ATL Telecom AM2048 - Manual (Page 12)

ascom
telecom USER GUIDE AM768A/AM2048A
12
Issue 8
3.3 DSL Transmission Frame
Both of the above transmission systems operate in a bit pump mode. The DSL frame is sent over this
‘data pipe’. The DSL frame contains the following
• Sync word
• Stuff bits
• Stuff control bits
• EOC channel
• Customer data
The nominal DSL frame is always 6ms long, regardless of the transmission rate of the individual
channels.
The DSL frame length is adjusted slightly by the use of stuff bits. There may be 4 stuff bits or 2 stuff
bits per frame. The stuff bits are used to adjust the effective payload bandwidth of the DSL frame.
If the user rate clock is slightly quicker than the line rate clock then less stuff bits are sent.
Alternatively, if the user rate clock is slightly slower than the line rate clock then more stuff bits are
sent. This mechanism allows the line rate bandwidth to be adjusted to match the user rate bandwidth.
At the receive end, the rate at which the stuff bits arrive is used to recover the user clock.
The transmit and receive paths may be operated independently from one another so that when a
G.703 user interface is present, the transmit and receive clocks are allowed to vary independently by
+50ppm. (Note. When operating with X.21 or V.35 interfaces, there is only a single clock at each user
interface.)
The Embedded Operation Channel (EOC) is carried in spare overhead bits in the transmission frame.
Packetised SNMP SET and GET messages are passed over the link from the ELU to the NTU, which
answers with the appropriate SNMP response.
Byte timing is maintained by the transmission system and is available if required.
3.4 Control Circuit
The control circuit is based on a micro-controller and determines the operational status of the unit
according to the state of the transmission system, the data interface and the configuration information
received from the terminal.
Configuration data and two copies of the application are stored internally in FLASH memory. On
power-up the application program is copied from FLASH to RAM from where it is executed. The main
application program is backed up so that if a corruption occurs it can be corrected. This also enables
the programming of a new application while the equipment is operating normally. Control is
transferred to the new application after an automatic restart at the end of the download. This
minimises the interruption of payload traffic. Also, if programming is interrupted, the previous version
of software will remain intact. A new application program may also be downloaded from the ELU to
the remote NTU over the line.