Roaming onto commercial 2G, 3G and LTE networks as well as trusted Wi-Fi networks is a primary issue addressed in the official set of minimum technical requirements released for the planned LTE-based nationwide public-safety wireless broadband network (NPSBN).
The 91-page document outlining requirements was prepared by the Technical Advisory Board for First Responder Interoperability and is intended for submission the First Responder Network Authority (FirstNet). Because the FirstNet board will not be seated until August, the FCC issued an Order of Transmittal, which will be delivered to the administrator of the National Telecommunications and Information Administration (NTIA) pending the FirstNet board's formation.
The minimum technical requirements recommended to FirstNet are an input to the RFPs that will eventually be issued by FirstNet for vendor bids and contracts.
The interoperability board said it aimed to create levels of interoperability for the NPSBN that are equivalent to those achieved in commercial service provider networks and to reflect how LTE technology would be used to meet public safety's unique mission requirements. The board said the NPSBN must fully embrace the technologies, standards and best practices used by commercial service providers.
In addition to addressing in-depth technical issues impacting basic network and user-equipment functionality and interoperability, the document also addresses broader topics such as roaming onto commercial LTE and 2G/3G networks, which the interoperability board said "will be especially important during the initial phases of deployment when Band 14 coverage is not yet ubiquitous."
To support roaming from the NPSBN onto 3GPP and/or 3GPP2 networks, a Band 14 LTE device will need to accommodate at least one additional 3GPP or 3GPP2 frequency band.
"FirstNet should ensure that its devices enable FirstNet to enter roaming agreements and public-private partnership arrangements with any commercial service provider and allow FirstNet users to obtain service in those commercial networks. A device that is capable of obtaining such service in certain bands shall operate on all FirstNet roaming partner networks operating in those bands and not be locked to a subset of FirstNet roaming partner networks operating in those bands," said the requirements document.
Among other things, the board directed that the new public-safety network shall support the use of mobile VPN technology to support mobility between the NPSBN and other networks.
One intriguing element offered up by the board is an allowance for different grades of service in different parts of the country due to budgetary constraints, a lack of base station sites in remote and other areas as well as other factors that "may make it difficult to provide a uniform grade of service, especially in the early years of the NPSBN."
The use of Access Network Discovery and Selection Function (ANDSF) was discussed as an option for roaming onto Wi-Fi networks. "ANDSF leverages the LTE credentials for authenticating users, allows seamless handover between LTE and trusted Wi-Fi and provides similar security as used in LTE. This may allow public-safety users to securely and seamlessly roam between NPSBN and their own Wi-Fi network," according to the requirements document.
The interoperability board also addressed the tricky issue of voice over LTE. If a user leaves the NPSBN while on a VoLTE call, "the call will drop, requiring the user to re-establish the call on the roaming network," said the document.
"The initial application of VoLTE on the NPSBN will therefore be less functional than the application of VoLTE in a commercial network where a service provider can leverage techniques such as Single Radio Voice Call Continuity (SRVCC) to hand over to other radio technologies. Consequently deployment of VoLTE may need to wait for a region to have significant NPSBN coverage. Also, the new network may not have the bandwidth available to continue previous services such as video sessions with the same QoS," it added.
The interoperability board said it has not determined how subscriber provisioning will be performed in the NPSBN, apparently leaving that for FirstNet to figure out.
The government expects FirstNet to participate in 3GPP standards processes to get desired public-safety capabilities--such as push-to-talk and off-network operation--implemented in future LTE standards.
The Spectrum Act, enacted on Feb. 22, 2012 as part of the payroll-tax extension, established FirstNet to oversee the construction and operation of a nationwide public safety wireless broadband network as licensee of both the existing public-safety broadband spectrum at 763-769/793-799 MHz and the spectrally adjacent D Block spectrum at 758-763/788-793 MHz.
- see this FCC webpage
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