This article is about the topology of communication networks. For the topology of electrical networks, seeTopology (electrical circuits). For the topology of transport networks, seeTransport topology. For notion of network from general topology, seeBase (topology).
Network topology is the arrangement of the elements (links,nodes, etc.) of a communication network.[1][2] Network topology can be used to define or describe the arrangement of various types of telecommunication networks, includingcommand and control radio networks,[3] industrialfieldbusses andcomputer networks.
Network topology is thetopological[4] structure of a network and may be depicted physically or logically. It is an application ofgraph theory[3] wherein communicating devices are modeled as nodes and the connections between the devices are modeled as links or lines between the nodes.Physical topology is the placement of the various components of a network (e.g., device location and cable installation), whilelogical topology illustrates how data flows within a network. Distances between nodes, physical interconnections,transmission rates, or signal types may differ between two different networks, yet their logical topologies may be identical. A network's physical topology is a particular concern of thephysical layer of theOSI model.
Examples of network topologies are found inlocal area networks (LAN), a common computer network installation. Any given node in the LAN has one or more physical links to other devices in the network; graphically mapping these links results in a geometric shape that can be used to describe the physical topology of the network. A wide variety of physical topologies have been used in LANs, includingring,bus,mesh andstar. Conversely, mapping thedata flow between the components determines the logical topology of the network. In comparison,Controller Area Networks, common in vehicles, are primarily distributedcontrol system networks of one or more controllers interconnected with sensors and actuators over, invariably, a physical bus topology.
Two basic categories of network topologies exist: physical topologies and logical topologies.[5]
Thetransmission medium layout used to link devices is the physical topology of the network. For conductive or fiber optical mediums, this refers to the layout ofcabling, the locations of nodes, and the links between the nodes and the cabling.[1] The physical topology of a network is determined by the capabilities of the network access devices and media, the level of control or fault tolerance desired, and the cost associated with cabling or telecommunication circuits.
In contrast, logical topology is the way that the signals act on the network media,[6] or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices.[7] A network's logical topology is not necessarily the same as its physical topology. For example, the originaltwisted pair Ethernet usingrepeater hubs was a logical bus topology carried on a physical star topology.Token Ring is a logical ring topology, but is wired as a physical star from themedia access unit. Physically,Avionics Full-Duplex Switched Ethernet (AFDX) can be a cascaded star topology of multiple dual redundant Ethernet switches; however, theAFDX virtual links are modeled astime-switched single-transmitter bus connections, thus following the safety model of asingle-transmitter bus topology previously used in aircraft. Logical topologies are often closely associated withmedia access control methods and protocols. Some networks are able to dynamically change their logical topology through configuration changes to theirrouters and switches.
A widely adoptedfamily of transmission media used in local area network (LAN) technology is collectively known asEthernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined byIEEE 802.3. Ethernet transmits data over both copper and fiber cables. Wireless LAN standards (e.g., those defined byIEEE 802.11) use radio waves, or others useinfrared signals as a transmission medium.Power line communication uses a building's power cabling to transmit data.
Fiber-optic cables are used to transmit light from one computer/network node to another.
The orders of the following wired technologies are, roughly, from slowest to fastest transmission speed.
Coaxial cable is widely used for cable television systems, office buildings, and other work-sites for local area networks. The cables consist of copper or aluminum wire surrounded by an insulating layer (typically a flexible material with a high dielectric constant), which itself is surrounded by a conductive layer. The insulation between the conductors helps maintain the characteristic impedance of the cable, which can help improve its performance. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.
Signal traces onprinted circuit boards are common for board-level serial communication, particularly between certain types of integrated circuits, a common example beingSPI.
Ribbon cable (untwisted and possibly unshielded) has been a cost-effective medium for serial protocols, especially within metallic enclosures or rolled within copper braid or foil, over short distances, or at lower data rates. Several serial network protocols can be deployed without shielded or twisted pair cabling, that is, with flat or ribbon cable, or a hybrid flat and twisted ribbon cable, shouldEMC, length, andbandwidth constraints permit:RS-232,[8]RS-422,RS-485,[9]CAN,[10]GPIB,SCSI,[11] etc.
Twisted pair wire is the most widely used medium for all telecommunication.[citation needed] Twisted-pair cabling consists of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer network cabling (wiredEthernet as defined byIEEE 802.3) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reducecrosstalk andelectromagnetic induction. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted pair (STP). Each form comes in several category ratings, designed for use in various scenarios.
2007 map showing submarine optical fiber telecommunication cables around the world
Anoptical fiber is a glass fiber. It carries pulses of light that represent data. Some advantages of optical fibers over metal wires are very low transmission loss and immunity from electrical interference. Optical fibers can simultaneously carry multiple wavelengths of light, which greatly increases the rate that data can be sent, and helps enable data rates of up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used forundersea communications cables to interconnect continents.
Price is a main factor distinguishing wired and wireless technology options in a business. Wireless options command a price premium that can make purchasing wired computers, printers and other devices a financial benefit. Before making the decision to purchase hard-wired technology products, a review of the restrictions and limitations of the selections is necessary. Business and employee needs may override any cost considerations.[12]
Terrestrialmicrowave – Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately 50 km (30 mi) apart.
Communications satellites – Satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically ingeostationary orbit 35,786 km (22,236 mi) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
Cellular and PCS systems use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.
Radio andspread spectrum technologies – Wireless local area networks use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread-spectrum technology to enable communication between multiple devices in a limited area.IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology known asWi-Fi.
Network nodes are the points of connection of the transmission medium to transmitters and receivers of the electrical, optical, or radio signals carried in the medium. Nodes may be associated with a computer, but certain types may have only a microcontroller at a node or possibly no programmable device at all. In the simplest of serial arrangements, oneRS-232 transmitter can be connected by a pair of wires to one receiver, forming two nodes on one link, or a Point-to-Point topology. Some protocols permit a single node to only either transmit or receive (e.g.,ARINC 429). Other protocols have nodes that can both transmit and receive into a single channel (e.g.,CAN can have many transceivers connected to a single bus). While the conventionalsystem building blocks of acomputer network includenetwork interface controllers (NICs),repeaters,hubs,bridges,switches,routers,modems,gateways, andfirewalls, most address network concerns beyond the physical network topology and may be represented as single nodes on a particular physical network topology.
AnATM network interface in the form of an accessory card. A lot of network interfaces are built-in.
Anetwork interface controller (NIC) iscomputer hardware that provides a computer with the ability to access the transmission media, and has the ability to process low-level network information. For example, the NIC may have a connector for accepting a cable, or an aerial for wireless transmission and reception, and the associated circuitry.
The NIC responds to traffic addressed to anetwork address for either the NIC or the computer as a whole.
InEthernet networks, each network interface controller has a uniqueMedia Access Control (MAC) address—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, theInstitute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is sixoctets. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.
Arepeater is anelectronic device that receives a networksignal, cleans it of unnecessary noise and regenerates it. The signal may be reformed orretransmitted at a higher power level, to the other side of an obstruction, possibly using a different transmission medium, so that the signal can cover longer distances without degradation. Commercial repeaters have extendedRS-232 segments from 15 meters to over a kilometer.[15] In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart.
Repeaters work within the physical layer of the OSI model, that is, there is no end-to-end change in the physical protocol across the repeater, or repeater pair, even if a different physical layer may be used between the ends of the repeater, or repeater pair. Repeaters require a small amount of time to regenerate the signal. This can cause apropagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters that can be used in a row, e.g., the Ethernet5-4-3 rule.
A repeater with multiple ports is known as hub, anEthernet hub in Ethernet networks, aUSB hub in USB networks.
USB networks use hubs to form tiered-star topologies.
Ethernet hubs and repeaters in LANs have been mostly obsoleted by modernswitches.
Anetwork bridge connects and filters traffic between twonetwork segments at thedata link layer (layer 2) of theOSI model to form a single network. This breaks the network's collision domain but maintains a unified broadcast domain. Network segmentation breaks down a large, congested network into an aggregation of smaller, more efficient networks.
Bridges come in three basic types:
Local bridges: Directly connect LANs
Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, have largely been replaced with routers.
Wireless bridges: Can be used to join LANs or connect remote devices to LANs.
Anetwork switch is a device that forwards and filtersOSI layer 2datagrams (frames) betweenports based on the destination MAC address in each frame.[16]A switch is distinct from a hub in that it only forwards the frames to the physical ports involved in the communication rather than all ports connected. It can be thought of as a multi-port bridge.[17] It learns to associate physical ports to MAC addresses by examining the source addresses of received frames. If an unknown destination is targeted, the switch broadcasts to all ports but the source. Switches normally have numerous ports, facilitating a star topology for devices and cascading additional switches.
Multi-layer switches are capable of routing based on layer 3 addressing or additional logical levels. The termswitch is often used loosely to include devices such as routers and bridges, as well as devices that may distribute traffic based on load or based on application content (e.g., a WebURL identifier).
A typical home or small office router showing theADSL telephone line andEthernet network cable connections
Arouter is aninternetworking device that forwardspackets between networks by processing the routing information included in the packet or datagram (Internet protocol information from layer 3). The routing information is often processed in conjunction with therouting table (or forwarding table). A router uses its routing table to determine where to forward packets. A destination in a routing table can include ablack hole because data can go into it; however, no further processing is done for said data, i.e., the packets are dropped.
Modems (MOdulator-DEModulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this, one or morecarrier signals aremodulated by the digital signal to produce ananalog signal that can be tailored to give the required properties for transmission. Modems are commonly used for telephone lines, using adigital subscriber line technology.
Afirewall is a network device for controlling network security and access rules. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase incyber attacks.
The simplest topology with a dedicated link between two endpoints. Easiest to understand of the variations of point-to-point topology is a point-to-pointcommunication channel that appears, to the user, to be permanently associated with the two endpoints. A child'stin can telephone is one example of aphysical dedicated channel.
Usingcircuit-switching orpacket-switching technologies, a point-to-point circuit can be set up dynamically and dropped when no longer needed. Switched point-to-point topologies are the basic model of conventionaltelephony.
The value of a permanent point-to-point network is unimpeded communications between the two endpoints. The value of an on-demand point-to-point connection is proportional to the number of potential pairs of subscribers and has been expressed asMetcalfe's Law.
Daisy chaining is accomplished by connecting each computer in series to the next. If a message is intended for a computer partway down the line, each system bounces it along in sequence until it reaches the destination. A daisy-chained network can take two basic forms: linear and ring.
Alinear topology puts a two-way link between one computer and the next. However, this was expensive in the early days of computing, since each computer (except for the ones at each end) required two receivers and two transmitters.
By connecting the computers at each end of the chain, aring topology can be formed. When anode sends a message, the message is processed by each computer in the ring. An advantage of the ring is that the number of transmitters and receivers can be cut in half. Since a message will eventually loop all of the way around, transmission does not need to go both directions. Alternatively, the ring can be used to improve fault tolerance. If the ring breaks at a particular link, then the transmission can be sent via the reverse path, thereby ensuring that all nodes are always connected in the case of a single failure.
In local area networks using bus topology, each node is connected by interface connectors to a single central cable. This is the 'bus', also referred to as thebackbone, ortrunk – alldata transmission between nodes in the network is transmitted over this common transmission medium and is able to bereceived by all nodes in the network simultaneously.[1]
A signal containing the address of the intended receiving machine travels from a source machine in both directions to all machines connected to the bus until it finds the intended recipient, which then accepts the data. If the machine address does not match the intended address for the data, the data portion of the signal is ignored. Since the bus topology consists of only one wire it is less expensive to implement than other topologies, but the savings are offset by the higher cost of managing the network. Additionally, since the network is dependent on the single cable, it can be thesingle point of failure of the network. In this topology data being transferred may be accessed by any node.
In a linear bus network, all of the nodes of the network are connected to a common transmission medium, which has just two endpoints. When the electrical signal reaches the end of the bus, the signal is reflected back down the line, causing unwanted interference. To prevent this, the two endpoints of the bus are normally terminated with a device called aterminator.
In a distributed bus network, all of the nodes of the network are connected to a common transmission medium with more than two endpoints, created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology because all nodes share a common transmission medium.
In star topology (also called hub-and-spoke), every peripheral node (computer workstation or any other peripheral) is connected to a central node called a hub or switch. The hub is the server and the peripherals are the clients. The network does not necessarily have to resemble a star to be classified as a star network, but all of the peripheral nodes on the network must be connected to one central hub. All traffic that traverses the network passes through the central hub, which acts as asignal repeater.
The star topology is considered the easiest topology to design and implement. One advantage of the star topology is the simplicity of adding additional nodes. The primary disadvantage of the star topology is that the hub represents a single point of failure. Also, since all peripheral communication must flow through the central hub, the aggregate central bandwidth forms a network bottleneck for large clusters.
The extended star network topology extends a physical star topology by one or more repeaters between the central node and theperipheral (or 'spoke') nodes. The repeaters are used to extend the maximum transmission distance of the physical layer, the point-to-point distance between the central node and the peripheral nodes. Repeaters allow greater transmission distance, further than would be possible using just the transmitting power of the central node. The use of repeaters can also overcome limitations from the standard upon which the physical layer is based.
A physical extended star topology in which repeaters are replaced with hubs or switches is a type of hybrid network topology and is referred to as a physical hierarchical star topology, although some texts make no distinction between the two topologies.
A physical hierarchical star topology can also be referred as a tier-star topology. This topology differs from atree topology in the way star networks are connected together. A tier-star topology uses a central node, while a tree topology uses a central bus and can also be referred as a star-bus network.
A distributed star is a network topology that is composed of individual networks that are based upon the physical star topology connected in a linear fashion – i.e., daisy-chained – with no central or top level connection point (e.g., two or morestacked hubs, along with their associated star connected nodes orspokes).
A ring topology is adaisy chain in a closed loop. Data travels around the ring in one direction. When one node sends data to another, the data passes through each intermediate node on the ring until it reaches its destination. The intermediate nodes repeat (retransmit) the data to keep the signal strong.[5] Every node is a peer; there is no hierarchical relationship of clients and servers. If one node is unable to retransmit data, it severes communication between the nodes before and after it in the bus.
Advantages:
When the load on the network increases, its performance is better than bus topology.
There is no need for a network server to control the connectivity between workstations.
Disadvantages:
Aggregate network bandwidth is bottlenecked by the weakest link between two nodes.
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints, is approximated byReed's Law.
In afully connected network, all nodes are interconnected. (Ingraph theory this is called acomplete graph.) The simplest fully connected network is a two-node network. A fully connected network doesn't need to usepacket switching orbroadcasting. However, since the number of connections grows quadratically with the number of nodes:
This makes it impractical for large networks. This kind of topology does not trip and affect other nodes in the network.
In a partially connected network, certain nodes are connected to exactly one other node, but some nodes are connected to two or more other nodes with a point-to-point link. This makes it possible to make use of some of the redundancy of mesh topology that is physically fully connected, without the expense and complexity required for a connection between every node in the network.
Hybrid topology is also known as hybrid network.[19] Hybrid networks combine two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, atree network (orstar-bus network) is a hybrid topology in whichstar networks are interconnected viabus networks.[20][21] However, a tree network connected to another tree network is still topologically a tree network, not a distinct network type. A hybrid topology is always produced when two different basic network topologies are connected.
Astar-ring network consists of two or more ring networks connected using amultistation access unit (MAU) as a centralized hub.
Snowflake topology is meshed at the core, but tree shaped at the edges.[22]
Two other hybrid network types arehybrid mesh andhierarchical star.[20]
Thestar topology reduces the probability of a network failure by connecting all of the peripheral nodes (computers, etc.) to a central node. When the physical star topology is applied to a logical bus network such asEthernet, this central node (traditionally a hub) rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. Allperipheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. Thefailure of atransmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the remaining peripheral nodes will be unaffected. However, the disadvantage is that the failure of the central node will cause the failure of all of the peripheral nodes.
If the central node ispassive, the originating node must be able to tolerate the reception of anecho of its own transmission, delayed by the two-wayround triptransmission time (i.e. to and from the central node) plus any delay generated in the central node. Anactive star network has an active central node that usually has the means to prevent echo-related problems.
Atree topology (a.k.a.hierarchical topology) can be viewed as a collection of star networks arranged in ahierarchy. Thistree structure has individual peripheral nodes (e.g., leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the functionality of the central node may be distributed.
As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.
To alleviate the amount of network traffic that comes from broadcasting all signals to all nodes, more advanced central nodes were developed that are able to keep track of the identities of the nodes that are connected to the network. Thesenetwork switches willlearn the layout of the network bylistening on each port during normal data transmission, examining thedata packets and recording the address/identifier of each connected node and which port it is connected to in alookup table held in memory. This lookup table then allows future transmissions to be forwarded to the intended destination only.
Daisy chain topology is a way of connecting network nodes in a linear or ring structure. It is used to transmit messages from one node to the next until they reach the destination node.
A daisy chain network can have two types: linear and ring. A linear daisy chain network is like an electrical series, where the first and last nodes are not connected. A ring daisy chain network is where the first and last nodes are connected, forming a loop.
In a partially connected mesh topology, there are at least two nodes with two or more paths between them to provide redundant paths in case the link providing one of the paths fails. Decentralization is often used to compensate for the single-point-failure disadvantage that is present when using a single device as a central node (e.g., in star and tree networks). A special kind of mesh, limiting the number of hops between two nodes, is ahypercube. The number of arbitrary forks in mesh networks makes them more difficult to design and implement, but their decentralized nature makes them very useful.
This is similar in some ways to agrid network, where a linear or ring topology is used to connect systems in multiple directions. A multidimensional ring has atoroidal topology, for instance.
Afully connected network,complete topology, orfull mesh topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are direct links. Networks designed with this topology are usually very expensive to set up, but provide a high degree of reliability due to the multiple paths for data that are provided by the large number of redundant links between nodes. This topology is mostly seen inmilitary applications.
^A. Hooke (September 2000),Interplanetary Internet(PDF), Third Annual International Symposium on Advanced Radio Technologies, archived fromthe original(PDF) on 2012-01-13, retrieved2011-11-12
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