User's Manual

23
AS4020 STs
AS4020 Subscriber Terminal Overview
The AS4020 Subscriber Terminal (ST) portfolio includes subscriber terminal types for voice,
and packet data. In general, the customer has access to the same range of facilities that are
supported by a conventional copper pair.
Type 42 subscriber terminals have the majority of the electronics in the Service Interface Unit
(SIU) located inside the customer premises. This is connected to the outdoor unit by an IF
drop cable. The ODU contains the RF components.
The iST subscriber terminal have the majority of the electronics in the Outdoor Unit (ODU)
and they are connected via a drop cable to the Indoor Unit (IDU) that contains the power
supply and the customer interfaces.
When an AS4020 system is planned, for overall RF capacity purposes an assumption will be
made of either the average modulation rate to be achieved across all subscribers or a specific
mix of modulation rates will be assumed across the subscribers. This introduces a new factor
into ST installation procedures: a check needs to be made of the actual modulation rate
achieved against the planned modulation. If this check is not carried out, the assumptions
made for modulation at the planning stage are not verified, with the potential result of the
achieved overall RF capacity being much less than the planned capacity, resulting in a
reduced level of service for subscribers.
In addition to establishing modulation rates for each subscriber, the RF Planning process will
ensure that adequate fade margins are present in order to achieve adequate link availability
figures. Fade margins are associated with two parameters – SNR and Receive (downlink)
/Transmit (uplink) Level. In an interference free, or island deployment, these two parameters
will be closely matched. However, in a deployment limited by interference levels (through
external interference or aggressive frequency reuse), The two parameters will be influenced
by different factors.
The output of an RF Planning exercise will generate the following requirements relating to an
individual ST deployment:
• Expected Modulation Modes (uplink & downlink) – based on the Rx SNR predicted
• Required SNR Margin above Modulation Mode switching threshold – to maintain
planned overall system throughput under shallow fade conditions.
Where average overall system throughput is a concern, a 0dB SNR Margin is considered
adequate for standard internet access services – this is because as some users SNR
degrades, others will improve maintaining an average system throughput. For a conservative
approach to system planning, a 2dB SNR margin would be adequate. However, if a particular
ST requires guaranteed high bandwidth for services such as video conferencing, then the
SNR margin would be increased.
The fade margin required to maintain link availability will depend on the actual availability
level required plus the propagation conditions. This is typically between 6 and 12dB.
Telephony
For normal telephony services, these could typically include:
• multi-frequency or loop-disconnect dialling
• subscriber private metering
• intrusion tone
• malicious call interception.
• caller line identification
Note. Line reversal is not supported.
All tones, tone cadences and announcements which are generated by the local exchange are
passed transparently over the AS4020 system. Line feed voltage, ringing current and ringing
cadence are generated by the ST equipment.
The Customer Premises Equipment (CPE) is connected to the line interface sockets on the
internal SIU.