06-DigitalDataComm

instead need to correct errors on basis of bits received error correction provides this
How Error Correction Works
adds redundancy to transmitted message can deduce original despite some errors eg. block error correction code
• e.g. modem, NIC
DCE transmits bits on medium DCE communicates data and control info with DTE
• Done over interchange circuits • Clear interface standards required
• if no remainder, assume no error
例6.4
1、给定 报文 D=1010001101（10比特）
预定比特序列P=110101（6比特）
帧检验序列R=由计算得出（5比特）
因此，n=15, k=10且(n-k)=5。 2、报文乘以25得到101000110100000
3、得到的数值除以P即：
Data Communications Interfacing
Characteristics of Interface
Mechanical
• Connection plugs
Electrical
• Voltage, timing, encoding
Functional
• Data, control, timing, grounding
Functional Specification
Circuits grouped in categories
• • • • Data Control Timing Ground
One circuit in each direction
• Full duplex
(See table in Stallings chapter 6)
Error Correction
correction of detected errors usually requires data block to be retransmitted not appropriate for wireless applications
• bit error rate is high causing lots of retransmissions • when propagation delay long (satellite) compared with frame transmission time, resulting in retransmission of frame in error plus many subsequent frames
two solutions to synchronizing clocks
• asynchronous transmission • synchronous transmission
Asynchronous Transmission
异步传输图例1
起始位 终止位 数据位
间隔不确定
Asynchronous - characteristics
• can use separate clock line • or embed clock signal in data
need to indicate start and end of block
• use preamble and postamble
more efficient (lower overhead) than async
burst errors
• contiguous sequence of B bits in which first last and any number of intermediate bits in error • caused by impulse noise or by fading in wireless • effect greater at higher data rates
Data and Computer Communications
Chapter 6 – Digital Data Communications Techniques
Objectives asynchronous versus synchronous transmission error detection and correction interfacing issues
110101 1101010110 101000110100000 110101 111011 110101 111010 110101 111110 110101 101100 110101 110010 110101 01110 Q 2n-kD
P
R
4、 25 D加上余数之后得到T=101000110101110，T被发送
EIA-232-F (USA)
• • • • • RS-232 Mechanical ISO 2110 Electrical v.28 Functional v.24 Procedural v.24
Mechanical Specification
Electrical Specification
DCE就绪 信号地
106 107 102
108
DTE就绪
Digital signals Values interpreted as data or control, depending on circuit More than -3v is binary 1, more than +3v is binary 0 (NRZ-L) Signal rate < 20kbps Distance <15m For control, more than-3v is off, +3v is on
Dial Up Operation (1)
Dial Up Operation (2)
Dial Up Operation (3)
RS232-DB25
RS232-DB9
RS232功能特性
计算机与调制解调器连接至少8根线路
1 2 3 4 保护地 发送数据 接收数据 请求发送 101 103 104 105 5 6 7 20 允许发送
• Also inhibits receive mode in half duplex
Modem responds when ready by asserting Clear to send DTE sends data When data arrives, local modem asserts Receive Line Signal Detector and delivers data
Procedural
• Sequence of events
V.24/EIA-232-F
ITU-T v.24 Only specifies functional and procedural
• References other standards for electrical and mechanical
• map k bit input onto an n bit codeword • each distinctly different • if get error assume codeword sent was closest to that received
Error Correction Process
Interfacing
Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE)
simple cheap overhead of 2 or 3 bits per char (~20%) good for data with large gaps (keyboard)
Synchronous Transmission
block of data transmitted sent as a frame clocks must be synchronized
• parity bit set so character has even (even parity) or odd (odd parity) number of ones • even number of bit errors goes undetected
Error Detection Process
Local and Remote Loopback
Procedural Specification
E.g. Asynchronous private line modem When turned on and ready, modem (DCE) asserts DCE ready When DTE ready to send data, it asserts Request to Send
Error Detection
will have errors detect using error-detecting code added by transmitter recalculated and checked by receiver still chance of undetected error parity