User guide

ASCOM USER GUIDE
High Speed Base Band Modems
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This wide choice of data rates allows the link to be set for optimal operation, the lower speed
rates allowing transmission over higher attenuation cables.
Note: any modem fitted with a G703 interface can only be set to 64k.
6.3.4 Internal / External Timing
If a modem is configured as MASTER then you also have the option to synchronise the modem
to an external clock. This allows your master modem to be set-up as a DTE. Note DTE set-up
will also require a switch over cable to configure the user interface connector in a DTE mode,
see section 8.
The G.703 interface is always configured for external bit timing.
6.3.5 Interface Control Line Transfer
The status of the user interface control lines can be communicated to the far end.
The control lines interchanged are:
X.21 V.11 - C circuit at local end to I circuit at remote end
(and vice versa)
X.21 bis V.28 / V.35 / V.36 / RS530 - circuit 105 at local end to circuit 109 at remote end
(and vice versa)
• Controls Off. This disables the transfer of control line information.
• Controls EOC. This sends the control line information by embedding the data into the
modems frame structure. This outband method is slowest but has no restricting effect on
the user bandwidth.
• Controls T/S. This allocates a 64k time slot in the transmitted data for transfer of control
information, and hence this 64k time slot is no longer available to the user. This inband
technique is fast but it does restrict the user rate by 64k. If this option is selected then the
maximum data rate will not appear in the list of choices, conversely, if the maximum data
rate is selected then this item will not appear here.
6.3.6 Transmit power
The modem can have two power settings:
• Auto. This is the normal mode of operation the modem transmits 13.5dBm of power into
a 135 ohm load.
• Power 7.5dBm. This is a low power setting to be used if problems are experienced on
short lines of less than 1 Km.
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28 ASCOM USER GUIDE
High Speed Base Band Modems
6.3.1 Modem Mode
When two modems are connected together to form a link one end must be configured as
SLAVE and the other as MASTER. The major configuration settings set at the MASTER modem
will be downloaded to the slave during synchronisation.
The modems can be set to IND SLAVE (independent Slave) and IND MASTER. Here the
configuration is not transferred between the modems, and must be setup individually.
Using independent modes will result in a faster sync up process; but at the expense of
flexibility in having to set up both ends of the link.
The G703 interface can only be MASTER or IND MASTER.
6.3.2 Number Of Line Pairs
If you have a two pair modem you have the option of selecting between one and two pair
operation. If one pair operation is selected you will be restricted to a maximum data rate of
512kbps. If two pair operation is selected then higher data rates up to 1024kbps are available
(depending upon the model). The single pair modems and any modem with the G703
interface fitted allow one pair operation only.
When two pairs are selected each pair carries half of the data, thus a 256kbps link will
consist of two 128kbps lines and so a performance advantage is gained over a single pair
256kbps link.
6.3.3 Line Rate
The modem can be set to operate at the following rates:
Data Rate Modem Type
64K AM64/128EA, AM64/384A, AM64/512A,
128K AM64/768A, AM64/1024A
192K AM64/384A, AM64/512A, AM64/768A,
256K AM64/1024A
320K
384K
448K AM64/512A, AM64/768A, AM64/1024A
512K
576K AM64/768A, AM64/1024A
640K
704K
768K
832K AM64/1024A
896K
960K
1024K