Network incumbents such as Cisco with IOS/ONE and Juniper with JUNOS plus its SDN software, can be a considered a NOS, but are dependent on the underlying network infrastructure. The SDN market is in its very early stages and there is wide variety in the capabilities of these SDN "NOS" product offerings. One of the few topics that Cisco and VMware agree on with regards to SDN is that there is NO such thing as a SDN network operating system!
The key question is whether there will be an independent market for SDN NOSes along the lines of the traditional network OS a la Novell Netware. Or whether the capabilities of the SDN NOS will be incorporated in a defined SDN control layer that is tightly tied to a specific cloud or network hardware architecture. This question will impact the emergence of SDN standards (e.g., northbound APIs) and ability of ISVs to develop SDN applications. Ultimately, the marketplace will decide the benefits (and drawbacks) of the various SDN NOS technologies and whether or not the SDN NOS market will emerge as a significant component of the overall networking market.
Read more about lan and wan in Network World's LAN & WAN section.