OSI Layer 1 - Physical Layer - Network Topologies

The choice of topology affects the performance and resilience of the infrastructure. Further, the physical topology of wires and devices may be different from the logical topology of how information is actually moved between systems. A security professional should be able to balance these considerations against the C-I-A triad characteristics of a particular topology.

Bus

A bus topology is a LAN with a central cable (bus) to which all nodes (devices) connect. All nodes transmit directly on the central bus, as shown in the figure. Each node listens to all the traffic on the bus and processes only the traffic that is destined for it. This topology relies on the Data Link Layer to determine when a node can transmit a frame on the bus without colliding with another frame on the bus.

Bus

Advantages of buses include:

• Adding a node to the bus is easy.
• A node failure will not likely affect the rest of the network.

Disadvantages of buses include:

• Because there is only one central bus, a bus failure will leave the entire network inoperable.

Tree

A tree topology is a hierarchy of objects, as in the figure. Instead of all the nodes connecting to a central bus, the devices connect to a branching cable. Like a bus, every node receives all the transmitted traffic and processes only the traffic that is destined for it. Furthermore, the Data Link Layer must transmit a frame only when there is not a frame on the wire.

Advantages of a tree include:

• Adding a node to the tree is easy.

Disadvantages of a tree include:

• A node failure will not likely affect the rest of the network but any node failure that provides additional branching will cause all dependent nodes to fail.
• A cable failure could leave the entire network inoperable.

A ring is a closed-loop topology. Data is transmitted in one direction only, based on the direction the ring was initialized to transmit in, either clockwise or counterclockwise. Each device receives and transmits data from only its upstream neighbor and downstream neighbors. Rings have been implemented using many different physical transmission media, ranging from coaxial cables and twisted pairs through fiber optics. Bidirectional ring systems have also been in use. By and large, ring technologies have been supplanted by others, although there are still legacy systems in use in the field.

Ring

Advantages of rings include: Disadvantages of rings include:

• Because rings use tokens, one can predict the maximum time that a node must wait
• Simple rings have a single point of failure. before it can transmit (i.e., the network is If one node fails, the entire ring fails. deterministic. Some rings, such as fiber distributed data interface (FDDI), use dual rings for failover.
• Rings can be used as a LAN or network backbone.

Mesh

In a mesh network, all nodes are connected to every other node on the network. A full mesh network is usually too expensive because it requires many connections. As an alternative, a partial mesh can be employed in which only selected nodes (typically the most critical) are connected in a full mesh and the remaining nodes are connected to a few devices. As an example, core switches, firewalls, and routers and their hot standbys are often all connected to ensure as much availability as possible.

The figure on the top shows a wired connection fabric, in which each node has the ability to switch incoming traffic to any combination of its output connections. These fabrics form the local backbone of many storage networks, server clusters, and high-performance computing systems.

The figure on the bottom shows a wireless mesh; note that no assumptions are made in this figure as to whether these radios are fixed bases, mobile units, or both. In either (left) or (right), mesh management functions can help ensure acceptable quality of service. Advantages of a mesh include:

• Mesh networks provide a high level of redundancy.

Mesh

Disadvantages of a mesh include:

• Physical mesh networks can be expensive, since for N nodes there need to be (N-1) connections with their connecting media.

Star

All nodes in a star network, as in the figure, are connected to a central device, such as a hub, switch, or router. Modern LANs usually employ a star topology.

Advantages of a star include:

• Star networks require fewer cables than full or partial mesh.

Disadvantages of a star include:

• Star networks are easy to deploy, and nodes can be easily added or removed.
• The central connection device is a single point of failure. If it is not functional, all the connected nodes lose network connectivity.

Star