Fibre Cables and SFPs – The Basics

Despite being in Networking for a while, I found the topic of SFPs and Fibre Cables difficult to fully grasp. There seemed to be endless jargon involved and standards that never made any sense to me.

After a challenging journey, I wrapped my head around them and wanted to compile a resource that would have helped me when I was first starting out.

Let’s begin.

What is an SFP? Why use one?

Let’s tackle a foundational concept that will help understand the purpose behind this technology.

Why do we use SFPs?

When it comes to Networking, we have 2 types of connections available for us to use:

  • Ethernet (also referred to as ‘Copper’ due to what it is made out of)
  • Fibre (uses glass inside of Fibre Optic Cables to send light down)

Ethernet is most suitable for most types of user connections and can carry high speeds for normal traffic. When it comes to designing a Network in an Enterprise environment however, Ethernet simply doesn’t provide enough bandwidth or speed for us to be able to carry large amounts of data in a datacentre environment. We need Fibre to carry more data at higher speeds.

But this brings about another problem.

The standard Ethernet cable we know and love uses the standard RJ45 connector to plug into a Network Switch. Fibre doesn’t have a connecter that fits into that same port. They are simply incompatible without any extra equipment or consideration.

This is where SFPs come in.

SFP stands for Small Form-Factor Pluggable. A Switch which accepts SFPs will have an empty slot for you to add an SFP into. Once plugged in, we will have an interface on that Switch which we can plug our Fibre Cable directly into.

A lot of the jargon around Fibre Cables and SFPs can be confusing. This is the reason I have written this post.

This post will look at some of the components used in a Fibre Optic deployment and help you acquire a basic understanding of the terms and technologies used.

We will look at 3 different aspects:

  1. The Switch
  2. The SFP
  3. The Fibre Optic Cable

These components are briefly illustrated in the diagram below:

Let’s get started.

Switches

As we mentioned previously, Switches often have ’empty’ ports that we can plug removable modules into. These are known as SFP ports. Depending on the type, purpose and model, a Switch can have SFP ports available only for uplinks to other Network Devices, with the rest being Ethernet, or it can have all its ports as SFP Ports

With Access-Layer Switches, it is usually the uplinks only which use SFP technology. On Datacentre Switches typically used more at the Distribution and Core layers, most (if not all) ports would be utilising SFPs on a modern Network.

These ports allow you to plug in an SFP which then connects to the internal motherboard/backplane of the Switch and function as an interface.

SFPs

Let’s take a look at SFPs themselves. Here we have 2 factors to consider

  1. Type
  2. Distance

Type

When it comes to SFPs, there are different types. Below are some you will commonly come across.

  • GBIC
  • SFP
  • SFP+
  • QSFP

It is easy to get lost with the jargon, so let’s go through each type one by one.

GBIC

Stands for Gigabit Interface Converter. These were the first type of ‘SFPs’ that were used in the Networking industry around 1995. They were quite large and have since been superseded by SFPs. More on that next. Speeds of up to 1G were supported.

SFP

The successor to the GBIC standard, SFPs (Small Form-Factor Pluggable) provided a smaller form factor, freeing up some much needed real-estate on Switches and quickly becoming the new adopted standard. Speeds of up to 1G are supported.

SFP+

As good as SFPs were, the Networks of the time simply needed more speed and bandwidth to meet their needs. This is where the SFP+ standard came in. It provides speeds of up to 10G and best of all, it is backward compatible in the same size slot that the standard SFP would fit into. This is pretty much the standard for what we use most of the time today.

QSFP/QSFP+

The Q in the name stands for ‘Quad’. As it implies, this means that up to 4 standard Fibre Optic data streams are combined to give bandwidth of up to 40G. Although this is a significant jump, the SFP module used for the QSFP/QSFP+ standard is of a different form factor to the standard SFP/SFP+ size. Switches that utilise QSFP/QSFP+ connections will have dedicated ports for it with the correct sizing.

Distance

Finally, we have designations for the type of SFP required in relation to distance. We will discuss more about how distance affects our Fibre Cabling requirements in the next section, but for SFPs, we need to understand if we are utilising a Long-Range or Short-Range connection.

In practise, these are subsequently abbreviated to LR and SR respectively.

Fibre Cables

When it comes to the Fibre Cables themselves, we have 2 aspects to consider:

  1. Connector Type
  2. Distance (and therefore Type and Standard)

Let’s walk through these one by one.

Connector Type

We mentioned that Fibre doesn’t use the same connector as Ethernet, the RJ45 Standard. Instead we have one of 2 options when it comes to Fibre Connectors at either end of the cable:

SC

  • Stands for Subscriber Connector
  • Also referred to as Standard Connector or Square Connector
  • As some of its alternate names reveal, this connector is square in shape and takes up more space
  • You will notice that this doesn’t fit in to the standard SFP or SFP+ modules, as such, it isn’t really used for the Switch side of Fibre connections, but it used to plug into a Fibre Panel
  • it is not used so much for new installations today as such Fibre Panels are switching to a new type of connector, LC. More on that next

LC:

  • Gets its name from the company who invented it, Lucent Corporation
  • Takes up half the space of the SC Connector
  • This is the connector type used for standard SFP and SFP+ connections

Distance

A main consideration when looking at Fibre Cables is to understand your requirements. The most important of these requirements is the distance the Fibre Cable will run.

  • For distances less than 2000m, Multi-Mode Fibre is usually recommended
  • For distances longer than 2000m, Single-Mode Fibre is used

Now that we understand the two types, both have their own Fibre Standards within them that its important to be aware of. Stick with me here.

Multi-Mode Fibre Standards

  • Within Multi-Mode Fibre, standards of cabling have been increasing to be able to carry higher bandwidth across longer distances
  • To this end, we have the standards OM1, OM2, OM3 and OM4, with OM4 being the latest and greatest

Single-Mode Fibre Standards

  • Single-Mode Fibre has 2 standards currently in use
  • These standards are OS1 and OS2

Misc.

Finally, it is important to note that there also exist Fibre Cables which have SFPs hardwired onto both ends. In this case all we have to do is plug both ends into our Network Devices. These are known as DAC/Twinax cables.

Example

Let’s take a look at 3 scenarios where we would have to choose the correct set of SFPs and Fibres. Keep in mind that financial cost here is not an issue.

Scenario #1

  • You have been tasked to connect a Fibre Cable across 35km from a Network Device into a Cisco Switch
  • The Bandwidth requirement is 10G

Scenario #2

  • You have been tasked to connect a Server to a Cisco Switch in the same rack, no longer than 5m apart
  • The Bandwidth requirement is 1G over 110m

Scenario #3

  • You have been tasked with connecting a Cisco Switch to an SC, Multi-Mode Fibre Patch Panel in your DC environment 100m apart
  • The Bandwidth requirement is 1G

Scenario #1: Answer

  • Since the range is 35km, we will be using Single Mode Fibre
  • The Fibre Standard we will use is OS1
  • Again, since the range is 35km, we will ensure that our SFP is suitable for Long Range, using an LR SFP
  • We will use an LC-LC connection type
  • Finally, our SFP will be the SFP+ flavour due to the bandwidth requirement being 10G

Scenario 2: Answer

  • Since the range is very short, we will be using Multi-Mode Fibre
  • The Fibre Standard we will use is OM4 (due to it being the latest and greatest available)
  • We will ensure to use a Short-Range (SR) SFP
  • We will use an LC-LC connection type
  • Finally, our SFP will be the standard SFP flavour due to the bandwidth requirement being 1G
    • Note however, that we can use an SFP+ module and set the bandwidth to be 1G also

Scenario 3: Answer

  • Since the range is very short, we will be using Multi-Mode Fibre
  • The Fibre Standard we will use is OM4 (due to it being the latest and greatest available)
  • We will use a Short-Range (SR) SFP
  • We will use an LC-SC connection type
    • The LC side will connect to our Switch/SFP. The SC side will connect to the Fibre Panel
  • Finally, our SFP will be the standard SFP flavour due to the bandwidth requirement being 1G
    • Note however, that we can use an SFP+ module and set the bandwidth to be 1G also

Conclusion

This is not an exhaustive list of every type of configuration, module or standard. It is a jumping point for you to gain a basic understanding of technology and terminology and allow you to dive deeper into your own research. This topic seemed very confusing to me for a while and I couldn’t find many explanatory resources for a wholesome, basic understanding. Hopefully this post can contribute towards that.

I hope this has helped.



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