Sub Wavelength Switching Solution

The Mismatch of GE to Existing DWDM

Ethernet can be transported over existing metro DWDM systems. However these systems use 2.5G or 10G wavelengths to transport their data. Neither of these rates is a bandwidth-efficient match for Gigabit Ethernet (GE) services leading to an inefficient use of transmission bandwidth and a lack of flexibility in delivering GE across a metro network. For example a Central Office might have 3 or 4 GE’s to connect into the network. This would only fill 30-40% of a 10G wave. Worst, these 3-4 GE’s are not likely to be go to the same end point, further fragmenting the capacity on the 10G wave. Clearly some form of grooming is required to make the transmission of these services cost effective.

The Xtera Solution: Sub Wavelength Switching (SWS)

Xtera’s 7200 Optical Switching Platform has the capability to deliver 3 types of switching fabrics (at Layers 0, 1 and 2) on the same platform. These can be installed as required to optimise the deployment of an operator’s metro DWDM network.

For deterministic delivery of GE services an operator would use the Sub Wavelength Switch (SWS) fabric. The SWS is a 320Gb/s Layer 1 switching fabric capable of cross connecting the data on the incoming 2.5Gb/s and 10Gb/s wavelengths to any of the outgoing wavelengths connected to the switch. This makes it possible to take the GE’s coming in from any of the access sites connected to the 7200 OSP (figure 1) and efficiently pack them into 10G wavelengths for transport into the core of the network. This is done without statistically multiplexing the GE streams thereby guaranteeing the availability of the bandwidth to the end customer and avoiding the delays inherent in Layer 2 multiplexing. Of course, since the 7200 OSP also has Layer 2 switch capability, the SWS can feed the GE signals into this fabric if the service delivered requires this.

Deployment

In a typical metro network the 7200 OSP would be deployed as hubs connecting a series of metro rings together (figures 2 and 3). The other nodes in each ring would normally have a GE access multiplexor (such as the 2610EAS) capable of delivering up to 8 GE’s or multiples of 8 GE’s on individual 10G wavelengths. Since the total Ethernet traffic in a small CO is normally much less than 8 GE’s, the 10Gb/s wavelengths are usually only partially filled. Even in larger CO’s the minimum granularity of 8 GE’s per 10G wave means that at least one 10Gb/s wave is only partially full.

In operation, the individual wavelengths from the access rings terminate on the 7200 OSP which sends the 10 Gb/s streams to the Sub Wavelength Switch. The SWS then connects each GE service to the desired output port. Up to 32 10Gb/s ports are available. This capability enables 3 key network functions:

• Network Efficiency - Individual GE’s from any node on the ring can be packed together to send highly filled 10Gb/s wavelengths from the access ring across the metro network. In the example in figure 1, five partially filled 10G wavelengths are reduced to two packed 10Gb/s wavelengths feeding into the metro network. If the wavelength traverses another 7200, these packed GE’s can be reconfigured and mixed with any of the GE’s at this new node further increasing network efficiency.
• Node to Node Connectivity - Any individual GE coming in on the access ring can be connected to any other node on the ring. In the example in figure 1, 9 GE’s have been interconnected around the ring.
• Protection Switching – Wavelengths can be sent in both directions around the access ring and terminated in the SWS. The SWS can then provide protection switching at a GE level.

In addition to the high packing ratio possible, because the SWS is fully non blocking it also acts as multi-degree switch allowing up to 32 different degrees. This makes it possible to link multiple rings together to provide ring to ring protection and improve the distribution of traffic around the network (figure 3).

Figure 1. Access network traffic demand

Figure 2. Typical access ring

Figure 3. Metro network