ATL Telecom AM64/512 - Manual (Page 6)

a telecom telecom User Guide User Guide High Speed Modems High Speed Modems
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2. 2. Constructional Details Constructional Details
2.1 2.1 Baseband Modems Baseband Modems
The BBM is housed in a professionally styled plastic case. On the front is a menu driven LCD with six keys
and five LED’s. On the rear panel there is a 3-pin IEC mains inlet socket, an 8 way, 8-loaded RJ45 line cord
socket, one data interface connector and a VT100 Management Port (9-way D-type). The modem’s line
connection is via an 8-way, 8-loaded RJ45 line connector cable. The line connections for pair 1 are to pins
7 (brown with white stripe) and 8 (white with brown stripe) and pins 4 (blue with white stripe) and 5
(white with blue stripe) for pair 2 in the RJ45 plug. The data interface connector type will be from one of
the six user specified interfaces available. They are detailed in section 5.3. Inside the plastic cover is a metal
case, which contains the main PCB. The metal case is for EMC screening purposes. There is also a front
panel assembly, comprising of the LCD, the keys and the LED’s which are attached to the main PCB by two
connectors feeding through the metal case.
The BBM is also packed with a 2.4 metre long mains lead, terminating on one end with a U.K. 3-pin mains
plug as standard (or other plug if specified) and 3-pin IEC connector on the other. Also included is a 3
metre long double-ended RJ45 connector 8 way, 8-loaded line cord or alternatively, an RJ45 to bare ended
cable for direct connection. These cables are both packed with the unit in separate tie-wrapped polythene
bags.
The overall dimensions of the unit are 274mm(L) x 251mm(W) x 55mm(H).
3. 3. System Overview System Overview
The AM High Speed baseband modems use the digital transmission system designed and developed by
Ascom Telecommunications Ltd. in Cardiff, U.K. It is intended for operation on 2-wire local telephone
network circuits, such as those meeting BT EPS-9 (2-wire), (Note: the AM64/768 and AM64/1024 require
two such circuits). They will operate satisfactorily on unloaded lines having a wide range of characteristics;
bridge taps can be tolerated, dependent upon their characteristics. Although the system requires a
baseband circuit, a continuous loop at DC is not required.
The system can transmit data at rates between 64k and 1024k. The lower rates are useful in situations
where transmission performance is being limited by attenuation or noise.
The line signal comprises of 2B1Q or 3B1O data and an embedded sync pattern, which is used for timing
control.
Echo cancellation is used to eliminate the unwanted reflections of the transmitted signal from the receiver
input. The echo canceller can eliminate echoes of up to 16 bit periods duration. To counteract the signal
distortion inherent in long lines, the system employs adaptive decision feedback equalisation to eliminate
trailing intersymbol interference; this also has a span of 16 bit periods.
A fixed linear equaliser is incorporated to control the received pulse waveform and to reduce the effect of
long tails of both echo and transmission responses. Digital signal processing is used throughout to achieve
high performance reproducibly.
The line is connected to the transmission circuit via a line transformer, which acts as a balun and provides
isolation; there is surge protection across the transformer line connections. The transmission circuit utilises
a custom IC and a number of proprietary components to perform the signal processing described above.
The interface circuit performs a number of functions: it converts between the user data from the
appropriate interface and the data which is transmitted down the line, it also links the control circuit to the
control lines of the data ports. Again, these functions are performed by a custom IC supported by a
number of standard components. Looking at each function in turn, user data between 64kbps and
1024kbps is sent as-is; byte timing is maintained by the transmission system and is available if required.