망의미래상 3 대인프라 : RFID/USN, ALL-IP(IPv6), BCN 다양한유무선엑세스망연동 인공위성 PSTN Media/ Signalling Gateway 디지털오디오 / 비디오방송 유무선액세스연동망 IS-95A/B/C(1x) MSC ALL IP 백본망 휴대인터넷 WLAN 게이트웨이 RFID/USN IP 기반무선액세스망 cdma EV/DO 이종시스템간연동 xdsl/ FTTH/ Cable Modem WCDMA/UMTS 4G 무선망? 3 김재현

4 유선통신기술 유선통신기술비교 xdsl DOCSIS FTTH MAC Protocol TDMA/TDD, FDMA/FDD TDMA S-ALOHA TDMA 전송속도 HDSL ADSL VDSL Mbps 하향 : ~ 6.1 Mbps 상향 : 16 ~ 640 kbps 하향 : 12.9 ~ 52.9 Mbps 상향 : 1.5 ~ 2.3 Mbps < 사용자측면 > 하향 : 1.5 ~ 3 Mbps 상향 : 500 kbps ~ 2.5 Mbps 하향 : 622 Mbps 상향 : 155 Mbps 비용저저저 HDSL : High bit-rate Digital Subscriber Line, ADSL : Asymmetric Digital Subscriber Line, VDSL : Very High Digital Subscriber Line, DOCSIS : Data Over Cable Service Interface Specification, FTTH : Fiber To The Home, 4 김재현

5 xdsl (Digital Subscriber Line) x signifies : various flavors of DSL always-on, high-speed data services over existing copper wires to residences & businesses 대표적인 xdsl 기술정리 DSL 형태설명데이터전송속도활용분야 HDSL High bitrate DSL Mbps duplex on two twisted-pair lines Mbps duplex on three twisted-pair lines - 서버와전화국간의 T1/E1 서비스 - WAN(Wide Area Network), LAN (Local Area Network), 서버액세스 ADSL Asymmetric DSL - 하향속도 ~ 6.1 Mbps - 상향속도 16 ~ 640 kbps - 인터넷및웹액세스, 동영상, VoD, 원격지에서 LAN 에접속하는등의용도에사용 VDSL Very High DSL - 하향속도 12.9 ~ 52.8 Mbps - 상향속도 1.5 ~ 2.3 Mbps - ATM 네트워크 - Fiber to the Neighborhood 5 김재현

6 DOCSIS (Data Over Cable Service Interface Specification) International standard developed by CableLabs High-speed data transfer with an existing cable TV (CATV) system DOCSIS Key Features Benefits / Services DOCSIS 2.0 (UL Data rate: 30 Mbps ) DOCSIS 1.1 (UL Data rate: 10 Mbps ) DOCSIS 1.0 (UL Data rate: 5 Mbps ) - Mandatory S-CDMA (Synchronous-Code Division Multiple Access)/ TDMA - Best of DOCSIS - QoS - Security - Spec d for retail - Standard spec - Symmetric services - Peer-to-peer - Business-to-business (20 T1 capacity) - Tiered service - Double UL data rate capacity - Lower op s costs - Better than competitor - High speed data - Internet access 6 김재현

7 DOCSIS 통신구조 Transmit Internet Protocol (IP) Packets Transparently Between the Head end and the Subscriber Location The DOCSIS System Consists of: Cable Modem Termination System (CMTS) located at the headed Cable Network Cable Modem (CM) located at the Customer Premise Wide-Area Network CMTS Network Side Interface Cable Modem Termination System (CMTS) Cable Network HFC Cable Modem (CM) CM Customer Premises Equipment Interface Customer Premises Equipment Transparent IP Traffic Through the System 7 김재현

8 DOCSIS MAC Protocol TDMA (Time Division Multiple Access) Ranging CM과 CMTS간동기화 Registration CMTS에연결정보를등록하는과정 UL_MAP 사용 경쟁슬롯, 데이터전송슬롯, Maintenance로구성 Binary Exponential Backoff Algorithm 8 김재현

9 FTTH (Fiber To The Home) PTP (Point To Point) 장점 : 안정적으로다양한서비스제공가능 단점 : 다량의광섬유가소요되어비경제적 AON (Active Optical Network) 스위치기술기반 장점 경제적이고일반적인구축방안 공동주택의경우가격, 성능, 유지보수등에서유리 단점 : Ethernet 기반의장비인능동장치사용, 전력공급필요 PON (Passive Optical Network) 광신호를가입자부근에서광스플리터를이용해분리함으로써여러명의가입자에게전송하는방식 장점 단독주택 농어촌지역구축에유리 전화국의송신, 수신장치및광섬유를많은사람이공유하게돼가입자당비용이절감 단점 : 상향신호의분배문제, 많은분기로인한전송신호의출력문제, 전송프로토콜및구현방법 9 김재현

무선통신기술 PAN - WAN WAN (Wide Area Network) 운전 IMT-Advance MAN (Metropolitan Area Network) 이동성 옥외 옥내 도보정지도보정지 2G cellular 3G cellular IEEE802.16e/20 IEEE802.11 802.15.

10 무선통신기술 PAN - WAN WAN (Wide Area Network) 운전 IMT-Advance MAN (Metropolitan Area Network) 이동성 옥외 옥내 도보정지도보정지 2G cellular 3G cellular IEEE802.16e/20 IEEE a/b a IEEE IEEE RFID LAN (Local Area Network) PAN (Personal Area Network) BAN (Body Area Network) 1000 Mbps 전송속도 10 김재현

11 무선통신기술 무선통신기술비교 전송범위 RFID/USN LAN < 100 m PAN, Access < 10 m MAN < 1Km Access < 10m (M/W) IMT- Advance 좁은영역 < 100m 넓은영역 >1km 이동성좁은지역좁은지역이동성보장 접속지역 넓은지역이동성보장 MAC Protocol CSMA/CA CSMA/CA, TDMA S-ALOHA TDMA/TDD W-DOCSIS SDMA/CDMA/ FDMA/TDMA/ Binary search TDD/FDD Peer-to-Peer 전송속도 1 54 Mbps < 1Mbps <480Mbps 15.3a < 50 Mbps < 1 Mbps 200 tag/antenna 좁은영역 ~ 1Gbps 넓은영역 ~ 100Mbps 비용저저중 저 고 CSMA/CA :Carrier Sense Multiple Access/Collision Avoidance, TDMA : Time Division Multiple Access TDD : Time Division Duplex, S-ALOHA: Slotted-ALOHA, W-DOCSIS : Wireles-Data Over Cable Service Interface Specification, SDMA : Space Division Multiple Access, CDMA : Code Division Multiple Access, FDMA : Frequency Division Multiple Access, FDD : Frequency Division Duplex 11 김재현

12 WLAN(IEEE ) 12 김재현

13 Wireless LAN Architecture Ad hoc WLAN Mode Peer-to-peer communication only Independent Basic Service Set IBSS Infrastructure WLAN Mode No peer to peer communication, always through AP Distribution system : Connect two or more BSS Distribution System AP-A BSS-A BSS-B AP-B Ad hoc network Infrastructure network 김재현

14 Service Types Asynchronous Service : Data service, FTP, Web, etc. Contention Traffic DCF (Distributed Coordination Function) Distributed controlled by STA (Station) Isochronous Service : Delay sensitive traffic, voice, etc Contention-Free Traffic PCF (Point Coordination Function) Centralized controlled by AP (Access Point) Super Frame Concept Superframe Contention-Free Burst Contention Traffic 14 김재현

15 Distributed Coordination Function (DCF) CSMA/CA protocol Use different Inter Frame Space (IFS) to differentiate traffic SIFS (Short Inter Frame Space) : High Priority PIFS (PCF Inter Frame Space) : Medium Priority DIFS (DCF Inter Frame Space) : Low Priority Sense channel during DIFS DIFS DIFS PIFS Contention Window Busy Medium SIFS Backoff-Window Next Frame Slot time Defer Access Backoff slot reduced when channel is idle 15 김재현

16 QoS Support in e HCF (Hybrid Coordination Function) Two channel access mechanism : Contention based channel access Controlled based channel access EDCA (Enhanced Distributed Channel Access) Using Arbitration IFS, differentiate QoS HCCA (HCF Controlled Channel Access) Channel access for parameterized QoS Transmission Opportunity (TXOP) Multiple frames can be transmitted during a TXOP with certain rules EDCA TXOP acquired by beacon Polled TXOP acquired by QoS CF poll 16 김재현

17 EDCA (Enhanced Distributed Channel Access) AIFS[AC] AIFS[AC] AIFS[AC] =DIFS PIFS CW=rand[1,CW i +1] medium priority AC low priority AC SIFS ACK SIFS high priority AC RTS defer access Contention Windows SIFS count down as long as medium is idle, Back off when medium gets bust again CTS AC Cwmin CWmax AIFSN AC_BK CWmin CWmax 3 AC_BE CWmin CWmax 7 AC_VO (CWmin+1)/2-1 CWmin 2 AC_VI (CWmin+1)/4-1 (CWmin+1)/2-1 2 CW i+1 [AC]=min[ ( ( CW i [AC] + 1 )*PF[AC] ) - 1, CW max ] With a aslottime: 9us SIFS: 16us PIFS: 25us DIFS: 34us AIFS: >=34 us PF : Persistent Factor 17 김재현

18 HCCA (HCF Controlled Channel Access) Additional polling based controlled contention scheme for HC to learn the TXOPs needed by the stations Contention Free Period, CFP(polling through HCF) Contention Period, CP (listen before talk and polling through HCF) Beacon QoS CF-Poll CF-end QoS CF-Poll Transmitted by HC Transmitted by (Q)STAs TBTT TXOP TXOP TXOP TXOP Time TBTT RTS/CTS Fragmented DATA/ACK (polled by HC ) RTS/CTS/DATA/ACK (after DIFS+backoff) RTS/CTS Fragmented DATA/ACK (polled by HC ) 18 김재현

19 IEEE IEEE (Bluetooth) - IEEE (UWB) - IEEE (Zigbee( Zigbee) 19 김재현

20 IEEE (Bluetooth) Overview Concept Short Range : 0.01m - 10m (100m) Low Power : 1mW, 2.5 mw, 100 mw Low Cost : < $5 Can be used for Data (max 753 kbps) / Voice(3 64kbps) Access Appliance Cable replacement Personal Ad-Hoc Connectivity Standard (Bluetooth SIG and IEEE ) 1999 : Version 1.0b 2001 : Version 1.1 (1Mbps) Current : Version 2.0 (2-11Mbps) Topology Piconet, Scatternet 20 김재현

21 Channel Allocation TDD/Single slot Multi-slot allocation 21 김재현

22 Piconet vs. Scatternet Piconet Standby state Scatter net Master Park state Slave Active state Sniff state Hold state Scatternet Stand by Parked Master Slave 22 김재현

23 IEEE (High Rate) Overview Objective Low complexity, Low cost, Low power, Short Range, QoS Capable, Peer to peer communication, High data rate (> 20Mbps) PHY 2.4GHz 5 Channel MAC Functionality Fast Connection Time Ad hoc Network QoS support Security Dynamic Membership Efficient data transfer Topology Piconet, Child piconet, Neighbor piconet Piconet Coordinator (PNC), Device (DEV) 23 김재현

24 IEEE a (UWB) Overview UWB (Ultra-Wide Band) : Bandwidth > 500 MHz Power spectral density (db) Narrow Band: (f H f L ) < 0.01*F C UWB: (f H f L ) > 0.25*F C f L f C Frequency (Hz) Characteristics Frequency : 3.1GHz~10.6GHz ERIP Emission level should be less than -41.3dBm/MHz Data rate : 110 ~ 480 Mbps, Range : < 10m Use IEEE MAC f H 24 김재현

25 IEEE a PHY layer (Proposed) Single Band vs. Multi-Band OFDM(MBOA) Single Band Multi Band OFDM Bandwidth BW: 2GHz, 4.8GHz, 1 or band (528 MHz) Modulation FDM+CDM+TDM (PSK) OFDM/ QPSK Data rate 28.5 Mbps 1.2Gbps Mbps Advantage Difficulty Vendors Simple HW Low cost, low power Time to Market ( 03 4Q) UWB Antenna Complex to Sync. XSI, SONY Easy to implement robust to interference Time to Market ( 05 1Q) Peak to average ratio Prob. Not verified yet Intel, TimeDomain, DTC, WisAir, GA, Femto Devices EIRP Emission Level (-41.3 dbm/mhz) - EIRP (Equivalent Isotropic Radiated Power) dbm/mhz Low Band High Band GHz 3 10GHz 25 김재현

26 IEEE a Superframe Structure Super frame #m-1 Super frame #m Super frame #m+1 Beacon #m CAP CFP (Contention Free Period) Asynchronous Isochronous Asynchronous Isochronous Beacon #m Contention Access Period MCTA1 MCTA2 CFP (Contention Free Period) CTA 1 CTA 2 CTM n-1 CTA n 1,000 ~ 65,535μs CSMA/CA Data/Control S-ALOHA Data/Control TDMA Data - MCTA : Management Channel Time Allocation 26 김재현

27 Piconet Independent piconet: PNC and DEV Dependent piconet Child piconet : # DEV > 255, extended area, Communication with PNC or DEV in parent piconet Neighbor piconet : when no available channel in parent piconet, communication with PNC or DEV in neighbor piconet Beacon Contention CFP Access Period CTA 1 CTA 2 CTA n Reserved time Bea con CAP CTA 1 CFP CTA 2 CTA n Reserved time Beacon DEV 1 C- DEV 2 DE V 1 P- PNC C- PNC C- DEV 2 27 김재현

28 IEEE (Zigbee( Zigbee) ) Overview Low Rate (20, 40, 250kbps), Low Cost, Low Power Short Range (< 10m) Dynamic device addressing Support for low latency devices Reliable by fully handshake protocol CSMA-CA channel access. Low power consumption Apply to u-smart Network : Energy save, Consumer Electronics, Toy, Security Health care check and monitoring System Topology Star or peer-to-peer topology 28 김재현

29 IEEE MAC overview The features of the MAC sub-layer Beacon management Channel access Guaranteed time slot management Frame validation Acknowledged frame delivery Association and disassociation Security mechanisms FFD (Full Function Device) A device capable of operating as a coordinator or device, implementing the complete protocol set. RFD (Reduced Function Device) A device operating with a minimal implementation of the IEEE protocol. 29 김재현

30 Star / Pear-to to-pear Topology Star topology PAN coordinator Cluster Head (CLH) Pear to Pear topology ex) Cluster Tree Network Full Function Device (FFD) Reduced Function Device (RFD) 30 김재현

31 IEEE Superframe Structure The LR-WPAN standard allows the optional use of a superframe structure. bounded by network beacons divided into 16 equally sized slots. Channel access mechanism Beacon enabled network slotted CSMA-CA A non beacon enabled network Un-slotted CSMA-CA TDD Contention access period (CAP) a slotted CSMA-CA mechanism transactions shall be completed by the time of the next network beacon. Contention free period (CFP) Included The guaranteed time slots (GTSs) The PAN coordinator may allocate up to seven of these GTSs Frame Beacon Contention Access Period Contention Free Period Inactive Period 31 김재현

32 휴대인터넷 (IEEE ) 32 김재현

33 IEEE Overview Wireless Metropolitan Area Network Broadband Wireless Access Coverage area : 1 Km Max Data Rate : 120Mbps~ IEEE Air Interface Standard IEEE : Air Interface (MAC and 10 ~ 66 GHz PHY) WiMAX forum coordinating interoperability testing Interoperability documentation in development P802.16a : amendment, 2 ~ 11 GHz Licensed Licensed-exempt Standard defines 4 PHY Mode WirelessMAN-SC (Single Carrier) WirelessMAN-SCa WirelessMAN-OFDM WirelessMAN-OFDMA SS Metropolitan Area SS BS SS SS 33 김재현

34 IEEE SC (Single Carrier) Binary Exponential Backoff Algorithm is applied when the collision is occurred The frames are divided by bandwidth allocation MAP message according to the usage. Uplink Frame Structure Gap between SSs Gap between transmission and reception Initial Ranging Period Contention Period Data Transmission Period of SS 1 Data Transmission Period of SS N Time Slot Access Collision Access Request Collision Bandwidth Request Bandwidth 34 김재현

35 Subchannel logical number s s+1 Preamble IEEE MC (Multi-Carrier) Frame Structure k k+1 k+3 FCH DL_MAP DL burst #1 (carrying the UL_MAP) DL burst #2 OFDMA symbol number DL burst #3 DL burst #4 DL burst #5 DL burst #6 Ranging subchannel UL burst #1 UL burst #2 UL burst #3 Preamble FCH DL_MAP UL burst #4 s+l Slot Slot DL Slot Slot Slot Refer to IEEE e std 2005 TTG UL RTG One subchannel DL PUSC : two OFDMA symbols DL FUSC : one OFDMA symbol UL PUSC : three OFDMA symbols DL/UL AMC : two, three or six OFDMA symbols PUSC : Partial usage of subchannels FUSC : Full usage of subchannels AMC : Adaptive modulation and coding FCH : Frame control header TTG : Transmit/receive transition gap RTG : Receive/transmit transition gap 35 김재현

36 IEEE MC (Multi-Carrier) Frame Structure The frame structure Preamble FCH (Frame Control Header) DL_MAP & UL_MAP DL/UL data bursts UL control channel for ranging 2 types of subcarrier permutation mode in OFDMA The distributed subcarrier permutation mode - PUSC, OPUSC, FUSC or OFUSC mode The adjacent subcarrier permutation mode - AMC mode 36 김재현

37 Service Classes Service Definition Application QoS Parameter UGS ert-ps rt-ps Support fixed size realtime service at periodic interval Support variable size real time service at periodic interval Support variable size real-time service based on polling access - VoIP without silence suppression - VoIP with silence suppression - Variable size vocodec - MPEG video - Maximum Sustained Traffic Rate - Maximum Latency - Tolerated Jitter - Maximum Sustained Traffic Rate - Minimum Reserved Traffic Rate - Maximum Latency - Minimum Reserved Traffic Rate - Maximum Sustained Traffic Rate - Maximum Latency nrt-ps Support non real-time service flows based on polling basis -FTP - Minimum Reserved Traffic Rate - Traffic Priority BE Best Effort -HTTP - Traffic Priority - UGS :Unsolicited Grant Service - ertps : extended real-time Polling services - rtps : real-time Polling Service - nrt-ps : non-real-time Polling Service - BE : Best Effort 37 김재현

38 RFID 38 김재현

39 RFID system What is the RFID system? The RFID system is a simple form of ubiquitous sensor networks that are used to identify physical objects Application of RFID system Asset tracking(e.g. libraries, animals) Automated inventory Stock-keeping Toll collecting Hospital Cloth ID, Credit card Pet, Cow Casino: i-coin Secret Document 39 김재현

40 RF-ID PHY (Carrier Frequencies) Frequency Range Max Range (Passive) LF 125 KHz HF MHz UHF MHz Microwave 2.45 GHz & 5.8 GHz < 0.5 m ~1 m ~3 m ~1 m Cost High (Antenna) Medium Low High Characteris- tics Good for Metal and Liquid Best for short range, single tag Moderate performance Fast read rates Bad for Metal and Liquid Tag Power Passive Passive Active/Passive tags Active/Passive tags Typical Applications Today Access control, animal tracking, vehicle immobilizers, POS applications including Speed Pass Smart Cards, Itemlevel tracking including baggage handling (non-us), libraries Pallet tracking, electronic toll collection, baggage handling (US) SCM, electronic toll collection Data Rate Slower Faster Metal/Water Better Worse Pa. Tag Size Larger Smaller 40 김재현

41 Anti-Collision Algorithm Tag collision problem in RFID system It is impossible to communicate among passive tags The reader broadcasts the request message to the tags If there are more than one tag response for the reader s request, their responses will collide We need an anti-collision algorithm to solve collision problem Tag collision problem 41 김재현

42 Multi-tag tag Anti-collision Algorithms in Standards Arbitration Air Interface (R->T / T->R) EPC Data rate (R->T / T->R) Security ISO TYPE A Framed Slotted Pulse interval ASK / FM0 not defined 33 kbps / 40 kbps None ISO TYPE B Probabilistic Binary tree Manchester-ASK / FM0 not defined 8/40 kbps / 40 kbps None AutoID Class 0 Bit-by-bit Binary Tree Pulse Width Mod./ FSK 64/96b 40/80 kbps / 40/80 kbps 24-bit kill AutoID Class 1 Binary tree using 8 bin slots Pulse Width Mod. / Pulse Interval AM 64/96b kbps/ kbps 8-bit kill EPCglobal Gen 2 Probabilistic Slotted Pulse interval ASK / Miller, FM0 96/256b 40 kbps / 640 kbps 32-bit kill, Access 42 김재현

43 IMT-Advanced (WINNER System) 43 김재현

44 WINNER System Challenge To make mobile communication systems more adaptable to user needs Constitution WINNER (Wireless World Initiative New Radio) is a consortium of 41 partners co-ordinated by Siemens under IST (information Society Technology). WINNER system concept Single new ubiquitous radio access system Self-contained, allowing WINNER to target the chosen requirements without the need for interworking with other systems Cooperation, interworking and infrastructure reuse may be used for mutual benefit (Cooperation) First deployment expected at the earliest in 2010, widespread from 김재현

45 Multitude of scenarios in WINNER system concept 45 김재현

46 Super-Frame structure (TDD) 8 frames with approximate duration 5.6ms Preamble DAC (Direct Access Channel) for the peer-to-peer communication 46 김재현

47 Multi-mode protocol architecture 47 김재현

48 전자정부통신망인프라기술 ALL-IP 기반망 유무선연동기술 액세스망의다양화및진보화 부서별전산망부서별전산망 무선무선통신통신망 통합통합처리처리시스템시스템 WiBro WLAN RFID/USN IMT-Adv, ALL IP 백본망 기술간연동관리 기술간연동관리 보안관리 보안관리 서비스품질관리 서비스품질관리 FTTH xdsl Cable Modem 유선유선통신통신망 48 김재현

49 미래전자정부인프라구축시고려사항 네트워크측면 ALL IP 망연동기술 (IPv6) 다양한엑세스망연동기술 유무선연동기술 /QoS 보장기술 보안기술측면 등급별사용자인증기술 해킹및네트워크공격 (DOS 등 ) 방지기술 소프트웨어측면 정부부처간다양한코드및체계의표준화 다양한자원 ( 문서, DB 등 ) 의 transparency 서비스측면 Always on 서비스 전자민원서비스홍보및교육 컴퓨터비전문가입장에서설계및개발 대중매체및공공기관에서의교육 49 김재현

50 맺음말 지속적개선형시스템 Continuous Quality Improvement(CQI) 국내기술의개발및적용필요 네트워크장비및통신기술 소프트웨어 보안기술 하나하나챙겨가며 감사합니다! 50 김재현

51 Reference [1] Data Over Cable Service Interface Specification : Radio Frequency Interface Specification," SP-RFI v1.1-i , April [2] [3] [4] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Medium Access Control (MAC) Quality of Service Enhancements, P802.11e-D13.0, January [5] IEEE (tm)-2002 Part 15.1: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Wireless Personal Area Networks (WPANs(TM))Gemma PauloWireless Cribs, "Living Large with a Wireless Home Network", October 2002 [6] IEEE Standard for Part 15.2: Coexistence of Wireless Personal Area Networks with Other Wireless Devices Operating in Unlicensed Frequency Bands [7] IEEE Part 15.3: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks(WPAN), February [8] IEEE P Standard for Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) pecifications for Low Rate Wireless Personal Area Networks (LR- WPANs), February [9] Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands, IEEE e-03/07r5, December [10] EPC, Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz 960 MHz Version 1.0.9, Jan., [11] ISO/IEC : 2005(E), Part 6C : parameters for air interface communications at 860 MHz to 960 MHz, [12] ETSI EN v1.1.1, Sep CTAN:http// [13] ETSI EN v1.1.1, Sep CTAN:http// 51 김재현

Microsoft PowerPoint ppt

Microsoft PowerPoint ppt Wireless LAN 최양희서울대학교컴퓨터공학부 Radio-Based Wireless LANs Most widely used method Adv: penetrating walls and other obstacles with little attenuation. Disadv: security, interference, etc. 3 approaches: ISM