Energy Efficiency Made Simple Vol IV

of the newer open standard redundancy protocols to be released.

Either HSR, PRP or HSR (High-Availability Seamless Redundancy) and

PRP (Parallel Redundancy Protocol) work by transmitting data along

two different paths to its destination. This means that in the event of

a failure on Path A, the data will arrive via Path B, without any further

delay or requirements for retransmission of the data. In other words,

bumpless redundancy does not require time to recover from a failure.

These two redundancy protocols provide zero recovery time, which

is great for mission critical, latency sensitive applications, however,

the capital cost will be more than required for standard redundancy

hardware.

When it comes to cable redundancy, it is best to look at it from

the perspective: How much redundancy can you afford not to have?

Will a cable break lead to complete shutting down of the entire grid

(or a section thereof) or is it simply going to mean a few people lose

out on some non-critical monitoring data. Installing an extra cable and

redundancy to prevent a countrywide blackout is definitely worthwhile;

however, installing the same cable to ensure a single user has 24/7

access to personal emails is not.

These are obviously extreme examples, but this is the decision

process for any redundancy. Weighing the cost (whether money, time

or effort) verses the potential losses if the redundancy is not in place

will determine if it is worth it. There is potentially no upper limit to re-

dundancy (although using every port on a switch to create a redundant

connection to another switch is obviously not useful at all). Budgetary

considerations will help determine the specific limit for your network.

Whilst the best option could be to run redundant hardware at every

point in the network, with redundant power supplies and multiple

redundant uplinks between sections of the network, this can quickly

deplete even the largest of budgets. Having little to no redundancy

could mean incurring huge losses by the simple accident of a cable

breaking. Once again, finding the balance is the biggest trick here, and

should not be undertaken by anyone without a working knowledge of

Ethernet and the redundancy options available.

Hardware redundancy

Cable redundancy is a topic that warrants special attention when

planning and designing a network. In fact redundancy is one of the

core decisions of the network as it can affect many other decisions.

Cable redundancy is not the only redundancy available, even though it

is the most commonly discussed. Another big redundancy point that

should be considered is hardware redundancy options. Whilst cable

redundancy protects against a communications link or cable breaking,

hardware redundancy is more concerned with what happens if a piece

of hardware or a component fails.

Power supply redundancy

One of the simplest of these, yet an overlooked or misused option, is

power supply redundancy. Units with dual redundant power supplies,

when installed properly, will prevent equipment from shutting down

in the event of one power supply failing (and provide a level of load

sharing between the two supplies, thus extending their lifetimes). It

is important that this is implemented correctly. For instance, daisy

chaining power from a single supply to power both redundant inputs

defeats the purpose. Although this will prevent shutdown if only a

single power input on the device fails (although in some cases even

Figure 2: Basic star topology – No redundant links available.

Figure 3: Basic ring topology with redundancy in place.

PC A

PC B

Switch A

Switch B

Switch C

Switch D

Figure 2

it can be seen that we have solved the issue of the broad-

cast storm and have changed the network backbone (connection

between switches) from a ring to a line/star topology. However, what

happens if we now lose the cable connecting switches B and C?

Communications between PC A and PC B will no longer be possible

at all, as there would be no communications path remaining between

their network segments. This leads us to the reason redundancy was

created and is used, especially on mission critical networks. While

cable redundancy mechanisms work in different ways, their outcome

is the same: they allow us to have physical loops on the network, yet

they logically disable connections so as to break any communications

loops on the network (such as in the image below, a redundancy

mechanism has effectively ‘broken’ the link between A and D, even

though the physical cable is still connected). In the event that another

cable break leads to communication interruption, the mechanism

will attempt to re-enable any redundant links held as back-up so that

communications are not interrupted.

There are many redundancy protocols available, a number of which

are proprietary to certain manufacturers. RSTP (Rapid Spanning Tree

Protocol) is one of the most commonly used open standards and is

supported by most hardware manufacturers. However, RSTP can take

up to 30 seconds to recover in a worst case example, and so can be

unsuitable for certain applications. In these cases one may need to

look at either a proprietary redundancy protocol, or alternatively one