Network Topologies

16. Hybrid

A hybrid network topology has characteristics of more than one standard topology. For example, different segments of a network may each use a different topology. Hybrid topologies can be complex to maintain because they typically incorporate a wide range of technologies. In a star-bus topology, you link the central nodes of two or more star networks using a common network cable, or bus. Each star network then becomes a subnetwork, and is a node on a larger bus network. In a star-of-stars topology, you connect the central nodes of two or more star networks with a new common node. In a star-ring topology, you connect the central nodes of multiple star networks in a ring. The data flow between different subnetworks passes through this ring.

18. Multipoint Connections

A multipoint connection links multiple nodes on a network. So each multipoint connection has more than two endpoints. A signal transmitted by any node on the connection is detected by all the other nodes, but only the intended node receives it. Multipoint connections are used, for example, in the physical bus network topology.

1. Network Model

A network model is a design specification that indicates how the nodes on a network are constructed to communicate with one another. This determines the extent to which communications and processing are either centralized or distributed. When selecting a network model, you need to consider specific requirements, connectivity methods, and technologies being used.

12. Star

A network with a physical star topology contains a central connectivity device, such as a switch, with individual physical connections to each node. The individual nodes send data to the connectivity device, and the device then either forwards it to the appropriate destination node, as in the case of a switch, or simply passes it through to all attached nodes, as in the case of a hub. Star topologies are easy to maintain and reliable -a single failed node doesn't prevent other nodes from communicating. However, if the central connectivity device in a star network fails, the entire network fails. A host-based network is a classic example of a physical star topology. Each node has a connection to the host computer and isn't aware of other nodes on the network.

8. Network Topology

A network's topology refers to the overall layout of its elements. A physical topology describes the layout, or shape, of a network's physical wiring and devices. A network's logical topology describes the configuration of the paths through which data moves on the network.

11. Bus Process

A node can only send data on a bus, if the communication channel is free. If it's free, the sender transmits a broadcast alert to inform all other nodes that a transmission is being done. This informs other nodes that the channel is being used, which prevents a collision of data packets from multiple users on the bus. If the channel is free, the data signal is sent out to all nodes, which are always listening passively to the channel to detect signals. The signal is detected by all the nodes on the network, but is only received by the destination device, which is marked by a MAC address. If two nodes try to transmit data at the same time, a collision occurs. Each node then waits for a random period of time before trying to retransmit its data. If none of the nodes accept the transmitted data - for example because the destination node is switched off, the data is dropped by the bus itself.

17. Point-To-Point Connections

A point-to-point connection is a direct connection in which one node transmits data directly to a specific device. For example, you can create a point-to-point connection by using a cable to connect one host's NIC directly to another host's NIC. Modern point-to-point connections implementations are used in both wired connections - such as coaxial and Ethernet cables - and wireless connections - such as microwave and laser links. Signals on wireless point-to-point connections may be degraded as a result of physical obstacles in the paths between endpoints.

19. Radiated Connections

A radiated, or broadcast, connection is a type of wireless point-to-point or multipoint connection between devices. For example, devices in a wireless LAN may use infrared or Wi-Fi connections to communicate. Wi-Fi connections enable users to move from place to place freely because devices don't have to be within line of sight of an access point. Wi-Fi provides good performance within the wireless access point coverage area, barring any signal interference.

6. Segment

A segment is made up of various nodes connected to an internetworking device - such as a switch or a router - which connects the segment to the rest of the network. All nodes attached to a segment have common access to that portion of the network, and all traffic in the segment is handled by the internetworking device belonging to the segment.

7. What can dividing a network into segments do?

Dividing a network into segments can improve network performance because it makes it possible to group nodes that communicate with each other most often and to keep the traffic between those nodes within a limited network area. However, performance can suffer if nodes in one segment regularly communicate with nodes on other segments.

2. Centralized Network

In a centralized network, a central host computer controls all network communication, and performs data processing and storage on behalf of clients. Users connect to the host via dedicated terminals, or terminal emulators. Highly centralized networks provide high performance and centralized management, but are typically expensive to implement.

3. Client/Server Network

In a client/server network, servers provide resources to clients. In response to client requests, servers provide authentication services and access to shared files, printers, hardware storage, and applications. In client/server networks, processing power, management services, and administrative functions can be concentrated where needed, and client computers can still perform many basic end-user tasks on their own.

21. Logical Bus Topology

In a logical bus topology, all nodes receive the data that's transmitted at the same time, regardless of the layout of physical wiring. In a logical bus with the physical star topology, for example, data appears to flow in a single, continuous stream, from the sending node to all other nodes - although nodes connect to a central switch and resemble a star. As an example, six PCs are connected to a central switch. Data flows from one node, to the switch, and then onto the other five nodes.

22. Logical Ring Topology

In a logical ring topology, each node receives data only from its upstream neighbor and retransmits data only to its downstream neighbor, regardless of the physical layout of wiring. A logical ring must be used with a physical ring topology or a hybrid such as a physical star-ring topology. Six PCs are connected in a ring. The sender passes data onto its downstream neighbor, who passes it onto its downstream neighbor. This is repeated until the recipient node receives the data.

23. Logical Star Topology

In a logical star topology, all nodes are wired onto the same bus cable, but a central device polls each node to check if it needs to transmit data. The central device also controls the time for which a node has access to the cable. This topology is less commonly used than the logical ring and bus topologies. Four PCs are connected to a central device. The sender nodes passes data to the device, which then forwards it onto the recipient device.

5. Mixed-Mode Models

In a mixed-mode network, elements from more than one of the three standard network models are incorporated. For example, a user workstation in a mixed-mode network may function as a client - requesting resources and services from a server - for day-to-day tasks. However, it may also use centralized or terminal emulation software to authenticate to a specific host system, for instance when accessing a particular application.

20. Logical Network Topology

In a network, the paths along which data flow don't always correspond to the layout of the network's physical wiring. Instead the flow of data is defined by a logical network topology. Logical network topologies provide information that physical topologies don't, such as the data transmission path between a sender and a receiver and the different places in which this path converges or diverges. This information can help you plan transmission links and resource sharing capabilities, and identify network routes for troubleshooting purposes.

4. Peer-To-Peer Network

In a peer-to-peer network, resource sharing, processing, and control of communications are completely decentralized. All clients on the network are equal in terms of providing and using resources, and each workstation authenticates its users. Peer-to-peer networks are easy and inexpensive to implement. However, they're practical only in very small organizations because they don't allow centralized data storage and administration. Each node has to be individually configured.

9. Bus

In a physical bus topology, nodes are arranged in a linear format, and a T-connector connects each node directly to a network cable. The cable is called the bus and serves as a single communication channel. Signals can reflect off the ends of the network cable, but you can attach terminators that impede or absorb these signals so they can't reflect onto the wire. Terminators also prevent signal bounce, in which signals endlessly move from one end of the wire to the other.

14. Mesh

In a physical mesh topology, each node connects directly to every other node, making it possible for each node to communicate with multiple nodes simultaneously Because all nodes have dedicated links with other nodes, there is no network congestion and data travels very fast. This topology is extremely reliable. However, it's difficult to implement and maintain because the number of connections increases exponentially as new nodes are added. The partial-mesh topology is a variation on the mesh topology in which only a few nodes have direct links with all the other nodes. This makes it less complex and less expensive than a full-mesh topology. A partial-mesh topology is commonly used in subnetworks containing small numbers of users and for which lower data transfer rates can be accepted.

13. Ring

In a physical ring topology, each node is connected to the two nearest nodes - its upstream and downstream neighbors - to form a ring. In the ring, data only flows in one direction to prevent data packet collisions. No central connecting device controls network traffic. Instead each node boosts the signals it receives, scans data packets, accepts packets destined for it, and forwards packets destined for other nodes. Ring topologies are potentially unreliable because the failure of a single node can bring down an entire network. A variation on the ring topology is the dual-ring topology, which includes one ring within another. The two rings transmit data in opposite directions. Dual-ring configurations support faster data transfer because data can be sent via the shortest path between a sender and the intended recipient. Also, if there's a breakage in the inner or outer ring, the topology automatically reconfigures to a single ring. This improves reliability.

15. Tree

In a physical tree topology, a central, or root, node is connected to one or more second level nodes, which in turn connect to other nodes at lower levels. The root node has a point-to-point link with each of the second-level nodes. Each of the second-level nodes has a point-to-point connection to one or more third level nodes. Each node in a tree network has the same number of lower-level nodes connected to it. This is known as the branching factor of the tree.

10. Bus disadvantages

• it's unreliable - the entire bus fails if there's a break in the network cable • it cannot support multiple pairs of terminals at the same time, and • it transmits data slower than the other topologies