基础计算机网络——链路层(Link Layer)笔记

【Data-link Layer 数据链路层】

1. responsibility: transfer datagrram from one node to adjacent node over a link

【Link layer servicesl 链路层服务】

1. framing, link access

2. reliable delivery between adjacent nodes

3. flow control, error detection and correction

4. half-duplex and full-duplex

【Link layer implementation 数据链路层的实现】

1. NIC: network interface card

2. Combination of software and hardware

【Error detection 错误侦测】

1. Parity checking:

    A. Single bit parity (even or odd parity)

    B. Two dimensional bit parity (divide d bits into i rows and j cols)

2. Internet check-sum 

3. CRC (Cyclic redundancy check)   

    D*2^rXOR R=nG

    R=remainder[( D*2^r)/G]

【Multiple Access Protocol 多路访问协议】

1. Distribute algorithm that determines how nodes share channel

2. IMAP: Ideal Multiple Acess Protocal (One node, rate=R; M node, rate=R/M)

【MAC Protocol 多路访问控制协议】

1. Channel partitioning: divide channel into smaller "pieces"

2. Random access: channel not divided, allow collision

3. Taking turns: nodes take turn, but node with more to send can take longer turns

【Channel partitioning 分道技术】

1. TDMA: Time Division Multiple Access

2. FDMA: Frequency Division Multiple Access

【Random Access 随机访问】

1. core: how to detect collisions and how to recovery from collisions

2. slotted ALOHA

    A. If no collision, node can send new frame into next slot; If collision, retransmit frame in each subsequent slot with probability p until           success

    B. Probability that any node has a success=NP(1-P)^N-1

3. Pure ALOHA

    probability that and node has a success=NP(1-P)^2(N-1)

4. CSMA and CSMA/CD

    Listen before transmit

【Taking turns 轮流访问】

1. Master node invites slave node to transmit in turn

2. Token passing concerns: token overheaded; latancy; single point failure

【Link layer address 链路层地址】

1. Mac address: get frame from one interface to another physically-connected interface

2. The length of mac address: 48 bits in hexadecimal （16进制）

【ARP 地址解析协议】

1. ARP-Address Resolution Protocol: address mappings for some LAN nodes in the form <IP addr; Mac Addr; TTL (time to live)>

2. Sender broadcasts ARP query and the receivers reply its mac address by identified IP address

【Ethernet 以太网】

1. start topology: active switch in center

2. connectionless and unreliable

3. protocol: unslotted CSMA/CD

4. delay: k*512 where k in {0,1,2,..., 2ⁿ}, n is the number of collisions

【Switch 交换机】

1. Hub: no frame buffering; no CSMA/CD

2. Switch: forward frame; transparent; plug and play; self learning

3. Allow mmultiple simultaneous transmissions

4. Own switch table

5. Frame destination location unknown: flood; destination location known: selective send

6. Switch vs Router

SwitchRouterlink layer devicenetwork layer devicemaintain switch tablemaintain routing tableimplement filtering, learning algorithmimplement routing algorithm