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by Dinesh Thakur Category: Computer Network

  Bus Topology

Bus topologies are multipoint electrical circuits that can be implemented using coaxial cable, UTP, or STP. Data transmission is bidirectional, with the attached devices transmitting in both directions. While generally operating at a raw data rate of 10 Mbps, actual throughput is much less.

This is employed frequently in the LANs with distributed control. In all nodes, as shown in Figure, share the common bus. Messages placed on the bus are transmitted to all nodes. Nodes must be able to recognize their own address in order to receive messages. However; unlike nodes in a ring, they do not have to repeat and forward messages intended for other nodes. As a result, there is none of the delay and overhead associated with re-transmitting messages at each intervening node. Because of the passive role node play in transmission on the bus, network operation will continue in the event of node failures. This makes distributed BUS networks inherently resistive to single point failures.

Bus networks employ a decentralized method of media access control known as CSMA (Carrier Sense Multiple Access), at allows the attached devices to make independent decisions relative to media access and initiation of transmission. This approach results in data collisions and requires frequent retransmission. But networks are specified in the IEEE 802.3 standard, and generally have a maximum specified length of 1.5 miles (2.5 km). Ethernet is based on a bus topology. A tree topology is a variation on the bus topology, with multiple branches off the trunk of the central bus. Bus networks also suffer from the vulnerability of the bus, as if, one node is down, all nodes in the bus will be down. Similarly, tree networks are dependent on the integrity of the root bus.

Examples of Bus Topology



Ethernet - Ethernet is the least expensive high-speed LAN alternative. It transmits and receives data at a speed of 10 million bits per second. Data is transferred between wiring closets using either a heavy coaxial cable (thick net) or fiber optic cable. Thick net coaxial is still used for medium-long distances where medium levels of reliability are needed. Fiber goes farther and has greater reliability but a higher cost. To connect a number of workstations within the same room, a light duty coaxial cable called thin net is commonly used. These other media reflect an older view of workstation computers in a laboratory environment. Figure shows the scheme of Ethernet where a sender transmits a modulated carrier wave that propagates from the sender toward both ends of the cable.

Ethernet was first designed and installed by Xerox Corporation at its Palo Atto Research Center (PARe) in the mid-1970. In the year 1980, DEC Intel and Xerox came out with a joint specification which has become the de facto standard. Ethernet from this period is often called DIX after its corporate sponsors Digital, Intel, and Xerox. Ethernet as the most popular protocol for LAN technology.

                                    Signal flow across an ethernet

Local Talk

LocalTalk is a LAN that employs bus topology. It was invented by Apple Computer Corporation for use with Apple's PCs LocalTalk uses ordinary telephone wire with standard RJ-l1 telephone connectors, the same as used to plug in a telephone and modem. Each connector plugs into a LocalTalk transceiver, which connects to a PC's LocalTalk interface card. LocalTalk transceivers that connect to a PC's parallel port are also available. Multiple computers can be connected in a daisy chain, like the way railroad cars are connected on a train. Hub can also be used for the same purpose.

LocalTalk does have some limitations over Ethernet. For one, LocalTalk is slower than Ethernet. LocalTalk has a bandwidth of230.4 Kbits/second, while Ethernet has a bandwidth of 10MB per second. In practice, Ethernet runs about 4-to-5 times faster, than LocalTalk. Another limitation of LocalTalk is that it cannot run TCP/IP protocols directly.

LocalTalk major advantages are that it is simple and very inexpensive. The hardware and software required for LocalTalk already built into every Macintosh computer to connect with a LocalTalk network with the exception of the iMac(tm). LocalTalk is also cheaper in terms of cabling than Ethernet. Many Macintosh computer owners use a printer cable to create a temporary network between two Macintosh Computers. There is no software to install beyond the Macintosh operating system itself and LocalTalk will work with any current version of the Macintosh operating system as well as versions from several years ago. For cabling of a LocalTalk network, standard RJ-II phone cable is used which is cheaper than the cable used for Ethernet. Many printers on the market come with built-in LocalTalk support making them easy to share on such a network.

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