S 5.2 Selection of an appropriate network topology

Initiation responsibility: Head of IT

Implementation responsibility: Building Services Manager, Planner

In the field of information technology, a differentiation between the physical and the logical network topologies is made. The physical and the logical topologies of a network are not necessarily identical. The logical network topology deals with the organisation of the data flows. Almost any logical network topology can be formed by configuring the active network components. Additional logical structures can be formed in the network using virtual local networks (VLAN).

In the following, the physical network topology, i.e. the routing of cables and the arrangement of the distributors in the building, will be discussed in more detail.

The physical topology, naturally, is almost always based on the spatial conditions the network will be installed in. These include, amongst other things:

• the locations of the network subscribers,
• the amount of space available for cable trays and cables (see S 1.21 Sufficient dimensioning of cable trays and channels),
• the required cable types (see S 1.20 Selection of cable types suited in terms of their physical/mechanical properties),
• the protection requirements for cables (see S 1.22 Physical protection of lines and distributors).

In general, there are two basic types of network topologies; the star topology and the bus topology. In addition, a tree-shaped structure can be formed from a star topology and a ring-shaped structure can be formed from a bus topology.

The star and the tree structures are the most commonly used structures when redesigning the IT cabling or adding IT cabling to buildings.

In the following, the advantages and disadvantages of the various topologies are listed. Other possible topologies not mentioned here can be considered as special cases of the structures examined below.

Star

In a star topology, all network subscribers are connected to a central node by dedicated lines. The "collapsed backbone" architecture in particular, in which one (logical) main switch connects all servers and terminal devices, results in a building with cables physically connected in the shape of a star.

This topology offers the following advantages:

• Damage to a line only affects the operation of the system connected to this line.
• Changes to the assignment of network subscriber to the central node connection point and disconnections of individual subscribers may be performed at a central location.
• With a physical star topology, any logical topology imaginable can be simulated.

The star topology has the following disadvantages, though:

• When the central node fails, all connected IT systems fail.
• Since each subscriber is connected individually to the central node, a large number of cables must be installed.
• A the number of individual lines increases, the risk of crosstalk can also increase.
• Due to the star-shaped cabling, range problems can arise depending on the type of cable and protocol used (see S 5.3 Selection of cable types appropriate in terms of communications technology). Amplifiers (repeaters) can be used to increase the range. The protocol used determines the number of repeaters possible per connection or, when operated in parallel, per cable. The additional investment and operating costs must be taken into account and compared to alternatives in the economic analysis. If range problems arise, the cabling can also be implemented in a tree-shaped structure as an alternative.

Tree

A tree structure is formed by connecting several stars to one central node. The network subscribers connected to the local network nodes are formed into groups. The local network nodes are in turn connected to one central network node using one or more dedicated lines.

The tree topology offers the following advantages:

• It has the same advantages in terms of connecting the systems to the local nodes as a star topology.
• Only the cabling on the local network node must be changed when new subscribers are added.
• When the local network nodes are designed accordingly, it is possible for the subscribers of such nodes to exchange data even if the other nodes fail.
• The number of cables needed is reduced by connecting the local nodes to each other using one line.
• To span large distances between nodes, repeaters only need to be installed on one line.
• It usually makes sense to use high-quality (usually more expensive) cable to connect the nodes to each other so that larger distances can be covered without requiring the use of repeaters. This often results in reduced costs and advantages in terms of reliability and investment/operating costs when the number of repeaters otherwise needed are taken into account.

The tree topology has the following disadvantages:

• When a transition point to another local network node malfunctions, operations are disrupted for all connected subscribers.
• The required documentation and management of the local network nodes may result in an increase in the total operating expenses of the network under some circumstances.

A typical example of an application of a tree topology is the topology formed by the connections of the floor distributors of a building (tertiary cabling in a star topology) to the main distributions of the building (secondary and tertiary cabling). When there are corresponding redundancy requirements, the floor distributors can also be connected to more than one building distribution.

Meshed network topology with star and tree structures

The additional connection of central and, if correspondingly required, local network nodes is referred to as an intermeshed topology. This establishes redundant connections, which are implemented to increase reliability and availability.

Bus

In a bus topology, all network subscribers are connected to one shared line. This is generally accomplished using a central cable that is then connected to the individual subscribers using breakout cables.

Newer cable types and specifications do not support bus topologies any more. This topology does not play a role any more for initial installations or modernisations of IT cabling.

Ring

From a topological point of view, the ring is a bus whose ends are connected together. A special type of ring is when the ring is formed twice into a double ring, such as the double ring used in FDDI.

Newer cable types and specifications do not support ring topologies any more. This topology does not play a role any more for initial installations or modernisations of IT cabling.

Cable types and maximum length

When cabling small buildings, a star cabling topology with a central node can be considered. One requirement, though, is that the IT cabling is routed in such a way that every terminal device connected is a maximum of 90 metres away when copper cable is used (according to EN 50173 for generic cabling systems). If this maximum length is exceeded, the standard requires that the required electrical transmission parameters are maintained. Selecting the appropriate products will ensure there is enough reserve to exceed the maximum installation lengths. Routing the cables separately to all terminal devices wherever possible also increases the reliability.

If terminal device connections cannot be established using copper cabling because of the distance or the presence of strong electrical interference, fibre optic cables (FOC) must be used. Depending on the transmission protocol and the quality of the fibre, up to 2 km can be spanned using multimode fibre optic cables. The higher the transmission bandwidth, the shorter the length you can install. Significantly larger distances can be attained when required through the use of single-mode fibre optic cable.

Cabling large buildings

When installing cables in large buildings, a tree structure is the most appropriate. Starting from a central distribution point (building distributor), the floors or sections of the building are connected in the shape of a star. The terminal devices are also connected from the technology rooms on each floor using a star topology. To increase reliability, it is recommended to install an extra building distribution that is connected to all floors or sections of the building for redundancy purposes. It must be ensured that the cabling is routed to the floors or sections of the building on separate routes. In addition, the building distributions should be intermeshed if possible so that external connections, for example for carrier lines, can be fed easily to both building distributions.

Review questions:

• Have the existing spatial conditions been taken into consideration when planning the physical network topology?
• Does the physical network topology contain regulations for identifying the locations of the network subscribers?
• Does the physical network topology contain regulations for dimensioning the spaces for trays and cables?
• Does the physical network topology contain regulations for the selection of suitable cable types?
• Does the physical network topology contain regulations for protection requirements of cables?
• Does the physical network topology contain regulations for the cable types used and the maximum lengths?
• Does the selected form of the line network correspond to the spatial conditions (physical network topology: star, tree, or a combination thereof)?
• Is the planned network topology suitable for the existing spatial conditions?
• Are redundant connections planned to connect the floors or sections of the building?