You ask – We Answer: Top SFP-10G-SR Questions Customers Are Asking

The SFP-10G-SR, as of 2017, is the leading mainstream optical module in terms of quantity used. Definitely, 40G and 100G optical transceivers are becoming increasingly popular, but because of stable technology (the first SFP+ modules arrived in the market already by 2006) and their reduced pricing, SFP-10G-SR is the dominant choice for short-range optical connections. Indeed, we even saw some market reports stating that SFP-10G-SR optics occupy about 25% of 10GbE ports shipped worldwide. Because of such wide adoption of SFP+ short-range modules by thousands of enterprises, Telco, and data centers, it is typical that many uncertain concepts and myths arise. We will try to clarify some of them based on the most popular doubts or questions our customers have expressed.

SFP-10G-SR: Basics

SFP-10G-SR is an optical transceiver based on the widely used SFP+ (Small Form-Factor Pluggable) MSA Standard. Every optical transceiver can be characterized by three groups of its properties: optical, electrical, and environmental.

SFP-10G-SR is an optical transceiver based on the widely used SFP+ (Small Form-Factor Pluggable) MSA Standard. Every optical transceiver can be characterized by three groups of its properties: optical, electrical, and environmental. Optical properties define such key factors as transceivers' operational center wavelength and their bandwidth, and compatible optical cable type. Transmission power and receiver sensitivity define the optical budget and, as a result, the maximum distance. Laser or photonic components are the most expensive items on every optical transceiver BOM, and the price of these components is a key price driver of the whole transceiver. SFP-10G-SR Transceivers use a VCSEL (Vertical-Cavity Surface-Emitting Laser) laser diode as a light emitter. Briefly – VCSEL lasers' main advantage is low cost and simple production process, but VCSEL can emit only a relatively wide beam of light, therefore it is adaptable only with multi-mode fiber. VCSELs on SFP-10G-SR transceivers use 850nm as the center wavelength and occupy a 20nm (840-860nm) spectrum. As a receiver or light detector, SFP-10G-SR modules use a PIN photodiode, which is relatively cheap but has rather low sensitivity, compared with another type of photodetectors used in long-range transceivers – APD (avalanche photodiode). Application of the VCSEL Laser diode and PIN photodetector delivers a minimum guaranteed optical budget of 3.8 dB. The distance that we can achieve with this optical budget will depend on the Cable Category Type we will use. Please check the table below for more information:

Electrical properties to most end users are interesting from the following four aspects: which link protocols the transceiver supports, power consumption, digital diagnostics function, and which devices the transceiver is compatible with. Most SFP+ SFP-10G-SR Transceivers can support baud rates from 0.6 up to 10.52 Gbps and a whole range of such applications as:

SFP+ SFP-10G-SR Transceivers support listed protocols in theory, but in many occasions, some of them are software-limited by settings in transceiver firmware, as in some choosy equipment platforms, which can cause incompatibility. Power consumption for all 10G SFP modules varies depending on their reach – for shorter-range modules, it is about 1W, but for greater distances like 40km to 80km, it is around 1.5W. Power supply for all SFPs is 3.3V as defined by MSA.

Digital Diagnostics Function (also called DOM/DDM) – its functionality is defined by MSA SFF-8074, which was first released in 2001, and later SFF-8472. This function relies on built-in allocated memory called A2h and defines access protocol and procedures, which allow the host device, which operates an optical transceiver or its network management system, to access the main Key Performance Indicators of the Transceiver in real time. The transceiver generates these diagnostic data by digitalization of internal analog data. It allows accessing such data as transceiver temperature, internally measured supply voltage, TX bias current, TX output power, and received optical power. This digital diagnostics memory also accommodates warning threshold levels for KPI’s, which can trigger alarms on the host system, in case of misconfiguration. Nowadays almost all optical transceivers are equipped with a digital diagnostic interface by default. 5 or 10 Years ago, digital diagnostic functions were considered as an additional feature, so it is still a chance that, in case of using some old transceivers, you may encounter items without this small but useful function.

Compatibility of transceivers is determined by the marketing policy choice of host systems vendors. The majority of vendors have simply decided to follow MSA (Multi-Source Agreement) guidelines and have not implemented any restricting functions in the software of their host devices. It means that these systems support transceivers of any producer, if their production and encoding comply with MSA. Some decided to take advantage of market conditions and permit the use of only their OEM Products. Here, you can read more about the implications of using 3rd party products on these systems. These protection algorithms are different vendor by vendor, but they all work on top of MSA guidelines. Some vendors use very basic identification by serial and part number; others have implemented quite advanced mathematical functions, like F=MD5 (Vendor Name + Serial + PN + Production Date). Anyhow, mathematics are mathematics, and the optical transceiver industry has found out how to provide desired results for host system mathematical functions, which allows using 3rd party transceivers with protected host systems and achieve huge investment savings.

Side effect of this protectionism policy is that industry is not using common terminology when speaking about optical transceivers, but many different part numbers and slang terms, which only generates confusion and an alphabet soup. For example, SFP-10G-SR, according to the IEEE 802.3ae standard, 10G optical Ethernet operating over MMF with 850nm wavelength, is 10GBASE-SR. So, why don't all equipment manufacturers have 10GBASE-SR-SFP, but such variations for the same thing as:

All listed part numbers describe technically the same SFP-10G-SR with the exact same functionality, and this is just the beginning of the list – we guess that if complete, this table could contain several hundred rows.

Because of biased perspectives on optical transceivers from the majority of equipment vendors, there is a lack of good and structured education on this topic. Many network engineers need to gather knowledge on this important part of the network by a trial–and–error approach. In order to contribute to our industry, we will try to shed some light on the most common questions we hear from our customers:

SFP-10G-SR and SFP-10G-LR: What is the Difference?

Well, both are SFP+ modules produced by the guidelines of MSA, both supporting the same network protocols or applications, and the same electrical interface properties. SFP-10G-SR uses 850nm photodiodes in order to be used with Multi-mode fiber, which allows it to reach Short Distance – up to 300m. SFP-10G-LR uses a DFB laser with a 1310nm wavelength suitable for single-mode optical cable, with a minimum guaranteed budget of about 3.8dB. 3.8dB budget allows SFP-10G-LR to reach about 10km, or as IEEE considers – Long Reach.

Is Cisco's original SFP-10G-SR and FET-10G the same?

Both SFP-10G-SR and FET-10G transceivers, from a hardware perspective, are exactly the same, just different coding. FET-10G has special coding features to be recognizable and usable only between the Nexus 2000, Nexus 5000, and Nexus 7000 series.

Is the Cisco SFP-10G-SR transceiver supported in other manufacturers' switches?

Well, it depends on the switch manufacturer. Most of the host system vendors have no protectionism policy and accept any MSA-compatible transceiver in their systems. All of the protection algorithms come on top of MSA guidelines, so Cisco SFP-10G-SR will work on all MSA-compatible systems. However, if it will be used with a system using a different proprietary algorithm, it will not be supported.

What is the difference between SFP-10G-SR= vs SFP-10G-SR-S=?

Many who read official vendor papers will answer this question that Cisco S-class optics are a more budgetary class of transceivers designed especially for enterprise and data center connections. As both SFP-10G-SR= and SFP-10G-SR-S= have almost identical specifications, economy is achieved by dropping unnecessary features / link protocols – such as OTN/SONET. We, however, would say that both these transceivers are the same, just different firmware configurations in order to switch off some protocols, and of course, the opportunity of marketing and revenue protection.

Why can’t SFP-10G-SR establish a connection over 200m of Multimode cable?

First, we would recommend testing these transceivers using a short-distance patch cable. If still no luck in establishing a link, there may be some actual problem with the transceiver hardware, but if you are able to successfully establish link, then the problem is the link budget. Check carefully the type of your multi-mode cable, because in case of older OM1 cable specification, the maximum distance is only about 33m in case of 62.5 micron core and 82m in case of 50 micron core. In order to reach 300m, you need a minimum OM3 specification multi-mode cable.

What is the best option for short-range 10G connections – DAC, Optical, or Cat5/6?

We would say that there is not always a universal answer that fits all scenarios. It can happen that you need to use 10GBase-T because of existing circumstances – you already have copper cables in the wall, or the equipment you need to connect already has 10G Copper ports. If so, 10G Base-T is the only option. If you have a totally free choice, optical connection is the best choice from cost and latency aspects. You can read more about this in our article -”10G Copper SFP+ – Necessary & Useful Option”. If we are talking about very short distances, then Direct Attach Cables would be the best option.

Do SFP-10G-SR modules support GE?

Yes – SFP-10G-SR modules can support GE speeds, but this also depends on the host system port properties. In the case of Cisco Catalyst switches, it will not mostly work, but in some systems, you can configure a 10G port to run in GE mode, and it will be supported by a transceiver module.