# 3. Network Access

* Physical Layer
  * Type&#x20;
    * Wired
      * Copper cable
        * Unshielded Twisted-Pair (UTP)&#x20;
          * Ethernet Straight-through&#x20;
          * Ethernet Crossover
          * Rollover
        * Shielded Twisted-Pair (STP)&#x20;
        * Coaxial&#x20;
      * Fiber-optic cable&#x20;
        * Single-mode (laser)
        * Multimode (LED)
    * Wireless
  * Connector
    * UTP
      * RJ45
    * Fiber
      * Straight-Tip (ST)&#x20;
      * Subscriber Connector (SC)&#x20;
      * Lucent Connector (LC) &#x20;
  * Function
    * **Encoding**
    * **Signaling**
      * Asynchronous
      * Synchronous
    * **Modulation**&#x20;
      * Frequency modulation (FM)
      * Amplitude modulation (AM)
      * Pulse-coded modulation (PCM)&#x20;
* Measurement
  * Bandwidth (Mbps)
  * throughput
  * goodput
* Data Link Layer
  * Logical Link Control (LLC)
  * Media Access Control (MAC)
    * Contention-based access
      * Carrier sense multiple access with collision detection (CSMA/CD)
    * Controlled access
      * Carrier sense multiple access with collision avoidance (CSMA/CA)
  * Frame (have MAC address)
    * Header
    * Data
    * Trailer
  * Ethernet
    * 802.3
    * MAC / Physical Address
      * 48 bits
    * Address Resolution Protocol (ARP)  通过IP地址获取mac地址
      * resolve IPv4 to MAC
      * threat
        * ARP spoofing&#x20;
  * Mode
    * Half Duplex
    * Full Duplex
  * Auto-MDIX

Data link layer is responsible for the exchange of **frames between nodes over a physical network media**. It controls **media access control and performs error detection**. Data link layer also **accepts Layer 3 packets and packages them into data units called frames**.

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