Computer Network - Hierarchical Architecture

Network concept

The computer network connects scattered computer systems with independent functions through communication equipment and lines, and realizes resource sharing and information transmission by software. It is also a collection of interconnected and autonomous computer systems.

Three ways to divide network planning

• Hardware, software, protocol
  Hardware includes host (end system), communication link (twisted pair, optical fiber), switching equipment (routing, switch) and communication processor (network card), etc. Software includes various software and resources
  sharing Various tools and software that are convenient for users to use.
  The protocol stipulates the specifications followed by network transmission data

• Edge part and core part
  The edge part is connected to the Internet and is composed of hosts used by users. The
  core part is composed of a large number of networks and routers of these networks, providing connectivity and switching services for the edge part

• Communication subnet and resource subnet
  The communication subnet is composed of various transmission media, communication equipment and network protocols to realize data communication between computers. The resource
  subnet is a collection of equipment and software for resource sharing, providing network users Services of hardware resources, software resources and data resources on
  

Network counting function

Data communication, resource sharing, distributed processing, improving reliability, load balancing

Network classification

Scope division

Wide area network WAN, metropolitan area network MAN, local area network LAN, personal area network PAN, ranging from large to small

Classified by transmission technology

Broadcast: Networked computers share a common communication channel. When a message is sent through this channel, all computers can hear it, and the receiving computer will check the destination to decide whether to accept it. Point-to-point: Each physical line connects a pair of
computers

Classified by topology

by user

public network, private network

Classified by switching technology

circuit switched network

The starting point and the end point establish a dedicated channel for data transmission, including establishing a connection, transmitting data, and disconnecting

message switching network

It is also a store-and-forward network. User data + source address + destination address + check code are packaged into a message, and the message is sent to the adjacent node. After all is stored, it is forwarded to the next node and the process is repeated. Until reaching the destination node, each message can independently select the path to the destination node

packet switched network

Also known as packet network, the data is divided into many shorter fixed-length data segments, and auxiliary information such as destination address and source address are added to each data block to form a packet, which is transmitted in a store-forward manner

Classified by transmission medium

Wired: twisted pair, coaxial cable, etc.
Wireless: Bluetooth, etc.

Performance

• Rate: data rate/data transmission rate/bit rate is
  the rate at which data bits are transmitted on the digital channel connected to the network planning host, and
  the unit is b/s kb/s Mb/s Gb/s Tb/s

• Bandwidth: the "highest data rate" that can pass from one point to another in the network per unit time, the highest speed supported by network equipment

• Throughput: The amount of data passing through a network per unit time, limited by the network bandwidth and rated rate

• Latency: the total time for data to travel from one end of the network to the other. Total Latency = Occurrence Latency, Propagation Latency, Processing Latency, Queuing Latency
  Send Latency: The time required for all the bits of a packet to be pushed to the link, from occurrence The time required from the first bit of a packet to the last bit of the packet pushed to the transmission link, also known as transmission delay Propagation
  delay: the time required for a bit to propagate from one end of the link to the other
  when queuing Delay: After the packet enters the router, it needs to be queued in the input queue to wait for processing, so there is a waiting delay.
  Processing delay: The time it takes for the data to perform some necessary processing for storing and forwarding at the switching node: extracting data, erasing error checking , looking for routes, etc.

• Delay-bandwidth product: When the first bit generated by the sender is about to reach the end, how many bits have been sent by the sender, also known as the link length in bits , delay-bandwidth product = propagation delay * channel bandwidth

-Round-trip delay RTT: The total delay RTT experienced from sending data to the sender receiving the receiver's acknowledgment
includes round-trip propagation delay (propagation delay*2) and
terminal processing time (processing delay of intermediate nodes, queuing delay and sending delay when forwarding data)

• Signal utilization rate: what percentage of a certain channel is data passing time/(with + no) data passing time
• Network utilization: weighted average of channel utilization

stratification reasons

Large problems are divided into small problems, and the levels are independent of each other

detailed concept

Entity: The active elements in the nth layer are called n-layer entities, and the entities on the same layer are called peer entities, that is, the same layer of two hosts.
Protocol: Rules established for data exchange between peer entities: syntax (what to do) + Semantics (structure of data and control information)+synchronization (event implementation sequence description)
interface: the upper layer uses the entry of the lower layer service
Service: the lower layer provides function calls for the adjacent upper layer

SDU Service Data Unit: Data transmitted
PCI Protocol Control Information: Information that controls the operation of the protocol
PDU Protocol Data Unit: Data unit transmitted between peer layers

The upper layer must exchange some instructions with the lower layer to use the services provided by the lower layer. These instructions are called service primitives in OSI.
● Request (Request): sent by the service user to the service provider, requesting it to complete a certain job.
● Indication (Indication): sent by the service provider to the service user, indicating that certain events have occurred.
● Response (Response): sent by the service user to the service provider, the response to the previous instruction.
● Confirmation (Confirmation): sent by the service provider to the service user, confirming the previous request.
These 4 types of primitives are used for different functions such as establishing connection, transferring data and disconnecting etc.
Responsive services include all 4 primitives
Unresponsive services have only requests and indications

We assume that the n+1 layer is the people who save money, and the n layer is the bank counter service staff. The person who saves money wants to deposit money at the counter. He will tell his request to the counter service staff. After the service staff knows it, they need your bank card and deposit amount cash and other documents, and instruct you to give the bank card and cash and other documents to the counter. She, and then the person who deposits money responds according to the instructions of the counter staff, and gives the bank card and cash and other documents to the counter staff, and finally the counter staff processes your request, saves money for you, and gives you a voucher to prove my Help you deal with some information such as deposit operations.