CCNA1 Modules 4-7 Study Notes

OSI Physical Layer Concepts

• Definition: The Physical layer transmits bits across local media and creates signals representing frames.
• Key Performance Metrics:     * Bandwidth: The capacity of a medium to carry data.     * Throughput: The measure of the transfer of bits across a medium over a given period ($10/100/1000\,Mb/s$), influenced by traffic, latency, and network devices.     * Latency: The amount of time, including delays, for data to travel from one point to another.     * Goodput: The measure of usable data transferred over a given period of time.
• Signal Terms:     * Modulation: The process by which one wave modifies another wave.     * Attenuation: The loss of signal strength as distance increases.

Transmission Media and Interference

• Copper Cabling:     * Uses unshielded twisted-pair (UTP), shielded twisted-pair (STP), or coax.     * Interference: Vulnerable to electromagnetic interference (EMI), radio frequency interference (RFI), and Crosstalk (distortion from signals in adjacent wires caused by magnetic fields).     * Mitigation: Twisting opposing wire pairs and proper grounding reduces crosstalk.     * Cables: Straight-through connects PC to switch; crossover connects similar devices; rollover is for console access.
• Fiber-Optic Cable:     * Uses light propagation; immune to EMI/RFI.     * Uses two strands for full-duplex connectivity.     * Offers greater bandwidth potential and longer distances but higher cost than UTP.
• Wireless:     * Uses air for microwave transmissions; susceptible to interference from cordless phones and microwaves.

Data Link Layer and Frame Structure

• Functions: Packages Layer 3 PDUs into compatible frame formats; manages media access (MAC); provides physical addressing.
• Sublayers:     * LLC (Logical Link Control): Implemented in software; identifies which Layer 3 protocol ($IPv4$ or $IPv6$) is being used; communicates with upper layers.     * MAC (Media Access Control): Responsible for data encapsulation and placement/retrieval of frames on/off the media.
• Frame Components:     * Header: Contains destination/source physical addresses; varies by protocol.     * FCS (Frame Check Sequence): Field in the trailer containing a CRC (Cyclic Redundancy Check) value to verify frame integrity/detect transmission errors.     * Trailer: Contains error detection (CRC).

Network Topologies and Media Access

• WAN Topologies:     * Mesh: High availability.     * Partial Mesh: Connects some, but not all, remote sites.     * Point-to-Point: Direct connection between two nodes.
• LAN Topologies:     * Extended Star: End devices connect to central intermediate devices, which connect to other central devices.
• Media Access Methods:     * CSMA/CD: Carrier Sense Multiple Access with Collision Detection, used by Ethernet. All devices listen before transmitting.     * CSMA/CA: Used by $802.11$ wireless.     * Duplexing: Full-duplex does not require media arbitration; half-duplex requires contention-based rules.

Ethernet Switching and MAC Addressing

• Switching Methods:     * Store-and-Forward: Receives the entire frame and performs a CRC check before forwarding; provides error checking but higher latency.     * Cut-Through: Lowest latency; forwards frame immediately after reading destination MAC. Includes Fast-forward and Fragment-free modes.
• MAC Table Operations:     * Builds tables using source MAC addresses.     * Forwards frames based on destination MAC addresses.     * If a destination MAC is unknown or is a broadcast ($FF-FF-FF-FF-FF-FF$), it floods the frame out all ports except the ingress port.
• Buffering: Shared memory buffering is ideal for switching between different port speeds (e.g., $1000BASE-T$ to $100Base-T$).
• Auto-MDIX: Features that automatically configure an interface for straight-through or crossover cable connections.
• Multicast Address: Example includes $01-00-5E-00-00-03$.