Topologies

Objectives

• Be able to define the term "network topology" and understand its significance in computer networks.
• Be able to describe the following common LAN topologies:
  • bus
  • star
  • ring
  • mesh
• Compare and contrast the characteristics, advantages, and disadvantages of different network topologies.
• Be able to identify and apply network topology concepts to real-world scenarios and understand the practical implications of choosing specific topologies based on requirements and constraints.

Network topology refers to the arrangement or layout of different elements (nodes, links, etc.) in a computer network.

The topology defines how these components are connected and how data is transmitted between them and can play a crucial role in determining the overall performance, efficiency, and reliability of a network. There are several common types of network topologies, including:

• Bus
• Star
• Ring
• Mesh

Note

Different GCSE exam boards require knowledge of different topologies but will be drawn from the list above

Bus Topology

In a bus topology, all devices share a common communication medium, typically a single communication channel or cable. Data is transmitted along the bus, and each device receives the data, but only the intended recipient processes it.

The bus serves as a single communication path for all devices on the network. Data transmitted by one device is accessible to all other devices connected to the bus. It as one of the implementations of an Ethernet network. To prevent signal reflections and ensure proper functioning of the network, the bus topology requires terminators at both ends of the central bus. Terminators absorb the signal and prevent it from bouncing back and causing interference.

Since all devices share the same communication medium, they contend for access to the bus when transmitting data. This can lead to potential collisions if two or more devices attempt to send data simultaneously. When a collision occurs the devices wait a random amount of time before retransmitting. The more devices connected to the network will inevitably mean higher demand beings placed on the cable and this will impact the performance of the network.

Each message (frame) will include the destination address for the intended recipient which picks up its message while being ignored by the other devices on the network.

If the central bus fails or encounters a break in the cable, the entire network may be affected. Troubleshooting and locating faults in the bus can be challenging.

The Bus topology is relatively simple to set up and is cost-effective for small to medium-sized networks. It requires less cabling compared to some other topologies, making it economical though, it is rarely used today.

Star Topology

In a star topology, all devices are connected to a central hub or switch. The central hub manages the communication between devices, and if one device wants to communicate with another, the data passes through the hub or switch.

• In this arrangement, each device on the network has its own dedicated connection to the central point. The central hub or switch manages the communication between devices by acting as a repeater, regenerating and forwarding data to the appropriate destination.

• In a traditional star topology with a hub, the hub is a basic networking device that receives data from one device and broadcasts it to all other connected devices. The layout of devices may resemble a star shape but the operation of the network is more akin to a bus network, and so sometimes called a logical bus network.

• In modern star topologies, switches are often used instead of hubs. Switches provide more efficient and intelligent communication. Unlike hubs, switches forward data only to the specific device for which it is intended, reducing network congestion.

• Devices in a star topology are isolated from each other in terms of communication which enhances network security. If one device wants to communicate with another, the data travels through the central hub or switch, and there is no direct communication between devices without involving the central point.

• Star topologies are easily scalable. Adding or removing devices does not affect the overall network structure, as each device connects directly to the central hub or switch.

• The failure of one device or cable in a star topology does not necessarily affect the rest of the network. Other devices can continue to communicate without interruption thus making the star topology more reliable than a bus topology. However, if the central hub or switch fails, the entire network may be affected.

Star topologies are commonly used in local area networks (LANs) and are favored for their simplicity, ease of installation, and scalability. The shift from hubs to switches has improved the performance and efficiency of star topologies in modern networking environments.

Hubs vs Switches

Hubs and switches can resemble each other but internally their operation is quite different. You can think of a switch as being an "intelligent hub".

A hub relays messages received to all devices on the network; a switch will send a message to its intended recipient.

The switch works by maintaining a table of known devices on its network i.e. those that are connected to its ports. The table contains the MAC address of these devices so when an incoming message is received the MAC address in the message is used to identify the correct destination and it can then be transmitted through the correct port to be sent to the device with that MAC address.

Ring Topology

In a ring topology, each device is connected to exactly two other devices, forming a circular or ring-like structure. Data travels in one direction along the ring, passing through each device until it reaches its destination.

• Ring topologies are less common in modern networking due to the prevalence of other topologies, such as star and bus, but they still have some specific use cases.

• Ring topologies can be either unidirectional (clockwise or counterclockwise) or bidirectional, depending on the network design and the capabilities of the networking equipment. Data travels along the ring from one device to the next until it reaches the intended destination. Each device in the ring regenerates and passes along the signal to the next device.

• Collisions are not an issue in ring topologies since data travels in a unidirectional or bidirectional manner. Unlike bus topologies, where collisions can occur when devices share a common communication medium, devices in a ring operate independently.

• Ring topologies can be more fault-tolerant than some other topologies. If a device or a section of the ring fails, the data can still travel in the opposite direction to reach its destination.

• The performance of a ring topology depends on factors such as the number of devices and the length of the ring. As the number of devices increases, the overall performance may degrade due to the increased propagation delay.

While ring topologies have advantages in terms of fault tolerance, they also have some drawbacks. For example, the failure of a single device or connection can disrupt the entire network, and the overall performance can be impacted as the network scales. Modern networking technologies often prefer other topologies like star or hybrid configurations due to their flexibility and scalability.

In a token ring network, a special signal called the token, circulates the network. Devices can only communicate when they have the token. Ring networks are used when guaranteed communication speeds are important. The time taken for a message to pass through the ring is consistent and reliable.

Mesh Topology

In a mesh topology, every device is connected, with no hierarchy, to every other device in the network via a dedicated link. This provides multiple paths for data to travel, enhancing reliability and fault tolerance. Mesh topologies can be either full mesh (every device connected to every other) or partial mesh (only some devices are interconnected).

• In a full mesh topology, every device is directly connected to every other device in the network. This results in a high level of redundancy and multiple communication paths between any two devices.

• In a partial mesh topology, only some devices are directly connected to every other device. This topology provides a balance between connectivity and cost, allowing for redundancy and alternate paths without the full connectivity of a mesh.

• Mesh topologies offer a high degree of redundancy, meaning that if one link or device fails, alternative paths can be used for communication. This redundancy enhances network reliability and fault tolerance.

• Mesh topologies can be scalable, but the number of connections grows rapidly with the addition of each device. Managing a full mesh network with a large number of devices can become complex.

• Mesh topologies generally provide high performance due to the multiple communication paths available. However, the overhead associated with managing numerous connections may impact overall network performance.

• The cost of implementing a mesh topology can be significant, especially in a full mesh configuration. The number of required connections and cabling can make it more expensive compared to other topologies.

Mesh networks can be wired or wireless but the latter is becoming more common and devices are available for installation in domestic environments.

Selecting the "right" topology

The choice of network topology will depend on multiple factors such as the size of the network, the communication requirements, cost considerations, fault tolerance, and scalability needs. Different network topologies offer different advantages and disadvantages, and understanding their practical implications is crucial for making informed decisions. When selecting a network topology for a real-world scenario, factors to consider include the number of devices, the distance between devices, the amount of data traffic, reliability requirements, ease of installation and maintenance, and scalability for future growth.

Examples:

Small Office Network:

A small office with a few computers, printers, and a central server would be advised to use a star topology due to its simplicity, ease of installation, and centralized management. A central switch can connect all devices, providing a dedicated connection for each and allowing for easy expansion.

University Campus Network:

A university campus with multiple buildings, departments, and thousands of users might use a combination of a backbone bus topology and a star topology. A backbone bus connects different buildings, while each building can have a star topology to connect individual departments. This setup provides scalability and fault tolerance.

Residential Wi-Fi Network:

A residential home with multiple floors and several family members would be best suited for a Wi-Fi mesh topology. Mesh networks provide seamless coverage throughout the home, even in areas with weak signals. Nodes can communicate wirelessly, and self-healing capabilities ensure reliable connectivity.

Questions

1. Explain what a network topology is and provide three examples of common network topologies. Compare and contrast the characteristics of bus and star topologies.

Network topology refers to how the network's physical layout looks and functions. Examples include mesh, grid, and wireless topologies.

Network topology is the arrangement of computers, and examples include tree, cellular, and point-to-point.

Network topology refers to the arrangement or layout of elements (nodes, links) in a computer network. Examples include bus, star, and ring topologies.

Network topology is a type of routing protocol in a network, such as RIP, OSPF, or EIGRP.

A network topology defines how devices are interconnected in a network. Bus topologies use a single cable shared by all devices, which can lead to collisions, while star topologies provide a dedicated connection between devices and the central hub, improving performance.

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2. Define a logical bus network and explain how it differs from the physical arrangement of devices in a network. Provide an example of when a logical bus might be preferred over a physical bus.

A logical bus network refers to how data flows in the network, not the physical connections.

A logical bus network is physically connected in a ring topology but behaves like a star topology.

A logical bus is another name for a star network with switches.

A logical bus network must have redundant cabling to prevent collisions.

A logical bus network can occur in a star topology when a hub broadcasts data to all devices. This differs from a physical bus, where all devices share the same physical connection.

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3. Describe the characteristics of a star topology. Discuss the advantages and disadvantages of using a hub versus a switch in a star topology. How does a star topology differ from a logical bus?

A star topology uses coaxial cables for central communication, similar to a bus network.

In a star topology, all devices are connected to a central hub or switch, making it easier to add or remove devices.

Star topologies only differ from logical bus networks in the way data is prioritized.

In star topologies, each device is directly connected to two others in a circular manner, enhancing redundancy.

A star topology offers centralized communication, but using a switch rather than a hub reduces network congestion. The logical bus describes data flow, not physical connections.

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4. Explain the concept of a mesh topology and its advantages. Differentiate between full mesh and partial mesh topologies. Provide examples of scenarios where a mesh topology would be beneficial.

A mesh topology creates dedicated point-to-point links between only some of the devices.

Full mesh connects every device to every other device, while partial mesh connects only some devices.

Mesh topologies provide limited redundancy and are prone to failure.

Mesh topologies are only used for small networks and are rarely scalable.

Mesh topologies provide multiple communication paths, increasing reliability. Full mesh is costly but offers the highest redundancy, while partial mesh is more cost-effective for medium-scale networks.

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5. Describe the key features of Wi-Fi mesh networks. How does self-healing contribute to the reliability of a Wi-Fi mesh network? Provide real-world applications where Wi-Fi mesh networks are commonly used.

Wi-Fi mesh networks only provide basic coverage and cannot scale to large areas.

Self-healing allows the network to reroute data if a node fails, maintaining connectivity.

Wi-Fi mesh networks are less reliable in large homes or office buildings.

Self-healing features make Wi-Fi mesh networks slower compared to traditional networks.

Self-healing ensures that if one node in a Wi-Fi mesh network fails, data can still travel through other nodes. This feature is crucial for maintaining reliable wireless coverage in large areas.

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6. Discuss the types of hardware used to establish connections in a mesh topology. Compare and contrast wired and wireless connections in a mesh network. Explain the role of routers and switches in a mesh topology.

Routers and switches are used to manage data traffic and maintain multiple paths between devices.

Wired mesh networks are cheaper than wireless mesh networks but offer more flexibility.

Wireless mesh networks only work with hubs and cannot use switches.

Mesh networks don't need routers or switches as data always flows directly between devices.

Mesh networks rely on routers and switches for directing data. While wired mesh offers higher performance, wireless mesh is more flexible. Routers ensure data is forwarded through optimal paths for reliability.

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7. Explain how fault tolerance is achieved in mesh networks. Describe a situation where a mesh network's self-healing capability would be particularly advantageous.

Mesh networks rely on a central hub to prevent data loss during a failure.

Self-healing allows mesh networks to automatically reroute data if a node fails, ensuring reliable communication.

Mesh networks don’t provide fault tolerance unless combined with bus topology.

Self-healing is only relevant to wired mesh networks, not wireless ones.

Fault tolerance in mesh networks is achieved through redundancy, allowing communication to continue even if links fail. Self-healing is particularly useful in disaster recovery or emergency situations, where continuous connectivity is vital.

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8. Define the CSMA/CD protocol and explain its relevance in network communication. In which network topology is CSMA/CD commonly used, and why is it less relevant in other topologies?

CSMA/CD is used in wireless networks to prevent interference between devices.

CSMA/CD is commonly used in bus topologies, where devices share the same communication channel.

CSMA/CD is primarily used in mesh networks to ensure data doesn't collide during transmission.

CSMA/CD is only used in modern Ethernet networks and isn't applicable to older systems.

CSMA/CD is used in Ethernet-based bus topologies to prevent data collisions when multiple devices transmit simultaneously. In topologies like star or mesh, collisions are minimized due to dedicated links.

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9. Discuss the scalability of a star topology. What challenges might arise as the number of devices in a star topology increases, and how can they be addressed?

Star topologies can't handle more than a few devices due to the complexity of the hub.

As the number of devices increases, network congestion and the load on the central hub may increase.

Star topologies scale well but are prone to frequent data collisions.

Star topologies are typically used for small networks and don’t support additional devices.

Star topologies are easily scalable, but as the number of devices grows, congestion may occur. Using switches instead of hubs or adding more switches can help manage increased network traffic.

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10. Identify specific applications or industries where mesh networks, particularly Wi-Fi mesh networks, are well-suited. Explain why these environments benefit from the characteristics of mesh topologies.

Wi-Fi mesh networks are ideal for large office buildings, campuses, or public spaces like stadiums.

Wi-Fi mesh networks work best in small homes and rarely support more than 10 devices.

Mesh networks are only used for specialized industries and aren’t common in public spaces.

Mesh networks offer limited reliability and are prone to network outages.

Mesh networks provide reliable and extensive coverage, making them suitable for large spaces like offices, campuses, and public venues. Their scalability and fault tolerance ensure uninterrupted connectivity.

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