Before I knew it, I was about to take the soft test. I had the same question as everyone when I signed up. What is the use of the soft test certificate? I think it is to add some color to my resume. Although the gold content is not too high, it is better than Not strong, right? (Mainly a certificate with a lot of gold, I can't afford it as a student, I know it all). So in order to pass the exam at a time, I watch the video explanation on station b (it is indeed a good learning website with strong functions) every day. Not much nonsense, prepare for the first day.
osi seven-layer model (open system interconnection model)
physical layer (unit: bit, bt)
data link layer (unit frame, representing mac address)
network layer (unit grouping, representing IP address)
transport layer (segment, famous port 1 ~1023, dynamic port 1024~65535)
Session layer (PDU protocol data unit, process number 5~7)
Presentation layer
Application layer
Encapsulation (layer 1—>layer 7) and decapsulation (7—>1, the reverse process of encapsulation)
TCP/IP protocol
Network interface layer (corresponding to osi data link and physical, ARP protocol RARP)
network layer (IP: TCP6 and UDP17, RIP: 520UDP protocol number, OSMP: 89, ICMP: 1, IGMP: 2 Internet groups Broadcast management protocol: protocol number)
transport layer (TCP:http:80,Telent:23,FTP:21,STMP25,DNS:53,POP:110)(UDP:DNS:53,TFTP:69,SNMP:161,DHCP :67,) Abbreviated as STDD
application layer (corresponding application, presentation and session)
transmission:
a waveform is a symbol, symbol rate: the number of waveform changes within a unit time. The bit rate is the information rate: the amount of information transmitted in a unit of time.
The relationship between baud rate and bit rate:
bit rate = binary digits modulated in baud rate unit + baud rate log2N (N is the symbol type)
channel width = highest frequency-lowest frequency
(no noise) maximum data rate = 2Wlog2N =Blog2N
B=Baud rate,
S/N ratio and decibel: 1dB=10 log10S/N
S/N
(noisy) Limit data formula=W log(1+S/N)
S is the signal power and N is the noise power.
Modulation: PCM
Sampling: Sampling frequency is greater than 2 times the maximum frequency
Quantization: 2 to the 7th power 128, effective frequency 7, channel 7 + 1 xbit
encoding: good bit rate = and bit relationship
bit rate = baud rate * bit bit
communication
Serial communication (I want to eat in the cafeteria, queue one by one)
Parallel communication (multiple exits, subway ticket checking),
single communication (data only communicates in one direction; TV)
half-duplex communication (communication in switching directions, but not simultaneous communication: Walkie-talkie)
Full-duplex: (Allow simultaneous transmission of data communication in two directions: mobile phone)
According to the data synchronization relationship: Synchronous communication: (Send at the same time at a specific time. Asynchronous communication: Data cannot be transmitted at the same time)
Data exchange
circuit exchange "advantage delay Small, high idle rate, no error control, early telephone
message exchange: cannot be disassembled, cannot establish dedicated communication, or a message can be sent to multiple locations, the shortcomings need to be stored in the middle, and it takes a long time to send. Package
Data packet: Advantages: no need to establish a connection, disadvantages each packet is independent, poor reliability (IP)
logic circuit (virtual link) advantages improve reliability, disadvantages inflexible
multiplexing:
time division multiplexing: synchronization Expenses (E1=2.048, T1=1.544, SDH=155155520)
Wavelength division multiplexing: optical fiber communication
Frequency division multiplexing: wireless LAN 2.4G, 5G ping segment low ping transmission voice, high ping transmission data.
E1 length T=125, divided into 32 gaps, each gap is 8bit (7 voices + one control), 8000 frames per second, data rate 2.048Mb/s, each voice data rate is 64Kb/s. The actual voice is 56Kb
ratio: 4:4:4
T1 has 24 voice channels, each gap is 8bit, a total of 193bit, data rate=1.544Mb/s, each voice data rate is 64Kb/s. The actual voice is 56Kb.
Ratio: 4:7:6
FTTH:
FTTC (fiber to the road)
FTTB (fiber to the building)
FTTH (fiber to the home)
data link layer:
error detection and correction (error detection: code distance >=d+1, error correction : Code distance> 2d d is an error)
Hamming code: original: 101101, m+k+1<=2K =>7+k<=2 to the k power m=6
CRC check code: Example. Suppose the original information is 10110, the multinomial formula of CRC is G(x)=x's 4th power + 4+1. To find the CRC check code
1, fill in 0 after the original, and the level of x is 4. Fill in 4 0s in the back, new The information is 10110 0000
2, using the generator polynomial to divide, and 101100000/10011 (without carry addition) the remaining 1111
3, the result check code = 10110 1111
point-to-point protocol:
collision detection:
1, electromagnetic wave propagation delay 5us in the 1Km cable.
2. Ethernet stipulates that the minimum frame length of 10Mpbs Ethernet is 64 bytes, the maximum frame is 1518 bytes, and the maximum transmission unit (MTU) is 1500 bytes. , Less than 64 bytes is invalid frame, (Gigabit and 10 Gigabit is 512 bytes)
Throughput rate: actual transmission rate per unit time = frame length / (time spent + 1 frame time spent) = frame length / (network segment length / Propagation speed + 1 frame length? Network data rate)
network utilization = throughput rate / network data rate
802.1 series
IEEE802.1d: STP
IEEE802.1q: virtual local area network vlan
IEEE802.1s: multiple spanning tree protocol MSTP
IEEE802.1w: fast generation Tree protocol RSTP
IEEE802.1x: Based on port access protocol control (802.11)
IEEE802.3ab: Gigabit-twisted pair
IEEE802.3u: 100M
IEEE802.3z: Gigabit-fiber
IEEE802.3ae: 10G