Transmission Modes Different ways of characterizing the transmission

Timing of the transmission of the data bits Serial –Data bits transmitted at different times –One bit after the other Parallel –Multiple bits transmitted simultaneously (same time) –Typically with different data “lines” for each bit

Alternating Interactions

Device 1Device 2 time data Simplex - one way (tv,radio, weather satellite)

Device 1Device 2 time data Half Duplex - alternate each way (telephone, cb, ham radio)

Device 1Device 2 time data Full Duplex - both ways same time (computer serial)

Timing between transmitter and receiver All transmissions are synchronized somehow –once per bit (Manchester) –once per byte –once per frame ….. Asynchronous (means without synchronization) but DOES synchronize once per BYTE. Awful name ASYNCHRONOUS

Serial (asynchronous) Encoding 0 StartBit Idle 1 Stop Bit FIRST LAST DataParity 0

Idle -> No information on the line Start Bit -> Defines the beginning of the byte Data -> Information (number of bits varies) Parity -> A check digit for correct reception (more later) Even/Odd/None Stop Bit-> A check for correct detection of start bit 1/1.5/2 bits long 0 StartBit Idle 1 Stop Bit DataParity 0

Start Bit Timing Bit Centers Clock -> 4 times faster that bit rate 2 ticks from beginning is bit center 4 ticks from there is next bit center

More Timing Details - Figure 3.5 Timer must be faster than the data rate Degree to which faster dictates how close to center you can get 16x (Fig 3.5.c.iii) shows only off 1/16th 4x (Fig 3.5.c.ii) shows only off 1/4th 1x (Fig 3.5.c.i) shows problems Page is misleading –1200/2400/9600 note is not for the diagram –i -> fastest data rate, iii -> slowest data rate –fastest clock ratio -> slowest data rate

Maintaining Synchronization Figure 3.9 DPLL digital phase locked loop count for transition if transition not seen as expected –adjust the timing interval by adding 1 tick subtracting 1 tick –on the next cycle. Used in many coding techniques. Book uses NRZI.. Forced change on 0, require a zero peridically.

Parity Counts number of ones in DATA Sets the parity bit to 1/0 –Even or –Odd May not choose to use at all (None) Not a good means of error detection Error in one bit … Error in 2 bits Assumes independence of bit errors … not always true

Parity examples DATA PARITY (even) = = 6 Use Second example and assume errors = 5 ERROR = 4 ??????? One can’t detect multiple bit errors properly!

Serial Transmission Many concepts in one byte Synchronization on a byte level Framing with start and stop bit Error detection with parity What does this cost us?

Efficiency A means of evaluating the nature of the cost to implements solutions to these fundmantal communication problems (data)/(data + overhead) Overhead is anything not part of that intended to be communicated for the receiver Be careful to pick a time which is repetitive when evaluating. Choose the right time is everything.

0 StartBit Idle DataParity 01 Stop Bit Efficiency Data Data + Overhead Efficiency = = = 8 11 = bps line modem = 1200 *.7272 = 872 bps ignoring idle!

Where would you see it? On a PC it is the COM1, COM2.. Port Typically RS232 interface –9 pin –25 pin –or others Modem, mouse, keyboard ASYNCHRONOUS because one can’t tell when the data will be transmitted from one byte to the next

Review Modem Communication Serial transmission

Figure 2.31 Interface between computer (dte) and modem (dce) Not all lines used Higher level controls present –request to send –clear to send asynchronous across xmit/recv interface can also be used for synchronous –line 16 one interface … both async and sync

Figure 2.32 Typical scenario Notice the timing between dce / dte Notice the timing between dce / dce Scenario describes half-duplex Full duplex minimizes the impact of “turnaround time” –revc –(turnaround) –send

Serial Summary Same name (asynchronous) used for two concepts –lack of timing –Serial (byte transmission) NOTHING in the name imples BYTE transmission but that is how it is used Synchronizes once per byte –assumes clocks will remain synchronized until the end of the byte Illustrates OVERHEAD

So what is Synchronous? Synchronizes –once per block of data not per byte Typically faster rates USB ports on a PC (find rates on www) –see –easier to understand after protocols More complex framing (each of these are bytes typically) (end) errordetect DATA control sync sync

Sync byte/string A pattern with which receiver can established synchronization The longer it is (to a point) the greater the reliability of the synchronization Like a start bit (e.g. ) No idle times between bytes(bits) in the frame.

Figure 3.12 SYN for character frames (idle) (start and end) synchronization maintained for frame duration

Character/bit stuffing What if timing/framing flags in beginning or ending? AND they are part of the data itself! Stuff extra symbols if in data Stuff even if “stuffed” version is in the data

Character stuffing SYN STX Data ETX SYN STX ETX Expected End of Data SYN STX ETX DLE ETX in data DLE ETX in dataDLE in data Receiver discards!

Bit Stuffing Basically the same! 5 zeros in a row, insert extra 0

Isochronous (periodic) Asynchronous –irregular gaps between bytes Synchronous –no gaps between bytes –gaps between blocks Isochronous –REGULAR gaps between blocks –telephone PCM –4000Hz -> 8000 samples/sec -> 8 bits/sample-> bps –What if on 1.5 Mpbs line?

Asynchronous Synchronous Isochronous Different arrival rates of bytes