Computer Network Reading Notes (3)

In computer networks, the division of various layers is mainly oriented to different objects. The physical layer is oriented to transmission media and solves the problem of how to transmit bit streams on physical media. The data link layer is oriented to the master
A machine-router or router-router can ignore the problem of how bit streams are transmitted on the physical media in the physical layer protocol, but instead solve the problem of data transmission between the above nodes, that is, how the computer
The IP packet at the layer is transmitted to the router or computer correctly.

There may be various networks, such as LAN (Ethernet) and Wan, between the host and the router or between the router and the router. To solve the problem of reliable data transmission between two nodes in the data link layer, that is, to solve the problem of reliable data transmission in the channel, you must first solve the problem of data transmission in each LAN or WAN.

In Ethernet, two protocols are involved: PPP protocol-point-to-point communication protocol and broadcast communication protocol. The former is mainly used for communication between computers and ISP of network service providers, and the latter is mainly used in Ethernet.

In Ethernet (LAN), the hub solves the problem of data broadcast communication protocol. Each computer is connected to an Ethernet network through a combination of hubs. The hub works on the physical layer and is responsible for data forwarding.
Sender. To transmit data over Ethernet, ensure that the addresses of each computer connected to the hub are different, that is, the MAC address (consisting of 6 bytes) is different. This problem is caused by the network adapter (NIC) laijie
Each network adapter ensures that the computer has a globally independent address (that is, when a computer changes the network adapter, its MAC address also changes ), the network adapter sends IP packets from the network layer
Wrap it into a MAC frame to control its transmission over the channel (ignore the problem of how the last bit stream is transmitted over the physical channel ).

The Physical Layer Protocol considers how to shield the actual physical transmission media so that bit streams can be smoothly transmitted without considering a specific transmission media, the data link layer protocol considers how to make MAC frames adjacent to each other.
Secure and reliable transmission between two nodes (computer-router or router-router. The so-called adjacent two nodes mean that there cannot be exchange devices in the middle, but there can be hubs (the hubs work in the physical layer,
Only forward bit streams ). A vro and a vro or a vro and a computer generally have various networks, such as LAN and WAN. Ethernet is widely used, and Ethernet is actually a LAN. Yes
Ethernet can be extended from the physical layer by adding a high-speed hub, or by connecting to a high-speed Ethernet switch (belonging to the data link layer, forwarding MAC frames through a forwarding table) on the data link layer.

Here, we must first understand the meaning of the data link layer. The so-called link is a physical line between two adjacent nodes (no switch in the middle), that is, the link is the actual physical channel, the data link is a complete set of protocols for secure transmission by adding various hardware and software protocols on the basis of the link.

A network consists of several nodes and links connecting to these nodes.

Networks and networks can be connected through routers to form the internet.

The network connects many computers, while the Internet connects many networks.

Composition of the Internet: edge and Core

Edge part: host. Most of the communication between hosts is in the client server mode.

Both the customer and the server refer to the two application processes involved in the letter. The client server mode describes the relationship between processes (programs) serving the services.

Core Part: Network and router (the most special and important, task: Forwarding received groups)

Exchange: From the Perspective of communication resource allocation, resources of transmission lines are dynamically allocated in some way.

Computer network performance indicators

Speed: Kb/s (k = 103 = 106) Mb/s (M = Mb)

Bandwidth

Intention: The frequency band width of a signal. The bandwidth of a Signal refers to the frequency range occupied by various frequency components of the signal.

In a computer network, bandwidth is used to indicate the ability of the network's communication lines to transmit data. Therefore, the network bandwidth indicates the"Highest Data Rate". (Bit/s)

Throughput

Latency

Transmission latency: data frame length (B)/channel bandwidth (B/s)

Propagation latency: channel length (m)/electromagnetic wave propagation rate (m/s) on the Channel)

Processing latency

Queue latency

Total latency
= Sending latency + propagation latency +
Processing latency + queuing latency

For high-speed network links:We only increase the data transmission rate rather than the bit transmission rate on the link. Increasing the data transmission rate only reduces the data transmission latency.

▲The high transmission rate of the Optical Fiber Channel usually refers to the high rate at which data is sent to the optical fiber channel, and the transmission rate of the optical fiber channel is actually a little lower than that of the copper wire.

Latency bandwidth product: Also known as the link length in bits. Transmission latency * bandwidth.

RTT(Round-Trip Time) Round-Trip Time

The total time that the sender has experienced after sending the data to the receipt of the confirmation from the receiver.

Utilization

Remember: a channel or high utilization rate can lead to a very large latency.

Computer Network Architecture

The so-called architecture:Is a set of layers of computer networks and their protocols.

OSIAnd the actual application of the TCP/IP System

 

Application Layer:

Directly provides services for your processes (programs.

Transport Layer:

Provides services for communication between processes on two hosts.

Network Layer:

1. provide communication services for different hosts on the group exchange network; 2. Find a suitable route

Data link layer:

Data transmission protocol between two adjacent nodes (host-router or router-router.

Adjacent nodes directly transmit data (point-to-point ).

Physical Layer:

Transmits a bit stream. Note: physical media is not included.

Entity, protocol, service, and Service Access Point

Entity:Program entity, Process

Protocol: a set of communication rules (horizontal)

Service:Functions that can be visible to a higher layer of entities. (Vertical)

Physical Layer

The physical layer is a protocol, not a specific transmission medium.

The role of the physical side: shielding specific transmission media and interfaces, providing the transfer protocol, and also serving as the role of switching from a serial port, (parallel transmission and serial transmission on the transmission media in the computer ).

What we call a modem is actually a transmitter, which performs encoding and modulation on the bit stream output by the computer and then sends it to the transmission system.

Modulation

Baseband modulation: Performs Waveform Transformation on the baseband signal. The transformed baseband signal

Band-pass modulation: The carrier is used to relocate the baseband signal to a high-frequency band for transmission, because the baseband signal usually contains many low-frequency components.