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
01011
1 1 0 1 0
01011
01011
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
Idle -> No information on the lineStart Bit -> Defines the beginning of the byteData -> Information (number of bits varies)Parity -> A check digit for correct reception (more later) Even/Odd/NoneStop Bit-> A check for correct detection of start bit 1/1.5/2 bits long
0
StartBit IdleIdle
1
Stop Bit
0 0001 1 1 1
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 10-6 … Error in 2 bits 10-12
• Assumes independence of bit errors … not always true
Parity examples
DATA
0 0 1 0 0 1 0 0 1 1 1 0 0 0 1 1
PARITY (even)
0
1
0 + 2 = 2
1 + 5 = 6
Use Second example and assume errors1 0 1 0 0 0 1 1
1 0 0 0 0 0 1 1
1
1
1 + 4 = 5 ERROR
1 + 3 = 4 ???????
One can’t detect multiple bit errors properly!
Serial TransmissionMany 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 IdleIdle
0 0001 1 1 1
DataParity
01
Stop Bit
Efficiency
Data
Data + OverheadEfficiency = =
8
1 + 1 + 8 + 1 =
8
11 = .7272
1200 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 www.pcs.cnu.edu/~dgame/cs335/topics/usb.ppt
– 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• 010101010101• 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-> 64000 bps
– What if on 1.5 Mpbs line?
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
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 ExampleTDM
SITE
1
SITE
2
A4A3A2A1
B4B3B2B1
C4C3C2C1
D4D3D2D1
..A2A1
..B2B1
..C2C1
..D2D1
D3C3B3A3D4C4B4A4...
Fully Utilized!
Data Transmission ExampleNOT Fully Utilized (9/16)
SITE
1
SITE
2
A4A3…A1
B4….B2B1
C4…….C1
……….D1
…..A1
..B2B1
…..C1
…..D1
……….A3...C4B4A4...
How Do We Make This More Efficient?
Under-Allocate
SITE
1
SITE
2
A4A3…A1
B4….B2B1
C4…….C1
……….D1
…..A1
..B2B1
…..C1
…..D1
A30001C4B4A40111...
4 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)
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