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What are the main components of MAN – Metropolitan Area Network?

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Computer network topologies have evolved from a bunch of hosts and servers that share resources in a clear defined space of a office building called Local Area Network – LAN,  to  the largest topology of networks called WAN’s – Wide Area Networks connecting dispersed geographically networks. The middle size networks are called MAN’s – Metropolitan Area Networks. The term is applied to the interconnection of networks in a city into a single larger network (which may then also offer efficient connection to a wide area network). It is also used to mean the interconnection of several local area networks by bridging them with backbone lines.
The diameter of such a network can range from 5 to 50 kilometers. As a rule, MAN does not belong to any particular organization, in most cases, a group of users or a provider who takes charge for the service own its connecting elements and other equipment. The transmission support for the MAN is represented by the links of fibre optic cable laid in a ring formation in a metropolitan area.
ATM (Asynchronous Transfer Mode) combines the characteristics of circuit switching and packet switching, which allows it to transfer even the real time data.
The MAN can be used to provide services including telecoms, Internet access, television and CCTV to businesses and citizens in these metropolitan areas.
Nowadays, MAN networks are connecting a number of Campus LAN networks which  are designed in hierarchical structure such as: access , distribution and core.
Next Generation MAN : The Metro Ethernet Solutions
Service providers use Metro Ethernet to provide Layer 2 Ethernet connections between customer sites in metro area networks. Driven by its relative simplicity, high bandwidth, and low-cost switches, Ethernet has become the transport technology of choice in metro area networks. There are numerous applications that require pure Layer 2 connectivity in the metro area network (MAN) for providing simple point-to-point, point-to-multipoint, or multipoint-to-multipoint services with a relatively low number of customer sites. However, Ethernet limitations become apparent in large MANs with thousands of access nodes. In this
case, service providers are more likely to offer Layer 3 Virtual Private Network (L3 – VPN) services based on multiprotocol label switch (MPLS) transport. When interconnecting hundreds or thousands of customer sites, this approach gives more flexibility, better scale, and ease of OAM.
MPLS is best and most widely used to interconnect data centers with branch offices and branches to other branches. Ethernet is best for interconnecting data centers. MPLS can handle any-to-any connectivity, including voice and video. Ethernet offers low-latency and high-throughput, which is ideal for disaster recovery. Using MPLS for WAN connectivity requires that all network devices and management tools be compatible with both MPLS and Ethernet. Because LANs use Ethernet, using Ethernet for the WAN gives organizations an all-Ethernet infrastructure, which simplifies network management.
For the above-mentioned reasons, operators are thirst for carrying all services over one network. Several networks coexist within the existing MAN, of which the IP MAN is the best choice. Nevertheless, the IP MAN also has some problems in handling these services, which can be explained in detail as follows:
The existing IP MAN is out of order, in which the Layer 2 switching and Layer 3 routing are mixed. So it cannot meet the requirements of providing QoS guaranteed services.
The IP MAN mainly implements L3 switching architecture. With a single access mode, poor access capability and without overall planning of service access points, this kind of access mode is hardly possible to support
“full-coverage full-service network”.
Highly positioned Provider Edge Router (PE) is a networking that cannot take the advantages of MPLS VPN technology. Layer 2 network too large, which is confined by the Virtual LAN (VLAN) ID resources and makes network troubleshooting difficult.
It causes serious waste of optical fibers and transmission resources. In order to attract more group users, IP MAN mainly employs the direct optical-fiber connection mode. Users are directly connected to PE. In some cases nearly half of the fibers in MAN are used only to connect group users, really being a great burden of carriers.
The IP MAN equipment has differentiated capabilities. Most IP MAN is unable to deploy either the QoS guaranteed services or the multicast services over the entire network.

Comparison of the construction costs of DWDM networks versus SDH networks

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The construction problem of multi-layer network design for a high-speed telecommunication network based on Synchronous Digital Hierarchy (SDH) and Wavelength Division Multiplex (WDM) technology, has to carry a certain set of demands with the objective of minimizing the investment in the equipment.
It all start with the design phase – where the effective costs of either optical SDH or DWDM networks has to be competitive.
Design processA network design method proceeds by generating cycles, evaluating the economics of building rings on those cycle, and building any economic rings. Generating a cycle involves picking two endpoints between which two disjoint link and node paths are desired — the two nodes selected are thus nodes on the candidate rings.
In a telecommunications network, a “ring” is a sequence of nodes arranged in a “cycle” so that no node is repeated. The “links” between nodes are places where fiber can be placed. Nodes are generally physical locations such as buildings where fiber bundles can be connected to each other and where equipment such as multiplexers, amplifiers, regenerators, transponders, etc., can be placed. Ring design entails in part the making of decisions as to ring placement, i.e., which nodes and which links are to be included. Ring design also concerns the selection of equipment, i.e., what types and rates of multiplexers, amplifiers, regenerators, transponders, etc., and where to place the equipment. Finally, ring design necessarily entails decisions as to what demand to place on the rings.
The models used for SONET/SDH provide for the following costs and parameters:
frame and installation,
regeneration loss thresholds,
maximum number of SONET ADMs on a ring, and
fiber material, sheath installation, and structure expansion cost.
Currently, Dense Wavelength Division Multiplexing (DWDM) is being installed largely on long-distance routes. The DWDM vintage normally used is point-to-point DWDM, or in other words, DWDM systems are utilized as fiber concentrators. The reason for this equipment being so prevalent for long-distance carriers is simple economics: DWDM can substantially reduce capital investment because of the ability to multiply the number of signals being carried by each fiber and thus avoid expensive cable or route upgrade and also save the cost of multiple regenerators.
DWDM is multiple signal transmit over a single fiber called DWDM or Different frequencies (colors/wavelengths/lambdas) for different connections over the single fiber. Full featured DWDM equipment can comprise the same range of cards as SDH. They can support fully configurable cross connect features. DWDM technology provides very high bandwidth long haul inter-connect links. DWDM is considered as one of the best technologies to increase bandwidth over an existing fiber plant. It enables one to create multiple “virtual fibers” over one physical fiber.
The DWDM layer is protocol and bit rate independent, which means that it can carry ATM (Asynchronous Transfer Mode), SONET, and/or IP packets simultaneously. WDM technology may also be used in Passive Optical Networks (PONs) which are access networks in which the entire transport, switching and routing happens in optical mode.
The differences between demand types mostly are caused by the design efficiency of these two technologies’ interface cards in terms of density and price.
Cost from IP – Internet Protocol – traffic approach
IP traffic is growing exponentially as customers migrate to IP-based applications. As these networks evolve to include bandwidth-intensive IP based voice, video, and data services, carriers must boost capacity in response to demand, knowing that the collected revenue will not scale at the same rate. Therefore, carriers must find ways to optimize the operating and cost efficiency of service networks and drastically reduce costs per bit.
Traditionally this was implemented using (IP) Internet Protocol over SDH approach which has the inconvenient of the optical to electrical to optical (OEO) conversion at the aggregate interfaces. The IP over DWDM is practically implemented as connection between DWDM router interfaces with an optically switched DWDM layer.

How Are Optical Fibers Patch Cords Manufactured?

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Since the Internet started changing people’s lives engineers have been constantly trying to find a way to provide the highest performance possible to their customers. In addition they have been focusing on providing the best possible service to their businesses. One of the main and most important developments in the Internet Era has been the optical network technology. This technology provided a huge leap forward for every Internet user and has been the main foundation of today’s high performance demanding businesses and Internet users.
This technology is based on the optical fiber which is the main part of optical networking. The optical fiber is defined as a single, hair thin, filament drawn from molten silica glass. These optical fibers have substituted the copper wires to produce high performance and high capacity transmission. They are optimized and pure so that light transmitted by optical devices like transceivers, can pass through them carrying the network traffic across a network architecture. The transceivers are the ones converting this light in electrical input and vice-versa, so various switches, routers, firewalls etc. can understand the traffic.
The main ingredient of the optical fibers is a chemical called silicon dioxide (SiO 2). In addition there are other chemical compounds found such as germanium tetrachloride (GeCl 4) and phosphorus oxychloride (POC1 3), however these are mainly used to produce the outer layer of the fiber, also known as the cladding. In the early days of this technology researchers were trying to connect the purity of the glass used to the attenuation of the signal and because in recent years this has been proved, the main focus today is developing optical fibers made from silica glass with the highest possible purity. One of the most important part of the composition of the glass is the fluoride content. It has been confirmed that glass with high fluoride content is improving the overall performance due to its purity along the whole fiber. This makes it suitable for deployment in multi-mode solutions because of the fact that multi-mode fibers transmit hundreds of discrete light wave signals concurrently.
In optic network architectures light travels across many individual optical fibers which are bound together around a high-strength plastic carrier for support. This is also called the core of the cable. In addition the core is then covered with a couple protective layers to protect it from outside stress. The protective layers are mainly made of Aluminum, Kevlar, and polyethylene which is the main ingredient of the cladding. The cladding plays a very important part of the network. This is mainly because light will constantly bounce off of it while traveling across the optical fiber. The amount of energy lost from the bouncing is called attenuation. The attenuation is measured in terms of loss (in decibels, a unit of energy) per distance of fiber. A high quality optical fiber should not lose more than 0.3 decibels per kilometer. This attenuation causes the light to lose power eventually therefore the signal must be repeated and strengthened with the help of laser repeaters. In today’s high performance networks these laser repeaters are deployed at every 30 kilometers in average. However, the good news is that recent studies showed that the newly developed ultra-pure glass will eventually provide the optical fiber to reach the 100 kilometer mark without the need of a laser repeater.
As with every electronical device found today, the manufacturing process is one of the most interesting parts of the whole picture. When it comes to optical fibers there are two methods in manufacturing them and each of those methods has its own purpose. To produce a multi-mode fiber where multiple light waves will pass through it and bounce off the cladding, reducing in shorter reach, the so called crucible method is used. This is the easier and simpler method out of the two because simply put the silica glass is melted and shaped to produce a fatter optical fiber.
The second method is called a vapor deposition process. Researchers developed three different vapor deposition techniques:
Outer Vapor Phase Deposition
Vapor Phase Axial Deposition
Modified Chemical Vapor Deposition (MCVD)
The most common technique used at present is the MCVD technique. With this technique a solid cylinder of core is produced and cladding material is put on top of it. After that process the core is heated and drawn into a thinner, single-mode fiber for long-distance communication. The step by step process shown below is far more interesting:
By depositing layers of specially formulated silicon dioxide on the inside surface of a hollow substrate rod, a cylindrical shape is formed. The deposition happens by applying pure oxygen, in gas form, to the rod. Together with the vaporized gas a couple of important chemicals are added including silicon tetrachloride (SiCl 4), germanium tetrachloride (GeCl 4) and phosphorous oxychloride (POC1 3). With the help of underneath flames the surface of the rod is kept constantly hot and when the oxygen contacts the rod a high purity silicon dioxide is formed inside the rod itself. This high purity silicon dioxide is the basis of the fiber optic core.
The second process of this technique starts by measuring the thickness of the formed silicon dioxide inside the rod. When the expected thickness is reached the rod will be put under a couple heating procedures to remove excess bubbles and moisture trapped inside. After this second step the formed silicon dioxide is usually 10 to 25 mm in diameter.
The solid shape of silicon dioxide is then transferred to an automatic fiber drawing system. This system can be up to two stories high and has the ability to produce continuous fibers of up to 300km.
In the above system the fiber first passes through a furnace where will be heated up to 2000 degrees Celsius. As the fiber is being pulled through the system the material in the original substrate rod forms the outer layer called the cladding.
As the fiber is pulled and drawn out, special sensors monitor its diameter and at the same time a separate device applies a protective coating on top. The process ends when the optical fiber reaches the desired thickness and is then sent to quality control.
It is safe to say that this process is the foundation to producing ultra-pure optical fibers. Today researchers try to find another solution which will offer even lower attenuation. They focus their hope on experimental fibers which are high in zirconium fluoride (ZrF 4) in content. These fibers have been tested and their attenuation results are astonishing, providing performance loss of only 0.005 to 0.008 decibels per kilometer. When these fibers enter in production and reach the networking market, they will provide a huge window to the future and honestly, we can’t wait for that to happen!

25G SFP28 Cable – Best for TOR Server Connection?

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During the past few years, there is a dramatic increase in the demand for bandwidth requirement for our communications.  Whether be in a communication service provider or in a public or private data centers, a development in connectivity that can cater higher speed, bandwidth is needed. That is why last July 2014, an industry consortium was formed to create a new Ethernet connectivity standard in data centers. This standard was called 25 Gigabit Ethernet or 25Bade-T, developed by IEEE 802-3 task force P802.3by. This standard was derived from the 100Gbe, however, its operation works as a four 25Gbps that are running on four fibers or coppers. Last June 2016, this technology was commercially released using new interfaces called SFP28 and QSFP28. This article will discuss about the SFP28.
The SFP28 was constructed in a four parallel 25Gpbs data lanes allowing a maximum rate of 100Gbps. This physical structure of the SPF28 is the same with the popular SFP and SFP+. This characteristic provides flexibility due to the fact that the 100Gbps can also be divided individually in to four 25Gbps connections. SFP28 uses a 28Gbps lane (25Gbps + error correction) specifically used for top-or-rack (TOR) switch to server connectivity. Moreover, SFP28 is available in both copper and fiber optic cables.
The copper cable version is manufactured in a single fixed-configuration module which means the copper cables are directly attached to an SFP+ module. This version is ideal to be used for short distances ranging from 1m to 5m. On the other hand, the optical fiber version functions in either an 850nm that utilizes a pair of multimode fiber and works to a maximum distance of 100m or in a 1320nm that is made with a pair of single mode fibers works up to 20km.
The development of 25G SFP28 has provided a wide range benefits especially in a web-scale data center environment where the trend is to toward a single port server due to cost.
Primarily, it gives way to efficiently utilize data and switch port density. The reason for this is that, existing 100G port can be used as a 4x25G with as QSFP to SFP28 break out cable instead of using for different ports. For example, a 25Gbe strand can provide 2.5 times more data than the popular 10G solution and can provide greater port density.
Moreover, it provided an extremely efficient increase in speed to server to top-of-rack(TOR) especially when using the Direct Attached Copper assembly. It also simplifies development of interoperability specification and system due to the fact that its backward compatibility and gives an easier upgrade path from an existing 10G ToR server configuration.
Furthermore, using 25G SFP28 for ToR servers are more economical. This is because it can provide higher port densities, fewer ToR switches and cables are needed. It allows a more cost-effective alternative top-of-rack server connection that uses point-to-point patch cords. It enables End of Row(EoR) or Middle of Row (MoR) by using the 30-meter structured cabling. As a result, it reduces the capital expense in the construction cost compared to other configurations like the 40GbE.
Ultimately, the 25G SFP28 assemble features a reduced power and smaller footprint requirements for data centers because it limits the power per port to under 3W.
Due to this benefits that the 25G SFP28 assembly provides, it is forecasted that it will be popular in the years to come. It is believed that the dominant next generation server connection is toward the 25Gbps speed in server and in the near future, there will be more equipment that will use the 25G SFP28 cable assembly.

How does bending effect a Fiber Patchcord?

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Fiber optic cables by their nature and the way they are manufactured are designed to be enduring the stress applied to them during their installation and maintenance, however as they are made of glass and are highly fragile it is highly advisable by the manufacturers to lower this stress to a minimum. Bending the fiber cables and the amount of quality loss depends on the type of the cable, if it’s Single- mode or Multi- mode cable, their design, their core diameter and their transmission wavelength. Usually longer wavelengths are more sensitive to stress and bending losses.
The process of bending or pulling loss starts inside the cable as the optical signal within the cable is not guided through the core of the fiber, instead a big part of the light itself is lost and bouncing in the walls and the cladding in the cable thus creating a high loss in optical light. Bending would most probably permanently damage the fiber cable by causing cracks in it. This would compromise the quality of the signal and the integrity of the data transmission. This is easily put to the test with the help of a visible laser put in the fiber itself and bending it at a certain point. The light loss will be visible where the cable is being bent.
During the last couple of years manufacturers and the Fiber Optic Association started developing a new type of cables that are more durable and can withstand higher stress and bending. This was firstly developed for the Single- mode fibers and after a couple of years for the Multi- mode fibers. The way they were testing the bending and the endurance of the cables was with the help of a piece of wood and bending the cable around it in front of a wide audience.
The bending of the fiber optic cables is measured by the bend radius. Only in the last couple of years this bending radius has been industry standardized by the Fiber Optic Association. In contrary before it was standardized the bending radius have been governed by the cable manufacturers. The new standard defined by the ANSI/TIA/EIA-568B.3 named “Optical Fiber Cabling Components Standard” sets exact performance specifications concentrated on the minimum bend radius and the maximum pulling tensions for 50/125 micron and 62.5/125 micron fiber optic cables. With the new standard introduced the manufacturers have the obligation to specify the minimum bending radius to which the cable could be safely bent during the installation. Most commonly the minimum bend radius of 1.6mm and 3.0mm fiber cables is around 3.5cm and the minimum bend radius for patch cable is around ten times the cable diameter. If referring to the manufacturer’s recommended bend radius is not possible, the general guideline for cable bending is no more than 20 times the diameter of the cable itself.
There are two types of bending radius: micro bends and macro bends. As the name suggests macro bends are larger than micro bends. Even though the two terms are very similar there is a significant difference in differentiating them. Macro bends are usually the bends that would be visible by the naked eye and micro bends are small microscopic deviations along the fiber itself.
However, it doesn’t take much for a micro bend to happen as it could be also caused by the fiber coating squeezing the cable because of very low temperatures. There is a standardized micro bend test procedure defined by the Fiber Optics Association named “FOTP-68 Optical Fiber Micro bend Test Procedure”. One way of developing and manufacturing more micro bend enduring fiber optic cables is by applying several layers of primary coating which would eventually protect the fibers of being bent.
Macro bends on the other hand, as aforementioned, is tested by wrapping the fiber cable around a specific material of a specified diameter. The standardized macro bend testing defined by the Fiber Optics Association is called “FOTP-62 IEC 60793-1-47 Measurement Methods and Test Procedures – Macro bending Loss”.
Another aspect of the bend radius that would affect the fiber cable performance is the path of the patch cable. This should be clearly defined by the manufacturer. If this is not properly done it would cause increased congestion in the termination panel possibly violating the band radius threshold. The patch cable should be easily accessed, easier to be maintained at all points of its path. Because the patch cables are commonly kept together with cable ties, manufacturers advise these cable ties be used with caution. Tightening the cable ties with an installation tool is harmful to the fiber optic cables and could very easily cause a full fiber breakage. Manufacturers advise the cable ties to be hand tightened but in the same time to leave them loose enough to be moved along the cable by hand.
The patch cable path should be well-defined and reduce the risk of stressing the cable. This way the patch cable path would be easier and quicker to be accessed by the engineer for maintenance works. The reduced fiber twists would ensure the optic light in the cable travels in the core of the cable thus minimizing the escape through the walls and the coating of the cable.
As the proper fiber management would affect the network’s reliability, performance and the cost, a well-defined cable paths could ensure a safe ground for future maintaining and network upgrading.

What interconnection solutions are available for QSFP28?

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The Quad Small Form-factor Pluggable (QSFP) is a compact, hot-pluggable transceiver. The data rates are from 4×1 Gb/s for QSFP and 4×10 Gbit/s for QSFP+  and to the highest rate of 4×28 Gbit/s known as QSFP28[3] used for 100 Gbit/s links.

The QSFP28 standard is designed to carry 100 Gigabit Ethernet, EDR InfiniBand or 32G Fibre Channel. This transceiver type is also used with direct-attach breakout cables to adapt a single 100GbE port to four independent 25 gigabit ethernet ports (QSFP28-to-4x-SFP28). Sometimes this transceiver type is also referred to as “QSFP100” or “100G QSFP”  for sake of simplicity.

QSFP28 transceiver not only have the same physical size as the QSFP+ used for 40G traffic, but the lowest power consumption among those that are capable of handling 100G traffic.

Basically, there are two types of transceivers: QSFP28-SR4 and QSFP28-LR4.

QSFP28-SR4 transceivers is specially designed to support connections of up to 100 meters over multimode fiber. This approach is similar to using AOC cables, but here it is possible to use structured cabling. They use more expensive non-standard MPO (multi push-on/pull-off cable) connectors which cancel out some of the cost savings of the transceiver.

QSFP28-LR4 versions support connections up to 10km over single-mode fiber. They use standard LC connectors and the existing structured LC cabling.

QSFP28 Cable Assemblies

QSFP28 cable (DAC or AOC cables) is the more convenient, low-cost method of connecting 100G equipment. Using cable assemblies removes many of the problems associated with dirty connectors. DAC is suitable for applications within 15m and AOC up to 70m. AOC cable assemblies provide similar performance to discrete transceivers and fiber cables.

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.

The main difference between active DAC and passive DAC is that there is a driving chip in the design of active DAC.

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 (eg. Cisco SFP-10G-AOC10M) 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. 100GbE QSFP28 AOC is composed of an OM4 multimode cable connecting two QSFP28 connectors on each end. 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).

Advantages of Active Optical Cables

The AOC assemblies provide the lowest total cost solution for data centers by having the key advantages as following:

  • Low weight for high port count architectures;
  • Small bend radius for easy installations;
  • Low power consumption enabling a greener environment.

For the 100G longer distance, the CFP and CFP2 offer DWDM Coherent technology and enable multi-channel long distance connectivity of more than 1000km. One thing we can’t miss is that the CFP is too big to be used in an Ethernet switch in volume.

Fan-out cable or breakout cable is considered as one of the the latest enabling technologies to help increase port densities and lower costs. Taking one (large bandwidth) physical interface and breaking it out into several (smaller bandwidth) interfaces, it has been highly recommended to be used in network migration. Breakout cables are also possible on most 100GbE QSFP+ ports where each of the 4 optical lines are broken out to 4 individual 25GbE or 10GbE interfaces. This solution requires either the deployment of a breakout cable that has 4 physical 25G / 10G endpoints, or the use of a breakout mux where an SR4 optic with MPO / MTP cable is deployed.