bus topology

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Image showing bus network layout

A bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet networks.

Bus networks are the simplest way to connect multiple clients, but may have problems when two clients want to transmit at the same time on the same bus. Thus systems which use bus network architectures normally have some scheme of collision handling or collision avoidance for communication on the bus, quite often using Carrier Sense Multiple Access or the presence of a bus master which controls access to the shared bus resource.

A true bus network is passive – the computers on the bus simply listen for a signal; they are not responsible for moving the signal along. However, many active architectures can also be described as a "bus", as they provide the same logical functions as a passive bus; for example, switched Ethernet can still be regarded as a Logical network, if not a physical one. Indeed, the hardware may be abstracted away completely in the case of a software bus.

With the dominance of switched Ethernet over passive Ethernet, passive bus networks are uncommon in wired networks. However, almost all current wireless networks can be viewed as examples of passive bus networks, with radio propagation serving as the shared passive medium.

The bus topology makes the addition of new devices straightforward. The term used to describe clients is station or workstation in this type of network. Bus network topology uses a broadcast channel which means that all attached stations can hear every transmission and all stations have equal priority in using the network to transmit^[1] data.

The Ethernet bus topology works like a big telephone party line – before any device can send a packet, devices on the bus must first determine that no other device is sending a packet on the cable. When a device sends its packet out over the bus, every other network card on the bus sees and reads the packet. Ethernet’s scheme of having devices communicate like they were in chat room is called carrier access/ collision detection (CSMA/CD). Sometimes two cards talk (send packets) at the same time. This creates a collision, and the cards themselves arbitrate to decide which one will resend its packet first. Only one PC on a bus network share a common wire, which also means they share the data transfer capacity of that wire – or, in tech terms, they share its bandwidth.

This creates an interesting effect. Ten PCs chatting on a bus each get to use a much higher proportion of its total bandwidth than, for instance, 100 PCs on the same bus (in this case, one – tenth compared to one – hundredth). The more PCs on a bus, the more likely you’ll have a communication traffic jam. ^[2]

Advantages and disadvantages of a bus network

Advantages

• Easy to implement and extend
• Well suited for temporary or small networks not requiring high speeds (quick setup)
• Cheaper than other topologies.
• Cost effective as only a single cable is used
• Cable faults are easily identified.
• Weight reduction due to less wires

Disadvantages

• Limited cable length and number of stations.
• If there is a problem with the cable, the entire network goes down.
• Maintenance costs may be higher in the long run.
• Performance degrades as additional computers are added or on heavy traffic.(shared bandwidth)
• Proper termination is required (loop must be in closed path).
• Significant Capacitive Load (each bus transaction must be able to stretch to most distant link).
• It works best with limited number of nodes.
• It is slower than the other topologies.

Standing Wave: As in bus topology there is necessity of proper termination if there is no termination the waves get reflected at the ends and gets added in other incoming waves causing cancellation. this addition and cancellation of waves leads to a standing wave. 1

References