How are Transceiver Modules Classified?
Types of transceiver modules can be classified into various categories based on their performance characteristics and end-use. Characteristics commonly used in fiber optic transceiver classification include: fiber mode, transfer rate, transmission distance, wavelength, and connector type. Each of these categories is discussed in more detail below.
Fiber optic transceiver modules with similar features are further grouped together and classified into packages, also known as form-factors. There are currently 6 primary form-factors of fiber optic transceivers in common use: SFP+, SFP, GBIC, X2, XFP, XENPAK, and becoming popular are QSFP+ for 40Gbps and CFP for 100Gbps transceivers.
Fiber Mode
Perhaps the most fundamental classification of fiber optic transceivers is the “mode type” of the fiber with which it is intended to be used. The two basic classifications of fiber mode types are: multimode and singlemode. Multimode fibers, with core diameters typically in the range of 50 to 62.5 microns, have substantially larger core diameters than singlemode fibers, which have core sizes in the 8 to 9 micron range.
Multimode fibers, with their larger core aperture, allow multiple modes of light to couple into the fiber. These various modes of light propagate at slightly different speeds as they travel down the fiber. The result is a ‘spreading’ of pulses referred to as Modal Dispersion. This type of multimode-specific dispersion severely limits the transmission distance achievable over multimode fiber as contrasted with singlemode fiber. Since multimode applications are always short reach, very inexpensive transmitters and receivers are typically used in multimode transceivers. So, while multimode fiber itself it not substantially different in price than singlemode, the price of multimode transceivers are typically a fraction of the price of singlemode ones.
As shown in the table below, there are several popular types of multimode fiber in use today. OM1 and OM2 fibers are appropriate for low speed transmission, such as 100Mbps to 1Gbps, which often utilize LED transmitters. OM3 and OM4 are referred to as laser-optimized multimode fibers, as lasers are used as optical sources at 10Gbps and faster.
| ITU Fiber Classification | Core Diameter (microns) | Bandwidth* Length Product (MHz*km) |
|---|---|---|
| OM1 (aka FDDI fiber) | 6.25 | 160-200 |
| OM2 | 50 | 400-500 |
| OM3 | 50 | 2000 |
| OM4 | 50 | 4700 |
Singlemode fibers, as their name implies, only allow a single mode of light to couple into the core. This completely eliminates the Modal Dispersion problem. Singlemode fiber transmission is limited by several other forms of dispersion, notably Chromatic and Polarization Mode. However, these are much ‘weaker’ dispersions, allowing singlemode fiber to support transmission distances several orders of magnitude longer than multimode fiber.
The most common type of singlemode fiber is termed “OS1” by the ITU and is also known as “standard singlemode fiber”. While there are other singlemode fiber types (e.g. dispersion shifted fiber, non-zero dispersion shifted fiber), most optical transceivers are simply specified for operation over OS1.
One final note, multimode transceivers have essentially no ability to achieve successful transmission over even short lengths of singlemode fiber. Singlemode sources can work over short distances of multimode fiber but are more expensive, so it makes little sense to use them in such applications.
Transfer Rate
Fiber optic transceiver modules are often categorized based on their data transfer rates. There are five popular rate categories used in fiber optic transceiver classification: 100GBase, 40GBase, 10GBase, 1000Base and 100Base. These rates refer to the speed at which a fiber optic transceiver is able to transmit data over Ethernet.
- 100GBase – 100 Gigibits per second (100GE, 100GbE, 100Gbps)
- 40GBase – 40 Gigabits per second (40GE, 40GbE, 40Gbps)
- 10GBase – 10 Gigabits per second (10GE, 10GbE, 10Gbps)
- 1000Base – 1 Gigabit per second (1GE, 1GbE, 1Gbps, 1000Mbps)
- 100Base – 100 Megabits per second (Fast Ethernet, FE, 100Mbps)
There are a number of other transfer rate hierarchies associated with certain subsectors of the market. Popular rates for Fibre Channel, historically used for high speed supercomputer interconnection and Storage Area Networking (SANs), are: 1Gbps, 2Gbps, 4Gbps, 8Gbps, and 16Gbps. Telecommunications networks have been using the SONET/SDH multiplexing hierarchy for many years with optical transmission rates of: 155Mbps, 622Mbps, 2.488Gbps, 9.953Gbps and 39,813Gbps.
Transmission Distance
Not all fiber optic transceiver modules can transmit data the same distance. As mentioned above, one major difference is multimode versus singlemode transceivers. For multimode applications, both transfer rate and the specific type of fiber affect the transmission distance. For singlemode applications, transfer rate is the dominant factor with respect to transmission distance.
Multimode applications are generally classified as “Short Reach”, usually with the nomenclature “SR” (older 100Mbps use “FX” and 1Gbps transceivers typically use “SX”). There are multimode transceivers marketed as “Long Range Multimode” or “LRM” supporting transmission distances slightly longer than SR parts, but these vary considerably from one vendor to the next and are still very much Short Reach modules. The table below shows typical transmission distances possible for the popular transfer rates over the most common four types of multimode fiber. Popular (though not all inclusive) designations used in describing and sometimes naming these parts is also provided.
| Transfer Rate | Typical Designation(s) | MULTIMODE FIBER CLASSIFICATION | |||
|---|---|---|---|---|---|
| OM1 | OM2 | OM3 | OM4 | ||
| 100Mbps | FX | 2km | 2km | 2km | 2km |
| 1Gbps | SX | 500m | 2km | 2km | 2km |
| 10Gbps | SR, USR, LRM | 33m | 82m | 300m | 400m |
| 40Gbps | SR, SR4, CSR4 | N/A | N/A | 100m | 150m |
| 100Gbps | SR4, SR10 | N/A | N/A | 100m | 150m |
Singlemode applications cover a wider range of distances over OS1 class fiber optic cables. The following table shows the typical transmission distances supported at the popular transmission rates in use today.
| 100Mbps | 1Gbps | 10Gbps | 40Gbps | 100Gbps | |||||
|---|---|---|---|---|---|---|---|---|---|
| DESIGNATOR | REACH | DESIGNATOR | REACH | DESIGNATOR | REACH | DESIGNATOR | REACH | DESIGNATOR | REACH |
| LX | 10km | LX | 10km | LR | 10km | LR4 | 10km | LR4 | 10km |
| EX | 40km | EX | 40km | ER | 40km | ER4 | 40km | ER4 | 40km |
| ZX | 80km | ZX | 80km | ZR | 80km | ||||
| ZX120 | 120km | ZX120 | 120km | ||||||
NOTE: Using devices such as optical amplifiers and dispersions compensators, transmission distances of 1000’s of kilometers are possible. The engineering of such links is highly specialized and is outside the scope of this article.
Wavelength
Infrared light is used in the transmission of data over fiber optic networks. A wavelength is the measurement of the distance between successive crests in the light wave. Fiber optic transceiver modules typically transmit data at one of three primary wavelengths: 850nm, 1310nm or 1550nm.
The reasons for the popularity of these three wavelengths are twofold: 1) fiber optic attenuation is much lower at these wavelengths; and 2) the US National Institute of Standards and Technology (NIST) provides metered calibration for testing fiber optics at these wavelengths.
Multimode fiber is designed to operate at 850nm and 1300nm wavelengths, while singlemode fiber is optimized for 1310nm and 1550nm wavelengths. In the singlemode domain, finer gradations of wavelengths are possible within both the 1310nm and 1550nm ‘windows’ using precision built transmitters. The two most common and standardized schemes are CWDM (Coarse Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing).
CWDM wavelengths are spaced at 20nm and are possible in the 1310nm window and, more commonly, in the 1550nm window, as described in the following table.
| Wavelength Window | CWDM Channels (nm) |
|---|---|
| 1310nm | 1270, 1290, 1310, 1330, 1350, 1370, 1390, 1410 |
| 1550nm | 1470, 1490, 1510, 1530, 1550, 1570, 1590, 1610 |
DWDM schemes are, as the name indicates, much more tightly space than CWDM. Rather than using wavelength (units: nm), frequency of light is typically used to describe the spacing of DWDM channels. The most common standard spacings of DWDM channels are 25GHz, 50GHz and 100GHz with 100GHz being the most widely deployed. These channels are allocated within a range from 190.1THz to 197.2THz or, in terms of wavelength, a range from 1577.03nm to 1520.25nm.
In both CWDM and DWDM systems, optical transceivers, each transmitting at their specific wavelength, are connected to wavelength division multiplexing devices. These devices combine and separate multiple wavelengths (or colors) of light onto/off-of a single fiber or fiber pair. CWDM systems tend to be popular and cost-effective in situations where the number of fiber strands is limited and adding fibers is expensive, even when distances are not particularly long. In long-haul systems where the system must be amplified and dispersion compensated multiple times, DWDM is widely deployed. Erbium Doped Fiber Amplifiers (EDFAs) and dispersion compensators act on all of the individual DWDM channels at once, without having to demultiplex and completely regenerate each channel every 80km or so.
Connector Type
Optical fiber connectors couple and align transceivers so that light can pass through the core. Transceiver modules can be classified into different groups based on their connector types. There are four main types of fiber optic module connectors used in conjunction with optical transceivers today: SC, LC, MPO, and ST.
| Connector | Description | Form Factors Using |
|---|---|---|
| SC | Subscriber Connector (snap-in connector) | GBIC, X2, XENPAK, some QSFP (40G) and CFP (100G) |
| LC | Lucent Connector (small form-factor version of the SC connector) |
SFP, SFP+, XFP |
| MPO | Multi-fiber Push-On (commonly 12 or 24 fibers per) |
Some QSFP (40G) and CFP (100G) |
| ST | Straight Tip Connector (bayonet mount connector) |
Not used on optical transceivers but popular at optical patch panels |
Most optical transceivers use duplex connectors, one for transmit and one for receive. There are Bi-Directional (BiDi) optical transceivers that are deployed in pairs with each end transmitting on a different wavelength (e.g., 1310nm and 1490nm). Each BiDi transceiver includes a 2-channel wavelength division multiplexer to separate/combine the two wavelengths.
For the newer QSFP and CFP modules that utilize a MPO connector, there is only a single connector but, as described in the table above, each connector may have 12 or 24 fibers, each of which connect to separate transmitters/receivers within the optical transceiver.
Connector types generally follow a color code system. If a connector is compatible with singlemode fiber, it will be yellow. Connector types compatible with multimode fiber will be orange, black or gray. If a boot is used over the connecter, then a blue boot symbolizes compatibility with singlemode fiber and a beige boot symbolizes compatibility with multimode fiber.
A variety of fiber optic cables are available on the market for connecting devices with dissimilar connectors.