[國考-電信類] 01 計算機概論-01 電腦網路 筆記(五)

第五章. DataLink Layer 資料鏈結層

1. Error Detection    Logical Link Control (LLC)

• Parity checks: 
• even(偶數個1), odd(奇數個1) -> detect single bit error
• two dimensional bit parity -> detect and correct single bit error
• 缺: 錯一串資料，不一定能偵測，無法更正
• checksumming:
• detect "errors" (e.g. flipped bits) in transmitted segment (only transport layer)
• 優: easy and fast to compute [in TCP/UDP]
• cyclic redundancy check
• 可偵測連續n bits的errors, e.g. CRC32 -> 32 bits

2. Multiple access protocols    Media Access Control (MAC)

• Channel Partitioning protocols -> 適合heavy loading
• TDMA (time division multiple access): 缺點是不能搶BW，不用浪費，無法達到rateR
• FDMA (frequency division multiple access): 缺點是沒用到的band浪費，無法達到rateR
• CDMA (code division multiple access): 所有的user分享一樣的frequency，但使用自己的code去編碼；通常用在無線寬頻(cellular satellite)
• Random access protocols -> 適合light loading
• Slotted ALOHA: collision之後，以P的機率決定下次slot是否傳送data；缺點是collision wasting slots，會有idle slot
• ALOHA: 有packet就立即送出，collision機會變大
• CSMA/CD (carrier sense multiple access with collision detection):
• channel idle -> 傳送；channel busy -> wait idle
• collision -> Jam signal, then exponential back off K={0,1,~,2^m-1}, m=min(n,10), t=K*512 bit times [slot time=0.1us * 512=51.2us]
• Taking-truns protocols: polling / token passing

3. Address Resolution Protocols (ARP): 建立IP/MAC的對照表

4. Ethernet: 46~1500 bytes (+18 bytes header), CSMA/CD

• 10Base2: 10Mbps, max 2m; 10BaseT: Twisted pair
• Hubs: physical-layer repeaters
• Switch: link-layer device, store and forward
• Router: network-layer device
• Manchester encoding: 1: up to down; 0: down to up, 讓sender和receiver同步，避免一連串0/1的資料

5. Point-to-point protocol (PPP): 點對點(2點), no multiple access control

6. Asynchronous Transfer Mode (ATM): 建立連線來保留資源，QoS