Networking Basics¶
Layers¶
| OSI | TCP/IP Stack | |
| 7 | Application | Application |
| 6 | Presentation | |
| 5 | Session | |
| 4 | Transport | Transport |
| 3 | Network | Internet |
| 2 | Data Link | Link |
| 1 | Physical | Physical |
Protocol Data Units (PDUs)¶
| Layer | PDU |
|---|---|
|
Segment |
|
Packet |
|
Frame |
|
Bit |
Network topologies¶
Star topology “central switch”¶
- if one link fails, the other links continue to function
- central device is a SPOF
- popular in modern networks
Mesh topology “multiple offices”¶
Allows for redundancy between each enpoint. In a full mesh, you need (n * (n - 1) / 2) links. Therefore, full mesh networks don’t scale well when the number of endpoints increases.
Links that are less useful can be removed. The network becomes a partial mesh. Endpoints can still reach each other by routing their traffic through other endpoints.
| Full mesh | Partial mesh |
|---|---|
| Optimal path | Might be suboptimal path |
| Not Scalable | More Scalable |
| More expensive | Less expensive |
Network architecture¶
Three Tiered¶
| Layer | Description |
|---|---|
| Access Layer | Connected to endpoints |
| Distribution Layer | Distributes the network to access layer |
| Core Layer | Connect several distribution layers together (buildings, BUs etc…) |
Collapsed Core¶
A collapsed core network is basically a three tiered network with the Distribution Layer and Core Layer merged into the so-called “Collapsed Core Layer”.
Cables and connectors¶
Copper¶
Nowadays we typically use Unshielded Twisted Pair cables with RJ45 connectors.
| Name | Type | Notes |
|---|---|---|
| RG59 | Coax | lossy, for short distances only |
| RG6 | Coax | typical TV cable |
| RG58 | Coax | 50 Ohms, used for 10 Base 2 |
| UTP | Twisted pair | unshielded twisted pair, typical cable today |
| STP | Twisted pair | shielded twisted pair, more expensive than UTP |
| Name | Type | Notes |
|---|---|---|
| F Connector | Coax | |
| BNC | Coax | Used with 10 Base 2 |
| DB9 | Serial | Often used for management |
| RJ45/RJ11 | Twisted pair | Most common today |
| Category | Ethernet |
|---|---|
| Cat 3 | 10 Base T |
| Cat 5 | 100 Base TX |
| Cat 5e | 1000 Base T |
| Cat 6 | 1000 Base T (thicker cables, better transmission) |
| Cat 6a | 10G Base T |
| Pin | T568A | T568B |
| 1 | white/green | white/orange |
| 2 | green | orange |
| 3 | white/orange | white/green |
| 4 | blue | |
| 5 | white/blue | |
| 6 | orange | green |
| 7 | white/brown | |
| 8 | brown | |
Fiber optic¶
- Immune to electromagnetic interference
- Can cover much larger distances than copper
- LED or Laser (more powerful)
| Type | Notes |
|---|---|
| single mode | one ray of light, up to 40km. More expensive |
| multimode | modal dispersion limits to ~2km |
| Type | Notes |
|---|---|
| ST | |
| LC | |
| SC | |
| MTRJ | 2 strands of fiber in one connector |
Troubleshooting¶
Approaches¶
| Name | Description |
|---|---|
| Top-down | Start at the app level, eg. can I reach website from browser? |
| Bottom-up | Start at the bottom, eg. is the cable plugged in? |
| Divide and Conquer | Start in the middle and bisect, eg. start with ping |
Steps¶
- Define the problem in a specific way
- Collect information
- Analyze information
- Eleminate unlikely causes
- Propose a hypothesis
- Test hypothesis (don’t interrupt people/systems if at all possible)
- Solve and document
If it didn’t work, start over at 2.