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The P25 Interface Standards as shown on the General System Model
are as follows:
RF Sub-System (RFSS)
Core Infrastructure
Common Air Interface (Um)
Radio to radio protocol
Inter-System Interface (ISSIg)
RFSS to all other system interconnections (In progress)
Telephone Interconnect Interface (Et) PSTN to RFSS defi nition
Network Management Interface (En) Network to RFSS defi nition (In progress)
Data Host or Network Interface (Ed)
Computer aided dispatch to RFSS defi nition
Data Peripheral Interface (A)
Radio to Data Peripheral defi nition
Fixed Station Interface (Ef)
Base station to RFSS / Console Sub-System defi nition (In progress)
Console Sub-System Interface (Ec)
Console to RFSS defi nition (In progress)
RF SUB-SYSTEM
The P25 interfaces bound the RF Sub-system (RFSS) infrastructure.
The RF Sub-system can be made from any collection of site equipment
(single station/site or multiple station/site), whose only requirement is
that the equipment supports the Common Air Interface, and contains
all necessary control logic to support the open intersystem interfaces
and call processing. The RF Sub-systems are the building blocks
for wide-area system construction and will connect with any other
confi guration of equipment or RF Sub-systems.
COMMON AIR INTERFACE
The Common Air Interface (Um) or CAI defi nes a standard (or
reference point) at which communications between P25 radios can
take place. The CAI is the core element of the P25 standard that
assures the ability of one company’s P25 digital radio to communicate
with another company’s P25 digital radio. Communications between
P25 radios are done at a gross bit rate of 9.6 kbps and with FDMA
channel access. Several processes take place to convert information
for transmission. The Common Air Interface uses an IMBE™ voice
coder (vocoder) to convert (compress) speech to a digital format for
communication. This voice information is then protected with error
correction coding to provide protection over the channel. The voice
information and error correction is then transmitted with additional
encryption information, unit identifi cation, and low speed data to fully
utilize the 9.6 kbps of channel capacity in the Common Air Interface.
A breakdown of the information contained in the Common Air Interface
can be found in Chapter 4: Anatomy of the Common Air Interface.
Chapter 5 contains some detailed information on the operation and
theory of the IMBE™ Vocoder.
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INTER-SYSTEM INTERFACE
The Inter-System Interface (G) is under development.
The Inter-System Interface (G) or ISSI permits multiple RF Sub-Systems
to be interconnected together into wide-area networks. The ISSI
defi nes a multi-channel digital interface supporting standard protocols
to enable interoperability utilizing mobility management and wide-area
service support functionality. The interface is designed to give system
designers the fl exibility to combine any number of RF Sub-Systems
of any size. The Inter-System Interface also provides a common
meeting place for RF Sub-Systems of different technologies (TDMA,
FDMA, micro-cell) and different RF bands. This interface is optional,
and need only be supported when intercommunication amongst and
across RFSS’s of Land Mobile Radio systems is desired.
Although a P25 subscriber radio may only operate freely among
systems with the standard P25 common air interface, the P25 ISSI has
the potential to connect between different radio or telecommunications
networks as long as they also support the ISSI interface.
The ISSI messaging defi nes the basic structures to be shared among
all equipped RFSS’s. The ISSI can be supported on any possible
networking confi guration, from a simple star confi guration to a full mesh,
to an intelligent network. This can consist of private links and network
support, or may be public links and network support confi gured as a
private network. Any intervening network supporting the information
of an ISSI link needs to preserve the ISSI messaging packet, but may
intermediately represent the ISSI packet in whatever convenient form
(e.g. ATM cell) is available.
The ISSI will support:
• mobility and data management,
• wide area service control,
• service transport,
• end to end protection of signaling information,
• other network interconnection.
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TELEPHONE INTERCONNECT
INTERFACE
P25 requires an open interface to telephone networks. The Telephone
Interconnect Interface (Et) supports both analog and ISDN telephone
interfaces, providing for selective use of proven standard telephone
interfaces currently in use.
The Telephone Interconnect Interface defi nes a 2-wire loop start
and a 2-wire ground start connection between the RF Subsystem
and the PSTN or a PABX. In addition, other optional interfaces may
be provided. The Telephone Interface deals only with voice service
because it has been assumed that circuit connected data services
would access a telephone network via a modem and connect to a data
port on the radio system.
NETWORK MANAGEMENT INTERFACE
The Network Management Interface (En) is under development.
The Network Management Interface defi nes a network management
interface to all RF-Subsystems. According to a single selected network
management scheme within any RF-Subsystem, all fi ve classical
elements of network management must be supported. It is expected
that a network management scheme will be selected that will bring
with it the ability to manage RF-Subsystems with available network
management system equipment. In addition, an existing network
management system, including computer and telecommunications
equipment, may well be able to encompass P25 radio systems.
DATA HOST OR NETWORK INTERFACE
The Data Host or Network Interface (Ed) defi nes four different types of
data connectivity. These include a native open interface for connecting
host computers, as well as the requirement to support three different
types of existing computer network interfacing (TCP/IP, SNA and
X.25).
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DATA PERIPHERAL INTERFACE
The Data Peripheral Interface (A) defi nes protocols by which mobile
and portable subscriber units will support a port through which
laptops, terminals, or subscriber unit peripherals may be connected.
It is required that the supported open interface protocols are passed
transparently into X.25, SNA, or TCP/IP computer networks at another
open interface on the fi xed equipment side. Transparency is listed
as a requirement, and it is expected that application layer standards
emerge for the connection of various peripheral devices.
FIXED STATION INTERFACE
The Fixed Station Interface (Ef) is under development.
The Fixed Station Interface will provide for communication between
a Fixed Station (FS) and an RF Sub-System (RFSS) operating in the
following modes:
a. Conventional Analog
b. Conventional Digital
c. Trunked Digital
d. Digital telephone interconnect
e. Circuit and Packet Data
The Fixed Station Interface defi nes a set of mandatory messages,
supporting digital voice, data, encryption and telephone interconnect.
These messages will be of a standard format passed over the interface.
Manufacturers can enhance this functionality using manufacturer
specifi c messages.
The analog confi guration for the fi xed station interface are two pairs
of signals in a 4-wire audio confi guration with both pairs of signals
in the audio frequency range 300-3000 Hz. One wire pair carries
signals to be transmitted by the FS and the other pair carries signals
received by the FS. The circuits are balanced with a nominal 600 ohm
impedance. Voice levels of each signal pair are nominally -10 dBm. It
is recommended that all inputs and outputs have lightning protection
isolation.
The digital confi guration for the fi xed station interface is an IP based
interface. The physical interface is an Ethernet 100 Base-T or 10
Base-T with an RJ-45 connector.
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The Fixed Station Interface can also provide several optional analog
interfaces as well:
2W Circuits
The Fixed Station Interface (analog) may be provided using 2W circuits
using a balanced 600 ohm termination.
E & M Control
The Fixed Station Interface (analog) may include standard E&M
signaling circuits.
Tone Control
The Fixed Station Interface (analog) may provide for remote control
using industry standard tone remote control equipment.
CONSOLE SUB-SYSTEM INTERFACE
The Console Sub-System Interface (Ec) is under development and
could possibly be integrated into the Fixed Station Interface in the
future.
The Console Sub-System Interface (CSSI) defi nes a multi-channel
digital interface. This interface is capable of supporting standard
protocols to enable interoperable support functionality. The CSSI
defi nes basic messaging structures to interface a console subsystem
to an RFSS.
The CSSI can be supported using a variety of networking technologies
and topologies, from a simple star confi guration to an intelligent
backbone network. The networks may be private, or public networks
confi gured as private networks.
The physical interface is an Ethernet 100 Base-T with an RJ-45
connector. The CSSI will support Ethernet 10 Base-T and 1000 Base-
T as an optional physical interface. The CSSI will optionally support
auto-sensing. Other interfaces may be installed as a manufacturer’s
option.
As a note, a console sub-system can connect directly to a fi xed station
and support one or more Fixed Station Interfaces. Manufacturers may
also optionally support a subset of the Data Host or Network Interface
in the Console.
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CHAPTER 3: P25 PRACTICAL
APPLICATIONS
ANALOG TO P25 TRANSITION
P25 equipment can be used in any confi guration that is typically
found in existing analog systems. Base Stations, remote bases,
repeaters, voting, and simulcast systems are all confi gurations of
P25 conventional systems. Transmitter RF power output levels and
receiver sensitivity levels of P25 equipment are very similar to those
of conventional analog equipment. P25 equipment can therefore be
used in a “one-for-one replacement” scenario of analog equipment.
This section will discuss some of the issues surrounding the transition
from an analog radio system to a P25 digital radio system as well as
supply general knowledge about P25 radio systems.
P25 FREQUENCY BANDS
The frequency bands in which P25 radio systems are available are
VHF (136 – 174 MHz) and UHF (403 – 512 MHz, 806 – 870 MHz). In
addition, P25, Phase 1 technology has been adopted by the FCC as
the digital interoperability standard for the new 700 MHz (746 – 806
MHz) digital public safety band.
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P25 DIGITAL CODE DEFINITIONS
A P25 digital radio system uses many different codes, identifi cations,
indicators and other digital information in the Common Air Interface.
Some of the codes are user accessible or programmable, while
others are meant for internal use inside of the CAI, or for specifi c
applications.
Frame synchronization
A special sequence of 48 bits marking the location of the fi rst bit of
the message provides frame synchronization. Frame synchronization
occurs at the beginning of every message (voice and data), and is
inserted every 180 ms throughout the voice message. This allows
receivers to pick up voice messages after the message has begun
(late entry of receivers). Late entry can occur when a subscriber unit
selects a channel (or talk group) while there is already an active signal
present. The subscriber unit was not active when the transmission
started, but is added when it detects the repeated frame sync function.
The frame synchronization is not accessible or programmable by the
user.
(See Chapter 4; Figure 4-3)
Network ID (NID)
Every P25 data unit packet contains the 64 bit NID fi eld. The NID is
composed of a 4 bit Data Unit ID and a 12 bit NAC code. The NID is
protected with a primitive BCH Code and a single parity bit is added to
fi ll out the NID code word to 64 bits.
(See Chapter 4; Figure 4-3)
Data Unit ID
The NID contains the 4 bit Data Unit ID fi eld. The Data Unit ID is used
to determine the “type” of packet information (eg. Header Data Unit,
Logical Link Data Unit 1, etc.). The Data Unit ID is not accessible or
programmable by the user.
(See Chapter 4; Figure 4-3)
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