Bus Topology
A Bus topology is a multipoint configuration. One long cable acts as a backbone to link all the devices in the network. Nodes are connected to the bus cable by 'drop lines' and 'taps'. Advantages of a bus topology include -
• all workstations connect to the same cable segment
• commonly used for implementing Ethernet at 10mbps
• the cable is terminated at each end
• wiring is normally done point to point
• a faulty cable or workstation will take the entire LAN down
• two wire, generally implemented using coaxial cable during the 1980's
The bus cable carries the transmitted message along the cable. As the message arrives at each workstation, the workstation computer checks the destination address contained in the message to see if it matches it's own. If the address does not match, the workstation does nothing more.
If the workstation address matches that contained in the message, the workstation processes the message. The message is transmitted along the cable and is visible to all computers connected to that cable.
There are THREE common wiring implementations for bus networks
• 10Base2 (thin-net, CheaperNet) 50-ohm cable using BNC T connectors, cards provide transceiver
• 10Base5 (ThickNet) 50-ohm cable using 15-pin AUI D-type connectors and external transceivers
• 10BaseT (UTP) UTP cable using RJ45 connectors and a wiring centre
Physical Bus Cable Limits
The above diagram shows a number of computers connected to a Bus cable, in this case, implemented as Thin Ethernet. Each computer has a network card installed, which directly attaches to the network bus cable via a T-Connector. It is becoming common to use 10BaseT (UTP) for implementing Ethernet LANS. Each workstation is wired in star fashion back to a concentrator wiring centre (hub). The hub is a multi-port device supporting up to about 32 ports. One of these ports is connected to a server, or the output of the hub can be connected to other hubs.
10Base2 THIN ETHERNET NETWORK LAYOUT
Limitations
• maximum number of trunk segments = 5
• maximum trunk segment length = 607 feet (185 meters)
• maximum network trunk cable = 3035 feet (925 meters)
• maximum number of stations on a trunk segment = 30
• minimum distance between T connectors = 1.5 feet (0.5 meters)
Rules
• each end of the trunk segment is terminated in 50-ohms
• one of the terminators is grounded
• connector splices are kept to a minimum
Cabling
• BNC-T type connectors
• RG58-AU 50-ohm cable, 0.2"
Bus Network Topology Summary
Advantages Disadvantages
Easy to implement Limits on cable length and Workstation numbers
Low Cost Difficult to isolate network faults
A cable fault affects all workstations
As the number of workstations increase, the speed of the network slows down
Wiring of the DIX Connector
10BaseT UTP NETWORK LAYOUT
Limitations
maximum segment length of 100 Meters
Hub to Hub or repeater to repeater links limited to 100 Meters
Rules
star topology 4 repeater/5 segment rule of 10Base5 is retained only two nodes per segment are allowed
Cabling
RJ-45 Connectors Category 3 UTP minimum, preferably Category 5
