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between data centres. In fact, the connections are much more diverse, not only in number, but also in terms of variance in speed and access type – whether enterprise location connections, mobile network connections or enterprise and consumers connected through Internet service provider connections. In most cases, the EDCI connection bandwidth requirements are lower than DCI bandwidth requirements. Specific data centre applications also place requirements on the EDCI. An example is the requirement for low jitter and latency, and a high-bandwidth connection required

ROADMs can best manage high bandwidth interconnection at lower cost points

for consumers watching streaming video service in real-time versus the much lower bandwidth required for a Google search instance. Te majority of end users are connected to the

cloud via services from CSPs. Note that these services are networking type services and are not necessarily ‘hard linked’ to specific cloud services, but instead, are probably part of an Internet access service. Tere are exceptions to this for some enterprise cloud-based services built as private networks where connections to end users may be included in a common network infrastructure. Unlike DCI connections that are in the 10–100 gigabit per second range, many of these EDCI connections range in the much lower megabit per second range. Common CSP services connecting end users include Carrier Ethernet, VPN, Internet access and mobile cellular data service.

Data centre operator DCI

Internet content provider (ICP)

Carrier service provider (CSP)

Carrier neutral provider (CNP)

Primarily point-to-point DWDM but expected to evolve to ROADM based DWDM solutions

Primarily ROADM- based DWDM

Both point-to-point and ROADM-based DWDM

Transport hardware for EDCI includes DWDM

packet transport systems, Carrier Ethernet equipment, MPLS-TP based packet transport nodes, DSL systems, cable data modem systems, mobile wireless solutions and IP/MPLS routers. It is also common to have a combination of platforms and technologies, with an edge platform feeding a packet-optical transport system for aggregation and core transport and routers at the data layer for support of switched VPN services. ROADM-based DWDM packet optical transport

can meet the requirements for EDCI, DCI and the varying requirements from the different data centre operators, both today and as they evolve. DWDM transport systems are suited to handle the large bandwidths required in data centre applications, but it is the addition of flexible ROADM capabilities that provides the next level of value.

How ROADMs add value ROADMs are DWDM optical components that enable wavelengths to be added, dropped and multiplexed locally or passed through, all under soſtware control. In terms of network architecture, multi-degree ROADMs enable optical express where wavelengths can be passed through optically, without an optical-electrical-optical (OEO) regeneration, to dramatically reduce the power consumption and cost of optical transport. Tere are various ROADM architectures with different levels of optimisation and flexibility to support a wide range of applications in terms of capacity and resiliency requirements. Essentially

there are three key types: l

Coloured ROADM – uses a broadcast-and-select architecture enabling optical express wavelengths from any degree to be passed to any other degree, but has limitations for local transponder add/drop wavelengths of fixed ports for specific wavelengths and fixed degree


Use CSP for interconnection of end users

ROADM-based DWDM and other access network technologies

Primarily use CSP or build their own

ROADM-based DWDM or Carrier Ethernet solution


Both point-to-point and ROADM-based DWDM

Use CSP or build their own ROADM-based or Carrier Ethernet solution

If applicable, ROADM- based DWDM

Table 1: How different types of data centre operators could use ROADM technology to support DCI and EDCI transport networks as well as combined applications

Both/common infrastructure

Not applicable l

direction tied to fixed ports. Tis technology is the most cost-effective type of multi-degree ROADM, but has limits on flexibility that may require manual intervention to change how transponders are connected.


Colourless and directionless (CD) – based on route-and-select architecture and adds the capability that any transponder add/drop wavelength can be assigned to any add/drop port or degree of the ROADM enabling changes to be made via soſtware for how transponders are assigned to wavelengths and degrees. CD ROADM architectures are more expensive than coloured ROADM architectures, but the flexibility offered proves in for larger, more mesh-based network architectures. Another important capability of CD ROADMs is support for flexible channel spacing, known as ‘gridless’ or ‘flexigrid’.

Colourless, directionless and contentionless (CDC) – also based on route-and-select architecture, but resolves the blocking problem with CD ROADMS when wavelengths of the same frequency are added/dropped from different degrees. (In the case of CD ROADMs, this has to be managed and can lead to stranded non-usable wavelengths in some directions.) CDC ROADMs also support flexible channel spacing. A CDC ROADM network enables non-blocking mesh architectures.

ROADM architecture and technology will continue to evolve to support additional flexibility in the future. One form of this will likely be additional modularity in terms of the capability to support pluggable optical components in terms of ROADM building blocks like wavelength selectable switches, splitters, and so on. With the support of optical express and


If applicable, ROADM- based DWDM

wavelength switching, ROADMs can best manage high bandwidth interconnections at lower cost points by maintaining the data signals optically for as long as possible in the network. Elimination of expensive electrical regeneration can save as much as 30 per cent in equipment costs. Tis becomes more important as the number of data centres increases and the trend towards distribution of more data centres towards the edge of the network continues. In addition, route-and- select (CD and CDC) ROADMs with gridless capabilities support the evolution to super- channels to transport very large DCI bandwidths greater than 100G. While the costs of these ROADMs are higher than point-to-point DWDM systems or broadcast-and-select ROADMs, when transporting 100G or greater bandwidths the relative cost of gridless ROADMs is much less a factor in the overall costs of the transport solution compared to the costs of electrical regeneration of

Issue 9 • Autumn 2015 FIBRE SYSTEMS 31


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