Computer network knowledge finishing __ computer network

Key knowledge of computer network

A comparison of circuit switching, message exchange and packet switching

1. Circuit Exchange

Public telephone Network (PSTN) and mobile network (including GSM and CDMA network) are used in circuit switching technology, its basic feature is the use of connection-oriented approach, before the two sides to communicate, the need for both sides to allocate a fixed broadband communication circuit, Communication between the two parties in the communication process has been occupied by the allocation of resources until the end of communication, and in the process of building and releasing the circuit need to use the relevant signaling protocol. The advantage of this approach is that in the communication process can be guaranteed to provide users with sufficient bandwidth, and real-time, time delay, low cost of switching equipment, but at the same time the disadvantage is that the channel utilization is low, once the circuit is established regardless of whether the communication between the two sides of the distribution circuit has been occupied

2. Message Exchange

Message exchange is a unit of data exchange, the message carries the information of the target address, the source address and so on, and the transmission mode of storage and forwarding is adopted in the switching node; Because the packet length is very different, the long message can cause a lot of delay; in order to meet the needs of various length messages and achieve efficient purposes, Nodes need to allocate different sizes of buffers, otherwise it is possible to cause data transfer failure, so the buffer allocation for each node is also more difficult. Another disadvantage is that when an error occurs, the entire message is transmitted again.

3. Packet switching

Packet switching is still used for store-and-forward transmission, but a long message is segmented into several shorter groupings, and the packets are sent individually, with the source, destination address, and number information. Using packet switching technology, you do not need to establish a connection before you communicate, each node will first receive the packet from the previous node and save it in a buffer, and then select the appropriate link to send it to the next node according to the address information in the packet header. In this way, the bandwidth can be allocated dynamically according to the requirements of the user and the ability of the network in the communication process. Packet switching is more efficient than circuit-switched channel, but has a large delay. The problem with packet forwarding: queuing delay and increasing the overhead associated with the head.

In short, if the amount of data to be transmitted is large, and the transmission time is much larger than the call time, it is more appropriate to use circuit switching, when the End-to-end path has many segments of the link composition, it is more appropriate to use packet switching data transmission. In view of improving the channel utilization of the whole network, packet switching and packet switching are better than circuit switching, in which packet switching is smaller than packet switching, especially for the abrupt data communication between computers.

II. interfaces, protocols and services

Interfaces: Low-level primitive operations provided to the upper level

Protocol: The rules that must be followed for information exchange between the two sides of the same layer

Services: Rules to be followed for exchanging information between different tiers

Third, the computer network system structure

Four, the role of the network layer

• Physical layer: Sending and receiving binary bit streams through the transmission medium.

A typical specification, such as RJ-45, that belongs to the physical layer definition.

L Data Link layer: Data encapsulation into frame, data transparent transmission, data error detection.

Data Link layer protocol representatives include: PPP, Frame Relay, etc.

L Network layer: Responsible for routing data packets between subnets, providing communication services for different hosts in packet switching network.

Network layer protocol representatives include: IP, ICMP, IGMP and so on.

• Transport layer: Responsible for the communication between processes in two hosts. The transport layer also deals with end-to-end error detection (different from the data link layer), congestion control, flow control, and so on.

Transport layer protocol representatives include: TCP, UDP and so on.

Application layer: An interface that provides access to network services for an operating system or network application.

Application layer protocol representatives include: FTP, HTTP, SNMP and so on.

V. How data is transmitted between layers of the network

The physical layer, the data link layer, the network layer belongs to the OSI model low three layer, is responsible for the creation network communication Connection link, the transport layer, the session layer, the presentation layer and the application layer is the OSI model high four layer, is responsible for the End-to-end data communication. Of course, not all traffic is going through all seven layers of OSI, such as the transfer between physical interfaces, only in the physical layer, and the connection between routers and routers requires only three layers below the network layer.

Six, in the network between each layer, the data is in what unit transmission

The transmission unit of the data in the physical layer is called the bit, the transmission unit of the data link layer is called the frame, the transmission unit of the data in the network layer is called the packet, and the transmission unit of the Transport Layer data is called the message segment.

Vii. What are the main differences between the Nyquist guidelines and the Shannon formula?

Nyquist criterion: In any channel, the rate of the code element transmission is capped, the transmission rate exceeds this limit, there will be a serious problem of crosstalk between the code, so that the receiver of the code-yuan sentence (that is, recognition) becomes impossible.

Shannon formula: The limit of the rate of information transmission is obtained.

Viii. What is the difference between synchronous communication and asynchronous communication

Synchronous communication: The two sides of the communication must first establish synchronization, that is, the clocks of both sides should be adjusted to the same frequency. Both sides send and receive continuous synchronized bit streams.

Asynchronous communication: When a character is sent by an asynchronous communication, the time interval between the characters being sent can be arbitrary. Of course, the receiving end must be ready to receive at all times. The sender can start sending characters at any time, so it must be marked at the beginning and end of each character, plus the start and stop bits so that the receiver can properly receive each character. The advantage of asynchronous communication is that the communication device is simple and inexpensive, but the transmission efficiency is low (because the cost of starting bit and stop bit is a large proportion).

Asynchronous communication can also be a frame as the unit sent, the receiver must be ready to receive frames at any time. At this point, the header of the frame must have some special bit combinations so that the receiver can find the beginning of a frame, which is also called the frame delimitation. The frame bounds also contain the end position of the frame, and there are two methods: one is to have a particular bit combination at the end of the frame to flag the ending of the frame, or a field with a frame length in the frame header. It is to be noted that when frames are sent asynchronously, it is not that the sender must add the start and stop bits to each character in the frame before sending it, but rather that the sender can send a frame at any time, and the time interval between the frames and the frames can be arbitrary. The sender does not need to coordinate with the receiver before sending a frame (no need for bit synchronization first).

Nine, the difference between bit synchronization and frame synchronization

Bit synchronization: makes each bit of information received by the receiving side synchronized with the sender.

Frame synchronization: Identifies the starting and ending positions of a frame.

X. Three basic problems of Data Link layer protocol

1, seal into frame

is to add a header and tail separately before and after a piece of data, thus forming a frame.

2. Transparent transmission

No matter what characters can be placed in the frame to transmit the past, such a transmission is transparent transmission. The solution to the problem of transparent transmission is: The data link layer in the sender has an escape character "ESC" inserted before the control character "SOH" or "Eoh" in the data, while the data link layer at the receiving end deletes the inserted escape character before sending the data to the network layer.

3. Error detection

The use of cyclic redundancy test (crc:cyclic redundancy check) error detection technology, if only the use of CRC, you can only do to the frame error-free reception, that is, received frames in the transmission process has not produced errors, and did not require the data link layer to the network layer to provide "reliable transmission" of services.

Xi. the function of adapter (NIC)

1, an important function of the adapter is to carry out the data serial transmission and parallel transmission conversion.

2, because the data rate on the network and the data rate on the computer bus is not the same, so in the adapter must be equipped with data cache memory chip.

3, the adapter will also implement Ethernet protocol.

12, CSMA/CD (carrier monitoring multi-point access/collision detection) The main points of the Protocol

Multi-point Access: Description This is the Total line network

L Carrier monitoring: is "before sending the first listening," that is, each site before sending data to check whether there are other sites in the bus to send data, if there is, then do not send data, to wait for the channel to become idle and then send.

L Collision Detection: is "side-send-side monitoring", that is, the adapter edge to send data to detect the channel signal voltage changes, in order to determine whether they are sending data in other stations are also sending data.

13, why Ethernet has a minimum frame length and maximum frame length

The minimum frame length is set to distinguish between normal and short frames that are aborted due to collisions.

The maximum frame length is set to ensure that all stations can compete fairly for access to Ethernet. Because if a station sends a special data frame, the other stations will have to wait a long time to send the data.

14, ARP Principle of work

1. First, each host establishes an ARP list in its own ARP buffer to represent the correspondence between the IP address and the MAC address.

2, when the source host to send data, first check the ARP list has the corresponding IP address of the destination host's MAC address, if so, send the data directly, if not, send the ARP packet to all the hosts of "This network segment", which includes: the source host IP address, the source host MAC address, The IP address of the destination host.

3, when all the hosts of this network receive this ARP packet, first check whether the destination host IP address in the packet is its own IP address, if not, ignore the packet, and if so, first remove the source host's IP and MAC address from the packet and write to the ARP list, overwriting if it already exists. It then writes its MAC address to the ARP response package and tells the source that it is the MAC address it is looking for.

4, the source host received the ARP response packet, the destination host IP and MAC address to the ARP list, and use this information to send data. If the source host has not received an ARP response packet, the ARP query failed.

The broadcast sends an ARP request and a single advertisement sends an ARP response.

It is assumed that computer A sends ARP requests in a local area network and wants to find out the hardware address of computer B. Then all computers on the local area network can receive the ARP Request packet sent by this broadcast. Which computer uses the ARP response group to tell computer B the hardware address of Computer A

This distinguishes between two situations.

If both computer B and computer A are connected to the same local area network, then Computer B sends the ARP Response group.

If computer B and computer A are not connected to the same local area network, then a router connected to the local area network must be forwarded to the ARP request group. At this point, the router sends an ARP answer group to computer A, giving its own hardware address.

16, the definition of IP protocol, what is the main role. What about TCP and UDP?

IP protocol (internet Protocol): Internet Protocol, which mainly provides the role of Internet interconnection.

TCP (Transmission Control Protocol, Transmission Control Protocol) is a connection-oriented agreement that must be connected to the other before sending and receiving data. UDP (User data Protocol, Subscriber Datagram Protocol) is a protocol that corresponds to TCP, it is a connectionless protocol, does not establish a connection to the other side, but sends the packet to the past directly.

17, Link layer broadcast and IP broadcast what is the difference

The link layer broadcast is to broadcast the MAC frame of all the hosts on the LAN by the Data Link layer Protocol (the second layer) on an Ethernet.

IP broadcasts are IP datagram broadcasts of all hosts on a network (that is, the destination network), implemented over the Internet using IP protocols.

18. ICMP (Internet Control Message Protocol) Internetwork Protocol

There are two kinds of ICMP message: Error report message and inquiry message.

19, when running ping 127.0.0.1, this IP datagram will be sent to WHO

127.0.0.1 is the loopback address. The host sends the tested IP datagram to the host's ICMP (instead of sending it to the Internet) for loopback testing.

20, the characteristics of the RIP protocol. The advantages and disadvantages of RIP. What the RIP uses to transmit.

1, features

L Exchange information only with neighboring routers

L The information that the router exchanges is all the information known to the current router, that is, its own routing table

L Exchange routing information at a fixed time interval

2. Advantages and Disadvantages

Advantages: Simple to implement and less expensive

Disadvantage: Limits the size of the network (because it can use the maximum distance of 15), bad news spreads slowly.

3, rip with UDP to transmit

21, the characteristics of the OSPF protocol. What the OSPF uses to transmit.

1, features