控制网络技术（英文三）Computer network foundation

一、development

1、terminal oriented

2、Circuit switching network
（1）Characteristics of circuit switching

Circuit switching must be connection oriented.
The three stages of circuit switching:
Establish a connection
Signal communication
Release the connection

（2）Problems in circuit switching

The computer's digital signal is burst and intermittent.
Circuit switching has long access time (10s to 20s).
different types, specifications and rates between terminals and computers of Reliability is not high

3、Packet switching network
（1） ARPANET is a milestone in the history of computer network development. Milestone marks the birth of modern computer network with the purpose of resource sharing.
Contribution to computer network technology：packet switching ，the layered network model，communication subnet and resource subnet
（2）The main features of packet switching

At the transmitter side, the long message is first divided into shorter and fixed length data segments.
Each data segment is added with a header to form a group.
Packet switching transmission unit
The head of each group contains control information such as address.
The first part was stripped from the group，finally restore to the original message

4、 Network architecture
（1） Different standardization
（2）The standardization of the network architecture
（3）Network function

data transmission--basic functions
sharing of resources-- the most attractive function
sharing hardware, software and data
improving the reliability and availability
carry out distributed processing

5、Network topology
（1）Network topology refers to the physical connection form of nodes in the network.
（2）Network topology --Geometric figures of network nodes and links
（3）The classification of computer network

the topology ：  star network, tree network, bus network, ring network, mesh network and so on.
users:： public network, private network
the way of information exchange: circuit switching network, packet switching network and integrated switching network.
medium： twisted pair network, coaxial cable network, optical fiber network, wireless network and satellite network.
the transmission type：  broadcast network, point-to-point network
size： local area network、metropolitan area network、wide area network

二、Network hardware

1、LAN
（1） LAN , Privately-owned networks, Geographic range is usually from tens of meters to a few kilometers. It belongs to a small network built by a department or unit. Broadcasts are widely used in local area network.
（2）The characteristics of LAN

size：  restricted in size
transmission technology: 10-100Mbps，low delay，few errors
topology：various topology (bus , star，ring )

2、MAN
MAN (Metropolitan area networks), A bigger version of a LAN, normal uses similar technology, between LAN and WAN, usually not more than a city or region. It is a network that can cover a city range.

3、WAN
WAN(Wide area networks), spans a large geographical area (a country or continent), also known as remote network, usually covers 10 kms-106 kms, such as crossing several cities or several national networks.

4、OSI/RM
（1）As long as we follow the OSI standard, a system can communicate with any other system that is following the same standard anywhere in the world.OSI failed in the market.OSI experts have no commercial driving force when they finish the OSI standard.The implementation of OSI protocol is too complex and inefficient.
（2）The establishment period of OSI standard is too long, so that the equipment produced by OSI standard can not enter the market in time.The level division of OSI is not very reasonable, and some functions are repeated at multiple levels.

5、Two international standards
（1）The legal international standard OSI has not been recognized by the market.The international standard TCP/IP is now the most widely used. TCP/IP is often referred to as the international standard.
（2）Protocol hierarchies

Data exchange in computer network must obey the rules agreed before.
These rules clearly specify the format of the exchanged data and the synchronization problem.
Network protocol, simple as protocol, is a rule, standard or convention for data exchange in network.

（3）The benefits of stratification
Each layer is independent.The flexibility is good.Structure can be split open.Easy to implement and maintain.
It can promote standardization work.

三、The OSI reference model

1、Physical layer—— How to make use of the physical media
（1）main function – provide a physical connection for the data link layer, to ensure that information is entered into the channel and taken down by the receiver, so that the bit stream can be transparently transmitted.
data unit – bit. the lowest level of OSI, the interface between network physical devices, not the transmission medium, but only the physical devices: the data terminal device DTE (such as displays, PC, workstations, etc.) and the data circuit end connection device DCE (such as a modem).
（2）Typical tasks of the physical layer
The voltage level of “0” and “1” . How long each one lasts . Full duplex synchronization . How to establish connections and disconnect connections . Mechanical interface, Pins; Transmission medium

2、Data link layer – how to go every step
（1）main function–establish, maintain and disconnect links between two adjacent nodes, and to transform unreliable physical links into error free data links through error control and flow control.
（2）data unit --frame

3、Network layer – which way to get to the other side
（1）main function – responsible for selecting an optimal path. how to select packets from source to destination.
（2）data units - packet
（3）Typical tasks of the network layer

routing – optimality、fairness、 simplicity、adaptability and stability.
congestion control – solve the problem of resource congestion in network.
network addressing -- routers are used to connect two or more different logical networks.
network interconnection -- when groups cross a network to reach their destination, the addressing methods, the limit of packet length, and protocols are different, network interconnections needs to be solved.

4、Transport layer transport layer – where is the other
（1）The main function – the level of main computer, the best use of the network resources according to the characteristics of the communication subnet, and the establishment of a transmission channel between the session layer of the source host and the destination host and economical way for transparent transmission of the message.
（2）Data unit – message
（3）The basic function is to accept data from the session layer, split it up into smaller units if needed, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.

5、Session layer–Who should speak and where to speak
（1）The main function–manage dialogue control, user interface. Although the session layer does not participate in the specific data transmission, it manages the data transmission. establishes, coordinates between two communication applications.
（2）Data unit – message
（3）The layers above the session layer are application oriented
The layers below the session layer are communication oriented, providing transparent and reliable data transmission for the upper level of the two session entities

6、Presentation layer–What does the other person look like
（1）The main function – data encryption and decryption、data compression and data transformation.
（2）Data encryption and decryption–In order to protect data security, the sender encrypts the message and the receiver decrypts it.
（3）Data compression – data compression can effectively use bandwidth, reduce delay and reduce communication cost.

7、Application layer–What to do
（1）The main function,–a direct user oriented layer that provides access to the process and system processes management. It contains a variety of protocols needed.
（2）The common network services – file service, e-mail service, virtual terminal service, network management service, printing service, integrated communication service, directory service, security service and so on.

四、5-layer protocol

1、point
5 (application layer)
4 (transport layer)
3 (network layer)
2 (data link layer)
1 (physical layer)

2、TCP/IP
（1）TCP/IP is the 4-layer architecture: application layer, transport layer, Internet layer and network interface layer.But the bottom layer of network interface is not defined.Therefore, combine the advantages of OSI and TCP/IP, propose a 5-layer protocol.
（2）Two services provided by transport layer

connection - oriented services(TCP)：Take the telephone system as a model. Establish connections, use connections, release connections
connectionless service(UDP)：Take the postal system as a model. Each message has a complete destination address, and each message is independent of other packets, passing through the selected route.

（3）Service primitives: the necessary information exchanged between the service user (N+1 entity) and the service provider (N entity) is used to notify the service user to take some action or to report the action of the service provider to the service provider’s peer entity.
（4）

  Request    ——An entity wants the service to do some workIndication ——An entity is to be informed about an eventResponse  ——An entity wants to respond to an eventConfirm    ——The response to an earlier request has come back

（5）Conclusion
In this way, the blue army on both sides is always unable to decide whether the last message has been received by the two sides.There is no agreement that will enable the blues to win 100%.

五、Composition of the Internet

1、The edge and core of the Internet
From the way of the Internet work, it can be divided into two parts.
（1）the edge part is composed of all hosts connected to the Internet. This part is directly used by users to communicate (transmit data, audio or video) and share resources.
（2）the core part is composed of a large number of routers connected to these networks. This part is to provide services for the edge part (providing connectivity and exchange).

2、The core part of the Internet
（1）most complex part of the Internet.
（2）provides connectivity to hosts，various forms of data.
（3）Routers play a vital role .
（4）Routers are key part to realize packet switching.

3、The role of the host and the router is different
（1）The host is to process information for users, send packets to the network and receive packets from the network.
（2）Routers store and forward packets, and finally deliver packets to the destination host.

4、The advantages of packet switching
（1）High efficiency: dynamic allocation of transmission bandwidth, and communication link is occupied by steps.
（2）Flexibility: group transfer unit
（3）Fast: no need to establish connection first
（4）Reliable: network protocol that guarantees reliability; distributed routing protocol enables network to have good survivability.

5、Problems brought by packet switching
（1）Packets need to be queued at nodes when they are stored and forwarded, which will cause a certain delay.
（2）Packets must carry the header (which contains essential control information), which also causes a certain cost.

6、Indexs of computer network
（1）rate

Bit is the unit of data in computer, and it is also the unit of information quantity used in information theory.
Speed, data rate or bit rate is the most important performance index in computer networks. b/s, or kb/s, Mb/s, Gb/s, etc.

（2）bandwidth

Bandwidth means the band width of a signal. The unit is Hz (or kHz, mHz, GHz, etc.).
Now bandwidth is synonymous with the highest data rate that the digital channel can transmit.

（3）The common mistake is to confuse two rates

There are two different rates in the network:The propagation rate of signal (electromagnetic wave) on transmission media (m/sec, or km/sec).The bit rate is sent to the network by computer.
The meaning and unit of the two rates are completely different.Broadband transmission:  refers to the higher rate of sending bits by computers .

（4）Broadband line: there are more bits per second from computer to line.The propagation speed of bits is the same on broadband and narrowband lines.Broadband and narrowband lines: the same speed. Broadband line: shortening of vehicle distance
（5）throughput

Throughput represents the amount of data that is passed through a network (or channel and interface) in a unit time.
Throughput is more often used for a measurement of the real world network in order to know how much data actually can be passed through the network.
Throughput is limited by the bandwidth of the network or the rated speed of the network.

（6）delay

Sending delay (transmission delay), when data is transmitted, the time that data blocks enter from node to transmit media.
That is, the time required from the first bit of the transmitted data frame to the last bit of the frame.
Propagation delay： electromagnetic wave needs to propagate a certain distance in the channel and the time it takes.
Signal transmission rate (i.e. transmission rate) and signal propagation speed on channel are completely different concepts.
Processing delay: the time spent by switching nodes for storing and forwarding necessary processing.
Queuing delay: the delay experienced by packet queuing in node cache queue.The length of queuing delay depends on the traffic in the network at that time.
The total delay experienced by data is the sum of transmission delay, propagation delay, processing delay and queuing delay.

（7） utilization ratio

Channel utilization indicates that a few times of a channel is utilized.
The utilization rate of the fully idle channel is zero.Network utilization is the weighted average of channel utilization of the whole network.
Channel utilization is not higher and better.

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