Day 2 Interfaces and Cables

1. Ethernet Standards (IEEE 802.3)

• Collection of network protocols/standards defined by IEEE in 1983

• Governs physical layer specifications including cabling

2. Data Measurement

• Bit: Smallest unit (0 or 1)

• Byte: 8 bits

• Speed measured in bits per second (Kbps, Mbps, Gbps, Tbps)

3. Cable Types

• UTP (Unshielded Twisted Pair): Copper-based, RJ-45 connector

• Fiber-Optic: Glass/plastic core, uses light signals

Bits and Bytes Conversion

• 1 Kilobit (Kb) = 1,000 bits

• 1 Megabit (Mb) = 1,000,000 bits

• 1 Gigabit (Gb) = 1,000,000,000 bits

• 1 Terabit (Tb) = 1,000,000,000,000 bits

UTP (Unshielded Twisted Pair)

• Twisted pairs protect against EMI (Electromagnetic Interference)

• Uses RJ-45 connector (Registered Jack)

• Maximum distance: 100 meters (for all copper standards)

STRAIGHT-THROUGH CABLE:

• Pin 1 → Pin 1, Pin 2 → Pin 2, etc.

• Use between DIFFERENT device types

• Examples:

  • Router ↔ Switch

  • PC ↔ Switch

  • Firewall ↔ Switch

CROSSOVER CABLE:

• Pins 1,2 swap with 3,6

• Use between SAME device types

• Examples:

  • Router ↔ Router

  • PC ↔ PC

  • Switch ↔ Switch

Straight-Through Cable (T568B on both ends):

1. White/Orange - Pin 1

2. Orange - Pin 2

3. White/Green - Pin 3

4. Blue - Pin 4

5. White/Blue - Pin 5

6. Green - Pin 6

7. White/Brown - Pin 7

8. Brown - Pin 8

Crossover Cable: End 1 (T568B):

1. White/Orange

2. Orange

3. White/Green

4. Blue

5. White/Blue

6. Green

7. White/Brown

8. Brown

End 2 (T568A):

1. White/Green

2. Green

3. White/Orange

4. Blue

5. White/Blue

6. Orange

7. White/Brown

8. Brown

In a crossover cable, pins 1&2 and 3&6 are swapped between the two ends.

Definition: Feature that automatically detects cable type and adjusts pin configuration

Key Points:

• Modern devices have Auto MDI-X

• Eliminates need for crossover cables

• Automatically adjusts Tx/Rx pins

• Allows straight-through cables for all connections

Fiber-Optic Structure (4 Layers)

1. Fiberglass core (center)

2. Cladding (reflects light)

3. Protective buffer

4. Outer jacket

Multimode Fiber (MMF)

• Wider core diameter

• Multiple light angles (modes)

• LED-based transmitters (cheaper)

• Shorter distances than single-mode

• Color code: Orange or aqua jacket

Single-Mode Fiber (SMF)

• Narrower core diameter

• Single light angle (mode)

• Laser-based transmitters (expensive)

• Longest distances

• Color code: Yellow jacket

1. SR = Short Range (Multimode, 400m)

1. LR = Long Range (Single-Mode, 10km)

2. ER = Extended Range (Single-Mode, 30km

1. Don't confuse bits and bytes - Speed is ALWAYS measured in bits/sec

2. All copper Ethernet = 100m max - Easy to forget this applies to all standards

3. Switch pins are opposite - Switches Tx on 3,6 (everyone else Tx on 1,2)

4. Auto MDI-X solves cable type issues - Old devices may not have this

5. Multimode ≠ better - Single-mode goes farther despite the name

6. 10/100BASE-T uses 2 pairs, 1000/10GBASE-T uses 4 pairs

7. 1000BASE-T and 10GBASE-T are bidirectional on all pairs (different from 10/100)

"Switch is Different"

• All devices Tx on 1,2 EXCEPT switches (Tx on 3,6)

"100 Rule"

• All copper Ethernet = 100 meters max

"Same needs Cross, Different needs Straight"

• Same device types = Crossover cable

• Different device types = Straight-through cable

• (Only applies to old devices without Auto MDI-X)

"SFP = Small Form-Factor Pluggable"

• Used for fiber-optic connections

• Transceiver module

"Fiber Distance Hierarchy"

• UTP (100m) < Multimode (hundreds of meters) < Single-Mode (kilometers)

"LED is Cheaper, Laser is Longer"

• Multimode = LED = Cheaper = Shorter

• Single-Mode = Laser = Expensive = Longer

Example 1: Office Floor Connection

Scenario: Connect 24 PCs to a switch, all on same floor

• Answer: UTP cables (straight-through)

• Why: Within 100m, cost-effective, standard for end hosts

Example 2: Building-to-Building (150m)

Scenario: Connect two switches in buildings 150m apart

• Answer: Multimode fiber

• Why: Exceeds 100m UTP limit, cheaper than single-mode, supports distance

Example 3: Campus Connection (3km)

Scenario: Connect two offices 3km apart

• Answer: Single-mode fiber

• Why: Only option that supports 3km distance

Example 4: Old Router to Old Router

Scenario: Two old routers (no Auto MDI-X) won't communicate

• Answer: Use crossover cable

• Why: Same device types need crossover without Auto MDI-X

The smallest unit of data, either 0 or 1

8 bits = 1 byte

In bits per second (bps), NOT bytes per second

Institute of Electrical and Electronics Engineers

IEEE 802.3 (established 1983)

Registered Jack-45 (connector for UTP cables)

Unshielded Twisted Pair

Electromagnetic Interference

The twisted pairs of wires

Small Form-Factor Pluggable (fiber-optic transceiver)

1,000 bits

1,000,000 bits

1,000,000,000 bits

1,000,000,000,000 bits

10 Mbps, 802.3i, 100m

100 Mbps (Fast Ethernet), 802.3u, 100m

1 Gbps (Gigabit Ethernet), 802.3ab, 100m

10 Gbps, 802.3an, 100m

100 meters

2 pairs (4 wires)

4 pairs (8 wires)

Pins 1 and 2

Pins 3 and 6

Pins 1 and 2

Pins 3 and 6 (OPPOSITE of other devices!)

Pins 1 and 2

Router, Firewall, PC (everything except switches)

Cable where each pin connects to the same pin on the other end (1→1, 2→2, etc.)

Between DIFFERENT device types (e.g., PC to Switch, Router to Switch)

Cable where pins 1,2 swap with pins 3,6

Between SAME device types (Router-Router, PC-PC, Switch-Switch) - only on old devices without Auto MDI-X

Straight-through (different device types)

Crossover (same device types)

Feature that automatically detects and adjusts pin configuration, eliminating need for specific cable types

Medium Dependent Interface Crossover

Yes! Auto MDI-X automatically adjusts the pins

Yes, most modern devices have Auto MDI-X

No, it will work with either straight-through or crossover

Device can send and receive data simultaneously

Pins 1,2 transmit; Pins 3,6 receive (or vice versa for switches)

Each of the 4 pairs is bidirectional (can both transmit and receive)

1) Fiberglass core, 2) Cladding, 3) Protective buffer, 4) Outer jacket

Reflects light back into the core

Fiber with wider core allowing multiple light angles (modes)

Fiber with narrower core allowing only one light angle (mode)

Multimode fiber

Single-mode fiber

Single-mode (due to laser-based transmitters)

Single-mode fiber

Orange or aqua

Yellow

UTP

Fiber-optic

UTP (fiber is NOT vulnerable)

UTP (fiber does NOT leak signal)

UTP (RJ-45 ports cheaper than SFP ports)

UTP cables (within 100m, cost-effective, standard for end hosts)

Multimode fiber (exceeds 100m UTP limit, cheaper than single-mode)

Single-mode fiber (only option supporting this distance)

Same device types need crossover cable (without Auto MDI-X)

Yes, Auto MDI-X will adjust automatically

Straight-through cable (different device types)

Cannot use copper (100m max). Must use multimode fiber (10GBASE-SR supports 400m, so need to stay within that or use single-mode)

Single-mode fiber (10GBASE-LR supports 10km)

Trick question! PCs don't typically connect directly to firewalls. Both transmit on pins 1,2, so if directly connected, would need crossover (or Auto MDI-X)

FALSE! Maximum is always 100m for copper Ethernet

Mostly TRUE for modern devices, but old devices without Auto MDI-X still need them

FALSE! Single-mode goes much farther (the name is misleading)

FALSE! 10/100BASE-T use 2 pairs; 1000/10GBASE-T use 4 pairs

They operate normally (Auto MDI-X adjusts automatically)

FALSE! Speed is measured in BITS per second

FALSE! Switches transmit on pins 3,6; all others on pins 1,2

FALSE! SFP ports are more expensive

FALSE! Single-mode is more expensive (laser-based transmitters)

To protect against electromagnetic interference (EMI)

Fiber doesn't emit signals outside the cable (no leakage to intercept)

Requires laser-based transmitters (vs. cheaper LED for multimode)

What to Memorize Cold:

1. All copper = 100m max

2. Switch Tx on 3,6 (everyone else 1,2)

3. Fiber distance hierarchy: UTP < MM < SM

4. 10/100 use 2 pairs; 1000/10G use 4 pairs

5. Same device = crossover (without Auto MDI-X)

6. SR=400m, LR=10km, ER=30km

Common Question Patterns:

• Distance selection (which cable type?)

• Cost optimization (cheapest option that works?)

• Old devices without Auto MDI-X

• Pin configuration problems

Red Flags in Questions:

• "Old devices" = probably no Auto MDI-X

• "Keep costs down" = choose cheapest option that meets requirements

• Distance over 100m = must use fiber

• "Same floor" or "nearby" = probably UTP

• Buildings apart = probably fiber