Chapter 6: Link Layer (LANs)

🔹 1. Link Layer (LANs): Overview & Goals

✅ What Is the Link Layer (LANs)?

• Responsible for transferring datagrams between adjacent nodes

• Includes:
• Error detection and correction
• Broadcast medium sharing
• Address resolution (e.g., ARP)
• Ethernet, switches, VLANs

• Key to understanding LANs, wireless, and datacenter networks

✅ Main Goal: Framing, Addressing, Multiple Access, Switching, VLANs, MPLS

🔹 2. Key Link Layer Functions

✅ Error Detection & Correction

✅ Multiple Access (MAC) Protocols

🧠 Random access vs channel partitioning – different trade-offs

✅ Switches, VLANs, and MPLS

• Switches: Mac addresses, flooding, learning, forwarding

• VLANs: Virtual LAN, port-based or MAC-based, traffic isolation

• MPLS: Label-switching, fast forwarding, Cisco, Juniper, Google B4

📌 Packet forwarding on switches is crucial for link-layer efficiency and LAN performance

🔹 3. Basic Link Layer Services

✅ Link Layer Responsibilities

• Encapsulates IP datagram into Ethernet frame

• Adds error detection (CRC)

• Performs MAC (Media Access Control): decide when to transmit

• Maintains a forwarding table (based on MAC address)

✅ Link Layer Addressing

🔹 4. MAC (Media Access Control) Protocols

✅ Four Main Classes of MAC Protocols

💡 CSMA/CD is used in wired LANs, CSMA/CA is used in wireless LANs (e.g., Wi-Fi)

✅ Efficiency Trade-offs

• TDMA, FDMA: very efficient at high load, but inefficient at low

• Random access: good at low load, but more collisions at high

• Polling, Token Passing: synchronized, structured, no collisions, but latency may be worse

🔹 5. Ethernet: The Most Important LAN Tech

✅ Ethernet Key Features

✅ Ethernet Frame Format

💡 Important Rule: Maximum frame size (MTU) is usually 1500 bytes – important for IP fragmentation.

🔹 6. Switches and Link Layer Forwarding

✅ Switch Key Functions

• MAC learning (builds forwarding table)

• Frame forwarding: only sends to the correct port

• No IP layer involvement, only Ethernet headers

• Operates in layer 2 (since it's not related to IP)

✅ Switch vs Router:

• Switch operates at MAC layer, not IP

• Router operates from application to link layer

✅ Switch Forwarding Table

• Mac Address: domain, target

• Interface: depends on MAC address

• TTL: holds entries for, say, 20 minutes

• Entry is persistent until changed or replaced

💡 Switching is fast, almost no delay, low overhead compared to routers.

🔹 7. Virtual LANs (VLANs): Centralized Network Management

✅ VLAN Purpose

• Isolate traffic (ARP, broadcast) between segments

• Security and management (prevent same VLAN abuse)

• Port-based, MAC-based: flexible options

• Span multiple switches using trunk ports and 802.1Q

✅ VLAN Frame Format

🧠 VLAN is logical, not physical distinction

✅ VLAN Example (Port-Based)

🔹 8. Ethernet Switching and ARP (Address Resolution Protocol)

✅ How Ethernet Switches Work

1. When frame arrives, switch reads MAC address.

2. If match, sends only on matching interface.

3. If no match, floods frame on all interfaces except source.

✅ ARP: Details

• Calculates MAC address of device given IP address

• Broadcast Ethernet frame: to all nodes

• Unicast ARP reply: to requesting node

• Used for initial IP address discovery, DNS, HTTP

⚠️ ARP Maintains IP-Mac mappings in switch or host ARP table

🔹 9. Ethernet Switching Example

✅ Example with Two Switches (A -> B)

✅ Self-learning switches reduce configuration overhead

🔹 10. Link Layer Traffic Classification

✅ Key Traffic Classification Details

🔹 11. Cyclic Redundancy Check (CRC)

✅ CRC: Error Detection

• Given data (D), polynomial (G), length (r)

• Find remainder (R) so that: $ (D \cdot 2^r) \mod G = R $

• Receiver divides (D, R) by G. Non-zero indicates error

💡 Can detect:

• All burst errors less than r+1 bits

• Single-bit errors

• Even more powerful than parity

🔹 12. MAC Protocols: ALOHA, CSMA/CD, CSMA/CA

✅ ALOHA Protocols

🔁 Random access vs centralized access – different trade-offs

🔹 13. Link Layer Protocols & Their Applications

✅ Key Link Layer Tech

🧠 Essential to remember: MPLS ≠ IP overload, but enhances IP forwarding!

🔹 14. Link Layer in Practical Networking

✅ Type of Information Network Activities

🧓 Logical layers used hierarchically in Networking Mathematics and Transport-Layer $ rr(r) $

🔹 15. Ethernet Standards – 802.3

✅ Key Specifications

🔁 Eth in wired environment vs Wi-Fi (wireless) – differing MAC protocols

🔹 16. Link Layer Exam Checklist

📌 Many key “link-layer” services are encapsulated into “data-plane” switches!

🧠 Pro Tip for Exams

You can now inspect and describe all aspects of a web request from IP to Link – that’s the goal of the 6th chapter! Here is a summary of how a web request is processed over 6 layers:

🧭 Most routing is done at network layer, switching at link! You already know about link-layer through “Dijkstra” and “Bellman-Ford” from previous chapters!

📚 Practical Tools for Chapter 6

• Wireshark to decapsulate:
• Ethernet frame
• ARP request
• DNS over UDP
• HTTP (ECN, flow table)

• Packet Tracer/Mininet to simulate:
• Ethernet switches
• VLANs
• MPLS

• Understanding network-layer interconnection via switches, routers, and bridges

🧾 Key Slide Quotes (Summary!)

✅ Now Prepare for Chapter 7: Wireless Networks

You have the basic understanding of the link layer now. Now prepare to master the wireless and MAC protocol in Chapter 7!