Chapter
1. The composition of the Internet
(1) Edge part: It is composed of all the hosts connected to the Internet. This part is directly used by users for communication (transmission of data, audio or video) and resource sharing.
(2) Core part: Consists of a large number of networks and routers connecting these networks. This part is to provide services for the edge part (providing connectivity and exchange).
2. Features of circuit switching and packet switching
Circuit switching:
(1) Circuit switching must be connection-oriented.
(2) Three stages of circuit switching: connection establishment, communication, and connection release.
Packet switching:
advantages:
high efficiency: dynamic allocation of transmission bandwidth, and the communication link is occupied section by section.
Flexible: use grouping as transmission unit and find routing.
Fast: You can send packets to other hosts without first establishing a connection.
Reliability: network protocol that guarantees reliability; distributed routing protocol makes the network have good survivability.
Disadvantage:
Packets need to be queued when they are stored and forwarded at each node, which will cause a certain delay.
The header that the packet must carry (with necessary control information in it) also causes a certain amount of overhead.
3. Delay
(1) Transmission delay (transmission delay)
① When sending data, the time required for the data block to enter the transmission medium from the node.
②It is the time from the first bit of the data frame to the end of the transmission of the last bit of the frame.
official
(2) Propagation delay
① The time it takes for electromagnetic waves to propagate a certain distance in the channel.
② The signal transmission rate (that is, the sending rate) and the signal propagation rate on the channel are completely different concepts.
formula
(3) Processing delay
The time spent by the switching node for some necessary processing for store and forwarding
(4) Queuing delay The delay
experienced by the queuing of packets in the node's buffer queue.
The length of the queuing delay often depends on the amount of communication in the network at that time.
(5) Total time delay
4. Protocol architecture
(1) OSI/RM seven-layer protocol
(2) TCP/IP four-layer protocol
(3) Five-layer protocol
Supplement: The architecture is abstract, while the implementation is concrete. It is the real running computer hardware and software.
Chapter
2. Basic concepts of the
physical layer The main task of the physical layer is described as determining some characteristics of the interface with the transmission medium. , Namely:
① Mechanical characteristics: specify the shape and size of the connector used in the interface, the number and arrangement of leads, fixing and locking devices, etc.
② Electrical characteristics: specify the range of voltages appearing on each line of the interface cable.
③ Functional characteristics: indicate what meaning the voltage of a certain level appears on a certain line indicates.
④ Process characteristics: indicate the order of occurrence of various possible events for different functions.
2. Data communication system The
data communication system consists of three parts as follows:
① source system (sender, sender)
a, source
b, sender
② transmission system (transmission network)
③ destination system (receiver, receiver)
a, Receiver
b, destination
3, common terminology
a, the purpose of communication is to transmit the message
b, the data is the entity that transports the message
c, the signal can be divided into two categories according to the value of the parameter representing the message in the signal:
a ) Analog signal (continuous signal)
b) Digital signal (discrete signal)
The basic waveform representing different discrete values is called symbol
d, baud (Baud) and bit (bit) are two different concepts
a) Baud is The rate unit of symbol transmission (how many symbols are transmitted per second). The symbol transmission rate is also called the modulation rate, waveform rate, or symbol rate.
b) Bit is the unit of information volume.
4. Several basic concepts related to signals
One-way communication (simple communication)-there can only be communication in one direction and no interaction in the opposite direction.
Two-way alternate communication (half-duplex communication)-both parties in the communication can send information, but not both parties can send at the same time (of course, they can not receive at the same time).
Two-way simultaneous communication (full-duplex communication)-both parties in communication can send and receive information at the same time.
5. Signal-to-noise ratio and Shannon’s formula
① Signal-to-noise ratio
② Shannon's formula:
The limit information transmission rate C of the channel can be expressed as
C = W log2(1+S/N) b/s
W is the bandwidth of the channel (in Hz);
S is the average power of the signal transmitted in the channel ;
N is the Gaussian noise power inside the channel ③Shannon’s
formula shows that
a) The larger the bandwidth of the channel or the signal-to-noise ratio in the channel, the higher the limit transmission rate of information.
b) As long as the information transmission rate is lower than the limit information transmission rate of the channel, some way can be found to achieve error-free transmission.
c) If the channel bandwidth W or the signal-to-noise ratio S/N has no upper limit (of course, the actual channel cannot be this way), then the limit information transmission rate C of the channel has no upper limit.
d) The information transmission rate that can be achieved on the actual channel is much lower than Shannon's limit transmission rate.
6. Baud rate and bit rate
7, oriented transmission media
① twisted
shielded twisted pair STP (Shielded Twisted Pair)
unshielded twisted pair UTP (Unshielded Log In the Twisted Pair)
② coaxial
50 coaxial
75 coaxial
③ cable
8, the channel Multiplexing technology ①Frequency
division multiplexing FDM, time division multiplexing TDM, statistical time division multiplexing STDM
- All users of frequency division multiplexing occupy different bandwidth resources at the same time (please note that the "bandwidth" here is the frequency bandwidth and not the data transmission rate)
- Time division multiplexing is to divide time into segments of equal length time division multiplexing frames (TDM frames). Each time-division multiplexed user occupies a fixed number of time slots in each TDM frame.
a) The time slot occupied by each user appears periodically (the period is the length of the TDM frame).
b) TDM signal is also called isochronous signal.
c) All users of time division multiplexing occupy the same bandwidth at different times.
d) Time division multiplexing may cause waste of line resources - Statistical time division multiplexing is an improved time division
multiplexing ②WDM
wavelength division multiplexing is optical frequency division multiplexing
③Code division multiplexing CDM
code division multiplexing is another method of sharing channels
Chapter 3
1, The channels used by the data link layer mainly have the following two types:
①Point-to-point channels. This channel uses one-to-one point-to-point communication.
②Broadcast channel. This channel uses one-to-many broadcast communication.
Three basic problems
(1) encapsulation into frames
(2) transparent transmission
(3) error control
2. Media sharing technology
① Static channel division
a) Frequency division multiplexing
b) Time division multiplexing
c) Wave division multiplexing
d) Code division multiplexing
② Dynamic media access control (multipoint access)
a) Random access
b ) Controlled access, such as multipoint line polling or polling.
3. Summary of CSM/CD
① Listen before
sending ② Send when idle
③ Detect while sending
④ Back off in case of conflict
4. Channel utilization
5. Extended Ethernet
①Extend the Ethernet at the physical layer (hub)
- Advantage
a) It enables computers on the local area network that originally belonged to different collision domains to communicate across collision domains.
b) Expand the geographic coverage of the local area network. - Disadvantages
a) The collision domain has increased, but the overall throughput has not been improved.
b) If different collision domains use different data rates, then they cannot be interconnected with a hub.
Hub features:
- Working at the physical layer;
- Expanded the geographical coverage of the local area network, and the total throughput rate remains unchanged;
- The Ethernet using the hub is still a bus network logically, and the workstations still use the CSMA/CD protocol and share the logical bus;
- Half-duplex working mode; the same operating speed;
- The hub is much like a multi-port repeater. ②Extend the characteristics of
LAN (network bridge)
bridge at the data link layer
Advantages: - Filter traffic.
- Expanded the physical scope.
- Improved reliability.
- It can interconnect LANs with different physical layers, different MAC sublayers, and different speeds (such as 10 Mb/s and 100 Mb/s Ethernet).
Disadvantages: - Store and forward increase the delay.
- There is no flow control function at the MAC sublayer.
- Network segments with different MAC sublayers will have a greater delay when bridging together.
- The bridge is only suitable for local area networks where the number of users is not too many (no more than a few hundred) and the communication volume is not too large. Otherwise, network congestion may sometimes occur due to the transmission of too much broadcast information. This is the so-called broadcast storm.
③The difference between bridge and hub - When the hub forwards the frame, it does not detect the transmission medium.
- The bridge must execute the CSMA/CD algorithm before forwarding the frame.
- If there is a collision during the sending process, you must stop sending and back off.
Advantages of VLAN - Split broadcast domain
- Facilitate network user management;
- Reduce network management overhead;
- Provide better security;
- Improve network service quality.
Classification of switches
- Straight-through
- Store-and-forward
Chapter 4
1. Two services provided by the network layer:
①Dummy telegram ②Datagram
2.IP
address
①Three types of commonly used IP addresses
②Unusual but special IP address
3. Subnet mask
①The default subnet mask of class A, B, and C IP addresses (left 1 and right 0 cannot be crossed)
②Subnet division selection of class B address (using fixed-length subnet)
4. CIDR without classification addressing (construction of super network)
①Summary
CIDR uses "slash notation", also known as CIDR notation, that is, add a slash "/" to the IP address surface, and then write the number of digits occupied by the network prefix (this value corresponds to the three-level address The number of 1s in the subnet mask). For example: 220.78.168.0/24
CIDR composes consecutive IP addresses with the same network prefix to form a "CIDR address block".
The address block represented by 128.14.32.0/20 has a total of 212 addresses (because the 20 after the slash is the number of digits in the network prefix, the host number of this address is 12 digits).
The starting address of this address block is 128.14.32.0.
When there is no need to indicate the starting address of the address block, such an address block can also be referred to as "/20 address block" for short.
②Commonly used CIDR address block
5. Types of ICMP message
① ICMP error report message
- End unreachable
- Time exceeded
- Parameter problem
- Change route (redirect)
②ICMP inquiry message
- Send back request and reply messages
- Timestamp request and reply messages
② Several common ICMP message types
6. Internet routing protocol
①Interior Gateway Protocol RIP
RIP is a distributed routing protocol based on distance vector.
RIP2 protocol message format
A RIP message can include up to 25 routes, so the maximum length of a RIP message is 4 20 25 504 bytes. If it exceeds, another RIP packet must be used to transmit
RIP protocol features: good news spreads quickly, and bad news spreads slowly.
A problem with RIP: When the network fails, it takes a relatively long time (for example, several minutes) to transmit this information to all routers.
②The advantages and disadvantages of RIP protocol
- Advantages:
a) Simple implementation and low overhead. - Disadvantages:
a) RIP limits the scale of the network. The maximum distance it can use is 15 (16 means unreachable).
b) The routing information exchanged between routers is the complete routing table in the router, so as the network scale expands, the cost increases.
c) "Bad news spreads slowly", which makes the convergence time of the update process too long.
7. IPv6
① The basic header of
IPv6, IPv6, still supports connectionless transmission, but the protocol data unit PDU is called packet.
Variety:
- Larger address space.
- Extended address hierarchy.
- Flexible header format.
- Improved options
- Allow the agreement to continue to expand.
- Support plug and play (that is, automatic configuration).
- Support the pre-allocation of resources.
- The IPv6 header is changed to 8-byte alignment.
②General form of IPv6 datagram
- The IPv6 datagram consists of two parts:
a) the base header and
b) the payload. Payload is also called net load. The payload is allowed to have zero or more extension headers, followed by the data part.
③The destination address of the IPv6 datagram
- Unicast: Traditional point-to-point communication.
- Multicast: point-to-multipoint communication.
- Anycast: This is a type added by IPv6. The destination station of anycast is a group of computers, but only one of them is delivered when the datagram is delivered, usually the nearest one
④ colon hexadecimal notation - Each 16-bit value is represented by a hexadecimal value, and each value is separated by a colon. For example:
68E6:8C64:FFFF:FFFF:0:1180:960A:FFFF - In hexadecimal notation, it is allowed to omit the 0 before the number. For example, omit the first three zeros in 0000 and write one zero.
⑤Zero compression - The colon hexadecimal notation can allow zero compression, that is, a series of consecutive zeros can be replaced by a pair of colons.
FF05:0:0:0:0:0:0:B3 can be compressed to:
FF05::B3 - Note: You can only use zero compression once in any address.
Chapter 5
1. The transport layer of TCP/IP has two main protocols:
①Connection-oriented TCP (Transmission Control)
②Connectionless UDP (User Datagram)
2. Commonly used port numbers
3. Features of UDP
a) UDP is connectionless
b) UDP uses best effort delivery
c) UDP is message-oriented
d) UDP has no congestion control
e) UDP supports one-to-one, one-to-many, many-to-one and Many-to-many interactive communication
f) UDP has a small header overhead
4. Channel utilization
① The advantage of the stop-and-wait protocol is simplicity, but the disadvantage is that the channel utilization is too low.
②Formula
5. TCP congestion control
6. TCP transport connection management
①TCP connection establishment: three-message handshake
②TCP connection release: four-message handshake