The communications industry is eagerly awaiting the implementation of 40Gbps and 100Gbps Ethernet cabling in major data centres.

The Institute of Electrical and Electronics Engineers (IEEE) 802.3ba 40Gbps and 100Gbps Ethernet Standards encompass a number of different physical layer requirements designed to suit various cable lengths and performance levels. These rely on using parallel paths for optics, either different fibres at the same wavelength (multi-mode) or different wavelengths in the same fibre (single-mode).

Table 1 shows that the 1m backplane objective uses four lanes of 10GBASE-KR physical layers (40GBASE-KR4), while the 7m copper cable objective is met with four or 10 differential lanes using SFF-8642 and SFF-8436 connectors.

It also shows that the 100m OM3 objective is achieved through parallel ribbon cable with 850nm 10GBASE-SR layers, like optics (40GBASE-SR4 and 100GBASE-SR10). The 10km 40G objective is met with four wavelengths around 1310nm of 10G optics (40GBASE-LR4) and the 10km and 40km 100G objectives with four wavelengths around 1310nm of 25G optics (100GBASE-LR4 and 100GBASE-ER4).

For multi-mode cabling, the emergence of parallel optics in the form of multi-fibre push-on (MPO) cabling systems means designers can achieve their current sub-40/100Gbps Ethernet performance and have a built-in migration path ready for use when demand for higher bandwidth comes along (see Figure 1).

TABLE 1:

 
    
For single-mode cabling inside the data centre, there is also a migration path to 40/100Gbps Ethernet through wave division multiplexing, which enables four or more wavelengths to be transmitted down one fibre. Both of these cost-effective solutions will suit the vast majority of data centres in the education, health, transportation and financial industries.

Meanwhile, the telecommunications carrier community has been working on a different solution to leverage their significant installation base of optical transport network (OTN), synchronous optical networking (SONET), synchronous digital hierarchy (SDH) and packet over SONET (POS) equipment.

Working with IEEE, they produced 802.3bg, a new 40Gbps Ethernet Standard for serial interface devices on single-mode fibre.

The new standardised interface, called 40GBASE-FR, supports transmission up to 2km over single-mode fibre and can enable equipment to be configurable to interoperate with an installed base of equipment conforming to existing telecom and ITU standards, such as OC-768, OTU3, STM-256 or 40G POS.

IEEE recently announced the ratification of IEEE 802.3bgTM-2011 40Gbps Ethernet single-mode fibre optic interface, a new standard enabling 40Gbps serial Ethernet interfaces to be deployed on single-mode fibre. It is anticipated that equipment suppliers will be able to support the new 40GBASE-FR interfaces in equipment that is already designed to support 40GBASE-LR4.

This second 40Gbps Ethernet standard, IEEE 802.3bg, paves the way for carriers to smoothly and rapidly adopt Ethernet technology in their networks. With the introduction of 40G, short range systems are the first choice for large backbone and campus installations. In data centres and any other application below 150m, multi-mode 40Gbps and 100Gbps are still the preferred options.

This is particularly true for storage area networks (SANs) in data centres where the number of connections is traditionally much higher. In this case the cost for single-mode fibre for the actives is much higher, without delivering as many benefits.

Many times in the evolution of Ethernet applications the question has been asked: single- or multi-mode? The answer remains the same.

Multi-mode is still cheaper with regards to light sources and gigabit interface converters are significantly cheaper than single-mode versions.

However, the gap is shrinking and the day may come where both technologies are priced equally.