Computer Technology and Software Professional Technical Qualification (Level) Examination-Soft Exam Intermediate Network Engineer Notes Two

Data communication method

1. According to the communication direction

1. Simplex communication
2. Duplex communication
3. Duplex communication

 

2 According to the synchronization method

1. Asynchronous transmission separates each character
2. Synchronous transmission

3. There are 3 types of data exchange technologies

1. Circuit switching (similar to making a phone call)
2. Message exchange (using store and forward, the difference from packet switching is that the message size is not fixed)
3. Packet switching

1. Datagram (IP)
2. Virtual circuit (X.25, FR frame relay, ATM)

 

 

Multiplexing technology

1. Frequency division multiplexing (radio broadcasting, CATV, ADSL)
2. Time division multiplexing TDM

1. Synchronous TDM--T1/E1 digital carrier, SONET/SDH (multiplexing technology in optical fiber)
2. Statistics TDM—ATM

1. WDM

 

 

Digital transmission system

Bell system T1 carrier (8kHz rate sampling, 128-level quantization [ie 7bit])

1. 24 channel multiplexing
2. The rate of each channel is 8000*7=56Kbps
3. 1.544Mbps after 24 multiplexing of T1 carrier
4. T2 carrier = 4 *T1 6.312Mbps
5. T3 carrier = 7 * T2
6. T4 carrier = 6 *T3

 

 

ITU-T E1 channel (8kHz rate sampling, 256-level quantization [ie 8it])

1) 32-channel multiplexing

2) The rate of each channel is 8000*8=64Kbps

    3) 30 channels are used to transmit data

    4) CH0 and CH16 are used to transmit control signaling

5) E1 carrier is 2.048Mbps after 32 multiplexing

6) E2 carrier=4*E1 8.448Mbps

7) E3 carrier=4*E2 34.368 Mbps

8) E4 carrier = 4*E3 139.264 Mbps

9) E5 carrier = 4*E4 565.148 Mbps

 

 

Synchronous digital series

SONET OC-1 channel is 50.112Mb/s (after each OC level is 3 times the previous level)

SDH STM-1 = 3*OC-1=155Mbps/s (STM-4, 16, 64 are 4 times of the previous level)

 

 

Hamming Code

The code distance is 2 (the redundancy bit and the data bit each have at least 1 change)--for example, 23 and 33 have at least one codeword different

    Can detect and correct errors

    Hamming code can only correct one error

 

1) 2n are all empty

2) Look at the seat number of the data bit and convert the seat number into binary.

Even parity is to make the corresponding data binary in the data bit into 0 (in reality, it can also be odd parity)

Check bit 2 corresponds to data bits 3, 6, 7, 10, 11, and the corresponding data in the corresponding data bit is

1, 0, 1, 1, 1 (see picture above), 4 even parity bits of 1 pair are 0

The parity bit 4 corresponds to the data bits 5, 6, and 7, and the corresponding data bits in the corresponding data bits are 0, 0, 1 (see the figure above), and a pair of even parity bits is 1.

 

 

 

m represents the number of bits of information

k represents the number of redundant bits

m+k+1≦2k

 

Cyclic Redundancy Code (CRC) has error detection and cannot be corrected

 

The highest power of the generator polynomial is the number of redundant bits

The coefficient is proposed as a divisor and the modulus is taken.

 

Low test point

PSTN-Public Switch Telephone Network

 

Data terminal equipment (DTE-Data Terminal Equipment, such as user computer)

Data Circuit Equipment (DCE-Data Circuit Equipment, interface used to manage the network)

 

The RS-232-C interface uses a 25-pin D-type connector

1. 1. (Electrical characteristics) Adopt V.28 standard circuit, data rate 20kb/s, distance 15m.
   2. (Functional characteristics) Adopt V.24 standard and use 44 connections.
   3. The signal source generates a 3-15V signal
   4. -3V--+3V is the signal level transition zone

RS-232-serial interface, using 9-pin connector

 

 

X.25 Public Data Network

Divided into three protocol layers, corresponding to the lower three layers of the IOS/OSI reference model

1. 1. Physical layer (X.21 protocol)
   2. Link layer (LAP-B protocol)
   3. Packet layer (PLP protocol)

Packet layer support:

1. Switched Virtual Circuit (SVC)
2. Permanent virtual circuit (PVC)

 

 

Three ARQ protocols

The stop-and-wait ARQ protocol is a combination of stop-and-wait flow control technology and automatic request retransmission technology

Select retransmission protocol: a combination of sliding window technology and automatic retransmission request technology

       W send =W receive ≦2k-1

Back N frame ARQ protocol (the size of the receiving window is always 1)

       W≦2k-1

Memory method (1-choose up and down)

 

 

 

HDLC protocol (Data Link Control Protocol)

    It is a synchronous link control protocol.

    PPP supports synchronous and asynchronous protocols

HDLC frame structure
F is the flag bit

A is the address

FCS check digit

 

 

 

The length of the frame relay information field is variable, and 1600 bytes is the default maximum length .

 

ISDN (Integrated Services Digital Network)

ISDN is divided into

1. 1. 1. 1. Narrowband ISDN

Mainly provide two interfaces

1. 1. 1. BRI (basic rate)=2B+D=144K (B is 64K, D is 16K)
      2. PRI (primary rate)=30B+D=2.048M (B is 64K, D is 64K)

2) Broadband ISDN

 

ATM (Asynchronous Transfer Mode) divides user data into groups, one packet is one cell, one cell is 53 bytes (5[control part]+48[payload])

ATM is based on virtual circuits (virtual circuits are connection-oriented and do not provide answers)

 

CBR—Fixed Bit Rate Service Fiber

VBR—Variable Bit Rate Service

    RT-VBR real-time  interactive compressed video signal transmission

    NRT-VBR non-real-time multimedia email

ARB—effective bit rate service

UBR—Indefinite bit rate service IP transmission

 

The 802 standard divides the data link layer into two sublayers

1. 1. Medium Access Control Sublayer (MAC)
   2. Logical Link Control Sublayer (LLC)

 

802.3 standard

CSMA/CD-Carrier Sense Multiple Access Control Detection

Monitoring algorithm

1. Non-persistent monitoring algorithm

Reduce the probability of conflict, reduce the utilization of the channel, and increase the transmission delay

1. 1- Persistent monitoring algorithm

High conflict probability, high media utilization, reducing channel idle time

1. P-persistent monitoring algorithm

 

After a conflict occurs in the Ethernet, the sender can try to send again after the JAM signal stops and waits for a random period of time

 

The binary exponential fallback algorithm takes into account changes in network load

 

 

Monitoring algorithm with conflict detection

   

 

 

 

 

The 7-byte preamble field is used for clock synchronization between the receiver and the sender

When the highest bit of the address is 0, it is a normal address, when it is 1, it is a group address

If the data part of the frame is less than 46 bytes, the padding field is used to achieve the required minimum length.

 

 

IEEE802.11 WLAN architecture

PLCP—physical layer convergence protocol

PMD—Physical medium related sublayer and PHY management sublayer

 

IEEE 802.11 defines 3 PLCP frame formats to correspond to 3 different PMD sublayer communication technologies

    FHSS

   

    Synchronization signal frame start symbol frame length data rate error 2 bit error

 DSSS

 

DFIR

 

2.4G

 

The 802.11 standard defines three access control mechanisms for the MAC sublayer

1. 1. Distributed coordination function DCF is carried out through CSMA/CA to support contention services. , To solve the problem of hidden terminals, the frame interval of DIFS (Distributed Coordination IFS) is adopted.
   2. Supports contention-free service through the point coordination function PCF, adopts PIFS (point coordination IFS) frame interval
   3. Support channel reservation through RTS/CTS, using SIFS (short IFS) frame interval

   

 

 

 

Answer: C

 

 

 

MANET network cannot use wired network protocols (such as rip, ospf), including

1. 1. Flat routing protocol

A priori/table-driven—mostly based on link state algorithms, suitable for little topology changes

Reactive/on-demand allocation-based on distance vector algorithm (DSDV [Distance Vector Protocol for Target Sorting])

1. 1. Hierarchical routing protocol
   2. Geographic Information Routing Protocol

 

 

 

AODV—Distribution-on-demand distance vector protocol

    Suitable for the rapidly changing Ad Hoc network environment