Introduction to LC Uniboot Fiber Patch Cables

In the past years, to meet the growing bandwidth needs, data center technologies and cabling structures have changed a lot. High density apparently becomes the trend. Data center has to install more and more fiber optic jumper cables in a given space, which makes cable management a more and more difficult problem. New products and technologies are applied to achieve high density in data centers. To find an easy-to-manage and space-saving method for high density cabling becomes an urgent issue for data center managers. In this post, a favorable high density fiber cabling solution—LC uniboot fiber patch cable, which is born to solve problems during high density cabling, will be introduced.

LC Uniboot Fiber Patch Cable vs Standard LC Fiber Patch Cable

LC fiber optic connector can offer higher density and performance in many different environments compared to other types of fiber optic connectors, which makes it a more popular choice for many applications. That is why uniboot fiber patch cables terminated with specially designed LC fiber optic connectors have been invented. With its unique structure, LC uniboot fiber patch cable has more advantages over traditional LC to LC cable in high density cabling environments. Differences between LC uniboot fiber patch cables and standard LC fiber patch cables are noticeable. The following picture shows an LC uniboot fiber patch cable (left) and a standard LC fiber patch cable (right) separately.

Less Cable Count to Cut Space Requirements

A traditional LC duplex fiber patch cable usually uses a two-cable design with two fibers separately enclosed in two different cables, and it is terminated on each end with a standard duplex LC fiber optic connector. LC uniboot fiber patch cable uses only one cable even it has two fibers. It has a single boot at the back of the duplex LC fiber optic connector. Two fibers for duplex transmission are firmly enclosed in a single cable, which can cut down the cable count up to 50% compared with traditional LC duplex fiber patch cords. Space requirements of data center cabling can be reduced significantly by LC uniboot fiber patch cables.

Easier Polarity Reversal to Increase Efficiency

For LC duplex fiber patch cables, polarity change can be really inconvenient, especially in high density cabling environments like data centers. Additional tools and fiber cable re-termination are usually required to change polarity of traditional LC duplex fiber patch cables, which wastes both time and money. And sometimes, improper handling could result in various faults. But the polarity reversal for LC uniboot fiber patch cables is much easier, which can be easily changed by several simple steps without additional tools. Currently, there are several different versions of LC uniboot fiber patch cables, and the polarity reversals of them might differ from each other. Two commonly used versions of LC uniboot fiber patch cables polarity reversal steps are shown in the following picture.

Special Feature to Achieve More Possibilities

With LC uniboot fiber patch cables, fiber optic network design can be more flexible without worrying about spaces and polarity problems. Other than space-saving and easy polarity reversal, LC uniboot fiber patch cable can also achieve more possibilities with its great features. Fiber optic cabling provides faster speeds with reliable quality, which saves a lot of time and money. And, the design and improvement of uniboot fiber patch cables have never stopped. LC uniboot fiber patch cable with push-plug tab has already been available in the market. This little change can make easier finger access and quicker latch release available, and it can also help to connect or disconnect a single fiber patch cable without affecting other surrounding links.

Conclusion

LC uniboot fiber patch cable can help solve problems in high density cable management with high efficiency. It is a more favorable solution compared to standard LC fiber patch cable. LC uniboot fiber patch cable can cut down fiber cabling spaces up to 50% and provide much easier polarity reversal solution. Different kinds of LC uniboot fiber patch cables are available for your applications, such as different cable lengths, different fibers and different cable jackets. You can choose the right one for your needs.

What Are Simplex, Half Duplex and Full Duplex?

When talking about fiber optic jumper cables, such as LC LC multimode duplex fiber cable or LC SC single-mode simplex fiber optic patch cable, we came across simplex and duplex, or full duplex. What are simplex and full duplex? They are two kinds of communication channels in telecommunications and computer networking, which provide pathways to convey information. In fact, other than simplex and full duplex, there is another communication channel named half duplex. These three communication channels are commonly used in telecommunication networks.

A communication channel can be either a physical transmission medium or a logical connection over a multiplexed medium. The physical transmission medium refers to the material substance that can propagate energy waves, such as wires in data communication. And the logical connection usually refers to the circuit switched connection or packet-mode virtual circuit connection, such as a radio channel. With communication channels, information can be transmitted without obstruction. In this article, a brief introduction to these three communication channel types will be given.

Simplex

A simplex communication channel only sends information in one direction. For example, a radio station usually sends signals to the audience but never receives signals from them, thus a radio station uses a simplex channel. And it is also commonly used in fiber optic communication. One strand is used for transmitting signals or for receiving signals. The good part of simplex mode is that its entire bandwidth can be used during the transmission.

Half Duplex

In half duplex mode, data can be transmitted in both directions on a signal carrier but not at the same time. At a certain point, it is actually a simplex channel whose transmission direction can be switched. Walkie-talkie is a typical half duplex device. It has a "push-to-talk" button which can be used to turn on the transmitter but turn off the receiver. Therefore, once you push the button, you cannot hear the person you are talking to but your partner can hear you. An advantage of half-duplex is that the single track is cheaper than the double tracks.

Full Duplex

A full duplex communication channel is able to transmit and receive data in both directions on a signal carrier at the same time. It is constructed by a pair of simplex links that allows bidirectional simultaneous transmission. Take telephone as an example, people at both ends of a call can speak and be heard by each other at the same time because there are two communication paths between them. Thus, using the full duplex mode can greatly increase the efficiency of communication.

Simplex Fiber Optic Cable vs. Duplex Fiber Optic Cable

A simplex fiber optic cable has only one strand of tight-buffered fiber inside the cable jacket for one-way data transmission. The aramid yarn and protective jacket enable the cable to be connected and crimped to a mechanical connector. It can be used for both multimode and single mode patch cable. For instance, single-mode simplex fiber optic cable is suitable for networks that require data to be transmitted in one direction over long distance. Different from simplex fiber optic cable, duplex fiber optic cable has two strands of fibers constructed in a zipcord style. It is often used for duplex communication between devices to transmit and receive signals simultaneously. Duplex fiber optic cable can be applied to all sorts of applications, such as workstations, fiber switches and servers, fiber modems and so on. And single-mode or multimode cable is also available with duplex cables.

Conclusion

To understand the operation of networking, you should at least know the concept of communication channels. Simplex, half duplex and full duplex are three modes of communication channels. Each one can be deployed for different applications. To make a cost-effective decision, you can choose the right fiber optic cable according to the channel mode that you need.

Solution for Fiber Patch Cable Bending

When you install fiber optic jumper cables, you should not bend them beyond their bend radius, for light may "leak out" when the fiber is bent. To install fiber optic jumper cables in tight spaces of high-density fiber patching areas in data centers, more cable bending are inevitably needed. As the fiber bends more acutely, more light leaks out (shown in the picture below). How to solve this problem? The answer is bend insensitive fiber patch cable. Bend insensitive fiber patch cable exhibits much lower optical power loss under bend conditions while remaining compatible with conventional cabling. What is bend insensitive fiber patch cable? This post will talk about this solution.

What Is Bend Radius?

To understand bend insensitive fiber patch cable, first you need to what bend radius is. Bend radius is the minimum radius one can bend a pipe, tube, sheet, cable or hose without kinking it, damaging it, or shortening its life. The smaller the bend radius is, the greater is the material flexibility. When installing fiber optic jumper cables, you need to be careful enough not to exceed the cable bend radius. Usually, if no specific recommendations are available from the cable manufacturer, the cable bend radius should be 20 times smaller than the cable's outside diameter when pulling the cable and 10 times the outside diameter when lashed in place. For example, while pulling a 2mm diameter cable, only a 40mm sweep is allowed; when lashed in place, make sure it's a 20mm sweep. For most of today's fiber patch cables, the bend radius is 30 mm. As we know, there are single-mode patch cable and multimode patch cord, and accordingly there are single-mode bend insensitive fiber patch cables and multimode bend insensitive fiber patch cables. These two kinds of bend insensitive fiber patch cables will be introduced below.

Single-mode Bend Insensitive Fiber Patch Cable

Single-mode bend insensitive fiber patch cables have been commercially available for a few years. ITU recommendation G.657 specifies two classes of single-mode bend insensitive fiber patch cables: G.657 A and G.657 B. Each category (A and B) is then divided into two sub-categories: G.657.A1, G.657.A2 and G.657.B1, G.657.B2. The minimum bend radius of G.657.A1 fibers is 10 mm, G.657.A2 and G.657.B1 fibers is 7.5 mm and G.657.B2 fibers is 5 mm. G.657.A1 and G.657.A2 fibers are fully compliant with ITU-T G.652.D fibers. Compared with minimum bend radius of the standard single-mode G.652 fibers, which is usually 30 mm, G.657 single-mode bend insensitive fiber patch cables are much more flexible, which thus can be confidently installed with a variety of installation methods and in the increasingly high-density application spaces of today's data center.

Multimode Bend Insensitive Fiber Patch Cable

Multimode bend insensitive fiber patch cables with a minimum bend radius of 7.5mm compares very favorably to the 30mm bend radius traditionally specified. To achieve this, an optical "trench" is added to the cladding area outside of the fiber core. This trench retains most of the light that would have escaped the core of a traditional multimode fiber. Requirements for a tighter bend radius have been developed based primarily on factors in the FTTH (fiber to the home) market. However, the benefits for premise markets have rapidly become apparent, particularly in data centers where more and more fibers are being installed in smaller areas. The expectation is that this new feature can enable deployment of multimode fibers in higher densities.

Conclusion

Bend insensitive fiber patch cables are made with solid trench which assists fiber optic cable to reduce optical loss when the cable is bent. They provide the same high quality, mechanical features and optical performance as standard fiber patch cords with the added capability of maintaining optical performance when bent or flexed. Bend insensitive fiber patch cables are available for multimode (OM2, OM3 and OM4) and single-mode (OS2) networks. Whether to choose single-mode bend insensitive fiber patch cables or multimode bend insensitive fiber patch cables, you can make a decision based on your applications.

Direct Attach Cable Assemblies for Data Center Interconnection

As direct attach cable assemblies use the same port as optical transceivers, but with significant cost savings and power savings in short reach applications, they are becoming more and more popular among data center operators. Direct attach cable assemblies are mainly used as media to support high transfer rates between servers, switches and storage devices in data centers. In this post, we will talk about usage of direct attach cable assemblies for data center interconnection.

Types of Direct Attach Cable Assemblies

Direct attach cable assemblies are terminated with transceiver-style plugs, such as SFP+ (enhanced small form-factor pluggable), SFP28, QSFP+ (quad small form-factor pluggable), QSFP28, and CXP, etc. Using the same port as transceiver optics, direct attach cables can support Ethernet, Infiniband and Fibre Channel but with independent protocols. In general, direct attach cable assemblies are divided into three families—direct attach passive copper cable, direct attach active copper cable and active optical cable (AOC).

Passive Direct Attach Copper Cable

Passive direct attach copper cables are without active circuitry component. They can achieve interconnections up to 7m at 10 Gbps or 40 Gbps with low power. Direct attach passive copper cable assemblies, like HP J9281B SFP+ passive direct attach copper cable, offer high-speed connectivity between equipment. They are compatible with hubs, switches, routers, servers, and network interface cards (NICs) from leading electronics manufacturers like Cisco, Juniper, etc.

Active Direct Attach Copper Cable

Active copper cables are designed in the same cable type as the passive one, but they contain low power circuitry in the connector to boost the signal and are driven from the port without additional power requirements. The active version provides a low cost alternative to optical transceivers, and are generally used for end of row or middle of row data center architectures for interconnect distances of up to 15 meters.

Active Optical Cable

Active optical cable (AOC) incorporates active electrical and optical components. It can achieve longer distance than the copper assemblies. In general, active optical cable can reach more than 100m via multimode fiber. Compared to direct attach copper cable, AOC, like Cisco SFP-10G-AOC10M SFP+ AOC, weighs less and can support longer transmission distance. It is immune to electromagnetic energy since the optical fiber is dielectric (not able to conduct electric current). And it is an alternative to optical transceivers and it can eliminate the separable interface between transceiver module and optical cable. However, it costs more than copper cable.

In addition, with the fan-out technology, both direct attach copper cable and active optical cable can be designed as breakout direct attach cable assemblies, like 40G QSFP+ to 4x10G SFP+ AOC, which can better satisfy the demands on network migration.

Direct Attach Cables for Data Center Interconnection

Direct attach cable assemblies are ideal choices for short-reach direct connection applications. Generally, they are used in the EDA (Equipment Distribution Area) where cabinets and racks house end equipment (servers) and where horizontal cabling from the HDA (Horizontal Distribution Area) are terminated at patch panels, as shown in the following picture:

For interconnection in racks and between rows of racks, direct attach cable assemblies are used to connect server to switch, storage to switch or switch to switch. Depending on different interconnect applications and distance requirements, direct attach copper cables, passive or active, active optical cable, or breakout direct attach cable assemblies with various length options can be used.

As 10G network is widely deployed in today's data center, 10G SFP+ DACs are commonly used in interconnect applications below 15m, such as server to switch or storage to switch interconnection in the same rack. And now 25GbE is popular and 25G direct attach cable assemblies, such as SFP28 DACs, are already available in the market. For 40GbE, 40G QSFP+ DACs and AOCs are used. Of course, higher speed and more bandwidth are needed for spine switches. Thus, 100G DACs, like QSFP28 DACs are used in this case.

Conclusion

There are a wide range of direct attach cable assemblies, including both direct attach copper cable assemblies and active optical cables which cover data rates of 10G, 25G, 40G, 100G and 120G. You can choose SFP+ direct attach cables for your 10G networks, or QSFP+ direct attach cables for your 40G networks. And both copper and optical fiber options are available.

Which One to Choose for FTTH Drop Cable Installation, Splices or Connectors?

When deploying an FTTH network, you must choose the right drop cable interconnect solution. This is for both ends of the drop cable—the distribution point and the home's optical network terminal (ONT) or network interface device (NID). There are two basic options: splices (permanent joint) and connectors (easily mated and unmated by hand). They both are widely used at the distribution point, but at the ONT/NID, a field-terminated connector or a spliced-on factory-terminated connector is used. How to choose? This post will discuss these two interconnect solutions for FTTH drop cables and their own pros and cons.

Splices: Pros and Cons

Pros: Splices can make sure excellent optical performance. Splicing can eliminate the possibility that the interconnection point becomes dirty or damaged. A dirty or damaged point may potentially compromise signal integrity, as may happen to a connector end face when it is handled while unmated. Contaminants will cause high optical loss or even permanently damage to the connector end face. And splice enables a transition from 250µm drop cable to jacketed cable.

Cons: Splice lacks operational flexibility. To reconfigure a drop cable at the distribution point, one splice must be removed, fibers rearranged, and new fibers spliced, which requires the technician to carry special splicing tools for simple subscriber changes. During this time, other customers' service may be disrupted by the fiber-handling process. 250µm fiber cable is usually used at the distribution point, which is easily bent and then causes high optical loss or even break the fiber. Besides, if a splice is used at the ONT, a tray is needed to hold and protect the splice, which increases the ONT size and potentially the cost.

Connectors: Pros and Cons

Pros: Connectors can be mated and unmated repeatedly, which makes them provide greater network flexibility. For example, to connect or disconnect an SC to ST fiber cable connection, without any tools, a technician can just easily plug or pull out the SC connector and ST connector on two ends. Connectors can also provide access points for networking testing.

Cons: Material cost is connectors' the most obvious downside. They cost more than splices, although network rearrangement with connectors is much cheaper. Providers need to weigh connectors' material cost and their potential for contamination and damage against the great flexibility and low network management expense.

Choose the Right Splice

Splice is appropriate for drops where there is no need for future fiber rearrangement, typically in a greenfield or new construction application where all of the drop cables could be easily installed during the living unit construction. Once the decision goes to splices, the type of splicing (fusion and mechanical) must be determined.

Fusion splicing has been the de facto standard for fiber feeder and distribution construction networks. Fusion splicer is used for FTTH drop splicing as it provides a high quality splice with low insertion loss and reflection. However, the initial capital expenditures, maintenance costs and slow installation speed of fusion splicing hinder its status as the preferred solution. Fusion splicing is suitable for companies which have invested in fusion splicing equipment and have no need to purchase additional splicing machines. Mechanical splices are successfully deployed around the world in FTTH installation, but not popular in United States because the index matching gel inside the splices can yellow or dry out, resulting in service failures. Great strides have been made in improving gel performance and longevity over the last 20 years.

Choose the Right Connector

Connectors could be used to connect different subscribers as needed for distribution points. It must be installed at the ONT and then offers flexibility both at the curb and at the home. Once the decision goes to connectors, factory-terminated connectors or field-terminated connectors must be decided.

Factory-terminated cables, such as LC to LC cable or SC to LC cable, which means the connectors you specify are pre-terminated for you, provide high-performance and reliable connections with low optical loss. By reducing installation time, factory termination keeps labor costs low. But factory-terminated cables are expensive compared to field-terminated alternatives. And they require a cable management system to store slack cable at the curb or in home. The installation of field-terminated connectors can be customized by using a reel of cable and connectors. Fuse-on connectors use the same technology as fusion splicing to provide the highest level of optical performance in a field-terminated connector. Mechanical connectors provide alternatives to fuse-on connectors for field installation of drop cables. Depending upon service provider requirements and living unit configurations, a hybrid solution of a field-terminated connector on one end of the drop cable and a factory-terminated connector on the other may be the optimal solution.

Summary

The drop cable interconnect solution is a key component of an FTTH network. Selecting the right connection option not only offers cost savings and efficient deployment but also provides reliable service for customers. Many FTTH drop cable installations have been field terminated on both ends of the cable with mechanical connectivity solutions.

What Affects the Optical Transmission Distance?

Optical network has now been more and more popular now, for it has several advantages, such as high speed, high bandwidth and high density. And fiber optic cables can support much longer distance than traditional copper cables (like twisted pair cables or coaxial cables). However, in practice, the distance that fiber optic cable can support is affected by many factors. Transmission distances of optical links vary from meters to hundreds of meters and kilometers, but optical signals may become weak over long distances. What affects the transmission distance? This article will talk about several factors that affect optical transmission distance.

Type of Fiber Optic Cable

Generally, the maximum transmission distance is limited by the dispersion in fiber optic cables. There are two types of dispersion that can affect the optical transmission distance—chromatic dispersion and modal dispersion. Chromatic dispersion is the spreading of signals over time resulting from different speeds of light rays. Modal dispersion represents the spreading of the signals over time resulting from different propagation modes.

For single-mode fiber optic cable transmission, it is chromatic dispersion that affects the transmission distance. The reason is that the core of a single-mode fiber optic cable is much smaller than that of a multimode optical cable, and only allows one mode of light to propagate. Single-mode optical cable can transmit signals over longer distance than multimode optical cable. Multimode optical cable transmission is largely affected by modal dispersion, because these optical signals cannot arrive simultaneously and there is a delay between the fastest and the slowest modes, which causes the dispersion and limits the performance of multimode fiber optic cables, as shown in the following picture.

Light Source of Fiber Optic Transceiver

Fiber optic cable is the path for the transmission of optical signals. However, most of the terminals are electronic based, so the conversions between electrical signals and optical signals are very necessary. And this process largely depends on fiber optic transceivers, which are commonly used in most of today's fiber optic networks. LED (light emitting diode) or laser diode inside are the light sources of fiber optic transceivers, which can also affect the transmission distance of a fiber optic link.

LED diode based transceivers can only support short distances and low data rate transmission. They cannot satisfy the increasing demand for higher data rates and longer transmission distances. For higher transmission data rates, laser diodes are used. Several commonly used laser sources in fiber optic transceivers are Fabry Perot (FP) laser, Distributed Feedback (DFB) laser and Vertical-Cavity Surface-Emitting (VCSEL) laser. For example, TRENDnet TEG-MGBSX SFP transceiver is a 1000BASE-SX SFP transceiver module with a VCSEL laser transmitter, which can support a data rate of 1.25Gbps and 550m transmission distance. The following table shows the main characteristics of these light sources.

Splices and Connectors

Splices or connectors are inevitable in many fiber optic systems. Signal losses can be caused when optical signals pass through each splice or connector. The amount of the loss depends on the types, quality and number of connectors and splices. For example, signal loss through an LC LC multimode duplex fiber cable may not be the same with an ST ST multimode duplex fiber cable.

Frequency of Transmission

As mentioned in the above table, different laser sources support different frequencies. The maximum distance an optical system can support is also affected by the frequency at which the signals are transmitted. Generally the higher the frequency is, the longer distance the optical system can support. Thus, choosing the right frequency to transmit optical signals is quite necessary.

Bandwidth

The bandwidth that fiber optic cable supports is another important factor that influences the transmission distance. As the bandwidth increases, the transmission distance usually decreases proportionally. For instance, a fiber that can support 100 MHz bandwidth at a distance of 5 kilometers will only be able to support 250 MHz at 2 kilometers and 500 MHz at 1 kilometer.

Summary

There are many factors that affect the optical transmission distance, such as the type of fiber optic cable, light source of fiber optic transceiver, splices and connectors, frequency of transmission, bandwidth that the network supports. All these factors need to be taken into consideration during the deployment of fiber optic networks to minimize the limitations on the transmission distance.

Why Choose 10GBASE-T for 10GbE Data Center Migration?

With the bandwidth migrating from 100M Ethernet to 1/10 Gigabit Ethernet (GbE), large enterprises have also been migrating their data center infrastructures accordingly. 10GbE technology has now been very mature and 10GbE in the data center is very common now. There are many interfaces options for 10GbE, such as SFP+ fiber, CX4, SFP+ direct attach copper (like HP J9281B SFP+ passive direct attach copper cable, or HP JG081C SFP+ passive direct attach copper cable), and 10GBASE-T. Among them, which one will you choose? Each one has its own advantages and disadvantages. In this post, we will talk about why choose 10GBASE-T in 10GbE data center migration.

Disadvantages of SFP+ in 10GbE Data Center

SFP+ has been adopted on Ethernet adapters and switches and supports both copper and fiber optic cables, which makes it a better solution than CX4. However, SFP+ optical transceiver (such as Cisco SFP-10G-LR-S 10GBASE-LR SFP+ transceiver) is not backward-compatible with the twisted-pair 1GbE broadly deployed throughout the data center. SFP+ connectors and their cabling were not compatible with the RJ-45 connectors used on 1GbE networks. Enterprise customers cannot just start adding SFP+ 10GbE to an existing RJ-45 1GbE infrastructure. New switches and new cables are required, which is a big chunk of change.

Advantages of 10GBASE-T in 10GbE Data Center

10GBASE-T is backward-compatible with 1000BASE-T, it can be deployed in the existing 1GbE switch infrastructures in the data centers that are cabled with CAT6, CAT6A or above cabling. As we know, 1GbE is still widely used in data center. Thus, 10GBASE-T is a great choice for gradual transition from 1GbE deployment to 10GbE. Other advantages of 10GBASE-T include:

Reach

Like all BASE-T implementations, 10GBASE-T works for lengths up to 100 meters, which gives IT managers a far-great level of flexibility in connecting devices in the data center. 10GBASE-T can accommodate either top of the rack, middle or end of the row network topologies, giving IT managers flexibility in server placement since it will work with the existing structured cabling systems.

Power

The challenge with 10GBASE-T is that even single-chip 10GBASE-T adapters consume a watt or two more than the SFP+ alternatives. More power consumption is not a good thing in the data center. However, the expected incremental costs in power over the life of a typical data center are far less than the amount of money saved from reduced cabling costs. Besides, with process improvements, chips improved from one generation to the next. The power and cost of the latest 10GBASE-T PHYs will be reduced greatly than before.

Reliability

Another challenge with 10GBASE-T is whether it could deliver the reliability and low bit-error rate of SFP+, or whether the high demands of FCoE could be met with 10GBASE-T. Cisco has announced that it had successfully qualified FCoE over 10GBASE-T and is supporting it on its newer switches that support 10GBASE-T in 2013.

Latency

10GBASE-T has a low latency range, from just over 2 microseconds to less than 4 microseconds. Latency for 10GBASE-T is more than 3 times lower than 1000BASE-T at larger packet sizes. Only the most latent sensitive applications such as HPC or high frequency trading systems would notice any latency.

Cost

When it comes to the cost, copper cables offer great savings. Typically, passive copper cables are two to five times less expensive than comparable lengths of fiber optic cables. In a 1,000-node cluster, with hundreds of required cables, that can translate into hundreds of thousands of dollars. Extending that into larger data centers, the savings can reach millions. Besides, copper cables do not consume power and because their thermal design requires less cooling, there are extensive savings on operating expenditures within the data center.

Conclusion

10GbE standards are very mature and reliable now. 10GBASE-T can save you a lot and at the same time provide you with easy cable installation. 10GBASE-T can also be backwards compatible with 1GbE networks, offering investment protection. Deployment of 10GBASE-T will simplify the networking transition by providing an easier path to migrate to 10GbE infrastructure in support of higher bandwidth needed for virtualized servers.