Optical transceivers were developed for mass production in the early 80s which was the early transition from embedded electrical analogue (wire) based communication systems across to early fibre-based systems. The advantages of lasers and optical fibre being speed and distance as well as power efficiency. Electrical wire-based communication systems still exist with these mostly transitioned across to digital/DOCSIS which we will discuss in separate article.
The purpose of ‘pluggable’ optical transceivers was to enable cost effective upgrades on a pay as you grow approach and to disaggregate the system from the optical component itself. Commercially available optical transceivers are based on industry standards (IEEE/ITU-T) and industry collaborations such as Multi-Source Agreements (MSAs) with the various equipment OEMs, module OEMs, and service providers to ensure there is commonality through the ever-expanding form factors and technology evolution.
There are some fundamental differences between SFP+, SFP28, QSFP+, SFP28, QSFP28, and QSFP56. These ‘form factors’ or pluggable optical transceiver types are all commercially available and being deployed in systems today. The diagram (Fig 1.) below shows a module with an optical interface and an electrical interface. The form factor being the physical size, how many electrical data lanes, and determines the maximum power dissipation.
Figure. 1 – the different parts and interfaces to an optical transceiver module (QSFP56 shown)
In addition to the form factor, there are other subtle but key differences, whether Multi-Mode Fibre or Single-Mode Fibre (MMF/SMF) is being used. As a high-level guide, MMF is typically used for applications up to 10km, and 2km and beyond (up to several 10km’s) will be SMF.
There are several modulation formats with the most common being: Non-Return Zero (NRZ or 1’s and 0’s), 4-state Pulse-Amplitude Modulation (PAM-4) and coherent (this typically is used for longer distances). The 10Gbps and 28Gbps single channel SFP+ and SFP28 modules also can tune the wavelength to enable efficient use the SMF for Wavelength Division Multiplexing (WDM) systems.
The acronyms are plentiful in the communications industry and as the technologies have evolved, this has opened new ways to package and miniaturise the capabilities. This summarises the main differences of the each of the optical transceiver form factors:
- SFP+: Small Form factor Pluggable running at 10Gbps and can be MMF or SMF. The ‘+’ is for 10Gbps.
- SFP28: Small Form factor Pluggable running at 25Gbps and can be MMF or SMF. The ‘+’ is for 10Gbps.
- QSFP+: Quad Small Form factor Pluggable running at 10Gbps per lane, 40Gbps aggregate speed. The ‘Q’ means quad or four electrical lanes.
- QSFP28: Quad Small Form factor Pluggable with each electrical lane running at 25Gbps.
- QSFP56: is the same as QSFP28 with the difference that each of the electrical lanes are running at 50Gbps.
| Form factor |
SFP+ | SFP28 | QSFP+ | QSFP28 | QSFP56 |
| Physical size (L x W x H in mm) | 97 x 35 x 19 | 97 x 35 x 19 | 18 x 8.5 x 52 | 18 x 8.5 x 52 | 18 x 8.5 x 52 |
| Power dissipation (Watts) | up to 1.7W | up to 1.7W | Up to 5W (class 7) or 10W (class 8) | Up to 5W (class 7) or 10W (class 8) | Up to 5W (class 7) or 10W (class 8) |
| Electrical data lane speed (Gbps) |
1 x 10Gbps | 1 x 25Gbps | 4 x 10Gbps (40Gbps) | 4 x 25Gbps (100Gbps) | 4 x 50Gbps (200Gbps) |
| Modulation format (available) | typically NRZ | NRZ/PAM-4 | NRZ/PAM-4 | NRZ/PAM-4 | PAM-4 (26.56 Gbd)/Coherent |
| Optical bandwidth /speed | 10Gbps (up to 16Gbps) | 25Gbps (up to 28Gbps) | 40Gbps | 100Gbps | 200Gbps |
| Distances (Kilometres) | 25m up to 80km | Up to 15km | Up to 20km | Up to 10km | Up to 5km |
| Fibre type | Available in MMF and SMF | Available in MMF and SMF | MMF & SMF | MMF & SMF | MMF & SMF |
| Optical connector | Typically, LC duplex | Typically, LC duplex | MTP/MPO-12 | MTP/MPO-12/LC duplex | MTP/MPO-12/LC duplex |
Table 1 – comparison of difference between optical transceiver functionality at high level
10Gbps SFP+ optical transceivers
10GBASE-SR
10GBASE-SR is a variant of the SFP+ module with the SR meaning ‘Short Range’. It is defined in the IEEE 802.3 Clause 49 standard, specially designed for MMF with 850nm lasers (transmitter). It has a data transmission rate of up to 10.3125 Gbps and can be used over multiple cabling options up to a link length of 300m with OM3/OM4 MMF fibre (or 26m across OM1 MMF fibre). The transmission distance may differ as the fibre cable changes. The optical connector is typically an LC duplex receptacle.
40Gbps QSFP+ optical transceivers
40GBase-SR440GBase-SR4 is probably the most used and popular 40G transceivers for short distance transmission for link lengths of 100m and 150m across OM3 and OM4 multimode fibres. It is an MPO-interfaced parallel fibre optic transceiver with a 4x10G transmission pattern. The 40GBase-SR4 can achieve simultaneous 40G signal sending and receiving with the help of four 10G SFP+ lanes in one module. You can usually see this type of transceiver being used in data centres to interconnect two switches with 12 lane ribbon cables.
40GBase-LR4
While the 40GBase-SR4 is used for short distance transmission, the 40GBase-LR4, is used for longer range transmission. They use the same 4-channel or multi-lane technology, however, LR4 differs in the sense that instead of using a single fibre strand for each lane, it multiplexes all four sending lanes onto one fibre strand and all four receiving lanes onto another single fibre strand. This transceiver type supports link lengths of up to 10km via a single-mode fibre with LC duplex optical connectors.
40GBase-CSR4
The principle behind the 40GBase-CSR4 modules is the same as the 40GBase-SR4 where they both have an MPO interface. The difference being the CSR4 can be considered as an ‘upgrade’ of the SR4 because it can support longer distance transmission, typically up to 300m using OM3 and 400m on OM4 optical fibre.
40GBase-LR4L
40GBase-LR4 Lite or more commonly known as 40GBase-LR4L has the same fundamentals as the 40GBase-LR4 in that it is used for long range 40G data transmission over a single-mode fibre cable. The difference though is that while the LR4 can support link lengths of up to 10kms, the LR4L only goes up to 2kms.
40GBase-ER4
The 40GBase-ER4 module can handle long distance transmission of data up to 40kms over SMF fibre with duplex LC connectors. It works by converting four 10G input channels to four CWDM optical signals, then multiplexes them into a single 40G channel. On the receiving end of the module, the single 40G input is demultiplexed into four CWDM signals, then transforms them to a four channel electrical data output.
100Gbps QSFP28 optical transceivers
There are four main types which are categorised by link length and explained in terms of the optical and electrical interface.
100G QSFP28 SR4
The 100GBASE-SR4 QSFP28 transceiver is a parallel 100G full-duplex optical transceiver which offers 4 independent transmit and receive channels, each capable of 25 Gbps operation for an aggregate data rate of 100 Gbps up to 100m across OM4 MMF. Generally, 100GBASE-SR4 QSFP28 transceiver converts parallel electrical input signals into parallel optical signals driven by a VCSEL array. The transmitter module accepts electrical input signals compatible with CML (common mode logic) levels. All input data signals are differential and internally terminated. The receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module outputs electrical signals are also voltage compatible with CML levels. All data signals are differential and support a data rate of up to 25 Gbps per channel.
100G QSFP28 PSM4
The 100GBASE-PSM4 QSFP28 transceiver is a parallel 100G single-mode optical transceiver with an MTP/MPO fibre ribbon connector. It uses 8 fibres (4 transmit and 4 receive), each lane transmitting at 25 Gbps, resulting in an aggregate data rate of 100 Gbps across 500m of SMF. The working principle of 100GBASE-PSM4 QSFP28 transceiver is like a 100G QSFP28 SR4 transceiver with the only difference being PSM4 works over SMF, while SR4 works over OM4 MMF.
100G QSFP28 LR4
The 100GBASE-LR4 QSFP28 transceiver converts 4 x 25 Gbps of electrical data into 4 channels of LAN WDM optical signals and multiplexes them into a single channel for 100G[LJ3] [rh4] optical transmission. On the receiver side, the module demultiplexes a 100G optical input into 4 channels of LAN WDM optical signals and then converts them to 4 output channels of electrical data. The central wavelengths of the 4 LAN WDM channels are 1295.56, 1300.05, 1304.58 and 1309.14 nm as members of the LAN WDM wavelength grid defined in IEEE 802.3ba. The 100GBASE-LR4 QSFP28 transceiver provides superior performance for 100G applications up to 10km over SMF and compliant to optical interface with IEEE802.3ba 100GBASE-LR4 requirements.
100GBASE-CWDM4
The 100GBASE-CWDM4 QSFP28 transceiver is a fully duplex 100G QSFP28 optical transceiver that provides a high-speed aggregated data rate of 100 Gbps across 2 km of SMF. The transmitter path converts four lanes of serial electrical data into optical signals which in turn are multiplexed and coupled into a single-mode fibre through an industry standard LC optical connector. The optical signals are engineered to meet the CWDM4 MSA specifications. On the receive side, the four incoming wavelengths are separated by an optical demultiplexer into four separated channels.