Monthly Archives: September 2014

SFP+ Direct Attach Copper Cables: Passive or Active?

New Post: Active or Passive SFP+ Cables?

 
 

 
 
Today’s question comes to us from Bill J. in Ohio:

 

Hello Cable Guy!  I could sure use your help…
I am a small business owner in a technology-heavy industry that is relying more and more on cloud-based application, storage and management systems; for both internal use as well as remote access by end-users, i.e. clients, subscribers and affiliates.  I am quickly realizing that our company runs the risk of being left behind our more tech-savvy competitors.  Needless to say — I cannot let this happen.

 

Bottom Line:  Our present network infrastructure has absolutely no headroom left from which we can grow.  It is basically nothing more than a 1000BASE-T network with some ancient 1-Gig Mini-GBICs (SFP’s) on the back-end.  I really need to scale this up by a factor of ten just to be on par with the competition and would like further scalability options in the near future without having to upgrade all of the hardware.

I think I have found the solution I’m looking for by using 10-Gigabit Ethernet SFP+ equipped switches with the possible aggregation of those switches via 40-Gigabit Ethernet QSFP+ at the top of the racks.  Here’s my dilemma — I’ve found several switches that are relative bargains from a price perspective, but their datasheets specifically call out the use of Active SFP+ Direct Attach Copper Cables.  They will NOT work with Passive SFP+ Direct Attach Copper Cables.

The manufacturer isn’t particularly clear as to why they absolutely must use Active SFP+ Cables.  I’ve noticed that the Active SFP+ Copper Cables sold by you folks at Amphenol Cables on Demand (www.CablesOnDemand.com) seem to cost several times as much as your passive cable variants.  So here’s what I need to know:

 

Why can’t I use your passive SFP+ cables with their switches?
Why do the active SFP+ cables cost so much more than passive?

Your help is sincerely appreciated!

 

Thanks Bill for your excellent questions.  I know you are certainly not the first customer to express interest in network upgrades using SFP+ as the cornerstone of the new network.  You have chosen the absolute best option for affordability, scalability and reliable performance, which incidentally sounds precisely what your business environment demands at this time.  Good work!  Now let’s get down to the nitty-gritty, shall we?

First, we need to establish exactly what Passive Cables are and what role they play.  A Passive Copper Cable, such as the 10-Gigabit Ethernet SFP+ Direct Attach Copper Cable product you are interested in, is effectively viewed as a transparent cable to the switch and requires little to no direct power to operate.  A passive cable therefore passes the signal through it without any modification or processing such as amplification or equalization.  

If a passive cable does nothing to the signal except act as a pass-thru transmission medium, then how exactly is the signal processed in the first place?  In most instances, this activity resides exclusively inside the electronic circuitry of the switch.  The switch will generally perform the following functions to the signal prior to its transmission through the cable assembly:

 

Signal Conversion:

Signal conversion occurs first in instances where electrical circuits must interface with optical circuits.  Since some switches operate select ports in the optical domain (i.e. fiber optic light pulses), that optical signal must then be converted over to an electrical signal that can be further modified and reliably sent over an electrical transmission medium such as copper cabling.  

Signal Conditioning:

Signal conditioning is in essence preparing the signal for the next step of the process.  This could involve formatting the signal using a standardized electrical modulation pattern that other down-stream equipment can recognize.  This could be conditioning the signal into a 10-bit/8-bit pattern where 8-bit segments of data require encapsulation in a 10-bit “packet” featuring additional header information that is used for error correction and re-transmission mechanisms.

Signal Amplification:

The next critical step, signal amplification, will raise the newly conditioned signal from a low-strength “machine level” signal to a far more powerful or amplified signal.  Signals are kept at lower strength levels inside the switch’s processing architecture for several reasons, including reduced power consumption, improved heat dissipation, reduced noise/interference characteristics and so on.  However, this internal “baseband” signal is far too weak to travel the typical distances commanded by cable assemblies. They’re only designed to travel a few inches around a PC-board or chipset!

 

Signal Equalization (Pre-Emphasis/De-Emphasis):

Now, if we were dealing purely with signals of past eras such as 10/100 Megabit (Fast) Ethernet, the final phase known as signal equalization would be totally unnecessary.  However, a signal such as the one sent over SFP+ Cables requires hundreds of MegaHertz (MHz) or even GigaHertz (GHz) of signal bandwidth to operate effectively.  The problem with such wide-banded signals is that the lower portion of the signal may arrive at the other end of the cable faster or stronger than the higher portion of the signal.  This causes an anomaly known as time delay and/or phase delay SKEW that absolutely wreaks havoc on the signal itself, sometimes to the point of being unrecoverable.

To avoid this, the signal must be equalized accordingly.  On the transmission channel of the link, the signal must go through a process called Pre-Emphasis.  It does exactly what it sounds like — it pre-emphasizes (boosts) the portion of the signal that tends to get skewed.  Similarly, on the receive channel of the link, the signal portion must be reciprocally de-emphasized (lowered).  This complete process of equalization equalizes the signal so that it is uniform and free of SKEW and therefore ready to be sent over a specific transmission medium, i.e. a SFP+ Direct Attach Copper Cable.

 

So, What are the Implications of Everything Above?
They are HUGE.

Approximately 7 out of every 8 SFP+ enabled switches on the market today perform all of the aforementioned signal processing/conditioning functions internally and automatically.  Because of this, customers who own these switches are free to use a wide selection of SFP+ Passive Direct Attach Copper Cables in lengths ranging from 0.5m all the way up to 10m (33 feet).  As you are already aware, these Passive SFP+ Cables are extremely affordable (prices start around $30.00).  The cables have only basic circuitry inside their integrated SFP+ connectors which tell the switch what brand and length they are — nothing more.

However, the switch manufacturers realized that some business owners, such as yourself, may prefer the option of lower up-front costs on the switches since they may not be populating all of the switch’s SFP+ ports with cables for some time.  The market has seen 1 out of every 8 of its SFP+ switches configured in this lower-cost arrangement.  In essence, all of these value SFP+ switches feature “dumb” SFP+ ports on them with little to no signal processing capabilities built in.

True, this may bring the upfront costs down because the signal processing chipsets in these ports are both complex and costly to integrate.  However, there is no such thing as a free lunch as they say.  If indeed we take all of that signal processing circuitry out of the switch, it has to be made up for somewhere else.

Well, where else exactly can that signal processing circuitry go?  There’s only ONE possible location available — THE CABLE. 

Switches with so-called “dumb” SFP+ ports therefore need “smart” SFP+ Cables to pick up the processing slack.  A passive SFP+ cable can be made “smart” by integrating signal processing circuitry into its built-in connectors and actively powering those circuits with DC power provided by the switch — Hence the name: ACTIVE SFP+ CABLES.

Since Active SFP+ Direct Attach Copper Cables require active smart circuitry in them to work, the cost of that circuitry, usually installed in the switch instead, transfers over to the cost of the cable.  This is precisely why Active SFP+ Cables cost up to 3x their passive counterparts.

 

The Cable Guy’s Final Thoughts:

If customers are in the same situation as Bill, I highly recommend performing a comprehensive cost analysis of two network development options along with appropriate deployment timelines.  For instance, let’s say you plan on buying 2 24-port SFP+ switches but only need to outfit 12 of the 24 ports in year one.

 

Passive Cable Option 1 (Year 1):

(2) Advanced SFP+ 24-Port Switches (Port Circuitry Built-In)

Cost:  $4,000

(12) Passive SFP+ Direct Attach Copper Cables @ $80 each

Cost:  $960

 

Total Cost (Year 1):  $4,960

 

Active Cable Option 2 (Year 1):

(2) Basic SFP+ 24-Port Switches (No Port Circuitry Built-In)

Cost:  $2,800

(12) Active SFP+ Direct Attach Copper Cables @ $180 each

Cost:  $2,160

 

Total Cost (Year 1):  $4,960

As you can see, in year one, both the Active SFP+ Cable Option and the Passive SFP+ Cable Option feature the same cost of $4,960.  However, the cost of the switch is a fixed cost and the cost of the cable is a variable cost.  Let’s say you wanted to outfit the remaining 12 open SFP+ ports with cables in year 2. 

For the more expensive upfront cost switches that support Passive SFP+ Cables, those 12 cables can be purchased for an additional $960.

However, for the cheaper upfront cost switches that only support Active SFP+ Cables, those 12 cables can be purchased for an additional $2,160.

So is it worth saving $1,200 on the switches in year one, only to have to spend double that amount more on cables by year two?  Probably not.  However, if you will likely not have to populate those extra ports ever in the future (i.e. wait for the next generation of switch technology altogether), maybe it’s worth it. 

However, these instances are atypical and therefore it is the opinion of this Cable Guy that it is always preferable to pay more upfront for the switch and benefit from drastically cheaper cables!

I truly hope this information will aid in your ultimate decision on this topic.  Please do not hesitate to contact me anytime at the email address below if you have additional questions:

cablesondemandblog@gmail.com

As always, I recommend Amphenol Cables on Demand (www.CablesOnDemand.com) for all of your 10-GbE SFP+ Cable needs.  Amphenol is one of only three companies in the world that actually manufactures all components of an SFP+ Cable including the raw copper wire and the SFP+ connectors (active and passive versions).  Other companies merely assemble some unspecified combination of SFP+ subcomponents, some of which may be of questionable quality and/or qualification.

When you want your network to work for you 100% of the time, you NEED the best quality network cabling and Amphenol is the global leader in quality network cabling products — period.  You can buy these cables factory-direct from Amphenol Cables on Demand at the best possible price — all with no lead-time and same-day shipping when ordered before 3PM Eastern!

Popular High Speed Products In-Stock & On-Sale Including:

10GbE-CX4 Cables / SDR & DDR InfiniBand Cables

10GbE SFP+ Direct Attach Copper Cables (Active & Passive)

40-Gigabit Ethernet & QDR InfiniBand QSFP / QSFP+ Cables

SAS Cables / Mini-SAS Cables / HD Mini-SAS Cables 

 

Sincerely,
The Cable Guy