Part 6: CCNA Training: What is a Network

Posted: October 21st, 2016

CABLING AND TOPOLOGIES

Topology: Physical Bus
A bus network is a single length of cable to which work Stations, printers, servers, routers etc. connect.

There have been two major physical bus technologies:
• 10Base5
• 10Base2
• 10Base5

This technology capable of 10Mbps and allowed a cable run of 500m with the maximum number of nodes connected to the wire limited to 100 devices at a distance of 2.5meters apart. 2.5 Meters is due to the wavelength of the signal used to transmit the data.

At each end of the cable run there was a terminator (Resistor of 50ohms), if this terminator was removed the network would fail as the AC signal rather than being dissipated would be reflected back down the wire, always a barrel of laughs that one.

Stations would connect to the network by having to pierce the actual cable. This type of connection is referred to as a vampire tap,

10Base2
This technology also referred to as “Thinnet” or “cheapnet”, is capable of 10Mbps and allowed a cable run of 185m with the maximum number of nodes connected to the wire limited to 30 devices per segment, with no more than 5 cable segments separated by 4 repeaters with no more than three of the 5 cable segments being mixing segments.

These mixing segments can either be 10Base2 or 10Base5, a mixing segment is one with connected stations, only a mixing segment can be populated with stations i.e. P.C’s etc.

Two of the segments can be Link segments either 10Base5 or 10Base2, these link segments connect the repeaters together

The minimum distance for cables between workstations must be at least a half-meter. Drop cables should not be used to connect a BNC connector to the network interface card (NIC) because this will cause signalling problems unless the NIC is terminated. The entire cabling scheme, including all five segments, can’t be longer than 925 meters.

The terminator (Resistor of 50ohms) is built into the connector of the last station on the bus network, if this terminator was removed the network would fail as the AC signal rather than being dissipated would be reflected back down the wire and the reflected signal is impossible to distinguish from a collision.

Terminators had a metallic chain used for grounding the network. Only one end of the network needed to be grounded, if both ends were grounded it would result in a “grounding loop” which results in network outages.

The Disadvantages of a physical Bus network are:
1. If there is a break anywhere along the cable run the entire network would stop functioning
2. One large broadcast and collision domain, the stations connecting to the wire the slower the network becomes
3. 10Base5 stations need to connect to the network by piercing the cable
4. Lots of collisions on the cable
5. No security
6. The number of work stations and cable length is limited