Basic knowledge of computer network

Source: Internet
Author: User
Overview of the Internet

1, three network integration: telecommunications network, cable TV network, computer network.
2, the Internet's two basic features: connectivity and sharing. Connectivity (connectivity): The Internet allows internet users, no matter how far apart, can be very convenient, very economical exchange of information (data, as well as various audio and video), as if these user terminals are directly connected to each other. Sharing is resource sharing: the meaning of resource sharing is manifold. It can be information sharing, software sharing, or hardware sharing. For example, download resources from the server.

3. The computer network consists of a number of nodes and links connecting these nodes (link). Nodes in a network can be computers, hubs, switches, or routers.
4, the network can also be interconnected through routers, which constitutes a larger coverage of the computer network. Such a network is called an interconnection network (internetwork or the Internet). Therefore, the Internet is the network of networks (net of networks).

5, the network connects many computers together, and the interconnection network connects many networks through routers. A computer that is connected to a network is often referred to as a host.

It is also important to note that network interconnection is not the mere physical connection of a computer because it does not achieve the purpose of exchanging information between computers. We also have to install many software on the computer that enables the computer to exchange information. So when we talk about network interconnection, it implies that the appropriate software has been installed on these computers, so that information can be exchanged over the network between computers.

6, two different meanings of internet and internet Internet interconnection, is a common noun. Refers to a computer network connected by multiple computer networks. Communication protocol can be any internet Internet or Internet, is a special term. The world's largest, open, and interconnected network of many networks. The TCP/IP protocol cluster is adopted as the communication rule. the composition of the Internet

(1), the edge part : By all connected to the Internet host composed. This is part of the user's direct use to communicate (transfer data, audio, or video) and share resources.
(2), the core part : A large number of networks and the routers connected to these networks. This section provides services for the Edge section (providing connectivity and switching).
the edge part of the Internet

The communication between the end systems at the edge of the network can usually be divided into two categories: the client-server approach (c/s) and the peer-to- peerapproach. 1, Customer one server mode (c/s mode)

The customer is the service requester and the server is the service provider.

2. Reciprocal mode (peer mode)

means that two hosts do not distinguish between the service requester and the service provider when communicating.

As long as two of the hosts are running peer connected software, they can communicate equally, peering.

the core part of the Internet

The core of the network is to provide connectivity to a large number of hosts on the edge of the network so that any host in the Edge section can communicate with other hosts.
In the core part of the network is the router (router), the router is a key component to implement packet switching (packet switching), whose task is to forward the received packet. 1. Circuit switching

(1), the process for establishing a connection (occupy communication resources) > Call (always occupy the channel resources) > Release (Release channel resources).
(2), an important feature of circuit switching is that in the full time of the call, two users of the call always occupy the end-to-end communication resources.
(3), circuit Exchange transmission efficiency is very low, because often communication is sudden, time is also very short, it may also cause a waste of resource consumption behavior. 2. Packet switching

(1), packet switching is the use of storage and forwarding technology .
(2), we call the whole piece of data to be sent as a message (message). Before sending the message, the longer message is divided into a smaller equal length data segment, in front of each data segment, plus some of the necessary control information consisting of the header (header), it constitutes a grouping (packet). Grouping is also referred to as "package", and the header of a group can be called "Baotou". Grouping is a unit of data that is transmitted over the Internet. The "header" in the group is very important, because the packet header contains important control information such as the destination address and the source address, each packet can select the transmission path independently in the Internet and be correctly delivered to the end of the packet transmission.

(3), the core layer is connected by many networks and routers, and the host is mainly in the edge portion, the core layer of routers and routers are connected with a high-speed link, and the edge router connected to the core layer is connected with a relatively low rate.
(4), the host is for the user to carry out information processing. Routers are forwarding groupings, which are packet-switched.

The router receives a packet, stores it temporarily, checks its header, finds the forwarding post, follows the destination address in the header, finds the appropriate interface to forward it, and forwards the packet to the next router. Such a step-by-step (sometimes through dozens of different routers) to store the forwarding way, the packet delivery to the final destination host.
Each router must frequently exchange routing information with each other in order to create and dynamically maintain forwarding in the router, so that the forwarding can be updated as the entire network topology changes.

(5), the advantages of packet switching

(6), packet switching of the shortcomings of the packet in each router storage and forwarding needs to queue, which will cause a certain delay. Therefore, it is necessary to try to reduce this delay. Because packet switching is not like circuit switching by establishing a connection to guarantee the various resources required for communication, it is not possible to ensure that the bandwidth required for the end-to-end communication. The control information that each packet must carry also creates a certain overhead (overhead). The entire packet-switching network also requires specialized management and control mechanisms. 3. Message Exchange

There is no requirement to establish a dedicated path between two communication nodes. The node organizes the information to be sent into a packet, which contains the address of the target node, and the complete message is transmitted forward from one station to the other in the network. Key features of the three switching modes in the data transfer phase:

Circuit switching-the bitstream of the entire message is continuously flowing from the source to the end point, as if it were being transported in a pipeline.

Message exchange-The entire message is first transmitted to adjacent nodes, all stored down to find the forwarding post, and forwarded to the next node.

Packet switching-a single grouping (which is only part of the entire message) is transmitted to adjacent nodes, stored down to find the forwarding post, and forwarded to the next node.

It can be seen from the graph that to transmit a large amount of data continuously, and its transmission time is much greater than the connection settling time, the transmission rate of the circuit exchange is faster. Packet switching and packet switching do not need to pre-allocate transmission bandwidth, which can improve the channel utilization of the whole network when transmitting burst data. Since the length of a packet is often much smaller than the length of the entire message, packet switching is less time-delay than message exchange and has better flexibility. Classification of computer networks according to the scope of the network

1. Wan Wan (Wide area Network): networks with a range of dozens of to thousands of kilometres, the WAN is a central part of the Internet, and its mission is to deliver data sent by hosts over long distances (for example, across different countries). The links connecting the nodes of the WAN are generally high-speed links with large communication capacity.
2, MAN (Metropolitan area network): Generally a city 5-50km. A metropolitan area network can be owned by one or several units, but it can also be a common utility used to interconnect multiple LANs.
3, LAN LAN (local area Network): a region such as the school around 1km
4. Personal region Pan (Personal area Network): A network that connects personal electronic devices with wireless technology. Its range is very small, about 10m or so. according to the users of the network

1, public network: refers to the telecommunications companies (state-owned or private) to build a large-scale network. "Public" means that all those who are willing to pay according to the requirements of the telecommunications companies can use the network.
2, private network: A department to meet the needs of the Unit's special business work to build the network. Such networks do not provide services to persons outside the organization. For example, the military, railways, banks, electricity and other systems have a dedicated network of the system. the performance index of computer network Rate

Rate refers to the rate at which data is transmitted, which is also known as data rate or bit rate.
The unit of rate is bit/s (bits per second) (or b/s, sometimes also written as BPS, that is, bit per second).

The signals sent by the computer are in digital form. Bit is derived from binary digit, which means a binary number, so a bit is a 1 or 0 of the binary number.

When it comes to the rate of the network, it often refers to the rated rate or the nominal rate, not the rate at which the network actually runs. Bandwidth

Bandwidth is explained in two ways
(1), a signal has a band width unit Hz
(2), the bandwidth in the computer network indicates the ability of a channel to transmit data in a net, Unit B/S. Therefore, the network bandwidth represents the "highest data rate" that can be passed by a channel in a network over a unit of time. Throughput

Unit time through a network of actual data volume Unit B/S

Throughput is limited by the bandwidth of the network or the rated rate of the network. For example, for a 1Gbids Ethernet, which means that its rated rate is 1 gbit/s, then this value is also the absolute upper limit of the throughput of the Ethernet. Therefore, for 1 gbit/s of Ethernet, its actual throughput may also be only about a few Mbit/s, or even lower, and does not reach its rated rate.
Note that sometimes throughput can also be represented by the number of bytes or frames transmitted per second. Delay

(1), delay (delay or latency) refers to the time required for data to be transmitted from one end of the network to the other.
(2), data in the network experience the total delay is the following four kinds of Shiyan and:
Total Delay = Transmit delay + propagation delay + processing time delay + queue delay

transmit delay (transmission delay) is the time required for a host or router to send a data frame, which is the time it takes from the first bit of the sending data frame to the last bit of the frame to be sent.

The transmit delay is also called the transmission delay (we try not to use the term transmission delay, because it is easily confused with the propagation delay below).

The formula for sending the delay is:

propagation delay (propagation delay) is the time it takes for the electromagnetic wave to propagate a certain distance in the channel.
The calculation formula for propagation delay is:

These two types of delay occur in different places.
The sending time delay occurs in the transmitter inside the machine (typically in the network adapter, with no relationship to the length of the transmission channel (or the distance transmitted by the signal).
However, the propagation delay occurs on the transmission channel media outside the machine and is independent of the transmit rate of the signal. The farther away the signal is transmitted, the greater the propagation delay.

processing delay : The host or router takes a certain amount of time to process when it receives the packet, such as analyzing the header of the packet, extracting the data part from the packet, making the error check or finding the appropriate route, etc., which results in a processing delay.

Queueing delay : Packets go through many routers when they are transmitted over the network. However, the packet is queued for processing in the input queue after it enters the router. After the router has determined the forwarding interface, it also waits to be queued for forwarding in the output queue. This creates a queue delay. The length of the queue delay often depends on the traffic at the time of the network. When the network traffic is very large, there will be a queue overflow, so that packet loss, which is equivalent to the queue delay is infinite.
Delay Bandwidth Product

As shown in the following figure, this is a cylindrical pipe representing a link, the length of which is the propagation delay of the link (note that the link length is now expressed as a unit of time), while the cross-sectional area of the pipe is the bandwidth of the link. Therefore, the time-delay bandwidth product represents the volume of the pipe, indicating how many bits such a link can hold.

For example, a link is set up with a transmission delay of ten MS and a bandwidth of S-Mbit/s. Figure out

This indicates that if the sending side sends data continuously, the sending end is sent 200,000 bits at the end of the first bit sent, and these 200,000 bits are moving forward on the link. Therefore, the delay bandwidth product of the link is also known as the length of the link in bits. round trip Time RTT


Computer network Architecture

Note: The architecture of Layer Five protocol is only designed to introduce the principle of the network, and the actual application is the TCP/IP four layer architecture. five-layer protocol

1. Application layer (Application layers)
(1), the task of the application layer is to complete the specific network application through the interaction between application processes .
(2), the Application layer protocol defines the rules for the application of inter-process communication and interaction.

The process here refers to the program that is running on the host. Different application layer protocols are required for different network applications. such as domain Name System DNS, HTTP protocol supporting Web application, SMTP protocol supporting e-mail, etc.

(3), the data unit of the application layer interaction is called the message (message).

2. Transport layer (transport layers)
(1), the Transport layer of the task is responsible for two hosts in the communication between the process to provide a common data transfer services . The application process uses the service to deliver the application beginning text.

The so-called "universal" refers to a particular network application, but multiple applications can use the same transport layer service.

Because a host can run multiple processes at the same time, the transport layer has the functionality of multiplexing and splitting.
Multiplexing is the service that multiple application-tier processes can use at the same time as the transport layer below,
split and reuse instead, the transport layer delivers the information it receives to the corresponding process in the application layer above.

(2), transport layer mainly uses the following two kinds of protocols: Transmission Control Protocol TCP (transmission-Protocol)-Provides connection-oriented, reliable data transmission services, the unit of data transmission is the message segment ( Segment). User Datagram Protocol UDP (User Datagram Protocol)-Provides a non-connected, best effort (best-effort) Data transfer Service (which does not guarantee the reliability of data transmission), the data transmission Unit is the user datagram.

3. Network layer
(1), the network layer is responsible for the packet switching network of different hosts to provide communication services . When sending data, the network layer encapsulates the packet or user datagram generated by the transport layer into a packet or packet for transmission.

In the TCP/IP system, because the network layer uses the IP protocol, the packet is also called an IP datagram, or is referred to as the number of reports.

Please note: Do not confuse the transport layer's "User datagram UDP" with the "IP datagram" of the network layer. In addition, regardless of the level of the transmission of data units, can be generally used as "grouping" to represent.

(2), the network layer of another task is to select the appropriate route , so that the source host transport layer of the packet down, to be able to find the destination host through the network router.

The internet is interconnected by a large number of heterogeneous (heterogeneous) networks through routers (router) . The network layer protocol used by the Internet is a non-connected Internet Protocol IP (Internet Protocol) and a number of routing protocols , so the network layer of the internet is also called the internetwork layer or the IP layer .

4. Data Link Layer
(1), when transferring data between two adjacent nodes, the data link layer assembles the IP datagram of the network layer into a frame (framing) and transmits the frame on the link between the two adjacent nodes.

Each frame includes data and necessary control information (such as synchronization information, address information, error control, etc.). When receiving data, the control information enables the receiving side to know which bit a frame starts from and to which end. In this way, the data link layer, after receiving a frame, can be extracted from the data part, handed over to the network layer. The control information also enables the receiving side to detect errors in the received frames. If there is a mistake, the data link layer simply discards the error frame, so as not to continue to send in the network to waste network resources in vain. If you need to correct the error that occurs when data is being transmitted in the data link layer (that is, the data link layer is not only to be checked and corrected), then a reliable transport protocol is used to correct the error. This approach makes the protocol of the data link layer more complex.

5. Physical layer (physical layers)
The units of data transmitted on the physical layer are bits. When the sender sends 1 o'clock, the receiver should receive 1 instead of 0.

The physical layer therefore considers how much voltage is used to represent "1" or "0", and how the receiver recognizes the bits sent by the sender. The physical layer also determines how many root pins should be connected to the cable's plug and how each pin should be connected. Of course, explaining what a bit means is not the task of the physical layer.

Note that some of the physical media used to deliver the information, such as twisted pair, coaxial cable, optical cable, wireless channel, etc., are not within the physical layer protocol but under the physical layer protocol. As a result, some of the physical media below the physical layer are considered the No. 0 layer. The process of data transfer between layers

Assume that host 1 of the application process AP1 ap_1 to host 2 of the application process AP2 ap_2 transfer data. AP1 ap_1 The data to the 5th tier (application layer) of the host. The 5th layer adds the necessary control information H5 H_5 becomes the next layer of data unit. The 4th Layer (transport Layer) received this data unit, plus the control information of this layer H4 h_4, and then to the 3rd layer (network layer), to become the 3rd layer of data units. And so on However, after the 2nd layer (the Data link layer), the control information is divided into two parts, respectively, the first layer of the data unit (H2 h_2) and the tail (T2 t_2), and the 1th layer (physical layer) due to the transmission of the bitstream, so the control information is no longer added.

Note that the transfer of the bitstream should begin with the header.
The OSI Reference Model refers to the unit of data transmitted between peer hierarchies as Protocol Data unit PDU (Protocol data units).

When this stream of bitstream leaves the host 1 through the physical media of the network to the router, it goes from the 1th layer of the router up to the 3rd layer in turn. Each layer carries out the necessary actions according to the control information and then strips the control information to the higher layer on the remaining data units of the layer. When the packet rises to the 3rd level, it finds the forwarding post in the router according to the destination address in the header, finds the interface to forward the packet, and then transmits it down to the 2nd layer, plus the new header and tail, then to the bottom 1th layer, then sends each bit out on the physical media.
When this string of bitstream leaves the router to reach the destination station host 2 o'clock, from the 1th level of host 2 in the manner described above, and then up to the 5th layer. Finally, the application process AP1 Ap_1 sent data to the destination station application process AP2 ap_2. entities, protocols, services, and service access points

1. Entity: represents any hardware or software process that can send or receive information.

In many cases, an entity is a specific software module.

2. The protocol is a collection of rules that control the communication of two peer entities (or multiple entities).

The three elements of the agreement:
(1) syntax, i.e. the structure or format of data and control information
(2) Semantics, that is, what kind of control information needs to be emitted, what action to complete and what response to make
(3), synchronization, that is, the sequence of event implementation detailed description
Two forms of the Protocol:
(1), text description (to people)
(2), program code (for computer)

under the control of the Protocol, the communication between the two peers enables this layer to provide services on a level up. To implement this layer of protocol, you also need to use the services provided in the following layer.

3, the Agreement and service have a very big difference
(1), the implementation of the Agreement ensures that the service can be provided up to a level. Entities that use this layer service can only see the service and cannot see the following protocol. In other words, the following protocol is transparent to the entity above.
(2), the agreement is "horizontal", that is, the agreement is the rule that controls the communication between peer entities. But the service is "vertical", that is, the service is provided from the lower level to the upper layer through the inter-tier interface.

In addition, all functionality that is not done within a layer is called a service. Only those functions that can be "seen" by a layer of entities can be called "services".

The services provided by the upper layer using the lower layer must exchange some commands with the lower layer, which are called the Service Primitives in the OSI.

The place where two layers of entities in the same system interact (that is, exchanging information), often referred to as the service access point, SAP (service access points). The service access point SAP is actually a logical interface.

The unit of data exchanged between layers and layers by the OSI is called the Service data unit SDU, which can be different from a PDU.
For example, you can have multiple SDU together as a PDU, or it can be a SDU divided into several pdus.

4, the relationship between two adjacent layers

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