Topologies
Network infrastructures may sometimes look like a mad spider's web, but they still have to be structured according to some logical and physical topology. The topology influences performance, scalability, and reliability of the network infrastructure for mySAP.com.
Bus Topology
It is still possible today to find installations using Ethernet networks based on the older bus topology with coaxial cables and BNC T-connectors. All client PCs are chained up to one single line, called a segment. Branches, or loops, are not permitted in such a daisy chain configuration. A bandwidth of 10 Mbps is shared within all clients.
A bus topology is easy to visualize on paper but can be a nightmare to implement and maintain in reality; it is difficult to find and repair failures. In many cases, coaxial cables lie around on the office floor like spaghetti and are targets for compression by roller-chairs or stretching by the cleaning service's broom. When a user simply pulls out a T-connector to move his or her equipment, the connectivity is disrupted for all users within that segment. In addition, the 50-ohm resistor mandatory at the end of the line has a tendency to disappear. Even worse, the 62-ohm connectors used for terminal connectivity look exactly like the 50-ohm versions and, if used on the same network segment, will kill the data traffic flow within that segment.
Any one of these types of failures impacts all users on the local network segment, and in many cases, the entire department isn't able to do work. The bottom line is that a network based on a bus topology is not recommended for mission-critical data traffic and should be replaced!
Star Topology
Because of the drawbacks of bus topology, state-of-the-art networks are based on a star topology. The star is formed by point-to-point connections with dedicated cables from every single wall outlet to a centralized cabling cabinet where an active network device connects them together. In an Ethernet network based on a star topology, this central concentrator is called a multiport repeater or hub. In Token Ring networks, it is called a multi-station access unit (MAU). Don't be puzzled by the name: Token Ring is actually implemented in what can best be described as a collapsed ring that looks like a physical star topology.
The term “hub” is generally associated with Ethernet networks, while the term “multi-station access unit” (MAU) is used to refer to the Token Ring wiring concentrator. Just as these two LAN technologies use different media access methods, hubs and MAUs perform different media access functions internally, but at one level they perform the same function: They are both network wiring concentrators.
Because of reasons discussed in detail in the next chapter, the total length of a single connection between the network interface of a computer and the hub using copper cables cannot exceed 100 meters (328 feet). Taking some reserves into account for footage up and down the wall, a cabling cabinet in a star network topology provides connectivity to an area of approximately 90 meters (300 feet) in diameter. In medium-sized installations, the cabling cabinets can be connected by high-performance uplinks directly to the SAP systems in the data center. On a large campus, the backbone itself is distributed and interconnected with even higher performance configurations.
The star topology guarantees that a physical failure at a single end connection or device impacts only a single user. Therefore, failure isolation and repair is much easier and can be done in less time. Such a topology is generally appropriate for mission-critical applications.
Hierarchical Star Topology—The Structural Standard
The drawback of traditional star topologies, however, is the high number of cables used, especially near central cabling cabinets. This can cause problems with space and weight restrictions of cabling channels. Therefore, a hierarchical star topology was developed.
The ISO/IEC standard 11801 (generic cabling for customer premises) as well as the European standard EN 50173 defined three levels of star topologies for the cabling structure (see Figure 9-6):
Primary, between buildings on the campus
Secondary, inside the building backbone between cabling cabinets
Tertiary, on office floor between wall outlet and cabling cabinet
Figure 9-6. Hierarchical Star Topology
Because of the multi-tier hierarchy of star topologies, the whole network topology begins to take on the structure of a tree. The network trunk lines are between buildings, the branches are throughout the building cores, and the twigs go to the workplaces. The primary and secondary levels act as backbones for the cabling to the individual workplace.
TIP
Don't Mix Cabling Components from Different Vendors
The quality of a network connection is not only determined by the category of the individual cable. Quality also depends on the integration of all components like plugs, wall outlets, patch panels, and patch cords. Vendor guarantees for compliance to the EN 50173 standard quality levels are valid only for complete cable systems from one supplier. Therefore, all components of the cabling infrastructure should be delivered by one certified vendor. Experience shows that the usage of cheap patch cables of a lower quality class often causes difficulty with isolating failures, which often appear later when migrating to higher bandwidths.