Chapter One Computer Network Overview
Section 1 Basic Concepts of Computer Network
1. The concept of computer network
Computer network is the use of communication equipment and communication link or communication network to interconnect computer systems with different locations and autonomous functions, and follow certain rules to realize the exchange of information before the computer system. A shorter, more general definition: A computer network is a collection of interconnected, autonomous computers.
Second, the network protocol
In the process of sending, receiving or forwarding information, the end systems, packet switching equipment or other network equipment interconnected in the Internet all need to follow some regulations or conventions. Network protocol
Three, the three elements of the agreement
Protocol is a rule or agreement that needs to be followed in the process of data exchange between network communication entities, and is an important guarantee for the orderly operation of computer networks.
Any agreement will have three basic elements: syntax, semantics and timing, called the three elements of the agreement
- Syntax: Define the format and structure of information exchanged between entities, or define the level of signals transmitted between entities (such as hardware devices).
- Semantics: Semantics is to define what control information needs to be sent (or included) in the information exchanged between entities, the specific meaning of this information, and how the receiving end should respond to control information with different meanings.
- Timing: Timing, also known as synchronization, defines the order in which entities exchange information and how to match or adapt to each other's speed.
Fourth, the function of computer network
The function of computer network is to realize rapid information exchange between different hosts. Through information exchange, computer networks can realize the core function of resource sharing, including hardware resource sharing, software resource sharing, and information resource sharing.
V. Classification of computer networks
The largest computer network at present is the Internet (Internet). According to different classification standards, these networks can be divided into different types
- Classified by coverage
(1) Personal area network (PAN) Personal area network is usually a small-scale network formed by personal devices through wireless communication technology to realize data transmission between personal devices. Personal
area networks usually cover 0-10m. (2) Local area network (LAN) Local area networks are usually deployed in local areas such as offices, office buildings, factories, and campuses. Use high-speed limited or wireless link to link the host to realize high-speed data transmission in a local area. Local area network usually covers 10m-1Km
(3) Metropolitan area network (MAN) Metropolitan area network refers to a network that covers a city, and the coverage area is usually 5-50km.
(4) Wide area network (WAM) The wide area network covers a range of tens to Several thousand kilometers, usually across a larger geographic space, which can realize the interconnection of a remote metropolitan area network or a local area network - Classified by topology
Network topology refers to the physical connection and layout between hosts and network devices in the network. According to the topology, computer networks can be divided into bus topology, ring topology, star topology, hybrid topology, tree topology and mesh topology, etc.
(1) Bus topology.
The bus-type topology network uses a broadcast channel as a common transmission medium, called a bus. All nodes are connected to the bus, and communication between nodes is carried out through a shared bus. Since the bus is a broadcast channel, when any node sends data through the bus, other nodes will receive the signal that carries the data.
Advantages:
① Less number of cables required
② Simple structure
③ Easy to expand
Disadvantages:
① Limited communication range
② Difficulty in fault diagnosis and isolation
③ Prone to conflicts
(2) Ring topology. The ring topology network uses communication links to connect all nodes into a closed ring. Data transmission in the ring is usually one-way (or two-way) transmission. Each node can receive data from the ring and forward data to the ring.
Advantages:
① The required cable length is short
② Optical fiber can be used
③ Easy to avoid conflicts
Disadvantages:
① The failure of a node can easily cause the entire network to be paralyzed.
② The process of joining or withdrawing a new node is troublesome.
③ There is a waiting time problem
(3) Star topology structure The
star topology structure network includes a central node, and the host in the network is connected to the central node through a point-to-point communication link. The central node is usually a hub, switch, etc. The communication between the hosts needs to be carried out through the central node.
advantage:
① Easy to monitor and manage
② Easy to diagnose and isolate
faults Disadvantages:
① The central node is the bottleneck of the network, once a fault occurs, the entire network is paralyzed
② The network scale is limited by the number of ports of the central node
(4) Hybrid topology
Hybrid topology It is a network formed by a mixture of two or more simple topology networks. Most actual network topologies are hybrid topologies.
Advantages:
① Easy to expand and can build networks of different scales
② Network structure can be optimized according to needs
Disadvantages:
① Complex network structure
② Complex management and maintenance
(5) Tree topological structure
tree The shape topology network can be regarded as the expansion of the bus topology or the star topology network. At present, many local area networks use this topology
Advantages:
① easy to extend
② easy fault isolation
disadvantages:
high reliability of the root node, the root node upon a fault, it may not result in a wide range of communications network
(6) mesh topology,
nodes in a mesh topology through a plurality Links are directly connected to different nodes.
Advantages:
① High network reliability
② The network can still be connected when one or more links fail.
Disadvantages
① The network structure is complex
② The cost is high
③ The routing protocol is complex
. 3. The classification of
data exchange by exchange means that the network is interconnected by each other The data transfer between the nodes, to realize the process and technology of sending data from the sending node to the destination node. According to the data exchange technology adopted by the network, computer networks can be divided into circuit switching, message switching network and packet switching network
. 4. Classification according to user attributes
(1) public network
(2) private network
Section 2 Computer Network Structure
1. The edge of the network
Computers, servers, smart phones, smart sensors, smart home appliances, etc. connected to the network are called hosts or end systems.
These end systems are located at the very edge of the network. Therefore, all end systems connected to the network constitute the network edge.
End systems on the edge of the network run distributed network applications, and exchange data between end systems to achieve application purposes.
2. Access to the network
The access network is a network that realizes the connection and access between the end system at the edge of the network and the core of the network.
Common access network technologies include telephone access, asymmetric digital subscriber line ADSL, hybrid optical fiber coaxial cable HFC access network, local area network and mobile access network
Third, the core of the network
The core of the network is a network composed of packet switching equipment interconnected by communication links, and its role is to realize data relay and forwarding between hosts on the edge of the network.
Section 3 Data Exchange Technology and Computer Network Performance
1. Basic concepts of data exchange
In order to connect a larger range and a larger number of hosts, many switching devices can be interconnected to form an intermediate network for data relay and forwarding, and
then the hosts are connected to the switching devices that are closer, and the data transmission between the hosts passes through the middle The network realizes relaying and forwarding.
This intermediate network does not need to care about the content of the transmitted data, but only provides the function of data relay and exchange for these data from one node to another node until reaching the destination node, which is called the data exchange network
that constitutes the exchange network. Nodes (ie, switching equipment) are called switching nodes, and the collection of switching nodes and transmission media is called the communication subnet, which is the core of the network.
2. Data Exchange Technology
Common data exchange technologies include circuit switching, message switching and packet switching
- Circuit switching
In a circuit switching network, a dedicated communication line is first established between two hosts through an intermediate switching node, called a circuit.
Then the circuit is used for communication. After the communication is completed, the circuit is removed and the
circuit switching is used for communication. Three stages of circuit, data transmission and circuit removal - Message exchange
Message exchange is also called information exchange. Its working process is as follows: The sender should add the sending/receiving host's address and other control information to
form a complete message, and then exchange the message in units of messages. The nodes of the network are transmitted in a storage-forwarding manner until they reach the destination host - Packet switching
(1) Basic principles of packet switching. Packet switching needs to divide the transmitted data (ie message) into smaller data blocks. Each data block is added with control information such as address and serial number to form a data packet. Each packet is independently transmitted to the destination, and the destination will receive the data. The packet is reassembled and restored to a message. The packet transmission process usually also uses store-forward switching
(2) Advantages of packet switching
① Low storage capacity requirements of switching equipment
② Fast switching speed
③ High reliable transmission efficiency
④ More fair
(3) Determination of
packet length ① Packet length and delay Time
The transmission of the packet switching network-the forwarding process can be abstracted as a queuing system. Based on the analysis of queuing theory, it is found that when the packets have the same length, the delay time of the packets in the switching process is smaller.
therefore. The message is divided into "packets" according to a certain standard length, so that the switching equipment can process the information as a unit, thereby shortening the delay time of the information in the exchange process.
② Packet length and bit error rate
The channel error rate of the communication link is another factor that needs to be considered when determining the packet length. Set the packet length to L bits, where h is the packet header length, and the data length is x bits, that is There is
x + h = L.
If the channel error rate is Pe, the probability of packet transmission is correct (only considering the bit error rate in the packet) is
Ps = (1-Pe) to the power of x+h
The probability that the packet transmission error requires retransmission is (1-Ps). Considering the possibility of multiple consecutive transmission errors, the optimal packet length Lopt can be obtained. The
highest channel utilization rate can be expressed as:
Third, the main performance indicators of computer networks
- Speed and Broadband
Speed refers to the amount of data transmitted per unit time on the network. It is used to describe the speed of data transmission on the network. It is also called data transmission rate or data rate
. The data transmitted by a computer network is binary data in units of bits. The basic rate is The unit is bit / s (bits per second), also known as the bit rate.
At the same time, broadband data is also used to describe the rate.
In the field of communication or signal processing, broadband originally refers to the bandwidth of the signal, that is, the difference between the highest frequency and the lowest frequency of the signal component, in Hz (hertz).
In computer networks When describing
the data transmission capacity of a link or channel, the term broadband is often used to indicate the highest data rate of the link or channel, and the unit is also bit/s - Latency
Latency refers to the time it takes for data to reach another node from a node (host or switching device, etc.) in the network.
The transmission process of each hop of the packet mainly produces four types of time delays
(1) Node processing delay
(2) Queuing delay
Packet transmission delay:
When a packet is sent on the output link, the time taken from sending the first bit to the last bit is called It is the transmission delay, also known as the transmission delay dt,
assuming the length of the packet is L and the
link width is R
(3) Transmission delay
(4) Propagation delay The time required for a
signal to come out of the transmitter and reach the receiver through a certain distance of the physical link is called the propagation delay dp.
If the physical length is D, the
signal propagation speed is V(m/s)
. Extended to dp: D/V
- Delay-bandwidth product The product
of the propagation delay of a physical link and the link bandwidth, called the delay-bandwidth product, is denoted by G, so
G= dp *R
To the favorite pig
Some people, because you are good to him, think you are good, he is the one you love; some people, because they understand your goodness, so want to be nice to you, he is the one who loves you; the end of happiness is The one you love becomes the one who loves you.