Energy Efficiency Made Simple Vol IV 2015

and their interconnecting cables. The logical topology, on the other hand, refers to the route that different data will take across the net- work, and can change depending on the source and destination of said data. While these two are closely tied to one another, it is important to realise that they are in fact different and equally important to consider for a truly efficient, stable and expandable network. Physical topology It is generally better to consider the physical topology first in this case, as you will generally be more limited on the physical topology due to geographic considerations (ease of laying cable, distance between sections/sites etc). When planning the physical topology it is important to cater for redundant links. For this you will need to take into account the type of redundancy you are going to run on the network. Some manufacturers will have proprietary redundancies that only cater for ring networks (as stated previously, it is recommended to avoid these proprietary protocols where possible). In this event, installing additional cables beyond those required for the ring is a waste of time and expenditure. On the flip side of the coin, installing too few cables to provide proper redundancy can lead to issues in the future if one or more of the cables does fail. Cost is also an important factor here, as the cost for provisioning and laying cables can be extremely high depending on the area in which they need to be installed. In some cases wireless links could be considered, depending on various factors (such as available line of sight, interference, distance and more). A full discussion of wireless communication is beyond the scope of this article; however, in short, wireless can be considered for non-critical information. Although in some unique cases it may be used for critical data transfer, this is not recommended as wireless is not nearly as stable or reliable as wired communications. For applications such as non-critical monitoring, wireless can definitely be a time and money saver. Logical topology Once we have a physical topology in place, the next step is to start planning the logical topology. The logical topology will be affected by configurations such as VLANs, redundancy, multicast control and routing (if required). The first step in planning the logical topology is to group various devices around the network together into ‘communica- tion groups’. In a nutshell, these are groups of devices that will need to communicate with one another on a regular basis. For instance, CCTV (Closed Circuit Television) devices on the network could all be grouped together into one group, VoIP (Voice over IP) into a second group and so forth. Alternatively one could group devices based on physical location (e.g. all devices in substation A will be in one group, devices in substation B in another group etc). Depending on the network, one may want to group devices based on a combination of these two components. For instance each substation could be its own group, with subgroups for devices with different functionality. While there is no set rule for grouping devices together, the most important point to keep in mind is that devices that will be communi- cating with each other constantly should be kept to the same group as much as possible. It is possible to route information between the logical groups (or VLANs); however, this puts increased strain on routing

hardware, and can lead to delays in data transmission as we get a ‘traffic jam’ (or bottleneck) at the routers interface to the network. One also wants to avoid routing any critical, latency sensitive data between VLANs, as once again this can add delays. At the same time, however, not separating devices at all means that the network will become very ‘noisy’ with background traffic, such as broadcasts. While this traffic is essential to correct network operation, too much background traffic means that critical traffic and relevant data may be delayed. For this reason, it is important to find a balance when grouping devices on the network. VLANs and proper traffic segregation are a big component of the IEC61850 standards, and should not be taken lightly. A well designed VLAN structure across the network will have a significant impact on providing a stable and reliable network and, next to topologies, is probably one of the most important components to design correctly. Once there is an idea of the devices that will be on the network and how they need to be grouped, IP address ranges for each VLAN can be considered. When dealing with IP ranges for a LAN, the selection must come from the private IP address range: - 10.0.0.0 to 10.255.255.255 - 172.16.0.0 to 172.31.255.255 - 192.168.0.0 to 192.168.255.255 These ranges of IP addresses are special in that they will NOT be routed across the internet. This means that these private ranges are free to use as they will never be exposed directly to the internet and thus we will not run into an issue with duplicate IP addresses on the internet. The different ranges are suitable for different networks depending on (a) the number of hosts (end devices) required on the network and (b) the number of sub-networks (different devices ‘groups’) allowed on the network. Selecting the correct range in the initial design phase is highly important, as changing an IP range at a later stage will generally either involve some downtime or alternatively can be extremely com- plicated (if downtime must be avoided) and will require a specialised solution and additional hardware for the change. One wants to select a range that caters for the number of devices in the initial network as well as any future expansion. At the same time, one needs to make sure that different subnets will be separated correctly and that the IP ranges comply with any requirements for the network. For instance, the new network design may be part of a larger network and thus you may be assigned an IP range to use. In this case care must be taken not to interfere with other ranges on the network, while still using your (limited) range to cover all current and future device requirements. This will be done by subnetting (dividing) the given range into smaller subnetworks. Generally these will correspond to the VLANs used, as in order to route between different VLANs they need to be assigned different IP subnet ranges. Properly splitting and allocating a subnet to cater for an entire network or network segment is an involved process and should not be undertaken by anyone without a working understanding of IP ranges, routing and VLANs. As an example A hypothetical new utility company for South Africa is in the process of setting up control centres and substations and has decided to use Ethernet as the communications technology to link all these different sites. The plan is to use a large IP subnet range for the entire

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ENERGY EFFICIENCY MADE SIMPLE 2015