1st, synchronous communication way characteristic:
When uses the synchronous communication, composes many
characters a message block, like this, the character may transmit one
after the other, but, (usually is called frame) in each group of
information the start must add on the synchronized character, when
does not have the information must transmit, must fill in the sky
character, because the synchronized transmission does not allow to
have the gap In the synchronized transmission process, a
character may correspond 5 ~ 8 Certainly, to the identical
transmission process, all characters correspondence same several, for
instance said n Thus, when transmission, according to every n
division is a time piece, the transmitting end transmits a character
in a time piece, the receiving end receives a character in a time
piece
When synchronized transmission, in an information frame contains
many characters, each information frame takes the start with the
synchronized character, generally uses the synchronized character and
the spatial character the identical code In the overall system,
uses the identical code by a unified clock controlling transmitting
end transmission and the spatial character The receiving end
certainly is should be able to distinguish the synchronized character,
when examines has string several matches with the synchronized
character, thought starts an information frame, thereupon, hereafter
several achievement actual transmissions information processes
2nd, face character synchronous protocol (IBM BSC agreement) This agreement had stipulated 10 special
characters (are called control character) to take the intelligence
transmission the symbol Its form is SYN
SOH
Title
STX
Block data
ETB/ETX
Block verification
SYN: Synchronized character (Synchronous
character), each coca 1 (single synchronization) or 2 (double
synchronization) synchronized character
SOH: Start of heading soh (Start of Header).
Title: Header, contains the source address
(transmission side address), the destination address (receiving end
address), the route instruction
STX: Start of text stx (Start of Text).
Block data: The main text (Text), is composed by
many characters
ETB: The block end of transmission (end of transmission
block), marks this end of data block
ETX: The full text finished (end of text), (full text
divides into certain transmissions).
Block verification: To starts from SOH, until
ETB/ETX field examination code
3rd, face bit synchronous protocol (ISO HDLC) An information may be the free position, combines
the marking frame with the position the start and the conclusion
The frame form is: F
AF field
C
I
FC field
F
F: Symbolizes the field; As a start
and the conclusion, the symbol character is 8, 01111110.
A: Address field, stipulation receiving end
address, but is 8 entire time of positions The receiving end
inspects each address byte 1st, if is "0", then behind with another
address byte If is "1", then this byte is last the address byte
C: Controlling field Instructs the information
field the type, 8 or 16 If 1st byte 1st is 0, then also has the
2nd byte also is the controlling field
I: Information field Must transmit data
FC field: Frame verification field 16 cyclical
redundancy check code CRC Besides F with automatic insertion
"0", participates in the CRC computation
4th, synchronous communication "0 insertions and deletion
technology"
In the synchronous communication, an information (or
several) the special character starts by, for example, F =01111110B.
But in the information frame other positions, completely
possibly appears these special characters, in order to avoid the
receiving end mistaking these special characters as the frame start,
the transmission side has used "0 insertions technologies",
correspondingly, the receiving end uses "0 deletions technologies".
Transmission side 0 insertions: Besides outset
character, when continuously appears 5 1, transmission side automatic
insertion 0. Causes in the entire information frame, only has
the outset character to include continual 6 1.
Receiving end "0 deletions technologies": The
receiving end receives continuously 6 1, took the frame the outset,
continuously appears 5 1 after 0 automatic deletions
5th, synchronous communication "byte packing technology"
Supposes the primary information frame which needs to
transmit is: SOT
DATA
EOT
The festival packing technology selects the
character replace method, causes the information frame in DATA not to
appear outset character SOT and conclusion character EOT.
Supposes presses down the table way to carry on the
replace: In DATA original character
The replace is
SOT
ESC X
EOT
ESC Y
ESC
ESC Z
Among them, ESC=1AH, X, Y, Z may assign for the
free character (except outside SOT, EOT, ESC).
Transmits Fang An to agree the way to the primitive
frame which needs to transmit to carry on the replace, after and
replace new frame transmission for receiving end
The receiving end carries on the opposite replace according to
the agreement way, may obtain the primitive frame information
6th, asynchronous communication and synchronous communication
comparison
(1) the asynchronous communication is simple, the
bilateral clock may permit the certain error Synchronous
communication more complex, the bilateral clock allowance error is
smaller
(2) the asynchronous communication is only suitable to selects
the <--> spot, the synchronous communication may use in to select
<-->. to be many
(3) corresponds the efficiency: Asynchronous communication
low, the synchronous communication is high
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