physical topology of a network refers to the configuration of cables, computersWritten by Gianfranco Fracassi
What is a Topology? The physical topology of a network refers to configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is method used to pass information between workstations. Logical topology was discussed in Protocol chapter .
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Main Types of Physical Topologies The following sections discuss physical topologies used in networks and other related topics.
Linear Bus Star Star-Wired Ring Tree Considerations When Choosing a Topology Summary Chart Linear Bus A linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals) are connected to linear cable. Ethernet and LocalTalk networks use a linear bus topology. Advantages of a Linear Bus Topology •Easy to connect a computer or peripheral to a linear bus. •Requires less cable length than a star topology. Disadvantages of a Linear Bus Topology •Entire network shuts down if there is a break in main cable. •Terminators are required at both ends of backbone cable. •Difficult to identify problem if entire network shuts down. •Not meant to be used as a stand-alone solution in a large building. Star A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator Data on a star network passes through hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of network. It also acts as a repeater for data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable. Advantages of a Star Topology •Easy to install and wire. •No disruptions to network then connecting or removing devices. •Easy to detect faults and to remove parts. Disadvantages of a Star Topology •Requires more cable length than a linear topology. •If hub or concentrator fails, nodes attached are disabled. •More expensive than linear bus topologies because of cost of concentrators.
What is a Protocol? the communications between computers on a networkWritten by Gianfranco Fracassi
What is a Protocol? A protocol is a set of rules that governs communications between computers on a network. These rules include guidelines that regulate following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer. See Topology and Cabling sections
of this tutorial for more information. The most common protocols are:
Ethernet LocalTalk Token Ring FDDI ATM Ethernet The Ethernet protocol is by far most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to cable before sending anything through network. If network is clear, computer will transmit. If some other node is already transmitting on cable, computer will wait and try again when line is clear. Sometimes, two computers attempt to transmit at same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, delay caused by collisions and retransmitting is very small and does not normally effect speed of transmission on network. The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps. Fast Ethernet To allow for an increased speed of transmission, Ethernet protocol has developed a new standard that supports 100 Mbps. This is commonly called Fast Ethernet. Fast Ethernet requires use of different, more expensive network concentrators/hubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet is becoming common in schools that have been recently wired. Gigabit Ethernet The most recent development in Ethernet standard is a protocol that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for backbones on a network at this time. In future, it will probably be used for workstation and server connections also. It can be used with both fiber optic cabling and copper. The 1000BaseTX, copper cable used for Gigabit Ethernet, is expected to become formal standard in 1999. LocalTalk LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through serial port. The Macintosh operating system allows establishment of a peer-to-peer network without need for additional software. With addition of server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps.