# 無線網路導論 期中考
###### tags: `大學部課程`
# Topic #1 MAC
- MACA: Multiple Access with Collision Avoidance (RTS/CTS)
- DCF: CSMA + RTS/CTS + ACK
- CTS的功能：
- 同意傳送
- 警告鄰居
- MACA: 聽到CTS安靜 -> 解決hidden/exposed terminal
- DCF: 聽到RTS/CTS都安靜 -> 沒解決exposed terminal
- Aloha: 想傳就傳
- slotted Aloha
- DAMA: demand assigned multiple access
1. Explicit reservation: 使用小slot進行競爭(ALOHA)，再依照預約使用通道
2. Implicit Reservation: PRMA (packet reservation multiple access)
直接競爭packet slot，搶到就可以一直用，當slot空了就搶下個slot
3. Reservation TDMA：
N mini slots，sta各自預約至多 K 個data slots，共N*K data slot
- Polling： Bluetooth and 802.11 PCF
- CDMA: 使用正交碼進行編碼解碼
- 怕蓋台
# Topic #2 WLAN (Part I)
### IEEE 802.11
- 包含PHY和MAC層
- 僅PYH層不同
- 使用CSMA/CA
- 可變速度
- AP、Ad hoc模式
- MAC Address: 6 Bytes
- Infrastructure:
- STA
- AP
- BSS: basic service set
- ESS: same subnet
- Ad hoc
- STA only
- IBSS: indepedent BSS
- Every STA act as a router
- Association
- AP 定期發Beacon (SSID, MAC)
- STA associate with AP
- Scanning
- Passive
- Active
### 802.11 MAC
+ Traffic services
- DCF
- PCF
+ Access methods
- DCF CSDM/CA: random back-off
- DCF w/ RTS/CTS: avoid hidden terminal
- PCF: polling
+ Priorities
- 先等IFS，若空直接送，不然等back-off
- SIFS: For ACK, CTS, polling response
- PIFS: For PCF
- DIFS: For DCF
- CFP and CP: PCF架於DCF之上
- Superframe: CFP+CP frames
- NAV: network allocation vector CFP長度
- Random Back-Off
- 若等完IFS，通道使用中，倒數RBO，避免Collision
- 當別人搶到通道，倒數暫停 (fairness and starvation avoidance)
- 通道空了後，再等IFS和剩下的back-off time
- DCF w/o RTS/CTS
- 傳送後等待ACK
- 沒收到ACK，增加RBO
- 接收端ACK只要等SIFS
- receiver透過CRC判斷要不要ACK
- Broadcast不用ACK
- DCF w/ RTS/CTS
- 等DIFS，透過RTS競爭通道
- Data超過RTS Threshold才用
- 搶到後，RTS、CTS包含NAV叫大家安靜
- 這時Data只要等SIFS就可以傳
- PCF: Point Coordination Function
- PC等PIFS後開始Poll，之後大家等SIFS傳資料/Poll
- PC等PIFS後沒回就換下個STA
- addressing (MAC addr)
- 透過addr1判斷是不是給自己的
- from AP:
1. DA (STA)
2. BSSID (AP)
3. SA (router)
- To AP:
1. BSSID (AP)
2. SA (STA)
3. DA (router)
- AP to Router
- Dest: router
- From: STA
- AP隱形了
### Power Managerment
- Active Mode (AM)
- Power-saving mode (PSM)
- AP 在Beacon中放 TIM
- TIM: traffic indication map
AP這裡有buffer哪些STA要收的資料
- STA定期醒來聽TIM，被點到就發PS poll並收資料
- DTIM = Delivery TIM
表示接下來要送Broadcast封包，都不准睡
- Rate Adaption
- 透過ACK收到狀況決定速率
- 連續10個ACK就升速
- 漏掉 2個ACK就降速
# Topic #2 (Part II) Bluetooth, Cellular
## Bluetooth
### 架構
- less than 10m
- fast frequency hopping: reduce interference
- short packets
- piconet: basic network
- 1 master upto 7 slaves
- master poll slave, slave 回 data or NULL
### Addressing
- BD_ADDR: 48 bits MAC addr
- AM_ADDR: 3 bits active slave addr
- PM_ADDR 8 bits parked slave addr
### 特徵
- 2.4GHz,79 sub-channels
- FHSS: frequency hopping at 1600 hops/s
- TDD: 奇數slot上行，偶數slot下行
- SCO、ACL
### 連接
- 透過inquiry和page交換BD_ADDR、CLOCK
- M: Inquiry -> S: Inquiry scan, response -> M
- M: page -> S: page scan, response -> M
- M: response -> S: connection -> M: connection
- Inquiry: M蒐集S
- Page: M提供資料
### Hop seq
- Master BD_addr -> hop seq
- Master Clock -> 大家同步
### Mixed Link types
- SCO: sync connection oriented
- circuit switching
- voice
- reserve time slot
- 保證頻寬
- ACL: async
- packet switching
- data
- 1,3,5 slots packet size
- 上下行可以不同大小
- 可能被別人用掉時間，造成延遲，所以只能傳非同步資料
- 可以廣播
### Low Power mode
- Hold Mode
- 小睡一下
- 仍持有AMA
- ACL Lost就算了
- Sniff Mode
- 定時醒來聽ACL
- 仍持有AMA
- ACL Lost就算了
- Parked Mode
- 定時醒來收beacon
- 歸還AMA
- 不能參與傳輸
## Cellular Internet
### Components
- cell
- BS
- FDMA/TDMA
- CDMA
- MSC(mobile switching center)
- manage multi BS
- connect to wired PSTN
- manages call
- handles mobility
- BS frequency planning
- PSTN
### Call setup
- control, traffic channel 使用不同channel
### 2G cellular (voice) network
- BSC
- serve tens of BTSs
- allocate BTS radio channels
- paging (finding cell)
- handoff (mobile user)
- MSC
- 收錢
- 建通話
- handles mobility (cross BSCs)
- Gateway MSC
- 整理 MSC資料
- 到公用電話系統
### 3G
- data network operates in parallel
- RNC (radio network controller)
- 除了voice還處裡data分給MSC和SGSN
- SGSN (serving GPRS Support Node)
- delever datagram
- interact with MSC 來收錢
- GGSN (Gateway GSN)
- similar to a gateway router
- connect multi SGSN to internet
- MSC
- GMSC
### 4G
- voice data皆用IP封包傳送
- 針對多媒體分配資源
- 把時間頻率分成小格(RU)，依照RUs收錢

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## Topic #3 (Part I)
### 自分で読む。