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Direct Link To This Post Topic: Base Band – Digital Communications
    Posted: 19/November/2008 at 16:13
Base Band – Digital Communications
 
Base band is the lowest ‘band’ of communications; we may have Ku Band Radio Frequency (RF) at the top, L Band Intermediate Frequency (IF) in the middle at the modem and then finally Base Band. The actual data that is being transported via satellite arrives at its intended destination from the modem.
 
Asynchronous RS-232 Communications
 
Asynchronous means the transmission of data without the use of an external clock signal. The timing required to recover the data from the communication link is encoded within the data link. Asynchronous communications allow for the transmitter and receiver clock generators to not be exactly synchronised or aligned.
 
If we assume the data being sent is text based characters then the following will make more sense:
 
Start Bits:
 
Asynchronous data is framed by use of start bits, character bits, parity bits and stop bits. This allows for receiver synchronisation to extract the data in the correct format.
 
[ Start Bit ] [ Data Bits ] [ Parity ] [ Stop Bits ]
 
        1                 8                 0               1
 
The start bit is followed by 8 data bits, no parity bit and one stop bit, for a 10-bit frame.
 
These days there is nearly always 1 start bit and so it is often not specified.
 
Data Bits:
 
The data bits are the number of bits per character. 7 bit characters are now rarely used and more often than not you will find that 8 bit characters are used. Both 7 bit and 8 bit characters are common in the American Standard Code for Information Interchange (ASCII), format.
 
Parity Bits:
 
A parity bit is added to ensure that the number of bits in a given set of bits is always even or odd. That is the number of bits with a value of 1. This is a simple form of error checking. 01101110 have 5 bits with a value of 1. If the parity is even the parity bit will also be 1.

If the parity bit is not used it may be called 'mark', where the parity is always 1, or 'space'  where the parity is always 0.

 
Stop Bits:
 
The minimum stop bit quantity can be more than 1. Some systems required 2 stop bits and some require 1.5 stop bits. Equipment that don't support fractional stop bits quantities can be set to 2 stop bits for transmit and 1 stop bit for receive.
 
Baud Rate or Speed:
 
Rather than go into the history and complexity of baud rates here, see the Wikipedia page: http://en.wikipedia.org/wiki/Baud_rate
 
 
Synchronous Data Links:
 
RS-232, RS-422, RS-423, X-21, RS-530, V11, V24, V35 and G703 are all examples of synchronous communications standards.
 
All this means is that they have both data and clock for both transmit and receive lines. Some of these standards are balanced and others are single ended (unbalanced). These differences in the interfaces are discussed later.
 
---------o Tx Data
---------o Tx Clock
 
---------o Rx Data
---------o Rx Clock
 
---------o Signal Ground
 
In it’s simplest form this is a synchronous un-balanced interface.
 
The transmit data has an associated transmit clock which is used to clock the data in or out of the sending device and the edges of the clock are aligned or synchronised with the edges of the data.
 
At the receiving device the received data is clocked in or out using the receive clock which is also aligned with the edges of the receive data.
 
The complexities arise with the directions of the clocks and data which are dependant on the devices in terms of DCE or DTE interfaces. DCE stands for Data Communications Equipment and DTE stands for Data Terminal Equipment. A modem is usually a DCE whereas a computer is usually a DTE. A DCE is normally connected straight to a DTE using pin to pin straight cabling. This means that the Tx Data from the DTE is connected to the Tx Data on the DCE because on the DTE this is an ouput and on the DCE it is an input.
 
Connecting two computers via a modem link would involve a DTE (computer #1) connected to a DCE (modem #1), then over a link to the DCE (modem #2) which is connected to the other DTE (computer #2).
 
Data and Clocking:
 
DCE data and clocks are always received on the transmit lines and transmitted on the receive lines. This is because the interfaces are always referenced to the DTE.
 
This is why a DTE to DCE interface cable is a straight pin to pin cable. A DCE to DCE or a DTE to DTE is a crossover cable because the DTE interfaces are expecting to receive the transmit data and clocks on the receive data and clock inputs and visa versa. The same goes for the DCE interfaces except they are expecting the receive data and clocks on the transmit data and clock inputs and visa versa.
 
A further complexity comes from clocking schemes:
 
 
Network Timing:
 
In standard point to point communications, that do not involve satellite links but are effectively over copper wires end to end, the interfaces are straight forward.
 
Data and clocks are sent from end A to end B and visa versa. There is little or no delay in the arrival of data and clock.
 
If a communications network (such as the BT network) is in-between end A and end B then the clocking scheme can become interesting. The network sometimes supplies both the transmit and receive clocks. It is a DCE. Data is clocked into it and out of it using the same clock. This clock is derived from a very stable atomic clock. There is no better clock.
 
End A and end B must use the clock to transmit the data and receive the data. For this reason most equipment has inputs for both transmit and receive clocks. This equipment may also be a DCE and that means that it is to be connected using a crossover cabling method as DCE to DCE.
 
END A                                                                                                                       END B
Tx Data  o---------<--------o Rx Data         Tx Data o---------<--------o Rx Data
Rx Data  o--------->--------o Tx Data         Rx Data o---------<--------o Tx Data
                                      [                                          ]
                                      [         NETWORK CLOUD     ]
                                      [                                          ]
Tx Clock o---------<--------o Rx Clock       Tx Clock o--------->--------o Rx Clock
Rx Clock o---------<--------o Tx Clock       Rx Clock o--------->--------o Tx Clock
                                                          
As you can see from this diagram the network (sometimes known as a cloud) is providing timing for both ends. This is ok as it is the same clock so all the data is being clocked in and out at the same time. This is synchronised or 'synchronous' data.
 
If you now put a satellite link at one end between the cloud and the user then there is a problem. The network clock has to be sent over the satellite link.
 
Satellite Modem Clocks:
 
END A                                                                                                                       END B
Tx Data  o--------->--------o Tx Data         Tx Data o---------<--------o Tx Data
Rx Data  o---------<--------o Rx Data         Rx Data o--------->--------o Rx Data
                                      [                                          ]
                                      [         SATELLITE LINK         ]
                                      [                                          ]
Tx Clock o--------->--------o Tx Clock       Tx Clock o---------<--------o Tx Clock
Rx Clock o---------<--------o Rx Clock       Rx Clock o--------->--------o Rx Clock
 
The clocks from end A need to arrive at end B with the data. This does not always happen because the end A clock is used by the satellite modem to clock the data in to the modem. At end B the modem uses either a ‘recovered clock’ derived from the received demodulated data on the satellite link or generated ‘internally’ by the modem, to clock out the data which is then hopefully in sync with the received data and this is fed to the receiver at end B.
 
If end B wants to supply the clock to the modem or rather the network clock to clock the data out from the modem then the modem will need to be able to use the clock from the network for both transmit and receive clocks.
 
Satellite Buffers:
 
If there is a buffer in between the modem and end B then the clock from end B is used to clock out the data from the buffer and the modem clocks its data into the buffer using its own clock. The buffer then takes up any differences and hopefully copes with the situation. Major differences between the clocks could result in the buffer overflowing and massive data errors reducing the availability of the link.
 
Thankfully, these days the satellite buffer is built-in to the modem which is an increasingly sophisticated piece of equipment. It may provide a host of clocking options and buffer settings to match and interface with the network.
 
Next we will look at the different standards in more detail.
 
 
 


Edited by Satcoms UK - 05/March/2015 at 18:02
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