# <center><i class="fa fa-edit"></i> Network Fundamentals: Building Ethernet LANs </center>
###### tags: `Internship`
:::info
**Goal:**
To gain a basic understanding of the components behind network technologies. Focus on vocabulary and systems overview.
- [x] Building Ethernet LANs

**Resources:**
[Juniper Online Learning Modules](https://learningportal.juniper.net/juniper/user_activity_info.aspx?id=769)
[Pre-Internship Notes by Jessica Chen](https://hackmd.io/@j-chen/SyfRATOmD)
[Daily Report by Lin Yen-Ting](https://hackmd.io/@8KbRc796SnuYA2Dvsvk_BA/SJU_CawIL)
:::
## Module 2: Building Ethernet LANs

### Overview
- Ethernet: the most common set of rules for connectivity between devices (for frames, communication, etc)
- 3 Components:
- Physical Layer Characteristics
- Access to Netowrk
- Frame Format
### Physical Layer Characteristics
- Ethernet Shared Bus: Ethernet used to connect multiple computers by the same wire/cable
- one computer transmits data, all other devices on the network receive
- Network Segment: a group of network devices that compete with each other for access to the wire
- Network Interface Controller (NIC): connects end-user devices to netowrk segment. (Others: Ethernet cabling/wireless NIC)

### Access to Network (How do devices know when it’s their turn to send data?)
- Media Access Control protocol (MAC): controls access to physical wire or segment.
- Carrier Sense Multiple Acccess with Collision Detection (CSMA/CD): MAC protocol for Ethernet
- Regulates "conversation" between devices; allows transmission when wire is idle
- Half-duplex communication: sends or receives data; only ONE direction at a time
- Collision detection (CD): if a collision occurs, both devices stop transmitting for a random amount of time before attempting again
- Full duplex communication: sends and receives data simultaneously

### Frame Format (How do devices know which data is for them?)
- NIC encapsulates data into an Ethernet Frame when devices need to send data
- To denote the start of a new frame, NIC adds a preamble and start delimiter (8-byte sequence)
- PROM chip (Programmable Read-Only Memory): unique address burned onto NIC by manufacturer
- Must Haves of Ethernet Frames
- Destination Layer 2 Address
- Source Layer 2 Address
- MAC address(AKA Layer 2 address, Hardware address, Physical address): 48-bit (6-byte) address that uniquely identifies a device’s NIC
- Expressed as a hexadecimal
- Bytes can be denoted by colons or dashes.
- Ex: 00:90:69:9f:ea:46
- First 3 bytes: identifies Organizationally Unique Identifier (OUI) manufacturer
- Last 3 bytes: identifies unique NIC
- Every device on network receives frame and compares destination address with its own
- if match, device checks if Cyclic Redundancy Check (CRC) Checksums match and determines Upper Layer Protocol
- discard if not match
- Types of MAC Addresses:
- Individual/unicast address: one recipient address
- Broadcast destination address: sends to all devices
- Group/multicast address: sends to a group of devices
### Early Ethernet Network Devices
- Collision domain/network segment: group of Ethernet devices that compete for access
- Repeater: Physical layer device used to connect separate physical cable segments together, acting like one to extends Ethernet and isolates failing cables
- Adds time to transmission
- 5-4-3 Rule: ensures repeaters don’t violate transmission time limitations.
- Total of 5 cable segments, connected through 4 repeaters, and only 3 of segments can contained end user devices (populated)
- There are 2 link segments and 1 collision domain
- Populated/user cable segments: contains end-user devices
- Unpopulated/link cable segments: connect repeaters together; do not contain end-user devices

- Coaxial cabling: used with Ethernetin early 1980s
- Twisted Pair Cabling: used since introduction of IEEE 10BASE-T standard
- AKA unshiedled twisted pair (UTP) cabling
- HUB: Replaced repeater with central hub in star configuration
- CSMA/CD
- Failure of one device or cable on the network can be easily unplugged due to invidual cables connected to the HUB
- Shared bus configuration with repeater, in contrast, are susceptible to failure if one device or cable causes problems
- Collisions still occur
- Bridge: connects two or more physical cable segments to create a larger network
- Used to decrease collision probability by breaking up large network into smaller collision domain
- Network segmentation: process of breaking up network
- Operates at Layer 2
- Functions of a Bridge
- Forwarding frames
- Learning MAC address
- Controlling traffic
- MAC Address Table: Bridge adds new frame addresses to table with its port. Drops frames if it already appears on table to reduce network traffic. Selectively forwards frames if the address already appears on table, but port is different
### Today's Network Devices
- While early Ethernet networks could only support shared bandwidths with speeds of 10Mbps, networks today have dedicated bandwidths that support 1 Gbps of speed
- bps: bits per second
- Network Bandwidth: data transfer rate or bit rate (bps). Max capacity of a network connection
- Network Throughput: actual data transfer rate. Takes into account network overhead and hardware/software bottlenecks
- More practical measurement; < bandwidth
- Networks grew --> needed more capacity to support users with higher bandwidth connection

- Switches: dedicated a single bridge port to one end-user device
- developed to overcomed the limitation and take full advantage of twisted pair cabling
- Hubs and bridges are no longer necessary today
- Collision domain
- End user device: no longer compete with each other for network access
- Switch
- Eliminates collisions
- Isolate each end-user device to its own collision domain
- Provide separate paths for transmission and receiving data
- Switches with VLANs
- VLAN: a group of network devices that behave as if they are connected to the same smaller LAN or completely separate switch; may be connected to same switch
- VLAN solves broadcast traffic (uses up network bandwidth) when transmitting to ports
- Essentially subdvides the LAN
- Devices at one VLAN do not receive broadcast from devices on another
- Router must be used to allow communicatin between devices on different VLANs
### Wireless LAN Architectures
- Autonomous Architecture (Fat Access Point/Fat AP): all functionality is coded into every access point or wireless determination point
- home networks
- Centralized (Switched WLAN System): access points become Thin APs with a central switch or access point controller that manages the system
- enterprise networks (has scalability)
- Distributed Architecture (Wireless Mesh System): used to create dynamic netowrk; environment with cabling would be impossible
- US military courses in field operations
- Switched WLAN systems are physically MORE SECURE than other architectures
- Access Point Controller Functionality
- Securety and access control
- VLAN management and manipulation
- User roaming between access points
- Quality of service
- 802.11 Group of Standards: only responsible for standards at Layer 1 (Physical Layer) and part of Layer 2 (Media Access Control Sublayer)
- Physical Layer: defines a variety of different connection types for wireless links
- Data Link Layer: primarily controls who may use the wireless channel at any point of time
- CSMA/CA
- Virtual Collision Detection: request-to-send and clear-to-send messages; client must receive a clear-to-send before sending and broadcasts to all clients in range
- Ethernet II Format VS Generic 802.11 Format
- 
- Finding an Access Point
- passively: listening to beacons
- actively: prowling for a particular network; any available access points will respond if available
- Sends association management frame
- Access point responds with association ID
- Re-association request to roam to another access point
- If incoming data queue foreign in old access point, data is forwarded across distribution system to a new access point
- Handling Collisions
- Hidden Node Problem: wen two clients are on either side of the access point and transmits. Neither will receive acknowledgement and back off by a random interval before attempting again
- Solutions:
- CSMA/CA
- 802.11 is Virtul Collision Detection Method
- Frame Control Settings