Photo by Eric Rubin
By Paul May
The goal of Project 25 (P25) is to provide an open standard that enables multiagency or regional LMR interoperability and promote full and open competition for the life cycle of a radio network. While the P25 Common Air Interface (CAI) and related trunking standards provide for interoperability on a subscriber radio level, network interoperability requires compliance with the P25 Inter Subsystem Interface (ISSI). The ISSI is the technology that provides network connectivity between P25 networks and allows network administrators the option to connect to other local, regional, statewide or federal networks in the future.
The development of the P25 ISSI standards is being completed in multiple stages or scopes. The ISSI Scope 1 defined the messages and procedures for subscriber mobility management and voice services between P25 networks. ISSI Scope 2 is the augmentation of the ISSI Scope 1 standards to primarily support packet data, console interfaces and supplementary data services. Within Scope 1, mobility management and voice services allow a P25 subscriber to roam to a foreign network, register on that network, and communicate both on the foreign network and back to their home network.
ISSI Scope 2 enhances the capabilities of Scope 1 by allowing a subscriber to have packet data services, including over the air rekeying (OTAR), on the foreign network. Scope 2 also defines ISSI support for trunked or conventional console interfaces and the ISSI support of supplementary data services for subscribers on a foreign network. Supplementary data services are P25-defined data messages for the subscriber and include emergency alarm, call alert, short messages, status query/update, radio unit monitor and radio inhibit/un-inhibit.
The ISSI Scope 1 standards are essentially complete, with the following documentation published as TIA standards:
• TIA-102 BACA; BACA-1; BACA-A; Messages and Procedures for Voice and Mobility Management Services
• TIA-102 CACA; ISSI Measurement Methods for Voice Services
• TIA-102 CACB; ISSI Performance Recommendations for Voice Services
• TIA-102 CACC; ISSI Conformance Test Procedures
The only remaining Scope 1 document, Interoperability Test Procedures, is currently at the comment resolution stage with balloting for publication anticipated by spring. ISSI Scope 2 has substantial standards development work continuing with revisions to the Scope 1 documents circulating in the APCO Project 25 Interface Committee (APIC) task groups to support the additional Scope 2 services. Beyond the Scope 2 efforts, the ISSI task group anticipates continuing standards development work to address security aspects of the ISSI, multicast on ISSI and IPv6 on ISSI.
The P25 ISSI is built on IP technologies originally developed for services such as streaming media applications across the Internet. The Internet Engineering Task Force (IETF) IETF session initiation protocol (SIP) and session definition protocol (SDP) are used to handle registration and call setup procedures for roaming subscribers. Using the real-time transfer protocol (RTP), ISSI-compliant systems can send communications from P25 unencrypted or encrypted talk groups bidirectionally from a home or foreign system. Both call setup and call communications are transported across a user datagram protocol (UDP)/IPv4 network stack.
In most cases, the ISSI connection between P25 networks will either be a private IP connection — such as 4.9 GHz WiMAX, licensed microwave or fiber — or a virtual private network (VPN) connection across a common carrier — T1 or multiprotocol label switching (MPLS). This IP connection must be in place and operational before ISSI communications between P25 networks can proceed; the ISSI isn’t intended to be an ad-hoc protocol that can be used to quickly set up communications on a real-time basis. In all cases, effort should be made to provision the IP connection with a sufficient quality of service (QoS) to maintain sufficient capacity and prevent call blocking of critical communications.
ISSI Call Flow for Voice Services
Subscribers, or radios, are associated with a home system. For example, a radio is linked with RF Sub-System (RFSS) 1. The radio can roam from RFSS 1 to RFSS 2, and using P25 protocols, register on an RFSS 2 site. RFSS 2 processes the P25 over-the-air registration and sends a SIP register message to the radio’s home RFSS.
The SIP register message is received and is acknowledged by the home RFSS, which provides information back to the serving (or foreign) RFSS with the radio’s characteristics. The serving RFSS assigns the visiting radio a temporary working subscriber unit or user ID. Temporary working group IDs are also assigned when the radio group affiliates using P25 over-the-air messages. Working subscriber unit IDs and working group IDs are required because the number schemes may be different on the two RFSS systems. When the visiting radio initiates a call with push to talk (PTT), the serving RFSS sends a SIP invite message back to the radio’s home RFSS.
The home RFSS grants the call back to the serving RFSS with a SIP OK message.
A SIP acknowledgement message is sent back to the home RFSS to finalize the call setup. Dedicated socket ports for the RTP streams are set up on each RFSS, and the call is transported using these dedicated socket ports for PTT, voice packets and unkey messages.
Calls can be terminated by either the home or serving RFSS using the SIP bye message. Typically, the home RFSS terminates the call. Note that these are simplified examples for the generic voice use case. However, the other features supported by the ISSI are performed in a similar manner.
Paul May is business development manager for Tyco Electronics Wireless Systems. May has more than 20 years of experience in marketing, product management and engineering products for the LMR industry. Contact him at firstname.lastname@example.org.