Yawn! Video #2

Video #2 is about CSMA/CD, ethernet types and cable types.

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. When multiple hosts connect via an old fashioned shared copper wire (bus), they need to be able to detect if another host is currently using the wire. If not, the host will send it's data. However, if two hosts decide to send data at exactly the same time, they need to monitor for collisions on the wire. A small change in voltage lets a host know if a collision has occurred. Once this happens the host sends a jam signal to each host on the network. Each host sets a random back-off timer or algorithm before attempting to transmit once again.

With the ethernet standard 10Base-T, the T stands for twisted-pair cable. The 10 refers to 10Mb/s. Twisting the pairs of wires reduces possibility of electromagnetic interference.

• Ethernet runs at 10Mb/s
• Fast ethernet runs at 100Mb/s
• Gigabit ethernet runs at 1000Mb/s or 1Gb/s

• Straight-through cables are used to connect a PC to a switchport
• Crossover cables are used to connect two similar devices together, typically two switches
• Rollover cables are console cables for a serial connection (DB9 -> RJ-45)

MAC/BIA/Physical/NIC/Ethernet/LAN Address is a 48bit address written in hex (aa-bb-cc-11-22-33). The first three sections are the OUI (Organisationally Unique identifier aa-bb-cc). The broadcast MAC address is FF-FF-FF-FF-FF-FF and the multicast MAC address is 01-00-5e[00-00-00 -> 7F-FF-FF].

WAN cabling, you can connect Cisco router serial interfaces directly with a DTE/DCE cable.

Any cable over 100 meters is a cause for alarm!