I. Overview
1. Computer network performance indicators
(1) Rate, transmission data rate, also known as data rate, bit rate, the units are: b/s, kb/s, Mb/s, Gb/s, Tb/s
(2) Bandwidth, the highest transmission rate, which is the highest value of the rate, the unit is the same as the rate
(3) Throughput, the amount of data passing through a network channel or interface per unit time, in units of b, kb, Mb, Gb, Tb
(4) Delay, sending delay (the time required for the host or router to send data), transmission delay (the time required for electromagnetic waves to transmit in the channel), processing delay, and queuing delay
(5) Delay-bandwidth product = propagation delay * bandwidth
(6) Round trip time RTT
(7) Utilization: channel utilization, network utilization
2. Computer network architecture
OSI: Open Systems Interconnection Basic Reference Model, Open Systems Interconnection Reference Model
Protocol: The rules, standards or conventions established for the exchange of data in a network are called network protocols
The benefits of layering:
(1) Each layer is independent, and the big problem is decomposed into multiple independent small problems
(2) Good flexibility, as long as the interface remains unchanged, the internal implementation can be modified without affecting the upper and lower layers
(3) The structure is easy to separate
(4) Easy to implement and maintain
(5) Promote standardization work
3. Five-layer structure (the seven-layer structure is to add a presentation layer and a session layer under the application layer, which is more of a theoretical structure, and is actually represented by a five-layer structure)
(1) Application layer
(2) Transport layer
(3) Network layer
(4) Data link layer
(5) Physical layer
2. Physical layer
1. The main task of the physical layer is to determine some characteristics related to the transmission media interface, including:
Mechanical properties, electrical properties, functional properties, process properties
2. The way of information interaction
Simplex communication, half-duplex communication, full-duplex communication
3. Channel multiplexing technology
Frequency division multiplexing: users occupy different resources at the same time
Time division multiplexing: all users occupy the same resources at different times
Wavelength Division Multiplexing: Optical Frequency Division Multiplexing
Code division multiplexing: Code division multiplexing is the use of a set of code groups containing mutually orthogonal code words to carry multiple signals
3. Data link layer
1. Data link: In addition to a physical line representing the physical layer, it also includes some communication protocols that control data transmission. The combination of the two is a data link
2. Network adapter: The data link layer protocol is generally implemented by the network adapter, which realizes the functions of the data link layer and the physical layer. Its main function is to realize the communication between the computer and the external local area network.
3. Protocol data unit of the data link layer: frame
4. Steps of point-to-point data link communication (point-to-point data link and data link using broadcast communication are two main data links):
(1) Node A adds the IP datagram handed over by the network layer of the node to the header and the tail and encapsulates it into a frame
(2) Node A sends the encapsulated frame to Node B through the physical layer link
(3) Node B submits the received frame to the network layer of Node B when checking that the received frame is error-free, otherwise, it will discard the frame
5. There are three basic problems to be solved by the data link layer protocol:
(1) Encapsulation into framing - frame start symbol SOH, end symbol EOT,
(2) Transparent transmission - to prevent the transmitted text from appearing in SOH or EOT, resulting in false start or false end
(3) Error detection - CRC cyclic redundancy detection is widely used
6. The PPP point-to-point protocol is the most widely used protocol in the data link layer. It provides unreliable datagram services; because the data link layer does not necessarily provide more functions than the network layer IP protocol, the PPP protocol does not require error correction. No serial number, no flow control, simplicity is the primary requirement and biggest feature of the PPP protocol.
7. Ethernet: The most widely used local area network specification today, using CSMA/CD technology, and running on various cables at a rate of 10M/s.
8. CSMA/CD protocol: carrier sense multipoint access/collision detection
9. The core points of CSMA/CD:
(1) "Multi-point access" , many computers are interconnected to a bus in a multi-point access mode, and only one computer can occupy the bus to transmit data at the same time.
(2) "Carrier Sense" , each station (computer) must constantly detect whether the channel is transmitting data, and can obtain the right to send if it is not occupied
(3) "Collision detection" , monitoring while sending, if it is detected that there are two stations on the bus transmitting data at the same time, stop the transmission immediately
10. MAC address: the physical address of the Ethernet
11. Bridge: It can forward and filter frames, can connect different physical layers, MAC sublayers and different Ethernets, and can expand Ethernet at the data link layer, but the disadvantage is that it increases the delay
12. Hub: can forward bit streams, work at the physical layer, and extend Ethernet at the physical layer
Fourth, the network layer
1. Address Resolution Protocol ARP: resolves the IP addresses of hosts and routers to hardware addresses.
2. ARP mechanism: each host has an ARP cache, which contains the mapping table from the IP addresses of the hosts and routers in the local area network to the hardware addresses, and this mapping table is often dynamically updated
3. Internet Control Message Protocol (ICMP): Provides host or router inquiries and reports errors or exceptions.
4. ICMP message type:
(1) Error report message (2) Query message
5. ICMP application:
Use the ping command to directly use the ICMP protocol to send back request and reply packets at the application layer over the transport layer to test the connectivity between hosts.
6. Router packet forwarding algorithm (network layer operation):
Extract the IP address of the destination host from the header of the datagram, and extract the network address according to the IP address class (class A, B, C, and D).
1. If the network address N is the network address connected to the router, the datagram is directly delivered to the destination host of the network. (This includes the process that the network interface software converts the IP address into a MAC address through the ARP protocol, encapsulates the datagram into a link layer MAC frame, and sends the frame to the destination host through the physical layer line)
2. If the network address is not the network address connected to the router, query the routing table