Codan Radio P25 Training Guide - Manual (Page 44)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 3: P25 Technical Information Page 36
P25 LOCATION SERVICES
P25 Location Services, such as GPS, provide a method of supplying a Location Service Host System
(mapping software for example) with the location of subscriber units within a P25 Radio System
coverage area. The Location Services can include information such as latitude, longitude, altitude,
Coordinated Universal Time (UTC), GPS quality, and many other location information messages. The
P25 standards break the location services into two tiers of service, Tier 1 and Tier 2.
Tier 1
Tier 1 is a simple subscriber to subscriber interface for Conventional direct (talkaround) or repeated
radio signals, without providing IP addressing, fi xed host routing, or more advanced confi guration
of triggering and reporting. Tier 1 is used for real-time fi eld incident applications where the Location
Service Host System (LSHS) is built-in to the subscriber. It does not provide location information to a
Fixed Data Host through an RF Sub-System. Tier 1 uses a dedicated Service Access Point (SAP) on
the Common Air Interface (CAI) to transmit location information formatted as described in NMEA 0183.
National Marine Electronics Association, NMEA 0183 is a combined electrical and data specifi cation
for communication between electronic devices (primarily marine) such as echo sounder, sonars, GPS
receivers and many other types of instrument. NMEA 0183 is a commonly used location protocol.
Tier 2
Tier 2 is a request/response protocol that allows an LSHS to make a request for location information
from a subscriber unit or Mobile Data Peripheral (MDP), and to receive a response from the subscriber
immediately, periodically or under various triggering conditions. The information can be transmitted
between subscribers in Conventional direct (takaround) or repeated mode, or between a subscriber
and a Fixed Data Host (FDH) in Conventional or Trunked Fixed Data mode. Tier 2 is used where there
is the necessary infrastructure to allow routing and transport in a customer data network using IPv4.
The Tier 2 approach utilizes Location Request/Response Protocol (LRRP) across the P25 Packet Data
Service to transport UDP/IP addressed location information packets from the LSHS to the subscriber,
and to transport location information from the subscriber to the LSHS. LRRP is an XML-based protocol,
and uses Effi cient XML Interchange (EXI) compression for effi cient transmission over the Common Air
Interface.