Computer Network Review

Computer Network Review

Computer network This course is this semester, "Software Technology Foundation" one of the course content, accounted for only 20 hours of teaching, scaled score is also about 30 points. But the knowledge of this course is too much, too miscellaneous, and often closely related to life, such as: why the router with the time is 192.168.0.1? Why is the intranet within the campus inaccessible outside the campus? Why the Internet is slow to set up a DNS this thing? These content let me want to know a little more about the knowledge, but the time is not much, can only listen to the class, the class repeatedly thought, by taking two books outside the Book 1 as a supplement. After all, the TCP/IP 4-layer protocol, and the 5-layer protocol in the book, each layer tells the process to "indecisive", but also "mastery", along with the most of the fields I have learned the nouns, it is not easy to understand. So in this review summary, will add some life-related ideas, not just a summary of knowledge.

Tomorrow will be the exam, today can not be summed up, dig a hole, back to fill.

Directory

Chapter One physical layer

Chapter II Data Link Layer

Chapter III Network layer

Fourth transmission layer

Fifth application layer (temporary space)

The sixth chapter related noun

The seventh chapter of the computer network in life

The eighth chapter analysis

Chapter One physical layer

Physical Layer Overview: Each layer has a responsibility for each layer, the physical layer of the overview is: How to send? How to collect? 2

2. The primary task of the physical layer is described as some of the features that determine the interface to the transport media, namely:

   • Mechanical properties
   • Electrical characteristics
   • Functional characteristics
   • Process characteristics

4. Quadrature amplitude modulated QAM

   • Using a variety of phase, amplitude, frequency, to generate a variety of code elements, so that each code element can represent a multi-bit 3

6. Shannon Formula

   C = W log2 (1+s/n) b/S

   • W for Channel bandwidth

     • S/n for signal-to-noise ratio
     • S is the average power of the signal transmitted within the channel
     • N is the Gaussian noise power inside the channel
   • Shannon's formula shows an ideal transmission rate, but the actual channel will be much lower.

8. Various cables

   • Twisted pair Wire
   • Coaxial cable
   • Optical cable

10. Channel Multiplexing Technology 4

    • Frequency Division Multiplexing

      • There is waste

    • Time Division Multiplexing

      • There is waste

    • Statistical Time Division Multiplexing

      • Reduce waste

    • Wavelength Division Multiplexing

      • Frequency Division multiplexing of Light

    • Code Division Multiplexing (CDM) 5

      • Commonly used: CDMA (Code Division Multiple Access)
      • The chip sequence should be orthogonal

12. Waveform

    • Mann Code
    • Differential Mann Code

Chapter II Data Link Layer

Data Link Layer Overview: How is it transmitted? How good is it?

The data link layer increases the frame head and tail at the first paragraph to increase the frame tail (CRC check)

2. Error checking

   • Parity check (this is a technology)
   • Internet checksum (This is the transport layer)
   • Cyclic check code (this is a technique)
   • CRC check (this is the data link layer, in the tail package)

4. MAC Protocol (media access control) 6

   • TDMA (Time Division Multiplexing)
   • FDMA (Frequency Division multiplexing)

   • Random Access Protocol (high utilization)

     • Time Slot Aloha

       To the time-phased, want to send each paragraph has a probability p sent, if the conflict, if not sent, continue to each time slot with the probability of p, until the conflict sent out.

     • Pure Aloha

     • CSMA (carrier detection multiplexing)

       • Features: Benfa side listening
       • Insist: A idle on the probability of a p re-hair
       • Non-insistence: a conflict, random wait a while to try again

     • Csma/cd

       • Sending mechanism with conflict detection
       • This introduces a minimum Ethernet frame length of 7
       • Application of CSMA/CD in Ethernet: exponential Backoff algorithm 8

6. LAN Technology

   • ARP9

     • Process:

       1. A know the IP address of B, need to understand the physical address of B
       2. A broadcast ARP query frame that contains the IP address of B all LAN hosts receive ARP queries
       3. b receives an ARP frame and returns its physical address to a
       4. A caches the received IP/MAC address pairs until the information expires (timed out)

   • RARP 10

     • For diskless workstations find native IP

   • Ethernet Frame Encapsulation

     • (8 bytes of pre-sync code, not in the etheric frame) + 6-Word program address + 6-byte send address + 2-byte-length +IP datagram + 4-byte CRC checksum

8. Hubs

   is essentially a repeater on the work-at-a-stream level: The received Bitstream is replicated on all other interfaces 11

10. Network Bridge

    The bridge has the function of filtering frames. When a bridge receives a frame, it does not forward the frame to all of the connections, but instead checks the destination MAC address of the frame before determining which interface to forward the frame to 12

    • Network Bridge to create a forwarding post

      1. If a frame from a is entered into a bridge from the interface x, then a frame must be transferred to a in the opposite direction from the interface.

         1. Once the bridge receives a frame, it writes down its source address and the interface into the bridge as an item in the forwarding post.
         2. When creating a forwarding post, the source address in the frame header is written below the "Address" column.
         3. When a frame is forwarded, it is forwarded according to the destination address in the received frame header. At this point, the "Address" column has been noted below the source address as the destination address, and the recorded entry interface as a forwarding interface.

    • Spanning Tree algorithm

      • In order to avoid the waste of the bridge ring, the minimum spanning tree algorithm is used to make root in one of the bridges, and the spanning tree is computed if there is a failure or removal of the bridge.

Chapter III Network layer

Network layer Overview: How can I find a place to go? How can I give it to the port I want to give?

1. IP datagram Format

   • Minimum of 20 bytes

   • IP grouping reorganization

2. IP Address

   • The IP address is only relevant to the interface, not much to the host router
   • All 1 addresses in the network are broadcast
   • 127.0.0.1 Native IP
   • The ABC class has a private network address

3. Sub-Network Division

   • Divide an IP segment into multiple subnets so that departments can use each
4. DHCP dynamically assigns IP addresses

Fourth transmission layer

Transport Layer Overview: How can I give it to the program?

2. Port

   • Range is 0~65535 (16 x bits)
   • 0~1023 is a low-end, reserved for some well-known applications or system applications
   • 1024~65535 for high-end

3. TCP Message Section header details

   
   

   • Source Port (16): Port number of the sending port

     • Destination Port (16): Port number of the receiving end

     • Ordinal number: Sequence the first byte of data in this article

       • the sequence number is randomly generated when the connection is established (pseudo-random)
     • Confirmation Number (32): Expecting to receive the first byte ordinal in the data of the next message segment sent by the other party
     • Data Offset (4): 4 bytes, which refers to the starting position of the data in this section of the article as to how long it began. The data can also be interpreted as: How many 4 bytes the header occupies.
     • reserved (6): It's useless, it's all 0

     • URG (1): Emergency (Urgent)

       • Emergency indicates that there is urgent data in this datagram to be shipped to
       • this tight Urgency does not refer to the urgency of the data, but some of the connection functions need to be delivered as soon as possible

     • ACK (1): Request bit (ask)

       • request bit is 1, indicating request data, confirmation number valid
       The
       • request bit is 0, which means no data is requested, that is, the confirmation number is invalid
     • PSH (1): Push (push) 1 does not wait for the cache to be full and then to the application layer
     • RST (1): Reset (Reset) 1 indicates a crash, and must be released and re-established
     • SYN (1): Synchronous (synchronized) 1 means a connection request or a connection acceptance request.
     • FIN (1): Terminating (Final) has 1 means send complete, request release connection
     • window (16): unit byte, which allows the sender to control the transmit rate based on this data (see Flow control)
     • Checksum (+):
     • emergency Pointer (16): unit byte, the length of the data portion of the emergency data
     • option (m):
     • padding (n): The entire header is a multiple of 32 because the data offset field is 4 bytes in the Unit

5. Three phases of TCP data transfer

   • Establish a connection

     1. SYN = 1, seq = x
     2. SYN = 1, ACK = 1, seq = y, ack= x+1
     3. ack = 1, seq = x + 1, ack = y+1
   • Data transmission

   • Release connection

     1. After sending the data, the party that wants to release a send fin = 1, seq = u
     2. At this time the other party can also send
     3. ack = 1, seq = V, ack= u+1 If the other person is still sending.
     4. Until Fin = 1, ACK = 1, seq = w, ack= u+1
     5. A must give answer ack = 1, seq = U + 1, ack = w+1
     6. When B is received, it can be closed, a must wait for two MSL to close

7. TCP's caching mechanism and traffic control

   A sliding window is used to cache, ensuring that the buffer is not overrun by confirmation and window size

9. Congestion control of TCP

   • Slow start and congestion avoidance
11. Udp

Fifth application layer (temporary space) chapter sixth related nouns

1. Protocol: Protocol defines the format and sequence of information sent and received between network entities, and the actions required to send and receive information 14
2. ASDL (asymmetric subscriber lines): Asymmetric digital Subscriber Line 15

3. Packet names for each layer:

   • Data Link layer: frame, ether frame, Ethernet frame
   • Network layer: IP datagram
   • Transport Layer: TCP segment, TCP message segment
4. Unidirectional communication (simplex) [can only be sent], bi-directional alternating communication (half-duplex communication) [cannot simultaneously receive \ Send], two-way simultaneous communication (full-duplex communication) [may simultaneously receive, send]
5. Baseband signal: Data signals such as text and images are baseband signals
6. Band-Pass signal: The baseband signal is modulated by the carrier signal. (signals that can be transmitted)
7. Modulation: amplitude modulation, frequency modulation, phase adjustment 16
8. IP Address: 32-bit, network layer address
9. MAC address (LAN address or Physical address): 48-bit to get a grouping (or frame) between interfaces that are physically interconnected (on the same network)
10. TCP: Built on a non-connected unreliable protocol (IP) that provides a reliable, connected delivery protocol

The seventh chapter of the computer network in life

1. Network Bridge

   This word appears in the bridging of Virtual machine network settings

2. What is going on in the intranet of campus network?

   The IP address of the intranet is the IP address of the private network, these IP addresses can only access the external network and cannot be accessed by the external network.

3. Why is the intranet site not on the outside?

   See outside the IP is a unified public IP, intranet IP can not find a public network host

4. Mobile Internet I also have an IP address ah? Why are there so many mobile IPs?

   Carefully confirm that these IP are also private network IP, not public IP

The eighth chapter analysis

1. CSMA/CD Minimum Ethernet frame length

   This Ethernet frame length to meet the transmission delay is twice times the longest propagation delay in the entire line, so as to ensure that a host a every time, even if the furthest host B in the impending conflict (the channel busy information) issued its Ethernet frame, a can also be sent after the Ethernet frame to receive the latest possible conflict signal.

2. Maximum Transmission Unit MTU

   • MTU refers to the maximum length of data that an Ethernet frame can encapsulate

   • MTU is a logical limitation

     • A larger MTU can cause a long interaction response delay, but the utilization is high
     • Smaller MTU reduces interaction response latency, but utilization decreases
   • The MTU of the Ethernet is 1500 bytes

3. Hubs, bridges, routers

   • A hub is a physical layer device: received on the hair 17
   • A network bridge is a data-link layer device: Received look at which group, the group of 18
   • Router is the network layer device: received to see which network, consider how to go, sent to the next stop 19

2. Reference books include the fourth edition of the computer network Shehiren

   The introduction of foreign textbooks of black skin in computer network

   "TCP/IP Detail: Volume One"?

4. Personal understanding, the summary of each chapter is so??
6. Suppose there are 16 kinds of code elements, then for each code element designator, 1~16, you can use different code element to indicate its label corresponding to the bits, that is, 4 bits. But the more kinds require more precision.
8. This actually does not belong to how to preach, is how to hair of the good content, the main view of the data link layer of this technology discussion??
10. Analogy: You communicate with the machine in Chinese, I communicate with the machine in English.
12. Mac here is not talking about MAC address that Mac, that is, the media Access Control??
14. Because if the etheric frame is too short to detect all possible collisions at the same time, the etheric frame has been sent out, and it is impossible to confirm that the Ethernet frame just emitted has been sent out?
16. 1 conflicts in {0,1} randomly selected a time to send, the second conflict {0,1,2,3}, exponential increment, 10 times and then 1024 in the selection of one??
18. Determine MAC address based on IP??
20. Determine IP based on MAC address??
22. Receive a data to send to all nodes, inefficient??
24. This look at the data is going to the heap, if the original pile of no matter, the other heap on the tube??
26. In other words, there will be two times 1 when the connection is established.
28. "In-depth understanding of the computer system," the description, information is a bit + context, the agreement stipulates the fixed meaning of the context, is the network can continue to operate without ambiguity on the basis of the operation??
30. Upstream speed and downlink speed is not equal, (PT station Brush upload when obviously feel the fax slow. ） ??
32. Just like the triangle function in high school?
34. The equivalent of it does not exist, he is the root line??
36. This is the MAC address that is written down, to see which pile of which on which to send??
38. Express in the Xi ' An interchange level, to go to Changchun Express? Send to Shenyang Transit station!??

Computer Network Review