The best of both worlds

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In the world of optical networking, coarse wavelength division multiplexing (CWDM) technology has recently appeared on the scene as a cost effective solution for building a metro access network. CWDM promises all the key characteristics of a network architecture that a service provider would wish for: transparency, scalability and low cost.

Until CWDM became an option, it was thought that the metro access network would be dominated by dense wavelength division multiplexing (DWDM). However, DWDM technology was primarily driven by the requirements of the long-haul network: hundreds of wavelengths built with expensive high-performance optics. Compared with CWDM, DWDM technology provides transparency and superior scalability, but at the price of increased cost-per-wavelength.

The typical metro access ring today requires fewer than 16 wavelengths with a total ring distance of less than 40km. The data rate is usually 1.25Gbit/s or 2.5Gbit/s per wavelength. Applications include remote storage services, intra-enterprise communication and high-speed data services. Connectivity is established through owned or leased fibre, or leased capacity from a local carrier or simply by pulling new fibre through existing conduits.

In building the metro access network telecommunication network designers have two clear approaches. One approach is to build a metro network with expensive DWDM optics with the advantage of having a clear path of growth in the future. The other approach is to build a network of low-cost CWDM optics — with the confidence that the network traffic will not exceed the capacity of 16 wavelengths in the near future.

If low-cost DWDM technology were to become available as traffic and services grow, then a third approach becomes possible: DWDM on CWDM networks.

A typical WDM metro access ring connecting four buildings using CWDM technology is shown in Fig.1. The figure shows a carrier hotel in one building connecting data services to three office buildings arranged in a ring network. This is a good example of how a service provider would provide data-centric services to customers in each of the three buildings. A variety of network equipment could be located in each building. The network equipment in each of the office buildings could be Ethernet switches, optical networking platforms or fibre channel storage switches.

The ring network in Fig.1 also shows four nodes connected together with a uni-directional (TX in one direction, RX in the other) pair of fibres. At the carrier hotel site are two CWDM 4-lambda multiplexer/demultiplexers. The multiplexers combine the wavelengths into a single fibre going out to the network. The demultiplexer separates the wavelengths from the network and routes them to individual CWDM optical transceivers plugged into the network equipment. For network protection, two multiplexer/demultiplexers are required to route signals to both the East and West directions around the ring.

Data travels from the carrier hotel to CWDM optical add/drop multiplexers (OADM) and CWDM transceivers in each of the office buildings' nodes. The OADM adds and drops a single wavelength from both the East and the West directions. All other wavelengths in the fibre pass through the OADM to the next node. Added/dropped wavelengths are connected via optical transceivers plugged into the customer equipment. The communication path leads from the office building optical transceiver to the corresponding optical transceiver (of the same wavelength) at the carrier hotel node.

Figure 2 shows a detailed view of how a CWDM OADM routes the optical signals through the office building node. The OADM terminates 1550nm wavelengths in the East and West directions to 1550nm optical transceivers plugged into the network equipment. All other wavelengths pass thought the OADM in each direction to the network. Each of the 1550nm transceivers communicates back to a corresponding 1550nm transceiver located in the carrier hotel. The OADM and optical transceiver wavelengths could be any one of eight wavelengths from 1470nm to 1610nm.

Protection is achieved by employing two transceivers at each the node. If a fibre break were to occur in one direction of the network, the transceiver going in the opposite direction would then be available to route the data.

The simple ring network shown in Fig.1 requires just three wavelengths to route data from each of three office buildings back to the carrier hotel. If the service provider wanted to add more bandwidth to the ring, CWDM technology would provide an additional five more wavelengths. CWDM optical networking systems have recently scaled to 16 wavelengths in the 1290-1610nm spectrum using zero-water-peak fibre (see LWE April 2002, p29, or on www.lightwave-europe.com). This fibre has low attenuation in the 1290-1610nm spectrum, which allow expansion to 16 wavelengths.

However, if traffic growth exceeds the limit of 16 wavelengths in the next few years, it would be possible to scale your bandwidth even further by adding DWDM to your network ring. Figure 3 illustrates from a spectrum view how DWDM on CWDM would work.

The CWDM spectrum grid at the top of the figure shows eight wavelengths out of the CWDM grid from 1470nm to 1610nm. The DWDM grid at the bottom shows eight wavelengths out of the ITU C band grid that are centred ±6nm around the 1550nm wavelength.

The middle spectrum depicts how the CWDM filter of the OADM creates a 12nm window over each of the CWDM grid wavelengths. The DWDM wavelengths are shown mapped into the 1550nm filter window. This illustration shows how a set of eight DWDM wavelengths could easily be mapped into each of the CWDM grid wavelengths.

The total wavelength count for a DWDM grid mapped into a CWDM grid would increase to 64 total wavelengths.

To achieve a hardware implementation of the spectrum view shown in Fig.3, a few changes are required to the CWDM OADM node shown in Fig.2. First, you need to add a DWDM multiplexer/demultiplexer between the CWDM OADM and optical transceivers. Second, change the two CWDM GBICs with the number of DWDM GBICs corresponding to the size of the DWDM mux that was added (four-lambda mux, eight GBICs). Finally, make the corresponding multiplexer and transceiver changes at the carrier hotel.

The new node will look like Fig.4. By adding the DWDM hardware the bandwidth has increased by a factor of four. However, adding the extra hardware has increased the optical losses through the communications path by 5–6dB. The optical losses reduce the total possible distance from the carrier hotel to the office building.

To accomplish the DWDM bandwidth expansion illustrated in Fig.4 requires pluggable optical DWDM transceivers built with lasers spanning the CWDM grid from 1470nm to 1610nm, with 100GHz spacing.

With the advent of pluggable DWDM transceivers of this design, network designers can build low-cost CWDM networks and then scale the bandwidth with DWDM technology as the network grows. With a 100GHz DWDM pluggable transceiver we could scale to 64 wavelengths on an eight-wavelength CWDM grid.

Future developments could include a 50GHz device that scales to 128 wavelengths or a 25GHz device that would scale the network to 256 wavelengths.

And what next? How about adding a low-cost optical amplifier to the DWDM mux to in-crease the distance of the ring?

Jim Aldridge
Senior Marketing Manager for WDM Products
Finisar
jim.aldridge@finisar.com

Jim Aldridge has spent four years at Finisar. His previous position was program manager of the Opticity Networking Platform.

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