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local area network

LAN knowledge framework; media access control (MAC: media access control, used to solve the problem of multi-channel access control), IEEE802 standards, data link layer, etc.

Edited at 2022-03-26 16:34:06

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local area network

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Outline

local area network

IEEE802 standard

IEEE802 protocol: defines several types of local area networks LAN, including the definition and description of the physical layer and media access control MAC sublayer

Ethernet (ETHERNET)

physical layer

The physical layer structure defined in the 802.3-2005 standard

mainstream structure

two parts

RS (coordination sub-layer, focusing on logical functions-protocol)

PHY (physical layer device, focusing on physical implementation - mechanical, electrical, functional)

PCS

physical coding sublayer

PMA

Physical media connection

PMD

Physical media related

two interfaces

xMII (media independent interface, parallel interface)

MDI (Media Dependent Interface, Serial Interface)

PHY specifications

data link layer

Serve

No connection: There is no connection establishment process between the network card that sends the frame and the network card that receives the frame.

Unreliable: The receiving network card does not send a confirmation frame after receiving the information. Error frames are discarded directly, and retransmission of discarded frames depends on the upper transport layer protocol. Otherwise, data frames are easily lost.

Provide a unified Ethernet frame format (The format of the frame depends on different technologies, and with the development of technology, the format of the frame is also constantly developing)

Ethernet II is DIx 2.0: The Ethernet standard frame format developed by Xerox, DEC, and Intel in 1982. Cisco name: ARPA (most Ethernet packets) The MAC frame of Ethernet V2 is relatively simple and consists of five fields. The first two fields are the 6-byte long destination address and source address fields respectively. The third field is a 2-byte type field, which is used to indicate what protocol is used by the upper layer so that the data of the received MAC frame can be handed over to the protocol of the upper layer. The fourth field is the data field, its length is between 46 and 1500 bytes. The last field is the 4-byte frame check sequence FCS (using CRC check)

mac frame anatomy mac address

Function: Used to distinguish a host on a LAN, equivalent to a unique identifier of a host, usually burned into the network card NIC.

Format: In order to ensure that MAC addresses are not repeated, IEEE, as the legal management organization of MAC addresses, is responsible for uniformly allocating the first byte (high 24 bits) of the address field to manufacturers. The lower 24 bits are allocated by the manufacturer

Obtaining method: You can know the MAC address of the host through the command: ipconfig/all

type

Unicast address (one-to-one): Frames for a unicast address are sent to the only site in the network specified by the unicast address. When the I/G bit in the MAC address is 0, it means that the MAC address is a unicast address.

Broadcast address (pair to all): When the destination address is a broadcast address, it means that the frame will be sent to all nodes. A broadcast address is a MAC address with all bits set to 1 (written in hexadecimal it is FF-FF-FF-FF-FF-FF).

Multicast address (one-to-many): When the destination address is a multicast address, it means that the frame will be sent to a part (a group) of sites in the network. A multicast address is a MAC address with the I/G bit set to 1.

IEEE 802.2/802.3 SNAP: The SNAP version of Ethernet 802.3 Ethernet frame format announced by IEEE in 1985. Cisco name: SNAP (BPDU, VLAN Trunk, and CDP between switches use the 802.3SNAP protocol)

The MAC sublayer provides two operating modes

Half duplex mode (half duplex): CSMA/CD algorithm competes for shared channel - traditional Ethernet

Full duplex mode (full duplex): exclusive physical medium, medium full-duplex channel, no CSMA/CD algorithm - modern Ethernet required

frame structure

Leading character ➕Start character SFD (8 bytes in total) The first 7 bytes are 10101010 appearing alternately, notifying the receiving end to prepare to receive data frames. It is mainly used by the receiving end for bit synchronization. The last 1 byte is 10101011B. Once two consecutive 1s are received, the frame starts Function: Make the receiving node synchronize and prepare to receive data frames 10baseT-Ethernet frames do not use end-of-frame delimiters. When the carrier on the bus disappears, the channel is idle, and the network interface cannot detect signals, it can be judged that a frame is over.

Destination address DA (6 bytes)

Use broadcast and multicast addresses, and also use unicast addresses.

Source address SA (6 bytes)

Only unicast addresses.

Data length: the length of the data field (46-1500B) (2 bytes) When 1500B is mainly formulated according to the DIX standard, the transceiver needs enough RAM to store a complete frame

Data: PDU from upper layer

Padding: Ethernet takes 51.2us as the length of the contention period and transmits 512bit (64B) Minimum frame length required to ensure normal operation of the CSMA/CD protocol When the data part is less than 46B, pad 0 to make up 46 bytes.

Checksum: CRC check

Data length and type

The type field of the Ethernet II frame identifies the upper layer data type carried by the Ethernet frame. 0x0800 represents IPV4 protocol data, 0x0806 represents ARP, and 0x86DD represents IPV6.

The segment value is 0x600=1536 or above, indicating type one DIXv2 frame

A field value of 1500 or less indicates the length --- 802.3 frame

Values between 1500 and 1536 are undefined.

Classification

Broadcast Ethernet

Switched Ethernet

Access control classification of shared media

Static allocation (channel partitioning—multiplexing)

Frequency Division Multiplexing FDM-Cable TV Network Principle: Allocate the frequency band equally to each user who wants to participate in communication

Time Division Multiplexing TDM Principle: Each user has a fixed channel transmission time slot

Advantages and disadvantages of static allocation Advantages: When the communication needs of each user are large (large load), the channel utilization rate is high; there is no conflict; suitable for a small number of users, the number is basically fixed Disadvantages: The network load is small and the channel utilization is low. Inability to flexibly adapt to changes in the number of sites and their traffic volume

Used in mobile communication technology: TDMA/FDMA/CDMA/VDMA

dynamic allocation

Controlled access: Only one site on the Internet sends information at any time No conflict, enjoy all bandwidth Centralized control (multi-point polling), decentralized control (token ring network)

Random access: Each site on the Internet can randomly access the channel according to its own wishes and utilize the full bandwidth of the channel. If two or more sites send information at the same time, a collision will occur. Conflict detection mechanism; conflict recovery mechanism (retransmission) Typical methods: ALOHA (18%); Slotted ALOHA (37%); CSMA; CSMA/CD; CSMA/CA Typical network: Ethernet

media access control

MAC: media access control media access control, used to solve the problem of multi-channel access control

MAC: multiple access control multiple access control

Packet-switched multiple access method for a typical wired network: CSMA/CD: Carrier- sense multiple access with collision detection Carrier sense multiple access/collision detection

Packet-switched multiple access method for a typical wireless network: CSMA/CA: carrier sense multiple access with collision avoidanceCarrier sense multiple access/collision avoidance

CSMA: carrier sense multiple access CSMA: carrier sense multiple access protocols, is proposed based on the ALOHA protocol •Main difference: A carrier monitoring (monitoring before sending) is added. Before sending a frame and accessing the transmission channel, the station first monitors whether there is a carrier on the channel to determine whether other stations are transmitting data. If there is a carrier, it means there are already users. While the channel is in use, frames are not sent to avoid collisions.

Listening strategy

Non-persistent CSMA (Non-persistent): Once it detects that the channel is busy, it will no longer monitor it, but will restart after a random period of time. New monitoring

Advantages: Reduces the probability of conflicts

Disadvantages: Increased channel idle time and increased data transmission delay

1-persistent CSMA (1-persistent): When the channel is busy, it still insists on listening. It always monitors that the channel is idle and sends immediately with probability 1.

Advantages: Reduced channel idle time

Disadvantage: If multiple nodes monitor the channel at the same time, the probability of conflict increases.

for EthernetEthernet

P-persistent CSMA (P-persistent)

Suitable for slotted channels

Send data with probability P and delay the next time slot with probability 1-p

Different p values can be set according to the amount of traffic on the channel, thus improving channel utilization.

Optimal P value: 1/n (n is the number of nodes)

For use in CSMA/CA or wireless packet-switched networks

improve proposals

Wired channel: Send data while detecting conflicts. If a conflict is detected, subsequent transmissions will be stopped to reduce channel waste. Usually used in Ethernet environments—CSMA/CD

Wireless channel: If the transmission medium is detected to be busy before transmission, the transmission will be delayed by a random interval. This random timing reduces the chance of collisions (multiple stations waiting to transmit), usually Used in WI-FI--CSMA/CA

Conflict detection implementation

Easy to implement in wired LAN: measure signal strength, compare transmitted signal and received signal

Wireless LANs are difficult to implement: received signal strength is overwhelmed by local transmit signal strength

CSMA/CD

Conflict detection method: Wired cable carrier detection technology: measure signal strength, the signal voltage swing is only higher than a certain value, it is considered that there are other stations on the bus, and the point is sending

Reason for introducing conflict detection (CD): propagation delay affects the accuracy of judgment

The propagation delay of electromagnetic waves in 1km coaxial cable is about 5us

When listening is idle, other stations may be sending information but the signal has not yet reached the physical location of the station on the medium. At this time, the station mistakenly thinks that idle

The greater the propagation delay, the greater the likelihood of collisions and the worse the protocol performance.

Working period

Transmission cycle

competitive cycle

idle period

Maximum conflict detection time: The conflict detection time refers to the period of time from when a station starts sending a frame to when a conflict is discovered. The maximum conflict detection time of the network is usually twice the end-to-end propagation delay of the network (2T). Ethernet IEEE802.3 takes 51.2ws as the length of the contention period (slot time). For 10 Mb/s Ethernet, 512bit, or 64 bytes, can be sent during the contention period.

Minimum frame length: Ethernet stipulates that the shortest effective frame length is 64 bytes. Any frame with a length less than 64 bytes is an invalid frame that is aborted abnormally due to conflict.

Channel utilization: data frame sending delay/time required to send a data frame: T0/nt T0=1/[1 n*(t/T0)]