By Guilherme Salvadori – Pre-Sales Engineer
In telecom projects for metropolitan networks, the issue of link capacity versus link distance is always one of the biggest dilemmas for operators and ISPs. It is not always possible to reach the final destination with direct fiber, sometimes it may be necessary to open intermediate Points of Presence to do the signal regeneration. However, the use of intermediate POPs is not always feasible, it may be due to cost or unavailability of an appropriate location for the installation of the equipment.
To solve this type of challenge, one of the possible solutions is to use DWDM systems, as we have already explained in the Datacom blog in the articles: Extend the capacity of your optical link in a practical way and New solutions for the expansion of 10Gb optical links.
However, even using market DWDM systems multiplexing Nx high throughput channels, sometimes the distance may still fall short of the project. In the case of Datacom solutions presented in the articles cited above, it is possible to have links of up to 110km with 8 10Gb channels, 95km with 16 10Gb channels or 80 km with 32 10Gb channels.
In order to bring more alternatives for high bandwidth applications over long distances, Datacom is launching the new model of the MPLS DM4770 switch family, the DM4770 16CX.
The highlight of the DM4770 16CX is the 4 100GbE interfaces that allow the use of QSFP-DD Coherent modules, high-tech transceivers that enable links with extended reach, as well as compatibility with DWDM systems.
In practice, using the DWDM solution from the previous articles (Mux + EDFA) together with a pair of DM4770 16CX switches equipped with QSFP-DD Coherent, it is possible to have a 400GbE link reaching 190km on a single pair of fibers. Or, if we only consider 1 port 100GbE of this same link, using EDFA amplification, we can have a link with 210km.
This switch has 12 40 / 100GbE ports on QSFP28 connectors, 4 40 / 100GbE ports on QSFP-DD connectors and 4 more 10 / 25GbE SFP28 ports, with all L2, L3, MPLS, filter and QoS packet switching performed in hardware, ensuring wirespeed operation without loss of performance. The power supplies are redundant AC or DC and have hot-swap ventilation modules. All of this in a device that is only 1U high.
In terms of software, the DM4770, as well as the latest releases from Datacom, is shipped with the DmOS operating system. For those who follow our products, you know that DmOS has all the main features that the market requires for switches of this size.
In the DM4770 16CX you will find features of MPLS L2 and L3 VPNs, MPLS with RSVP and also Link Agregation with dynamic traffic balancing mechanisms - DLB for example. The equipment also has dynamic routing protocols such as BGP and OSPF, among other features that can be consulted in the DmOS datasheet.
QSFP-DD Coherent Module
As the bit rate per channel increases beyond 10Gb, fiber deficiencies that degrade transmission performance and signal quality also increase. Chromatic dispersion, polarization mode dispersion (PMD), optical noise and non-linear effects, become significant problems.
Coherent systems are able to extract information from amplitude, phase and polarization, allowing the use of two orthogonal polarization modulation formats. Using digital signal compensation techniques, it is possible to reduce some of the effects that affect optical transmission, such as chromatic dispersion.
Let's take an industry standard 10Gb SFP + module as an example. These modules generally have a chromatic dispersion tolerance of around 1600ps / nm. The chromatic dispersion compensation module that Datacom has in its portfolio (DCM40), expands this tolerance to 2400ps / nm, which allows reaching a distance of up to 110km together with the optical amplification. This means that sometimes the link still has the power budget to go a distance greater than 110km, but the chromatic dispersion tolerance (around 18 ps / nm.km in the 1550nm window) ends up limiting the distance of the link .
The QSFP-DD coherent module, on the other hand, has a chromatic dispersion tolerance of 5000ps / nm, minimizing this effect in link projects. Theoretically we would not have a dispersion problem with this module, the link limiting would be only its power budget.
With the DM4770 16CX using coherent QSFP-DD modules, what we have in practice is the possibility of establishing 100GbE point-to-point links of up to 115km considering 3dB of margin, without the need for amplification and chromatic dispersion compensation:
It is also possible to amplify a point-to-point link with Datacom's DM936 EDFA amplifier solution. In this scenario, it is possible to reach an incredible 210km, considering 3dB of margin:
The QSFP-DD module is tunable, that is, it is possible to tune it in several channels in the C band of the DWDM (functionality that will be available in version 6.4 of the DmOS in Q4 / 2021). This makes it possible to use the DM4770 16CX in DWDM systems, allowing to pass multiple channels of 100Gb on a single pair of fibers.
The example below shows an application in conjunction with Datacom's 8-channel passive DWDM (DM936 8CH33), allowing for an 800Gb link at 96km (with 3dB margin) over a single optical fiber pair.
It is still possible to improve this application with a DM936 EDFA amplifier from Datacom, which can reach a distance of up to 190km!
With Datacom's passive 32-channel DWDM (DM936 32CH19), it is possible to transmit 32 channels of 100Gb, that is, 3.2Tbps in a single pair of fibers, up to a distance of 80km without amplification.
With EDFA amplification, it is possible to reach 32 channels of 100Gb up to 152km:
It is also possible to combine 10Gb and 100Gb channels in the same DWDM system, for this, it is necessary to calculate the link in order to consider how many channels of each capacity will be present in the DWDM. This is an excellent alternative for those who already have existing DWDM equipment in their network and want to further expand the bandwidth capacity by inserting 100Gb channels into the link already deployed (just recalculate the system).
Note: All the calculations presented are made considering fiber with attenuation of 0.2dB / km
For solutions involving DWDM and amplification, we suggest consulting Datacom's pre-sales support, which will do this dimensioning properly: firstname.lastname@example.org.
In addition to long distance solutions and high density transport of 100Gb channels, the DM4770 16CX is ideal for performing the aggregation of high speed ethernet traffic (Nx 100GbE) in the Core of the network of operators and internet providers, as well as the interconnection of servers in Datacenters.
The DM4770 16CX is also manageable through DmView, which is Datacom's Management Software. With DmView it is possible to supervise and configure Datacom equipment, providing functions for supervisory management, failures, configuration, provisioning, auditing, performance, security, inventory, network discovery and topological maps. The Enterprise Lite option (up to 15 elements) is free, and is available for download on the Datacom support self-service website (ask for a user on the website for access, if you do not have access yet): supportcenter.datacom.com.br.
In addition to the DM4770 16CX, Datacom has a complete line of MPLS switches, interoperable with equipment from the largest manufacturers in the market, which include the DM4360, DM4370, DM4000, DM4100, DM4170, DM4270 and DM4380.
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