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

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

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?

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

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 No idle times between bytes(bits) in the frame.

Isochronous 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

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)

Sharing the medium Many users One channel

Multiplexing Space - division –physically separate channels (wires) Time - division –sharing a CPU in multiprogramming OSs –telephone connections to a switching station Frequency - division –tv channels on a cable line –telephone conversations on a TRUNK line –radio stations sharing the airwave

Space division User 5 User 6 User 7 User 8 User 1 User 2 User 3 User 4

Time and Frequency division User 5 User 6 User 7 User 8 User 1 User 2 User 3 User 4 Medium

Time Division time frequency User 1 and User 5 User 2 and User 6 User 3 and User 7

Frequency Division time frequency User 1 and User 5 User 2 and User 6 User 3 and User 7

Statistical Multiplexing Making the use of the medium more efficient Examples –cars on the highway –seats reserved on an airline flight –lines for making phone calls All overbook. Do not provide sufficient capacity to meet maximum demand. Provide less capacity. Save money. Usually good enough!

Data Transmission Example TDM SITE1SITE1 SITE2SITE2 A4A3A2A1A4A3A2A1 B4B3B2B1B4B3B2B1 C4C3C2C1C4C3C2C1 D4D3D2D1D4D3D2D1..A 2 A 1..B 2 B 1..C 2 C 1..D 2 D 1 D3C3B3A3D3C3B3A3 D4C4B4A4D4C4B4A4... Fully Utilized!

Data Transmission Example NOT Fully Utilized (9/16) SITE1SITE1 SITE2SITE2 A 4 A 3 …A 1 B 4 ….B 2 B 1 C 4 …….C 1 ……….D 1 …..A 1..B 2 B 1 …..C 1 …..D 1 ……….A 3...C 4 B 4 A 4... How Do We Make This More Efficient?

Under-Allocate SITE1SITE1 SITE2SITE2 A 4 A 3 …A 1 B 4 ….B 2 B 1 C 4 …….C 1 ……….D 1 …..A 1..B 2 B 1 …..C 1 …..D 1 A C 4 B 4 A bits overhead per frame saves wasted slots. Less capacity required. Unable to meet Maximum Demand. Overhead

Queueing Statistical multiplexing generates a whole new science Underallocating generates potential waiting lines –gas station –bank tellers –on-ramps at interstate –your personal “to-do” list ……………. Computer simulation –when to change resource amount (more tellers)

Multiplexing a subtle distinction Users trying to make calls –Statistical –Some users have to wait to gain access Calls actually on the line –Not Statistical –Once on, you consume the line as long as you are connected