Physical Layer and Data Link Layer
In computer networks, the Physical Layer and the Data Link Layer sit at the bottom of the OSI 7-layer model. They define how data is actually transmitted across the medium. In this chapter, we will go through Ethernet, NICs and cables, hubs, and switches.
01. Ethernet
1) Ethernet Standards
The most widely used technology in Local Area Networks (LANs)
Based on the IEEE 802.3 standard
Evolution of transmission speeds:
10 Mbps: Original Ethernet
100 Mbps: Fast Ethernet
1 Gbps: Gigabit Ethernet
10 Gbps and beyond: Data centers, backbone networks
2) Transmission Media Notation
Example: 100BASE-TX
100: Transmission speed (100 Mbps)
BASE: Baseband signaling (digital transmission)
TX: Type of transmission medium
T: Twisted Pair
F: Fiber optic
💡 Examples:
10BASE-T → 10 Mbps using UTP cable
1000BASE-SX → 1 Gbps using short-range multimode fiber
3) Ethernet Frame Structure
| Preamble | Destination MAC | Source MAC | Type/Length | Data | FCS |
Preamble: Synchronization signal (alternating 1s and 0s)
Destination MAC / Source MAC: Hardware addresses of sender and receiver
Type/Length: Identifies the upper-layer protocol (e.g., IP, ARP)
Data: Payload (minimum 46 bytes, maximum 1500 bytes)
FCS (Frame Check Sequence): Error detection using CRC
4) [More to Know] Token Ring
Developed by IBM
Only the device holding a token can transmit data
Advantage: No collisions
Disadvantage: If one device fails, the entire network is affected → rarely used today
📌 Key Takeaways (4 items)
Ethernet standards / BASE notation / Frame structure / MAC address
02. NIC and Cables
1) NIC (Network Interface Card)
Appearance: Installed as a card on the motherboard, or built-in as a chipset
Roles:
Connects a computer to the network
Provides a unique MAC address
Converts digital data into electrical/optical signals and vice versa
💡 Examples: Wi-Fi NIC, Ethernet NIC
2) Twisted Pair Cable
Appearance: Copper wires twisted together to reduce interference
Types:
UTP (Unshielded Twisted Pair): No shielding, low cost
STP (Shielded Twisted Pair): Shielded against electromagnetic noise, more expensive
Categories:
Cat5e: 100 Mbps ~ 1 Gbps
Cat6: 1 ~ 10 Gbps
Cat7/8: High-performance, used in data centers
3) Fiber Optic Cable
Appearance: Glass or plastic fibers transmitting data using light
Single Mode Fiber (SMF):
Straight path, uses laser light
Supports several kilometers to tens of kilometers
Ideal for long-distance communication
Multimode Fiber (MMF):
Multiple paths for light
Limited to hundreds of meters
Suitable for LANs and building-level connections
📌 Key Takeaways (5 items)
NIC / MAC address / Twisted pair / Fiber optic / Cable categories
| Type | Characteristics | Distance | Example Use Case |
| UTP | Low-cost, no shield | Tens to hundreds of meters | Home/office LAN |
| STP | Noise-resistant | Tens to hundreds of meters | Industrial sites |
| Fiber (SMF) | Long distance, high speed | Several km ~ tens of km | Backbone, ISP |
| Fiber (MMF) | Shorter range, cheaper | Hundreds of meters | Building LAN |
03. Hub
1) Features of a Hub
Operates at the Physical Layer
Forwards incoming signals to all ports (broadcast)
Simple but inefficient (many collisions)
2) Collision Domain
A network segment where packet collisions can occur
With hubs, all ports share one collision domain → high collision rates
3) CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
A method to reduce collisions in Ethernet:
Carrier Sense: Check if the channel is idle before sending
Multiple Access: Many devices can attempt to use the channel
Collision Detection: Detect if a collision occurs (signal corruption)
Backoff and Retry: Wait a random time and retransmit
📌 Key Takeaways (5 items)
Hub / Broadcast / Collision domain / CSMA/CD / Physical layer
04. Switch
1) Features of a Switch
Operates at the Data Link Layer
Forwards frames based on MAC addresses
Supports independent communication between ports → fewer collisions
2) MAC Address Learning
The switch records the source MAC address of incoming frames into its MAC address table
When sending, it looks up the destination MAC and forwards only to the correct port
3) VLAN (Virtual LAN)
Logically separates one physical network into multiple sub-networks
Port-based VLAN: Divides based on port numbers
MAC-based VLAN: Divides based on device MAC addresses
Benefits: Improved security, traffic separation, simplified management
📌 Key Takeaways (4 items)
Switch / MAC learning / VLAN / Data link layer