7.1
Point to Point
MTT Deploys the following Point to Point communication
systems for its access network.
·
MDS radios (Microwave Digital radio Systems)
·
LEDR
·
Ericsson mini links
7.2
MDS
MDS 450D radios are used in
MTT. These are used to provide data links with 64 Kbit/s and 128
Kbit/s data rates for the customers. A pair of radios is used in a
data link. Only difference is the radio at customer premises is
supplied with A/C 220 V main supply and radio at base station is
supplied with D/C 48 V supply, which is available at base
stations. MDS 450D uses seven-level frequency shift keying (FSK).
The Fig 3-1 shows the basic layout of a MDS radio.
 
Fig 7.1 - MDS
Radios
The radio consists of five main parts
namely :
·
Power supply unit
·
Transmitter unit
·
Receiver unit
·
Interface unit
·
Duplexer unit
Power Supply Unit
This unit is used in
customer premises equipment to convert A/C main power supply to
D/C 48 V. The switch mode concept is used in this power supplies.
This unit is not required at the base station radio because direct
48 V supply is available at base stations.
Transmitter Unit
Transmitter card does all
the processes involved in transmission such as adjusting
transmitting frequency, transmitting power and modulation.
Receiver Unit
This unit is
responsible for all the processes involved in receiving signals.
Receiving frequency is programmed in to this unit.
Interface Unit
This unit provides all the interfaces to the radio. There are
several switches in this unit, which can be used to select clock
modes and loops. The clock modes, which can be used in this radio,
are internal, external, or receiving clock modes. TX loop, remote
loop back, and loop back are the loops that are used in this
radio. TX loop is used to check the interface unit. The Fig 3-2 shows a diagrammatic
representation of the interface unit.
This unit provides interface for the Hand Held Terminal (HHT),
which is used to program and control the radio.
Duplexer Unit
This
is a cavity resonator type filter, which is used to direct the
relevant signal frequencies in the correct paths. There are three
cavities in the filter. Receiving and transmitting connections are
connected to the two end cavities and cable to the antenna is
connected to the center cavity. This enables the transmitting
signal frequencies to travel only in the antenna path and the
receiving signals from the antenna path to travel towards the
receiver path.
Antennas Used In MDS
There
are various kinds of antennas, which can be used with MDS radios
but in the MTT; they use Yagi antennas for MDS links.
HHT (Hand Held Terminal)
This device is used to program the
radio unit. Main functions of the HHT are listed below.
·
Programming transmitting frequency
·
Programming receiving frequency
·
Keying the radio
·
Examining status of the radio
Interfaces Used In MDS Radios
The interface used in MDS
450D is the EIA-530. There fore different interface adapters have
to be used when MDS is used with equipments, which have different
interfaces.EIA-530 to V.35 interface adapter is shown in Fig
.
Fig 7.2 - EIA-530 To V.35 Interface Adapter
7.3 Training
Experience in MDS
The
MDS radio is a less complexity device used in MTT network. These
are widely used in point to point low rate data transmission. So
we had much opportunity to get practical experience in MDS radios.
My practical experience can be divided in to two sections. One is
in testing & fault detection of existing links and other
aspect in installation of MDS links.
Testing
& Fault Detection
I involved in testing MDS in
several ways. Various test setups used in testing are described in
details below.
·
RF Loop back Test Setup
Using this setup, we tested the radio by itself. The
Fig shows the test
setup, which was used. Normally this setup is used to test the
interface unit.
Fig 7.3 - Loop
back Test Setup
Back-to-Back Test Setup
This setup is used when both the radios are
available for bench testing. In this setup both radios are
connected back-to-back with an attenuator as shown in the Fig.7.4
Fig 7.4 -
Back-to-Back Test Setup
LEDR is the latest version of the MDS
series. There are few improvements in LEDR when compared with MDS.
This has a reduced bandwidth of 25 MHz for 64kbits/s radios and
50MHz bandwidth of 128kbits/s radios where in MDS it is as twice
as this. LEDR has LCD display and menus driven controlling system,
which facilitates programming the radio directly without using a
HTT as in MDS radios. The details of the remote radio can also be
checked using this controlling system.
Front Panel of an LEDR Consists of the following
POWER—Main Power is applied to radio.
ACTIVE—This is the Primary unit in a redundant
configuration.
ALARM—A general alarm condition is present
RX ALARM—The modem is not locked to a
receive signal
TX ALARM—There is a problem with the
transmitter
I/O
ALARM—There
is a payload data interface error
LEDR also uses the Yagi
antennas as in MDS radios but the maximum transmitting power is 1
W (30 dBm). This is some what less when compared with MDS radios.
LEDR has better performances than MDS and application areas are
same as in MDS radios.
ERICSSON mini links are
point to point systems, which are used to transmit multiple of 2
Mbit/s signals. They are in different categories.
4
x 2 Mbit/s
8
x 2 Mbit/s
16
x 2 Mbit/s
The
Ericsson minilinks consists of following parts.
·
MMU
·
SMU
·
RF Unit
·
Antenna Unit
The RF unit is fixed to the antenna. There fore only the IF
signal is coming to the ground. This avoids requirements of
wave-guides from antenna to the ground. MTT use Ericsson minilinks
at Mullariyawa and Aluth Mawatha base stations.
· Chapter
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