Week 6 Topic 2: Layer 1 and 2 protocol – ethernet

• Ethernet is a set of layer 1 and 2 protocols, typically using copper wire but also supporting fibre optic cable

Physical Mediums

Unshielded Twisted Pair (UTP)

• Wires are twisted to reduce interference
• Category 5e: Max distance of 100m at 1 Gbps.
• Category 6: Max distance of 100m at 1 Gbps, or 55m at 10 Gbps.
• Category 6a: Max distance of 100m at 10 Gbps.
• Category 7: Max distance of 100m at 10 Gbps.
• New installations should use at least category 6 cable.

Shielded Twisted Pair (STP)

• Similar to UTP but includes extra shielding (foil) to further reduce interference.

Fibre Optic

• Transmits light pulses through a thin glass tube.
• Multi-mode fibre optics have varying path lengths, causing signal blurring at high speeds, limiting the maximum distance to around 550m at 10 Gbps.
• Single-mode fibre optic cables have a smaller core, that allows only one path for light, enabling high speeds over long distances (up to 40 km at 10 Gbps).
• Fibre is immune to electromagnetic interference and can achieve higher speeds over greater distances than copper.
• Fibre optic cable, termination, specialized training, and equipment are more expensive than copper, making it cost-effective for distances over 100m or when high speeds (10 Gbps) are required.

Coaxial Cable

• Used for radio frequency signals, including computer networks and cable television.

802.11 Wi-Fi

• Wireless technology to connect devices to the internet.

Ethernet Evolution

• Initially used a bus topology with a single coaxial cable connecting all hosts.
• Bus topology issues:
  • Network failure upon node malfunction or cable disconnection.
  • High coaxial cable cost.
  • Limited signal distance.
  • Collisions due to simultaneous message sending, causing interference.
• Collision detection: a special signal is sent to notify hosts, and hosts wait a random "back-off" time before resending data.
• Half-duplex: data can be sent in both directions, but not simultaneously.

Star Topology

• Replaced bus topology with a hub in the middle, implementing a spoke-hub distribution paradigm
• Involves every host connecting to a central hub.
• Unlike bus topology, a single host failure does not down the entire network.
• Replaced coaxial cable with cheaper unshielded twisted pair cables.
• Collision is still an issue.

Ethernet Hubs

• Layer 1 devices that send data received on one port to all other ports.
• Hubs can participate in collision detection and act as signal boosters.
• Hubs do not have memory and are limited to 10 Mbps.

Frames

• A frame is a packet of data at layer 2 consisting of a header, payload, and CRC checksum.
• The header contains metadata, including destination and source host information, and data type.
• The payload contains the actual data being transmitted.
• The CRC checksum is used to verify data integrity.
• MAC addresses are unique identifiers for each network interface card (NIC).
• When a host receives a packet, it checks if the destination MAC address matches its own; if not, it ignores the packet.
• Hubs can cause security and performance issues due to sending data out of all ports

Ethernet Switches

• Layer 2 devices with memory to store frames and eliminate collisions.
• Switches operate in full-duplex mode, allowing simultaneous sending and receiving of data.
• They learn and store MAC addresses in a MAC table, sending data only to the intended port.
• Switches break up collision domains.
• Managed switches can be configured, with a 'console' port for configuration.
• Switches can support different speeds on different ports.
• Switches do not have MAC addresses unless managed.

Collision Domain

• Network segment where simultaneous transmissions result in collisions.
• Hubs create a single collision domain, while switches limit collision domains to individual connections.

Ethernet Speeds

• Copper wire medium:
  • 10BASE-T: 10 Mbps (most hubs).
  • 100BASE-T: 100 Mbps (older switches).
  • 1,000BASE-T: 1,000 Mbps (gigabit), common in new PCs.
  • 10GBASE-T: 10 Gbps over UTP.
• Fibre optic cable:
  • 10GBASE-LR: 10,000 Mbps over fibre.

802.11 (Wi-Fi)

• Allows wireless devices to interact with Ethernet networks with compatible layer 2 MAC address schemes.
• Short-range wireless with a maximum distance of around 50 meters.
• 802.11b: Up to 11 Mbps.
• 802.11g: Up to 54 Mbps.
• 802.11n: Up to 300 Mbps, operates on 5 GHz or 2.4 GHz.
• 802.11ac: Over 1,000 Mbps.
• Wireless Access Point (WAP): connects wireless devices to wired networks.
• WAPs allocate time for each client to transmit, preventing collisions.
• Ethernet Limitations:
  • Not ideal for long-distance or very large networks due to broadcast noise.
• MAC table size limitations in switches (around 8,000 MAC addresses).