Practical LAN Topologies

There are a variety of topologies which are used to implement networks responsible for sending messages between machines. This article is a discussion on the practical considerations and uses of common Local Area Network Topologies.

The vast majority of networks in the world fit into the category of Local Area Networks (LANs). These networks include very simple cases with a single machine connected to an Internet Service Providers modem, and slightly more complicated networks which use commodity routers from Linksys and D-Link to provide what is known as a private LAN shared amoungst a variety of computers. LANs become even more complicated with systems such as virtual networks which have the ability to communicate data between distinct private networks as if they were local.

Types of Network Topology

Since this article covers practical LAN topologies it is important to distinguish the important types of network topology and classify them as either practical or impractical. There are six basic types of network topology which are again subdivided into two subcategories. The six basic network topologies are:

The subcategories for each major topology have to be considered when deciding whether a topology is practical. The physical topology is the subcategory which represents the real world implementation of how the network is connected and considers the various transports needed to implement the design. The logical topology is typically represented via a visual graph of the implementation and represents the conceptual design of the network.

Basic topologies can be bridged and intermixed to create hybrid topologies which take on the properties of multiple basic topologies when viewed through different physical transports, and take on the different properties when viewed from a logical or physical perspective. For instance, a common wireless home network logically functions as a star network, and physical operates as a bus network.

Point to Point

Point to Point is the the simplest network topology from a logical and physical perspective and offers the highest level of network performance. Point to point topologies are a subset of every other topology. This topology is implemented via a single connection from one network node, directly to another.

Bus

Bus networks are implemented via a common transmission medium in which nodes "listen" in on the common medium for signals addressed to that node. Bandwidth is shared across all nodes attached to the to the bus, and collision handling is a critical component of network transmission. Modern physical implementations of this topology include standard IEEE802.11 wireless networks where each wireless device on the network listens and transmits on the common wireless medium (channel) and collision detection is built into the protocol. Modern physical LAN topologies rarely include a bus topology for wired networks since the advent of switched ethernet networks.

Modern commodity routers bridge a wireless IEEE802.11 bus topology with a switched ethernet star topology which is technically a hybrid physical topology, but from a logical perspective is simpler to represent as a star topology.

Star

Star topologies are implemented via a common switching node which routes data from one connected node to another as if it were configured as a point to point topology. From the nodes transmitting and receiving data, there is no practical distinction between a star network and a point to point network, however the inclusion of the central switch allows a network to grow to N nodes, where N is the number of leaf nodes attached to central switch. From a logical perspective N can be arbitrarily high, however from a physical perspective N is limited to the number of physical connection ports available on the switch.

Combining more than one star topology results in a tree topology and is known as daisy-chaining which is a very common practice. It is theoretically possible to connect star networks into a ring or mesh topology, however from a physical topological perspective most common hardware will fail to handle these cases, and either fail to function entirely or degrade to a tree topology.

Tree

The tree network topology is the most common physical and logical topology in LAN architectures. The principle behind a tree topology is that each node on the network graph can have child nodes, and those child nodes are distinct.

A common example of this topology in practice shows up in most home networks. There is a parent node, such as a cable or DSL modem, which usually has one child node which again is usually a commodity router from linksys, dlink or netgear. The router has any number of children which include laptops, desktops, network printers, game consoles, etc. In circumstances where the number of client devices on the network exceeds the physical number of ports on the router, there may be a network switch node which in turn is provides switched messaging to all its child nodes.

From a routing perspective tree networks are the simplest and most commonly implemented topology. One reason for this is that there is only one routing path to send data between any two nodes on the network.

Mesh

Mesh networks are a superset of tree networks, and are distinguished by allowing nodes to act independantly as routers, are self healing and can be static or ad-hoc. Common LAN environments deploying commodity hardware rarely include the concept of mesh networking since from a wired LAN perspective there is little necessity for this topology. On the other hand, wireless mesh networks are gaining considerable traction as a deployed topology since they provide network extensibility beyond the normal range of a single wireless transmitter and can reduce the cost of network expansion in comparison to wired topology expansions.

One emerging standard for wireless mesh networks is the IEEE 802.11s which is currently in draft and aims to amend the common 802.11 protocol to define how WLAN devices can create a mesh network.

Ring

Ring topologies are implemented by connecting each node to two other nodes on the network, such that there is a contiguous connection between all nodes in the network. This topology has some physical implementations however is almost completely unused in modern LANs.

Impractical LAN Topologies

Impractical topologies include designs which are avoided due to complexity, substantial risk, lack of physical hardware, insufficient performance, cost, managability or necessity.

Something is impractical if it under, or significantly over reaches requirements. As with all projects, it is important to determine the requirements both now and in the reasonable future. For instance almost no home networks include point to point fiber optic connections since the cost and complexity of deploying fiber network adapters on typical home computers and devices vastly outweighs the benefits of the technology. This may be very different in workplace environments where computation and rapid data transfer is critical.

Wired Bus and Ring topologies are impractical since the hardware necessary to implement these topologies has is rare. These have been replaced with ubiquitous star and tree network topologies since performance, and simplicity are markedly improved in those technologies.

Topologies That Work

The properties that make a topology practical are the degree of simplicity, serviceability, availability of hardware and the absence of errors. For instance, in the case of a home network which is streaming high definition video to several network users, a wired network is more practical than a wireless network since the bandwidth of common wireless interfaces may easily be saturated, especially if transmission errors are common due to interference or distance. The consequence of effectively saturating the wireless network is dropped video segments which typically obviates the reason for watching high definition video.

Industrial circumstances may require thorough packet filtering between industrial control systems which may run software with unserviced vulnerabilities. This would be impractical in many commercial or residential scenarios, but are a firm requirement in a factory with expensive equipment.

Mesh network topologies work very well in wide area scenarios such as campgrounds, warehouses and campuses.

Residential home networks can usually benefit immensly from a thorough examination of local devices and services which should be available on the network. Often running multiple network drops to workspaces, televisions, bedrooms from a centralised distribution point in a house vastly improves reliability, performance and obviates the need to install ethernet switches in these locations later to service additional devices.

Conclusion

Designing a network requires that both the logical and physical topology be taken into account during the requirements phase of project. In general it is worthwhile to put more than a token amount of effort into designing a network since this can have a significant impact in the long term maintenance and the need to install adhoc expansions later.

Topology designs vary based on the scenario they are intended for. The following set of articles deal with the practical implications of various topologies in a variety of common scenarios:

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