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By manderson0310 Oct 08, 2014 6378 Words
Matthew Anderson
October 1, 2014
Unit 3 Assignment 1
“A bus network topology is a network architecture in which a set of clients are connected via a shared communications line/cables” (Abu, 2013). There are several common instances of the bus architecture, including one in the motherboard of most computers. Bus networks may be the simplest way to connect multiple clients even though many may have problems. When two clients want to transmit at the same time on the same bus, this may cause some problems. Thus systems use bus networks normally some may have scheme for collision handling or collision avoidance for communication on the bus. Though this may be quite often in using Carrier Sense Multiple Access or the presence of a bus master’s who controls access to the shared bus resource.  “A ring network is a network topology in which each node connects exactly to two other nodes, forming a single continual pathway for each node - a ring” (Conjecture Corporation, 2003). In a ring network data travels from each node to the other node/s with each node handling every packet. A ring network provides only a pathway between any two nodes at one time. Ring networks may be disrupted by the failure of a link. A node failure or cable break will isolate every node attached to the ring. A malfunctioning that any workstation can create problems for the entire network. “A hierarchical network or a tree network resembles a star network in that several computers are connected to a central host computer usually a mainframe” (Miss). “However, these "client" computers also serve as host computers to next level units” (Miss). Thus, the hierarchical network can theoretically be compared to a standard organizational chart or a large corporation. “Typically, the host computer at the top of the hierarchy is a mainframe computer. Lower-levels in the hierarchy could consist of minicomputers and microcomputers” (Miss).

This paper is about networking structure and what this structure is all about, what it does and what happens when it doesn’t work. Bus Topology: 
In a bus network, there is a use of a backbone or main line that connects all devices. The single cable functions as a shared communication media that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. Ethernet bus topologies are relatively easy to install and don’t require much cabling compared to the alternatives. 10Base-2 (Thin Net) and 10Base-5 (Thick Net) both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computer are added to a network bus, performance problems will likely result. In addition, if the main cable fails, the entire network crashes. Advantages:

1. It’s easy to setup, handle and implement.
2. It’s best suited for small networks.
3. It very low in cost.
1. The cable length is limited. This limits the number of networks nodes that can be connected. 2. This network topology can perform well only for a limited number of nodes. When the number of devices connected to the bus increases, the efficiency decreases. 3. It’s suitable for networks with low traffic. High traffic increases load on the bus and the efficiency drops.  4. It’s heavily dependent on the center main cable. A fault in the bus leads to network failure. 5. Each device on the network “sees” all data being transmitted, thus posing a security risk.

Ring Topology:
In a ring network, every device has exactly 2 neighbors for communication purposes. All messages travel through a ring in the same direction (either “clockwise” or “counter-clockwise”). A failure in any cable or a device...

Bus Network none as one of the most commonly used topologies. With this network all devices are connected to a common central cable. The cable is referred to as a bus. Some pros of this particular network are that it is inexpensive, and easy to install. Another pro to this network is that adding new devices are simple. All that is required to add a new device is connecting the device to the bus. Unfortunately the cons of this network can be somewhat fatal. If the main cab e goes bad the whole network will shut down, and it is not always easy to find out why it failed. Still due to the low cost and easy configuration; the bus network is the most widely used.

Star network is very different from the bus network. Each device is connected to a hub. A hub is a specialized type of hardware. The main function of the hub is to receive data transmission and route them to the proper destination. The star network is easy to install and update because; all nodes are connected directly to the hub. When the network needs s configuring changes are made to the hub instead of rewiring the network. Pros are since each node connects to the hub it is easy to diagnose the problem. Another pro would be if one computer or laptop fails only that computer or laptop will not be able to send or receive data. Cons are if the hub fails the network will shut down. Another con would be the hub could bottleneck. Bottleneck means to slow down or to stop working completely. Basically there is not enough capacity to handle the current volume of traffic.

In a ring topology each node is connected to two other nodes creating a ring. The signal travels around the loop in one direction, passing through each computer. A good thing is that multiple LAN’s can be connected to each ring. One node will hold the data and transmit it to the other computers. Some pros would be that the ring is more reliable than the bus and a star. The ring topology is more reliable because if one...

Bus network – Components such as computers, printers, servers, and a common cable called a bus make up the bus network. The common cable or bus connects all devices to the network. The bus network is inexpensive and easy to configure. All one as to do is plug or remove a device from the bus cable to reconfigure the network. This makes the bus network widely used in businesses. The disadvantage is that the entire network is dependant on the bus cable. If the cable went bad, the network goes down and it is hard to determine the cause of the problem. Think of the bus network like a power strip. It easy to plug and unplug devices in to the power strip but if the breaker on the strip kept tripping or the strip went bad it would be hard to determine the cause.

Ring network – The Ring network has the same components as the bus. It has computers, printers, servers, and cables. In the ring network, all the devices on the network are connected to two other devices completing a circle or ring. The ring network is more reliable than the bus or star because it is not dependant on just one component to keep the network up. If one of the devices on a ring network went bad the systems administrator could just change the flow of data so that it would go in the opposite direction until it reached its destination. This makes the ring network expensive and difficult to install.

Star network – The Star network has the same components as the bus and ring but it uses a hub to connect all devices on the network to the server. This makes it very easy to add and remove devices or reconfigure the network. Instead of reconfiguring the entire network all one has to do is reconfigure the hub. It also makes very easy to diagnose problems if one device on the network goes down it does not bring the entire network down but because like the bus network it is dependent on one component the hub to connect the network if the hub goes bad then the entire network goes...

Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer network.[1][2] Essentially, it is the topological[3] structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ between two networks, yet their topologies may be identical. A good example is a local area network (LAN): 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. Conversely, mapping the data flow between the components determines the logical topology of the network. Contents

1 Topology 
1.1 Point-to-point
1.2 Bus
1.3 Star
1.4 Ring
1.5 Mesh
1.6 Tree 
1.6.1 Advantages
1.6.2 Disadvantages
1.7 Hybrid
1.8 Daisy chain
2 Centralization
3 Decentralization
4 See also
5 References
6 External links
There are two basic categories of network topologies:[4]
1. Physical topologies
2. Logical topologies
The shape of the cabling layout used to link devices is called the physical topology of the network. This refers to the layout of cabling, the locations of nodes, and the interconnections 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 telecommunications circuits. The logical topology in contrast, is the way that the signals act on the network media, 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. A network's logical topology is not necessarily the same as its physical topology. For example, the original twisted pair Ethernet using repeater hubs was a logical bus topology with a physical star topology layout. Token Ring is a logical ring topology, but is wired a physical star from the Media Access Unit. The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies but describes the path that the data takes between nodes being used as opposed to the actual physical connections between nodes. The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or by the flow of the electrical signals, although in many cases the paths that the electrical signals take between nodes may closely match the logical flow of data, hence the convention of using the terms logical topology and signal topology interchangeably. Logical topologies are often closely associated with Media Access Control methods and protocols. Logical topologies are able to be dynamically reconfigured by special types of equipment such as routers and switches.

Diagram of different network topologies.
The study of network topology recognizes eight basic topologies:[5] Point-to-point
Ring or circular
Daisy chain
The simplest topology with a permanent link between two endpoints. Switched point-to-point topologies are the basic model of conventional telephony. 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 as Metcalfe's Law. Permanent (dedicated) 

Easiest to understand, of the variations of point-to-point topology, is a point-to-point communications channel that appears, to the user, to be permanently associated with the two endpoints. A children's tin can telephone is one example of a physical dedicated channel. Within many switched telecommunications systems, it is possible to establish a permanent circuit. One example might be a telephone in the lobby of a public building, which is programmed to ring only the number of a telephone dispatcher. "Nailing down" a switched connection saves the cost of running a physical circuit between the two points. The resources in such a connection can be released when no longer needed, for example, a television circuit from a parade route back to the studio. Switched: 

Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically, and dropped when no longer needed. This is the basic mode of conventional telephony. Bus

Main article: Bus network

Bus network topology
In local area networks where bus topology is used, each node is connected to a single cable. Each computer or server is connected to the single bus cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted. Since the bus topology consists of only one wire, it is rather inexpensive to implement when compared to other topologies. However, the low cost of implementing the technology is offset by the high cost of managing the network. Additionally, since only one cable is utilized, it can be the single point of failure. If the network cable is terminated on both ends and when without termination data transfer stop and when cable breaks, the entire network will be down. Linear bus

The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.[1] Note: When the electrical signal reaches the end of the bus, the signal "echoes" back down the line, causing unwanted interference. As a solution, the two endpoints of the bus are normally terminated with a device called a terminator that prevents this echo. Distributed bus

The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are 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 (i.e., all nodes share a common transmission medium). Star

Main article: Star network

Star network topology
In local area networks with a star topology, each network host is connected to a central hub with a point-to-point connection. In Star topology every node (computer workstation or any other peripheral) is connected to central node called hub or switch. The switch 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 nodes on the network must be connected to one central device. All traffic that traverses the network passes through the central hub. The hub acts as a signal repeater. The star topology is considered the easiest topology to design and implement. An 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. Extended star 

A type of network topology in which a network that is based upon the physical star topology has one or more repeaters between the central node (the 'hub' of the star) and the peripheral or 'spoke' nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes beyond that which is supported by the transmitter power of the central node or beyond that which is supported by the standard upon which the physical layer of the physical star network is based. If the repeaters in a network that is based upon the physical extended star topology are replaced with hubs or switches, then a hybrid network topology is created that is referred to as a physical hierarchical star topology, although some texts make no distinction between the two topologies. Distributed Star 

A type of 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 more 'stacked' hubs, along with their associated star connected nodes or 'spokes'). Ring

Main article: Ring network

Ring network topology
A network topology that is set up in a circular fashion in which data travels around the ring in one direction and each device on the ring acts as a repeater to keep the signal strong as it travels. Each device incorporates a receiver for the incoming signal and a transmitter to send the data on to the next device in the ring. The network is dependent on the ability of the signal to travel around the ring. When a device sends data, it must travel through each device on the ring until it reaches its destination. Every node is a critical link.[4] Mesh

Main article: Mesh networking
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 by Reed's Law. Fully connected network

Fully connected mesh topology
A fully connected network is a communication network in which each of the nodes is connected to each other. In graph theory it known as a complete graph. A fully connected network doesn't need to use switching nor broadcasting. However, its major disadvantage is that the number of connections grows quadratically with the number of nodes, per the formula

and so it is extremely impractical for large networks. A two-node network is technically a fully connected network. Partially connected

Partially connected mesh topology
The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link – this makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network. Tree

Tree network topology

This section may be confusing or unclear to readers. (June 2011) This particular type of network topology is based on a hierarchy of nodes. The highest level of any tree network consists of a single, 'root' node, this node connected either a single (or, more commonly, multiple) node(s) in the level below by (a) point-to-point link(s). These lower level nodes are also connected to a single or multiple nodes in the next level down. Tree networks are not constrained to any number of levels, but as tree networks are a variant of the bus network topology, they are prone to crippling network failures should a connection in a higher level of nodes fail/suffer damage. Each node in the network has a specific, fixed number of nodes connected to it at the next lower level in the hierarchy, this number referred to as the 'branching factor' of the tree. This tree has individual peripheral nodes. 1. A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central 'root' node and only one hierarchical level below it would exhibit the physical topology of a star. 2. A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology. 3. The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large – this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible. 4. The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network. 5. If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy. Such a type of network topology is very useful and highly recommended. Advantages

It is scalable. Secondary nodes allow more devices to be connected to a central node. Point to point connection of devices.
Having different levels of the network makes it more manageable hence easier fault identification and isolation. Disadvantages
Maintenance of the network may be an issue when the network spans a great area. Since it is a variation of bus topology, if the backbone fails, the entire network is crippled. definition: Tree topology is a combination of Bus and Star topology. An example of this network could be cable TV technology. Other examples are in dynamic tree based wireless networks for military, mining and otherwise mobile applications.[6] The Naval Postgraduate School, Monterey CA, demonstrated such tree based wireless networks for border security.[7] In a pilot system, aerial cameras kept aloft by balloons relayed real time high resolution video to ground personnel via a dynamic self healing tree based network. Hybrid

Hybrid networks use a combination of any 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 a tree network connected to a tree network is still a tree network topology. A hybrid topology is always produced when two different basic network topologies are connected. Two common examples for Hybrid network are: star ring network and star bus network A Star ring network consists of two or more star topologies connected using a multistation access unit (MAU) as a centralized hub. A Star Bus network consists of two or more star topologies connected using a bus trunk (the bus trunk serves as the network's backbone). While grid and torus networks have found popularity in high-performance computing applications, some systems have used genetic algorithms to design custom networks that have the fewest possible hops in between different nodes. Some of the resulting layouts are nearly incomprehensible, although they function quite well.[citation needed] A Snowflake topology is really a "Star of Stars" network, so it exhibits characteristics of a hybrid network topology but is not composed of two different basic network topologies being connected. Daisy chain

Except for star-based networks, the easiest way to add more computers into a network is by daisy-chaining, or 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. A linear 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, a ring topology can be formed. 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. When a node sends a message, the message is processed by each computer in the ring. 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. Centralization

The star 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 as Ethernet, 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. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission 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 is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way round trip transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems. A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networks arranged in a hierarchy. This tree 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. These network switches will "learn" the layout of the network by "listening" on each port during normal data transmission, examining the data packets and recording the address/identifier of each connected node and which port it is connected to in a lookup table held in memory. This lookup table then allows future transmissions to be forwarded to the intended destination only. Decentralization

In a mesh topology (i.e., a partially connected mesh topology), there are at least two nodes with two or more paths between them to provide redundant paths to be used in case the link providing one of the paths fails. This 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 a hypercube. The number of arbitrary forks in mesh networks makes them more difficult to design and implement, but their decentralized nature makes them very useful. In 2012 the IEEE published the Shortest path bridging protocol to eases configuration tasks and allows all paths to be active which increases bandwidth, and redundancy between all devices.[8][9][10][11][12] This is similar in some ways to a grid network, where a linear or ring topology is used to connect systems in multiple directions. A multidimensional ring has a toroidal topology, for instance. A fully connected network, complete topology, or full 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 n(n-1)/2 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 in military applications. See also

Broadcast communication network
Computer network
Computer network diagram
IEEE 802.1aq
Internet topology
Network simulator
Relay network
Rhizome (philosophy)
Scale-free network
Shared mesh
Switched communication network
Switched mesh
Tree structure
1. Groth, David; Toby Skandier (2005). Network+ Study Guide, Fourth Edition'. Sybex, Inc. ISBN 0-7821-4406-3.  2. ATIS committee PRQC. "mesh topology". ATIS Telecom Glossary 2007. Alliance for Telecommunications Industry Solutions. Retrieved 2008-10-10. 3. Chiang, Mung; Yang, Michael (2004). "Towards Network X-ities From a Topological Point of View: Evolvability and Scalability". Proc. 42nd Allerton Conference. 4. Inc, S., (2002). Networking Complete. Third Edition. San Francisco: Sybex 5. Bicsi, B., (2002). Network Design Basics for Cabling Professionals. City: McGraw-Hill Professional 6. "Meshdynamics : Highest performance Voice, Video and Data Outdoors". Retrieved 2008-02-23.[dead link] 7. Robert Lee Lounsbury, Jr. (September 2007). Optimum Antenna Configuration for Maximizing Access Point Range of an IEEE 802.11 Wireless Mesh Network in Support of Multimission Operations Relative to Hastily Formed Scalable Deployments (PDF). Retrieved 2008-02-23. 8. "Avaya Extends the Automated Campus to End the Network Waiting Game". Avaya. 1 April 2014. Retrieved 18 April 2014. 9. Peter Ashwood-Smith (24 February 2011). "Shortest Path Bridging IEEE 802.1aq Overview". Huawei. Retrieved 11 May 2012. 10. Jim Duffy (11 May 2012). "Largest Illinois healthcare system uproots Cisco to build $40M private cloud". PC Advisor. Retrieved 11 May 2012. "Shortest Path Bridging will replace Spanning Tree in the Ethernet fabric." 11. "IEEE Approves New IEEE 802.1aq Shortest Path Bridging Standard". Tech Power Up. 7 May 2012. Retrieved 11 May 2012. 12. D. Fedyk, Ed.,; P. Ashwood-Smith, Ed.,; D. Allan, A. Bragg,; P. Unbehagen (April 2012). "IS-IS Extensions Supporting IEEE 802.1aq". IETF. Retrieved 12 May 2012.

Network Topologies
The Trouble with Supply Chains (
Bus, ring, star, and other types of network topology
By Bradley Mitchell
Computer Network Design
Cloud Networking
Network Topology
Wireless Mesh Network
Network Diagram

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Get the Network White Papers, Videos and More. Enjoy 99.9999% Network Uptime. Cheap High Speed 60% Off On High Speed Internet Fantastic July 2014 Deals In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.  Topology in Network Design

Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there. Network topologies are categorized into the following basic types:  bus 

More complex networks can be built as hybrids of two or more of the above basic topologies.  Bus Topology
Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.  Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.  Illustration - Bus Topology Diagram 

Ring Topology
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.  To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.  Illustration - Ring Topology Diagram 

Star Topology
Many home networks use the star topology. A star network features a central connection point called a "hub node" that may be a network hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.  Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)  Illustration - Star Topology Diagram 

Tree Topology
Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the root of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.  Illustration - Tree Topology Diagram 

Mesh Topology
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.  A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.  Illustration - Mesh Topology Diagram 

Topologies remain an important part of network design theory. You can probably build a home or small business computer network without understanding the difference between a bus design and a star design, but becoming familiar with the standard topologies gives you a better understanding of important networking concepts like hubs, broadcasts, and routes.  Next page > Network Design and the OSI Model > Page 1, 2

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The Open Systems Interconnection (OSI) reference model has been an essential element of computer network design since its ratification in 1984. The OSI is an abstract model of how network protocols and equipment should communicate and work together (interoperate).  The OSI model is a technology standard maintained by the International Standards Organization (ISO). Although today's technologies do not fully conform to the standard, it remains a useful introduction to the study of network architecture.  The OSI Model Stack

The OSI model divides the complex task of computer-to-computer communications, traditionally called internetworking, into a series of stages known as layers. Layers in the OSI model are ordered from lowest level to highest. Together, these layers comprise the OSI stack. The stack contains seven layers in two groups:  Upper layers - 

7. application
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5. session 
Lower layers - 
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More - OSI Model Layers 
Upper Layers of the OSI Model
OSI designates the application, presentation, and session stages of the stack as the upper layers. Generally speaking, software in these layers performs application-specific functions like data formatting, encryption, and connection management.  Examples of upper layer technologies in the OSI model are HTTP, SSL and NFS.  Lower Layers of the OSI Model

The remaining lower layers of the OSI model provide more primitive network-specific functions like routing, addressing, and flow control. Examples of lower layer technologies in the OSI model are TCP, IP, and Ethernet.  Benefits of the OSI Model

By separating the network communications into logical smaller pieces, the OSI model simplifies how network protocols are designed. The OSI model was designed to ensure different types of equipment (such as network adapters, hubs, and routers) would all be compatible even if built by different manufacturers. A product from one network equipment vendor that implements OSI Layer 2 functionality, for example, will be much more likely to interoperate with another vendor's OSI Layer 3 product because both vendors are following the same model.  The OSI model also makes network designs more extensible as new protocols and other network services are generally easier to add to a layered architecture than to a monolithic one.  Next page > OSI Model Q&A > Page 1, 2

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