Selecting the right cabling plant for data center connectivity is critically important. The wrong decision could leave a data center incapable of supporting future growth, requiring an extremely costly cable plant upgrade to move to higher speeds. In the past, due to high cost of single-mode fiber (SMF), multimode fiber (MMF) has been widely and successfully deployed in data center for many years. However, as technologies have evolved, the difference in price between SMF and MMF transceivers has been largely negated. With cost no longer the dominant decision criterion, operators can make architectural decisions based on performance. Under these circumstances, should we choose SMF or MMF? This article may give you some advice.
MMF Can’t Reach the High Bandwidth-Distance Needs
Based on fiber construction MMF has different classifications types that are used to determine what optical signal rates are supported over what distances. Many data center operators who deployed MMF OM1/OM2 fiber a few years ago are now realizing that the older MMF does not support higher transmit rates like 40GbE and 100GbE. As a result, some MMF users have been forced to add later-generation OM3 and OM4 fiber to support standards-based 40GbE and 100GbE interfaces. However, MMF’s physical limitations mean that as data traffic grows and interconnectivity speeds increase, the distance between connections must decrease. The only alternative in an MMF world is to deploy more fibers in parallel to support more traffic. Therefore, while MMF cabling has been widely and successfully deployed for generations, its limitations now become even more serious. Operators must weigh unexpected cabling costs against a network incapable of supporting new services.
SMF Maybe a Viable Alternative
Previously, organizations were reluctant to implement SMF inside the data center due to the cost of the pluggable optics required, especially compared to MMF. However, newer silicon technologies and manufacturing innovations are driving down the cost of SMF pluggable optics. Transceivers with Fabry-Perot edge emitting lasers (single-mode) are now comparable in price and power dissipation to VCSEL (multimode) transceivers. Besides, Where MMF cable plants introduce a capacity-reach tradeoff, SMF eliminates network bandwidth constraints. This allows operators to take advantage of higher-bit-rate interfaces and wave division multiplexing (WDM) technology to increase by three orders of magnitude the amount of traffic that the fiber plant can support over longer distances. All these factors make SMF a more viable option for high-speed deployments in data centers.
Comparison Between SMF and MMF
10GbE has become the predominant interconnectivity interface in large data centers, with 40GbE and 100GbE playing roles in some high-bandwidth applications. Put simply, the necessity for fiber cabling that supports higher bit rates over extended distances is here today. With that in mind, the most significant difference between SMF and MMF is that SMF provides a higher spectral efficiency than MMF, which means it supports more traffic over a single fiber using more channels at higher speeds. This is in stark contrast to MMF, where cabling support for higher bit rates is limited by its large core size. This effectively limits the distance higher speed signals can travel over MMF fiber. In fact, in most cases, currently deployed MMF cabling is unable to support higher speeds over the same distance as lower-speed signals.
|Name||Interface||FP (SMF)||VCSEL (MMF)|
|Link Budget (dB)|
|—||4 to 6||2|
|Reach (in meters) (Higher value is better)|
As operators consider their cabling options, the tradeoff between capacity and reach is important. Network operators must assess the extent to which they believe their data centers are going to grow. For environments where users, applications, and corresponding workload are all increasing, single-mode fiber offers the best future proofing for performance and scalability. And because of fundamental changes in how transceivers are manufactured, those benefits can be attained at prices comparable to SMF’s lower performing alternative.
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