無線網路CH6
學校 : 中華大學 系所 ： 通訊工程學系 老師 ： 余誌民教授
1
無線通訊網路
Outline I.無線通訊網路簡介 II.無線通訊的多重存取(multiple access)
II.無線通訊的多重存取(multiple access)技術
本章的重要觀念
多重存取(multiple access)技術是什麼 ? 有那些多重存取的技術 ? 各種無線通訊系統使用什麼樣的多重存取 技術 ?
本章的重要觀念 由於頻寬(bandwidth)是有限的資源，為
了使有限資源得以發揮最大效益，最好讓 更多使用者共同享用頻寬，這也就是所謂 的多重存取技術 在無線通訊網路中，多重存取技術的優劣 會影響頻寬的使用效率，決定系統的服務 容量(capacity)，因此是相當重要的關鍵 技術
雙工特性(Duplexity) FDD可以同時提供兩個單工頻道
兩個頻道分別稱為前向頻道(forward channel)及反向頻道(reverse channel)，兩頻 道間以隔離頻段(guard band)分開 前向頻道用於基地台到行動台的流量，也稱 為下傳連接(downlink) 反向頻道用於行動台到基地台的流量，也稱 為上傳連接(uplink)
TDD可以在同樣的頻率下提供兩個單工的 時槽(simplex time slots)
TDD利用時間來分隔前向頻道與反向頻道， 不需要雙工器
頻率領域雙工(FDD)
雙工器將訊號分開到前向頻道與反向頻道
共用傳輸媒體的原理
有線區域網路中最有名的乙太網路使用 CSMA/CD通訊協定就是規範傳輸媒體 (transmission medium)的共用問題 探討該協定前，先認識ALOHA，ALOHA最早 是為了封包無線電 (packet radio)網路發展出來 的
共用傳輸媒體的原理
ALOHA的運作原理： 1.
2. 3.
網路節點送出資料框(frame)後，必須等待回應 (acknowledgement)，等待的時間約是封包預期在網 路上來回一趟所需時間(round-trip propagation delay)，再加上一段固定長度的時間 假如在時限內收到回應，表示傳送成功，否則就需重 傳(resend)，若是數次傳送都失敗，則放棄 接收端檢查收到的資料框是否有效(valid)，含資料框 目的地位址是否與接收端相符，及資料框中的資料框 檢查順序(frame-check-sequence)欄位，兩者檢查無 誤，就可以送出回應，若是檢查發現無效，接收端直 接忽略收到的資料框
共用傳輸媒體的原理
ALOHA造成碰撞(collision)的機率極高，為改善 網路效能，有人提出slotted ALOHA，將通訊頻 道的使用分成一小段時間，約是將一個資料框 送上介質的時間，所有的網路節點在時間上必 須同步，只有在固定時段上才能傳送資料框， 也表示只有在這些時段上才會發生碰撞 由於傳統的區域網路上傳輸延遲常小於資料框 送上介質的時間，因此，若有一節點在傳送資 料，其他節點幾乎是立即察覺，假如節點在傳 送資料前能先感測是否有節點已經在使用介質 ，則碰撞情況應該可以降低
共用傳輸媒體的原理
CSMA(Carrier Sense Multiple Access)，就是 根據該原理改進，其運作原理如下： 1. 2.
網路節點送出資料框(frame)前必須感測介質是否有 其他節點在使用(即carrier sense)，假如使用中，則 必須等待 若是介質可使用，則立即傳送，若是同時有多個節 點傳送資料框，則發生碰撞會造成資料失誤，因此 網路節點送出資料框以後，要等待回應，超過時限 ，必須重新傳送
共用傳輸媒體的原理
由於碰撞仍會造成介質使用率降低，有人提出 CSMA/CD改善碰撞造成的問題，其運作原理如 下： 1. 2. 3.
假如介質可使用，則直接傳送資料框 假如介質忙碌中，繼續等待，直到介質可用為止 假如發生碰撞，送出簡短的壅塞訊號(jamming signal)，告知其他節點，然後繼續等待一段時間
從CSMA/CD運作原理可以發現碰撞偵測很重要 ，由於訊號傳送會衰減，影響碰撞的偵測 IEEE標準才規定10Base5網路的最大傳送距離 不超過500m，10Base2網路的最大長度不超過 200m，10BaseT不超過100m
MAC技術 ALOHA
Asynchronous ALOHA Slotted ALOHA
CSMA
CSMA/CD : collision detection CSMA/CA : collision avoidance Non-persistent P-persistent
存取控制技術的分類
MAC技術
隨機存取(random access)的方法，會有碰撞產 生的可能性，無線網路也運用類似的技術，例 如WLAN就是使用CSMA/CD 決定性存取(deterministic access)方式是無線通 訊網路中最常使用的
有趣的隱藏節點問題技術(一)
A、B、C三個節點，A與B或B與C都能互相通 訊，但是A與C距離太遠，無法通訊，因此A對 於C或是C對於A而言，都是對方的隱藏節點 (hidden node) 隱藏節點的問題是：A在範圍內沒有感測到有其 他的傳輸，所以送封包給B，此時C在範圍內也 沒有感測到有其他傳輸，所以也送封包給B，這 兩個封包在B發生了碰撞(collision )
有趣的隱藏節點問題(二)
有趣的隱藏節點問題(三)
假設A想送資料封包給B，則A先送RTS(request to send)封包給B，其內記載何時會出多少資料 給B，B送回CTS(clear to send)封包給A，這時 C會感測到CTS封包，等於間接感受到A的存在 CSMA/CA程序如下：
檢查載波是否存在，就是有無節點正在使用介質 假如載波不存在(no carrier)，檢查CTS表格看是否 有涵蓋範圍外的節點要傳送資料 假如載波不存在且CTS表格也顯示介質可以使用， 則開始傳送資訊
無線存取技術簡介(一)
用手機撥號之後，不久就可以連線通話，其間 發生很多事件是跟存取技術有關 所謂存取技術(access technology)是指使用者 連上通訊管道或網路的方式 以無線通訊來說，FDMA、TDMA與CDMA是最 常見的存取技術 各種不同的存取技術配合雙工(duplex)特性就產 生FDMA/FDD、TDMA/FDD、TDMA/TDD等技 術
無線存取技術簡介(二)
FDMA(Frequency Division Multiple Access)
使用不同的頻率來區別不同的傳輸通道 傳輸通道分成上傳(uplink)與下傳(downlink)兩大類 使用FDMA來存取的網路，不管是上傳或下傳，都 先取得一個目前沒有被使用的頻率，指定給該段通 訊(session)使用 FDMA的缺點是指定的頻率相近時容易彼此產生干 擾(interference)， 指定頻率與頻率再用的程序也比較沒有效率
無線存取技術簡介(三)
TDMA(Time Division Multiple Access)
CDMA(Code Division Multiple Access)
把同一頻率用時間間格分隔給多個用戶使用 使用展頻(spread spectrum) 技術，將訊號分成片段 (segment)，然後分到整個頻寬裡 使用封包交換的資料傳輸技術，對無線傳輸指定唯 一的辨識碼，可以用來決定傳輸發生的時間與地點
WCDMA(Wideband CDMA)
WCDMA是CDMA的延伸 支援封包交換與電路交換的資料傳輸，由於使用較 大的頻寬，因此傳輸速率較高
無線多重存取技術的簡單分類
無線多重存取技術多屬於所謂的決定性的存取 (deterministic access)技術，當節點需要頻道容 量(channel capacity)時必須向控制點(control point)提出請求 FDMA、TDMA與CDMA都是決定性的存取技術 ，請求的程序雖是系統的負擔，但能保證請求 者能得到所需要的頻寬 從頻段(band)對無線多重存取技術做簡單分類 窄頻段系統(narrowband):可用的無線電頻譜 會分成很多窄頻段的頻道 寬頻段系統(wideband):多個傳送器可以在同 一個頻道上傳訊，
媒體存取控制協定
通訊頻道是可以共享的，但需要適當的管理機 制，網路協定中的媒體存取控制協定(MAC protocols, Medium Access Control protocol) ， 負責訂定通訊頻道的共享規則 無線電媒體具有廣播的特性，當無線電涵蓋範 圍中有多個訊號源在相同頻率下同時送出訊號 ，會發生碰撞 CDMA、FDMA、TDMA與輪詢屬於預約類型不 發生衝突的媒體存取控制方式 ALOHA與CSMA則屬於雜亂隨機的存取控制方 式
各種多重存取技術
有線網路使用的存取協定不一定適用於無線網 路 Token bus與Token ring網路使用順序式 (ordered)MAC技術 ALOHA與CSMA則屬於雜亂隨機的存取控制方 式
無線多重存取技術的簡單分類
頻率分割多重存取
頻率分割多工(frequency division multiplexing) 將行動台與基地台間的空氣介面(air interface) 頻寬分割成數個類比頻道(analog channel)， 例如一個15MHz的頻譜可以分成多個200KHz 的頻道 假如雙向的傳訊各用一個FDMA頻道，則稱為 FDD(frequency division duplex)或FDD(full-full duplex) 頻率分割多重存取(FDMA, Frequency Division Multiple Access) 把頻寬分割成多個不同頻率的 頻道(frequency channel)，不同使用者利用不 同的頻道來傳訊
FDMA的基本原理
頻率分割多重存取
靜態的FDMA對於資料的傳送不是很好的選擇 FDMA的基本特徵：
一個FDMA頻道一次只能承載一通電話的電路，假 如頻道目前沒有使用中，無法讓其他使用者共享 一旦FDMA的語音頻道指定，基地台與行動台可以 在頻道上同時且連續地傳訊 FDMA常見於窄頻系統，因為FDMA頻道的頻寬通 常很窄，例如AMPS使用30KHz FDMA系統的複雜度比TDMA低，由於FDMA提供連 續的通訊，所以所需的額外控制資訊也較少 FDMA行動台需要使用雙供氣(duplexer)，因為接收 與傳送會同時作業，因此設備成本會增加
時間分割多重存取
TDMA(Time Division Multiple Access)將頻道 以時間切割，分成多個時槽(time slots) 每個時槽內只有一個用戶可以傳送或接收資料 例如一個封包含有N個時槽，則每個時槽就像 使用個別的頻道，對使用者而言，傳訊的過程 是不連續的 因此FDMA可以使用類比的FM(frequency modulation0，但TDMA僅能用於數位資料與數 位調變的情況
TDMA的基本原理
時間分割多重存取
TDMA封包結構顯示，一個封包含有數個時槽 以TDMA/TDD而言，有一半的時槽用於前向頻 道，另一半時槽用於反向頻道 在TDMA/FDD中，相同或類似的封包結構個別 使用於前向頻道與反向頻道，但兩個頻道的載 波頻率是不同的 TDMA frame中的preamble欄位含有基地台與 行動台的位址與同步資訊，讓彼此間能相互辨 識
時間分割多重存取
對使用者而言，資料的傳輸不是連續，而是分 段發生(burst)，也讓行動台的電能消耗降低， 因為用戶傳送器沒有作用時可以暫時關閉 TDMA的傳輸是片段的，所以接收端在每次傳 送中必須同步，且不同使用者之間要用保護時 槽(guard slots)分隔 TDMA可以把不同數目的時槽分給不同的使用 者，達成所謂的頻寬隨取(bandwidth on demand)的功能
CDMA的基本原理
無線頻道的共用存取技術
OFDM傳送端的處理(transmitter chain)
OFDM中的tone
其他的多重存取技術 Hybrid FDMA/CDMA (FCDMA) Hybrid direct sequence/frequency hopped multiple access (DS/FHMA) Time division CDMA (TCDMA) Time division frequency hopping (TDFH)
Q&A 1 G與2 G行動無線系統的差別在那裡 ? 骨幹化(trunking)與多重存取(multiple access)的涵義有何差異 ?
IEEE 802.11技術簡介
Outline IEEE 802.11 Family IEEE 802.11 Standard’s Evolution Management features of 802.11 How does a station join an existing cell Competing technologies to IEEE 802.11
Adv. vs. Disadv. Advantages
Without cabling for client devices Low cost “Wi-Fi Certified” Widely available …
Disadvantages
Spectrum assignments not consistent worldwide Power consumption Limited range Wi-Fi pollution …
802.11 Family 802.11 a
High-speed Physical Layer in the 5GHz Band
Finished
b
Higher-speed Physical Layer Extension in the 2.4GHz Band
Finished
d
Specification for operation in additional regulatory domains
Finished
e
MAC Quality of Service (QoS) Enhancements
f
Inter-Access Point Protocol (IAPP)
Finished
g
Further Higher Data Rate Extension in the 2.4 GHz Band
Finished
h
Spectrum and Transmit Power Management Extensions in the 5 GHz band in Europe
Finished
i
MAC Security Enhancements
Finished
j
4.9 GHz – 5 GHz Operation in Japan
Finished
k
Specification for Radio Resource Measurement
m
Revision of 802.11
n
Enhancements for Higher Throughput
p
Wireless Access for the Vehicular Environment
r
Fast Roaming/Fast BSS Transition
s
ESS Mesh Networking
t
Recommended Practice for Evaluation of 802.11 Wireless Performance
u
Interworking with External Networks
v
Wireless Network Management
Standards Evolution of 802.11 Mesh Extensions
802.11s
QoS Extensions
802.11e, 802.11r
Security Extensions Radio & Regulatory Higher Data Rates
802.11i 802.11d, 802.11h, 802.11j, 802.11k 802.11b, 802.11a, 802.11g, 802.11n
Standard Evolution Goal
Quality of Service Security Interconnection Performance & Coverage Performance improvement Modulation Scheme Channel Bonding Frame Burst mode MISO, MIMO Coverage extension Smart Antenna MIMO
802.11 Family 802.11: the original standard with 1Mbit/s and 2Mbit/s in the 2.4GHz band. 802.11a: High-speed physical layer (OFDM) operates in 5 GHz band with a maximum raw data rate of 54 Mbit/s
802.11b: Enhancements to 802.11 to support 5.5 and 11 Mbit/s FHSS, DSSS Operates in 2.4 GHz band
802.11d: International (country-to-country) roaming extensions (2001)
802.11e: Medium Access Control (MAC) Quality of Service (QoS) enhancement
802.11f: Inter-Access Point Protocol
802.11g: Applied OFDM to the 2.4 GHz band Data rate up to 54 Mbit/s backwards compatible with b
802.11h provides Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) to the 802.11a MAC solves problems like interference with satellites and radar using the same 5 GHz frequency band
802.11i Medium Access Control (MAC) security enhancements
802.11j designed specially for Japanese market allows operation in 4.9 to 5 GHz band
802.11n Higher throughput improvements using MIMO (multiple input, multiple output antennas)
Primary IEEE 802.11 Specifications 802.11b
802.11a
802.11g
802.11n
Standard Approved
Dec-99
Jan-00
Jun-03
Dec-07 (Expected)
Maximum Data Rate
11 Mbps
54Mbps
54 Mbps
600 Mbps
Different Data Rates Configurations
4
8
12
576
Typical Range
70m
30 m
50m
60m
Modulation techniques
DSSS, CCK
OFDM
DSSS, CCK, OFDM
DSSS, CCK, OFDM (HT)
RF Band
2.4GHz
5GHz
2.4GHz
2.4/5GHz
Number of Spatial Streams and Antenna
1
1
1
1-4
Channel bandwidth
20MHz
20MHz
20MHz
20 or 40MHz
Number of Non-Interleaving Channels
3
23
3
26
Pros & Cons Standard
Pros
Cons
802.11b
lowest cost; signal range is good and not easily obstructed
slowest maximum speed; home appliances may interfere on the unregulated frequency band
802.11a
fast maximum speed; regulated frequencies prevent signal interference from other
highest cost; shorter range signal that is more easily obstructed
802.11g
fast maximum speed; signal range is good and not easily obstructed
costs more than 802.11b; appliances may interfere on the unregulated signal frequency
802.11n
fastest maximum speed and best signal range; more resistant to signal interference from outside sources
standard is not yet finalized; costs more than 802.11g; the use of multiple signals may greatly interfere with nearby 802.11b/g based networks
Introduction of 802.11n SISO and diversity antennas Single Input and Single Output is common architecture.
Introduction of 802.11n Antenna Diversity To reduce the effects of multipath, dropouts on signal quality and throughput. Two independent antennas and radio systems are used for transmitting and receiving signals. Voting processor choose which radio has better signal path to client.
Introduction of 802.11n Diversity Reception
Antenna A
Antenna B
Reflected Path LOS Path
Receiver A
Receiver B
Voting Processor
or
How does MIMO works?
A data stream is divided into multiple unique streams. Data streams are transmitted at the same time. MIMO takes the advantage of multipath. MIMO receiver combines all streams.
Radio
Radio
DSP
DSP Radio
MIMO enabled client
Radio
MIMO enabled AP
Advantage of 802.11n Overcoming the multipath effect of RF Higher performance More coverage
What is 802.11n
New IEEE Standard (not approved yet) Uses MIMO radio technology Provides “wire-like� performance Data rate is from 100Mbps to 600Mbps which depends on implementation Supports both 2.4 GHz and 5 GHz Includes advances in QoS & Power Saving High definition video mode at 5 GHz Uses multiple streams Less interference and more channels
Major components of 802.11n Features
Definition
Standard Status
Better OFDM
Supports wider bandwidth & higher code rate (max. data rate -- 65Mbps)
Mandatory
SpaceDivision Multiplexing
Multiple streams transmitted through multiple antennas
Optional Up to 4 spatial streams
Diversity
Exploits the existence of multiple antenna to improve range and reliability
Optional Up to 4 antennas
MIMO Power Save
Limit power consumption by utilizing multiple antennas only on as-needed basis
Mandatory
Major components of 802.11n (cont) Features
Definition
Standard Status
40MHz Channels
Doubles data rates by doubling bandwidth from 20MHz to 40MHz
Optional
Aggregation
Allowing bursts of multiple data packets between overhead communications
Required
Reduced Inter-frame Spacing (RIFS)
Provides a shorter delay Required between OFDM transmissions than in 802.11a or g
Greenfield Mode
Eliminates support for 802.11a/b/g devices in an all 802.11n network to improve efficiency
Optional
Time to transfer 30 min HD video.
Legacy 802.11a/g AP
<5Mbps
5-10Mbps
10-15Mbps
20-30Mbps
15-20Mbps
>40Mbps
30-40Mbps
MIMO Technology
<5Mbps
5-10Mbps
10-15Mbps
20-30Mbps
15-20Mbps
>40Mbps
30-40Mbps
Management Issues Without tethering a wired network, the wireless medium is unreliable. Users can take advantage of the lack of physical boundaries. For a mobile devices, power consumption is always a critical problem.
Management Architecture MAC
MLME MAC MIB SME
PHY
PLME PHY MIB
Relationship between management entities and components of 802.11 specification
MLME: MAC layer management entity PLME: PHY layer management entity SME: Station management entity MIB: Management information base
How to join an existing cell Scanning Authentication Association
Scanning What is “scanning”?
The802.2 process of identifying existing networks in the area. IEEE What are theControl parameters Logical Link (LLC) used in the scanning OSI
procedure?
IEEE 802.2 (independent, infrastructure, or MAC BSSType both) Media Access Control (MAC) BSSID (individual or broadcast) Frequency Infrared 802.11a Direct SSID (network name) 802.11b Hopping Sequence (IR) 802.11g ScanType (active or passive) Spread Spread PHY … Spectrum Spectrum ChannelList (FHSS) (DSSS) With DS products, it is a list of channels. With FH products, it is a hop pattern
Layer 2 (Data Link)
OSI Layer 1 (Physical)
ProbeDelay MinChannelTime The minimum time to spend on each channel when scanning. â&#x2030;§ ProbeDelay
MaxChannelTime The maximum time to spend on each channel when scanning. â&#x2030;§ MinChannelTime
Passive Scanning Passive scanning saves battery power. In passive scanning, a station moves to each channel on the channel list and waits for Beacon frames. AP1
AP2 Found BSSs: BSS 1, AP1 BSS 2, AP2 BSS 2, AP2
AP3
Active Scanning For each channel to be scanned, Move to the channel and wait for either an indication of incoming frame or for the ProbeDelay timer to expire. If an incoming frame is detected, the channel is in use and can be probed. The timer prevents an empty channel from blocking the entire procedure.
Send Prove Request frame. Wait for MinChannelTime and check if channel is busy If idle, there is no network. Move to next channel. If busy, wait until MaxChannelTime and process any Probe Response frames.
Probe Probe response request
AP1
Probe response
AP2
DIFS
STA
AP1
AP2
Min Response SIFS Time
Probe Request
DIFS
SIFS
Ack
Ack
Probe Response
Probe Response
Scan Report The report lists all the BSSs that the scan discovered and their parameters.
BSSID BSStype SSID Beacon intervals (integer) DTIM period Timing parameters: Timestamp, an offset PHY, CF, IBSS parameters BSSBasicRateSet: Stations must be able to receive data at all the rates listed in the set.
Joining After compiling the scan results, a station can elect one of the BSSs to join. Choosing which BSS to join is an implementationspecific decision and may even involve user intervention. Common criteria: Power level, signal strength.
The joining process involves matching local parameters to the parameters required by the selected BSS. One of the most important tasks is to synchronize timing information.
Authentication Wireless networks are attractive in large part because physical
access is not required to use network resources. The authentication process only proves the identity of one station (not mutual authentication). Network administrators may wish to authenticate mobile stations, but mobile stations can not authenticate the AP. One-way street: stations authenticate to network. A man-in-the-middle attack: a rogue AP could send Beacon frame to steal Authentication information. Two approaches: open-system and shared-key authentication.
Open-system authentication. 1: From-source Authentication algorithm - 0 (open system) Sequence number - 1
AP 2: Authentication algorithm - 0 (open system) Sequence number â&#x20AC;&#x201C; 2 Status code
Open-system authentication exchange
The identity of any station is its MAC address.
Shared-key Authentication Shared-key authentication makes use of WEP (Wired Equivalent Privacy). It requires that a shared key be distributed to stations before attempting authentication. The challenge text is composed of 128 bytes generated using the WEP keystream generator with a random key and IV.
1: From-source Authentication algorithm - 1 (shared key) Sequence number - 1 2: Authentication algorithm - 2 (shared key) Sequence number – 2 Status code – 0 (successful) Challenge text (clear)
AP
3: Authentication algorithm - 2 (shared key) Sequence number – 3 Challenge text 4: Authentication algorithm - 2 (shared key) Sequence number – 4 Status code – 0 (successful)
Shared-key authentication exchange
The challenge text is composed of 128 bytes generated using the WEP keysteam generator with a random key and IV.
Preauthentication
AP1
AP2
Authentication
BSS1
BSS2
Stationsignal AP2â&#x20AC;&#x2122;s is associated moves begins isright using stronger, into with the network the so AP1 station overlap BetweentoBSS1 decides moveand association BSS2 to AP2
Data
Reassociation
AP1
AP2 Preauthentication
BSS1
BSS2
Stationsignal AP2â&#x20AC;&#x2122;s is associated moves begins isright using stronger, into with the network the so AP1 station overlap BetweentoBSS1 decides moveand association BSS2 and todetects AP2 the presence of AP2
Data
Reassociation
Association Association is recordkeeping procedure that allows
the distribution system to track the location of each mobile station. After association completes, an AP must register the mobile STA on the network so frames for the mobile STA are delivered to the AP. One method of registering is to send a gratuitous ARP. 802.11 explicitly forbids associating with more than one AP.
Associated procedure 1: Association request 2: Association response â&#x20AC;&#x153;AIDâ&#x20AC;?
AP
3: Traffic
Association request & reply are unicast frames. (ACK is required)
Reassociated procedure Old AP
3: IAPP “Please send any buffered frames 1: Reassociation request for…” “My old AP was…” 2: Reassociation response “I am your new AP, and here is New AP your new AID” 5: “Here are some frames buffered from your old AP.”
4: IAPP “send buffered frames for…”
Reassociation with the same AP
BSS
2: Reassociation exchange
AP
Other Authentication Methods RADIUS (Remote Authentication Dial In User Service) Account/Password
Registration of MAC address of mobile station 802.1x (WIRE1x)
States of Authentication and Association Unauthenticated and
State 1 Unauthenticated, Unassociated
Successful Authentication
DeAuthentication Notification
DeAuthentication Notification
DeAssociation Notification
unassociated
The node is disconnected from the network and not associated to an access point.
Authenticated and unassociated
State 2 Authenticated, Unassociated Successful Association or Reassociation State 1 Authenticated, Associated
The node has been authenticated on the network but has not yet associated with the access point.
Authenticated and associated The node is connected to the network and able to transmit and receive data through the access point.
Power Conservation Powering down the transceiver can lead to great power savings in wireless networks. sleeping, dozing, PS mode vs. awake, active, on Power conservation Minimizing the time spent in the awake stage. Maximizing the time in the PS mode.
Power Management in Infrastructure Networks In infrastructure networks: APs remain active at all time. APs are aware of the location of mobile STAs. STA can communicate its power management state to its AP. All traffic must go through APs. APs are an ideal location to buffer traffic.
APs play a key role on power management in infrastructure networks.
Power Management in Infrastructure Networks (cont) AP have two power management-related task: Buffering frames AP can determine whether a frame should be delivered to the wireless network. Frames should be buffered if the station is asleep.
Periodic announce buffer status AP should periodically announce which stations have frames waiting form them. A station only needs to power up the transmitter to transmit polling frames when there are buffered frames for it.
Power Management in Infrastructure Networks (cont) Listen Interval is the number of beacon periods for which the mobile station may choose to sleep. Longer listen intervals require more buffer space on the access point.
AP must agree to wait for at least the listen interval before discarding frames. If a mobile station fails to check for waiting frames after each listen interval, they may be discarded without notification.
Unicast frame buffering and delivery When frames are buffered, the destination nodeâ&#x20AC;&#x2122;s AID provides the logic link between the frame and its destination. (Multicast or Broadcast: AID = 0) APs assemble a TIM and transmit it in Beacon frames. TIM is a virtual bitmap composed of 2008 bits (251 B). Offsets are used so only a small portion of the virtual bitmap needs to be transmitted. Each bit corresponds to a particular AID.
Unicast frame buffering and delivery (cont) To retrieve buffered frames, mobile STA use PS-Poll Control frame. When multiple stations have buffered frames, all STAs with buffered data must use random backoff algorithm to transmit the PS-poll. Each PS-Poll is used to retrieve one buffered frames. ACK is required.
Buffered frame retrieval process Listen Intervals: 2 for STA1, 3 for STA2 BI TIM: Frame for 1
TIM: Frames for 1 & 2
TIM: Frame for 2 TIM: Frames TIM: No Frame TIM: No Frame for 1 & 2 Busy
AP PS-Poll
PS-Poll
STA1
Busy
STA2 CW Defer
PS-Poll frame retrieval AP PS-Poll Time
Frame 1, more data ACK PS-Poll Frame 1, more data
ACK PS-Poll Frame 2
ACK
STAs may switch from a PS mode to active mode at any time. If a STA switches to active mode, frames can be transmitted without waiting for a PS-Poll. APs use an aging function to determine when buffered frames are old enough to be discarded. Aging function should not discard frames before he listen interval has elapsed.
Delivering multicast and broadcast frame: DTIM Buffered broadcast and multicast frames are saved using AID 0. DTIM period Buffered broadcast and multicast traffic is transmitted after DTIM beacon. The AP may choose to defer the processing of incoming PS-Poll frames.
Delivering multicast and broadcast frame: DTIM Listen Interval: 3 for STA1 DTIM interval
BI TIM
DTIM BC
TIM MC
TIM
DTIM
TIM BC
MC
AP PS-Poll BC
MC
BC
MC
STA1
Multicast and broadcast buffer transmission after DTIMs.
Timer synchronization Wireless network technologies depend a great deal on the distribution of timing information. Timing information is especially important in frequency-hopping networks.
Timer synchronization Timing information is especially important in frequency-hopping networks Timing synchronization function (TSF) is based on a 1-MHZ clock. (one tick = 1 micro sec)
Infrastructure timing synchronization APs are responsible for maintaining the TSF time, and any STAs associated with an AP must simply accept the APâ&#x20AC;&#x2122;s TSF as valid. Associated STAs maintain local TSF so that they can still remain roughly synchronized with the global TSF when missing a Beacon frame. Network timer
Timestamp + Local offset
Beacon/ Probe Response Local offset
Local timer Save TSF value
Begin join process
Matching the local timer to a network timer
Competing technologies Bluetooth
Version
Data Rate
Version 1
1 Mbps
Version 2.0 + EDR
3 Mbps
WiMedia Alliance (3.0?)
53 ~ 480 Mbps
ZigBee IEEE 802.15.4 operation in the unlicensed 2.4 GHz, 915 MHz and 868 MHz ISM bands 250kbps
Competing technologies UWB FCC authorizes the unlicensed use of UWB in 3.1–10.6 GHz. 100 Mbps ~ 400 Mbps
HomeRF 2.4 GHz, WBFH (Wide band frequency hopping) 10 Mbps, 150 ft.
3GPP The 3rd Generation Partnership Project 3.9G (HSDPA – 3.5G) 100Mbps
WiMAX
Road Map of Wireless Communication FWBA 802.16a/b/d
Distance
2008/9
MWBA 802.20
2006
10km
2.5G
3.5G
3G
WAN
2010
4G/LTE 2006/7
M WiMAX 802.16e
5km
MAN
2km WLAN 802.11b
100m
WLAN 802.11g
WLAN 802.11n
2007
802.11a HyperLAN2
Zigbee 2004 802.15.4 2005
10m
802.15.4a
Bluetooth 802.15.1/1a
1Mbps
Bluetooth 2.0
2005
50Mbps
LAN
2006
UWB 802.15.3a
100Mbps
PAN
Bandwidth
References IEEE 802.11 standard 802.11 Wireless Networks The Definitive Guide, Oreilly Demystifying MIMO and 802.11n, Peter Reinders, Bluesocket, Inc 802.11n: Next-Generation Wireless LAN Technology, Broadcom 802.11 Technologies: Past, Present and Future, TROPOS networks