So what exactly is the difference between the line side and client side of the network? The client side of the network is where traffic will come on-ramp into the optical backbone. In this case the client side interface will be used to interconnect between the metro networks and the optical backbone. So why is that important?Anytime there is this type of interconnection it is implied that devices of different types are going to be interconnected. For example a router might be connected to the transport core, or a metro network scaled device might be connecting to the transport core. In either of these cases not only could the device be different but they could be from separate vendors. The other use case for the client interface is exactly what it says in the name the client connection. When a service provider is providing connectivity to a CPE premise this will again be on the client side of the network.Bearing all this in mind it is imperative that there is an agreed and standardised interface that is being used for the network interconnection. Therefore when we consider a client interface we can be sure that we are going to be presented with a standardised interface of some description. Typical examples could be a CFP, SFP or XFP interface presentation and of course these would depend on the service being carried and at what bit rate they are being offered, So what exactly is the line side? The line side interface is the higher rate interface that is used for the backbone network. Consider a typical optical backbone network. It will have generally have particular characteristics regardless of where it is deployed It will utilise proven transport technologies such as SDH or OTN It will utilise high capacity optical links between major metro hubs The network vendor will be consistent for each network Metro networks will interconnect across the backbone from a client interface Consider the third item: The network vendor will be consistent for each networkThis is a crucial difference when compared to the client side. As there will be no other network vendors or network devices on this part of the network it is no longer a requirement for a standardised interface to be deployed. Therefore a vendor will be free to deploy any technologies they develop on this interface in order to meet the needs of their customer. Depending on technology deployed this could include utilising non-standard FEC algorithms, out of band signalling or other proprietary technologies.In terms of 100Gb/s networks the big differentiator is the modulation technology and DWDM channel scheme used on this line side interface. These technologies will be discussed in later articles.