Overview of Part One
(1) Division of network development stages
The four stages of computer network development: the first generation of computer networks---the remote terminal connection stage; the second generation of computer networks---the computer network stage; the third generation of computer networks---the computer network interconnection stage; the fourth-generation computer Network---internet and information superhighway stage.
①The first stage: computer technology and communication technology are combined to form a primary computer network model. The main purpose of network application at this stage is to provide network communication and ensure network connectivity. Strictly speaking, the network at this stage is still a variant of the multi-user system.
②The second stage: On the basis of computer communication network, a computer network with complete network architecture and protocol is realized. The main purpose of network applications at this stage is to provide network communication, ensure network connectivity, network data sharing and network hardware equipment sharing. The milestone in this phase is the ARPAnet network of the US Department of Defense.
③The third stage: The computer solves the problem of computer networking and interconnection standardization, and proposes the "Open System Interconnection Reference Model (OSI RM)" that meets the international standards of computer networks, which greatly promotes the development of computer network technology. At this stage, network applications have developed into the information service era that provides enterprises with information sharing services.
④The fourth stage: The computer network is developing towards interconnection, high speed, intelligence and globalization, and it has rapidly gained popularity, realizing the widespread application of globalization. The representative work is the Internet.
(2) Definition and classification of computer networks
A computer network is a collection of autonomous computer systems interconnected in a way that can share resources with each other.
The main purpose of network establishment is to realize the sharing of computer resources;
Classification of computer networks
①Classification according to network transmission technology: broadcast network, point-to-point network
② Classified according to the scope of the network: local area network (LAN) wide area network (WAN) metropolitan area network (MAN) personal area network (PAN)
③Classification according to the switching function of the network: circuit switching, packet switching, message switching, hybrid switching
④ Exchange according to network users: private network, public network
Knowledge expansion:
The use of packet store-and-forward and routing mechanisms is an important difference between point-to-point networks and broadcast networks.
Circuit switching must be connection-oriented. The three stages of circuit switching: establishment, communication, and release
Features of circuit switching :
① Circuit switching is the switching technology used in traditional telecommunication networks.
②Two telephones can be connected to each other with only a pair of wires.
③But if you want to connect 5 telephones in pairs, 10 pairs of wires are required.
④N telephones are connected in pairs, requiring N(N – 1)/2 pairs of wires.
This kind of networking that must go through the establishment of connection → communication → release connection is called connection-oriented (connection-oriented).
Circuit switching must be connection-oriented. But connection-oriented is not necessarily circuit switching.
Disadvantages of circuit switching:
①Not flexible enough: as long as at any point in the path established by both parties
If something goes wrong, the call will be interrupted and you must redial to establish a new connection
Pick up.
②Low efficiency: computer data is very bursty, which will cause network
Serious waste of resources.
③In addition, circuit switching is used, between terminals and computers of different speeds
It is difficult to communicate with each other.
Features of packet switching:
①The characteristic of packet switching is label-based.
②The networking mode that can send data at any time without establishing a connection is called connectionless.
③ Packet switching can also use connection-oriented mode. Therefore, a connection-oriented network is not necessarily a circuit-switched network.
Advantages of packet switching:
① High efficiency: The transmission bandwidth is dynamically allocated, and the communication link is occupied section by section.
②Flexible: Take grouping as the transmission unit, and select the forwarding route independently for each grouping.
③Rapid: You can send packets to other hosts without first establishing a connection; the network uses high-speed links.
④Reliable: perfect network protocol; distributed multi-route packet switching network makes the network have good survivability.
Comparison of three exchange messages
| project |
Circuit switching |
Message exchange |
Packet switching |
| Data transmission unit |
Whole message |
Segmented message |
Grouping |
| Link establishment and teardown |
need |
Not needed |
Not needed |
| Routing |
When the link is established |
Per message |
Each grouping |
| Link failure impact |
Unable to continue transfer |
no effect |
no effect |
| Transmission delay |
A certain link establishment time is required, but the data transmission delay is the shortest |
No link establishment is required, but storage and forwarding will cause delays due to queuing and slow data transmission
|
No link establishment is required, but store-and-forward queuing will cause delay, and data transmission is faster than message transmission
|
The composition of the Internet
From the perspective of the way the Internet works, it can be divided into the following two major parts, the edge part and the core part
Two communication methods: client-server method (C/S method) peer-to-peer method (P2P method )
The router is an important component to realize packet switching.
router
① There is no direct connection between the input and output ports in the router.
②The process of the router processing the packet is: put the received packet into the buffer (temporary storage); search the forwarding table to find out which port should be forwarded to a certain destination address; send the packet to the appropriate port and forward
Access Network AN (Access NetWork) is also called local access network or residential access network
(3) The composition and performance of the computer network;
The composition of computer network basically includes: computer, network operating system, transmission medium, application software
The computer network is composed of three parts: resource subnet, communication subnet, and protocol.
Resource subnet: consists of several hosts, which provide services to various users;
Communication subnet: It is composed of some dedicated node switches and communication links connecting these nodes;
A series of protocols: These protocols are used for communication between hosts or between hosts and subnets.
Computer network performance:
① Speed
Bit: The unit of data volume in a computer, which is also a unit of information volume used in information theory.
Speed, namely data rate or bit rate, is one of the most important performance indicators in computer networks. The unit of rate is b/s, or kb/s, Mb/s, Gb/s, etc.
Speed often refers to rated speed or nominal speed.
②Bandwidth
"Bandwidth" (bandwidth) originally refers to the bandwidth of the signal, the unit is hertz (or kilohertz, megahertz, gigahertz, etc.).
Now "bandwidth" is synonymous with the "highest data rate" that a digital channel can transmit, and the unit is "bits per second", or b/s (bit/s).
The more commonly used bandwidth unit is
Kilo ratio per second, namely kb/s (103 b/s)
Megabit per second, namely Mb/s (106 b/s)
Gigabit per second, namely Gb/s (109 b/s)
Tera ratio per second, namely Tb/s (1012 b/s)
Please note: In the computer world, K = 210 = 1024
M = 220, G = 230, T = 240。
Changes in digital signal flow over time
The width of the signal on the time axis narrows as the bandwidth increases.
③ Throughput
Throughput: indicates the amount of data passing through a certain network (or channel or interface) in a unit time.
④ Delay
Transmission delay: When sending data, the time required for the data frame to enter the transmission medium from the node.
That is, the time from sending the first bit of the data frame to the end of sending the last bit of the frame.
Propagation delay: The time it takes for electromagnetic waves to travel a certain distance in the channel.
Processing delay: the time it takes for the switching node to perform some necessary processing for store and forward.
Queuing delay: The delay experienced by the queuing of packets in the node buffer queue.
The length of queuing delay often depends on the amount of communication in the network at that time.
Key point: total delay = transmission delay + propagation delay + processing delay + processing delay
Error-prone concept
For high-speed network links, we only increase the data transmission rate rather than the bit propagation rate on the link.
Increasing the link bandwidth reduces the data transmission delay.
⑤ Delay bandwidth product
Delay bandwidth product = propagation delay * bandwidth
The delay-bandwidth product of the link is also called the link length in bits.
⑥Utilization rate
Channel utilization: The channel utilization indicates what percentage of the time a certain channel is used (there is data passing). The utilization of completely idle channels is zero.
Network utilization: The network utilization is the weighted average of the channel utilization of the entire network.
Channel utilization is not as high as possible.
The relationship between delay and network utilization
Question: If D is the current delay of the network, D0 is the delay when the network is idle, U is the utilization of the network, and the value of U is between 0 and 1, what is the current network delay D?
Under appropriate assumptions, the following simple formula can be used to express the relationship between D and D0:
The relationship between the current network delay D and the network utilization U
It can be seen from the figure that as the utilization rate U increases, the time delay D also increases sharply
Reduction method: increase the bandwidth of the line.
⑦Round trip time (RTT)
In computer networks, the round trip time is also an important performance indicator. It represents the total elapsed time from when the sender sends data to when the sender receives the confirmation from the receiver (the receiver sends the confirmation immediately after receiving the data).
(4) Elements of the network protocol
Elements of network protocol: syntax, semantics, synchronization
Syntax: The structure or format of data and control information.
Semantics: What kind of control information needs to be sent, what action to complete, and what kind of response.
Synchronization: A detailed description of the sequence of events.
(5) Knowledge development
Benefits of layering: each layer is independent, flexible, separable in structure, easy to implement and maintain, and can promote standardization
①System with five-layer agreement:
TCP/IP is a four-layer architecture: application layer, transport layer, internet layer, and network interface layer
OSI is a seven-layer architecture: application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer
②Entity, agreement, service and service access point
The protocol is "horizontal", that is, the protocol is the rule that controls the communication between peer entities.
The service is "vertical", that is, the service is provided by the lower layer to the upper layer through the interlayer interface.
The place where entities of the two adjacent layers of the same system interact is called the Service Access Point (SAP).
③TCP/IP architecture
Note: The router only uses the network layer when forwarding packets, and does not use the transport layer and application layer.
IP over Everything: IP can be applied to all kinds of networks
Everything over IP: refers to under the TCP/IP architecture, various network applications are built on the basis of IP
