Page 346 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
P. 346
338 CHAPTER 13 Communications
Idle time
Idle time
Data Idle Idle Flag Data
Next block
Bits
Idle Flag Data Data Data
One block
(a)
Incoming electrical signal
(b)
FIGURE 13.9
Synchronous transmission: (a) depicts data blocks and (b) displays bit recovery.
Sampling circuit
Clock extraction circuit
• Synchronous transmission mode (STM): While similar to ATM, STM (Figure 13.9) is carried out under the auspices of a timing source. All bit transactions are carried out with clock synchronization regardless of the bytes they belong to. There are not startstop bits and the data is transmitted in a continuous stream. While this method is quite effective for large mainframe computers with scheduled data transfers, ATM remains the method of choice in the subsea industry.
Figure 13.10 provides a human voice analogy of synchronous versus asynchronous transmission. Data is transmitted in discrete information packets (called “frames”). Within each frame (Figure 13.11) are the various tags, flags, and addresses used for messaging and delivery along with the actual message wrapped in the frame.
Data is transmitted to its destination through a data transmission channel. These data paths transport the electrical signal from the transmitter to the receiver. The channel is characterized by its bandwidth and signal-to-noise ratio (SNR) parameters. Further, baseband transmission is cen- tered on this data channel and is modulated via a signal coding scheme (discussed above) and is typically a DC signal modulated by one of these schemes.
Anything that is nonrandom can be compressed through some type of data compression method. The purpose of data compression is to allow for reducing the number of actual bits transmitted while maintaining the message integrity, making the compression function a part of the transmission.
As a last consideration, communications may be further broken down into point-to-point trans- missions or point-to-multipoint transmissions. These configurations will determine if a two-way communication link can be established or a more wide area “broadcast” arrangement is made.
