The communications industry is awash with jargon, acronyms, and sometimes what might appear to be conflicting references.
It is worth taking a step back to provide some context to how the industry has evolved. There are two main standards bodies such as the International Telecommunication Union – Telecommunication (ITU-T) and Institute of Electrical and Electronics Engineers (IEEE). The ITU-T has been around since 1865 and provides information for organisations to work to set common ‘Recommendations’. These usually end up evolving as standards through collaboration with industry professionals/organisations around the world.
Industry bodies and influences
The ITU-T focuses on telecommunications and is there to ensure the efficient and timely production of recommendations covering all areas of telecommunications and Information Communication Technology (ICTs) on a worldwide basis. It also defines tariff and accounting principles for international telecommunication services ensuring a level of consistency in financials. These industry bodies consist of industry and academic technical specialists who understand the principles and potential impacts when proposing the recommendations. It is then up to the industry to form working groups and agreements to agree on the specifics which is then adopted as standards.
Westbury Photonics consists of industry professionals who have been in the photonics and communications industry for over 25 years. The experience covers a vast area of knowledge from understanding which standards and recommendations are applicable, what they mean, and the materials and components typically used within these systems at the PHY and MAC layers of the OSI model.
The IEEE started producing standards in 1973 and focuses mainly on relevant data communications. IEEE 802.3 was published in 1983 and it is a working group and a collection of Institute of Electrical and Electronics Engineers (IEEE) standards produced by the working group defining the physical layer and data link layer’s media access control (MAC) of wired Ethernet. This is generally a local area network (LAN) technology with some wide area network (WAN) applications. The majority of optical transceivers are used in data communications today and will comply with different parts of the IEEE 802.3 standards.
In addition to standards and recommendations, there are MSAs (multi-source agreements) and the OIF (optical internetworking forum) with each having a published list of companies who collectively agree a set of common interface protocols, physical form factor, power dissipation, and other technical details to ensure an SR (Short reach) optical transceiver purchased from a company will also work with a different company’s and be recognised by a switch or other terminal network.
To recap, the IEEE and ITU-T collectively work together to form recommendations and standards for the physical layer and the data link layer of the media access control (or MAC address as some of you will be familiar with) for communications. Although these are more common with LAN some are also used for WANs. In addition to the IEE and ITU-T, the MSA and OIF collaborate to produce documentation describing the interface protocols, physical dimensions (and tolerances), form factors, and reach (SR, LR, ER, ZR and so on) in more detail.
Understanding Optical Transceivers Acronyms
Having some understanding of how the recommendations, standards, MSAs, and the OIF work together, let’s look into further detail and explain the acronyms, such as the module form factor, power consumption/class, laser type/wavelength, and fibre type.
Communications systems today consist mainly of two fibre types, multimode (MMF) and single-mode (SMF). Copper is still used today and is defined under 10GBASE-CX4/KR/KX4/-T using SFP+ or QSFP form factors.
For the purposes of this article, we will focus on the two commonly deployed optical fibre types; MMF which is used for shorter links such as LAN and Datacentres, and SMF which is suited for longer link lengths, typically from 1km (0.6 miles) upwards. SMF transceivers are more expensive than MMF transceivers due to the components used, tolerances/equipment to manufacture, and testing, and are produced in the tens of thousands per year. MMF transceivers on the other hand use readily available materials (including silicon photonics) and are manufactured in the tens of millions per year.
Here are some of the suffixes and abbreviations for different link lengths at 10Gbps Ethernet rates. These standards are aligned to IEEE 802.3, and define the requirements for full-duplex point-to-point links between network switches with different PHY (physical layer 1) types such as SFP+ (Small Form Factor Pluggable at 10GbE):
- Short Range (SR) or 10GBASE-SR: These support link lengths up to 300m over MMF with 850nm lasers. Transceivers of this type are of the original multimode optical specification and are widely available. Typical applications include intra-datacentre and within a building.
- 10GBASE-LX4: this is used for MMF and SMF types. The difference with this approach is the optical transceiver integrates 4 separate lasers each operating at 3.125 Gbps and are spaced widely apart (coarse wavelength division multiplexing) in the 1310nm optical spectrum. The coding and 8B10B PCS and defined further in IEEE 802.3 under clause 48. The link length is up to 10km over SMF or 300m over FDDI-grade OM1/2/3 MMF. You will need to couple this transceiver through an SMF offset-launch mode conditioning patchcord.
- 10GBASE-PR: Although originally specified in IEEE 802.3av standard, PR is a 10G Ethernet PHY for passive optical networks (PON) with 1577nm lasers in the downstream (DS) direction and 1270nm lasers for the upstream (US). These optical transceivers are typically used in point-to-multi-point configurations from the central office (CO) to the home or office premises. There are three power budgets for consideration depending on the link length and power budget available.
- Long Reach (LR) or 10GBASE-LR: Support distances up to 10km over SMF typically using 1310nm lasers. There is no minimum distance for LR. Typical applications would be from a central office (CO) to an office building and one side of the city to the other.
- Extended Reach (ER) or 10GBASE-ER: These transceivers are suitable for link lengths of up to 40 kilometres across engineered links and 30km over standard SMF links. The wavelength used is in the 1550nm range and usually with Externally Modulated Lasers (EMLs) as the transmitter source in the optical transceiver. Due to the emitted laser power, attenuation is required for links less than 20km in distance. Typical applications include CO-to-CO and metro to point-to-point links. In some instances, wavelength division multiplexing (WDM) is used which is where different wavelengths spaced in the 1550nm band is used across a single pair of optical fibres to increase the utilisation of the fibre bandwidth (which can be from 10Gbps for a single channel up to 960Gbps with 96 channels)
- 10GBASE BiDi: There are several manufacturers and operators who have introduced a 10GBASE SFP+ optical transceiver with a single optical port. These are commonly known as ‘BiDi’ or ‘bidirectional’ and use a single strand of fibre (typically Latched Connector type). This is accomplished be using a passive prism inside each optical transceiver and a matched pair of transceivers using two different wavelengths such as 1270 and 1330 nm. These transceivers are available in varying transmit powers and reach distances ranging from 10 up to 80 km. BiDi transceivers allow companies to extend the life of existing Latched Connector (LC) across MMF by enabling it to support 40 Gbps throughputs instead of requiring an infrastructure upgrade to multi-fibre push on (MTP/MPO) connector types
- Extended Reach (ZR) or 10GBASE-ZR: These transceivers are suitable for link lengths up to 80km across single-mode fibre and typically use 1550nm lasers. The ZR suffix PHY is not specified within the IEEE 802.3ae standard and manufacturers have adopted their own specifications based upon the 80 km PHY described in the OC-192/STM-64 SDH/SONET specifications. Due to the very high optical transmit power, significant optical attenuation is needed for shorter links. Although ZR is an unofficial format and not technically part of the IEEE 802.3ae standards is accepted and used within the industry. The use of ZR based optics should be preceded with an optical power test of the fibre span to ensure a problem-free deployment. Several hardware manufacturers including Cisco have their own implementations of ZR which is not always compatible with other manufacturers. Applications include city-to-city and can often be used to span across countries with multiple hops.
The explanation above covers part of the 10Gb Ethernet standards, recommendations, and form factors, and fibre types for the SFP+ (small form factor pluggable 10Gbps) PHY level transceivers.
Written by Westbury Photonics
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