Shared LAN refers to the LAN technology in which all nodes share a common communication transmission medium. Shared media LAN can be divided into Ethernet, token bus, token ring, fiber distributed data interface FDDI, and high-speed Ethernet and FDDI II developed on this basis. Wireless local area network is the product of the combination of computer network and wireless communication technology. Like wired LAN, it can be shared.
Switched LAN refers to the switched LAN technology with frames or smaller data units in the data link layer as data exchange units and Ethernet switches as the core. Switched LAN can be divided into switched Ethernet, ATM network and virtual LAN developed on this basis. Due to the high networking cost of ATM networks, in recent years, ATM technology has rarely been used to set up local area networks, and more is switched Ethernet.
LAN reference model-IEEE 802 reference model was formulated by the 802 Committee of the Institute of Electrical and Electronics Engineers of America, which corresponds to the lowest two layers of OSI reference model, namely physical layer and data link layer.
Students may wonder why the IEEE802 reference model only corresponds to the lowest two layers of the OSI reference model. This is because the topology of LAN is relatively simple, generally bus-type, ring-type, star-type and tree-type, and there is only one link between two contacts, so there is no need for routing and flow control, so the model does not need to consider the network layer. As for other high-level applications, they are often related to the specific implementation and are usually included in the network operating system, so the model has no corresponding description for other high-level applications.
From the figure, we can see that the lowest layer of the IEEE802 reference model corresponds to the physical layer of the OSI reference model, and its main functions are to encode and decode signals, generate and remove preambles, and send and receive bits.
There are many kinds of local area networks, using various transmission media and different access methods, so the IEEE802 reference model divides the data link layer into two layers: media access control MAC and logical link control LLC.
The logical link control LLC sublayer has nothing to do with the transmission medium and is suitable for various types of local area networks. It completes the establishment, maintenance and release of communication links, provides a unified interface between high-level protocols and MAC layers, and completes the sending, receiving and flow control of frames.
The MAC sublayer of media access control is related to the transmission medium, and it is directly related to the topology of the network and the type of transmission medium. It is responsible for media access control, reasonable channel allocation, and realizing that multiple devices in the LAN share a channel resource? And data frame assembly and disassembly, MAC address identification and error detection.
The IEEE802 reference model contains a series of standards: from IEEE802. 1 to IEEE802.22, many local area networks have been proposed. After years of use and elimination, the most important survivors are now only 802.3 Ethernet, 802. 1 1 wireless local area network (wiFI) and 8021
The story of Ethernet begins at 1976. It was designed and implemented by bob metcalfe, a student at MIT, and his colleagues. They connected all the computers with a long and thick coaxial cable. As we can see in the picture, they named the system "Ethernet".
The classic Ethernet is 10M Ethernet, which uses four transmission media-thick cable, thin cable, twisted pair and optical fiber, namely 10base-5 thick cable Ethernet, 10base-2 thin cable Ethernet, 10base-T twisted pair Ethernet and/kloc.
Classical Ethernet has a maximum cable length limit, and signals within this range can propagate normally, while signals beyond this range will not propagate. In order to build a larger network, multiple cables can be connected through repeaters/hubs. On these cables, information is transmitted in Manchester code. Ethernet can contain multiple cable segments and multiple hubs, but the distance between any two transceiver terminals is not allowed to exceed 2.5 kilometers, and no more than 4 hubs pass between any two transceiver terminals.
Traditional Ethernet uses CSMA/CD protocol with collision detection to control the use of the network. The agreement has three basic points:
(1) Multi-point access: Multiple computers are connected to a bus through multi-point access, and the bus topology is adopted, and the physical topology can be star.
(2) Carrier monitoring
When a station has no data to send, it first listens to the channel to determine whether other stations are transmitting data. If the channel is idle, it sends data; If the channel is busy, the workstation will wait for the channel to be free before sending the frame.
(3) Collision detection
That is, listening and sending. If there is a conflict, the station waits for a random time and then starts the above process from the beginning.
These three points can be summed up in four sentences-listen first, then send, send while listening, stop the conflict, and delay the retransmission.
With the wide application of switches and the demand for high bandwidth, IEEE802.3U Fast Ethernet was born. Fast Ethernet 100BASE-T is a star Ethernet for transmitting 100Mb/s baseband signals. By replacing the network card with 100Mb/s switch, users can easily upgrade from 100BASE-T to 100 base-t without changing the network topology.
100base-t4: 4 pairs of Class 3 UTP twisted pair are used, of which 3 pairs are used for data transmission and the fourth pair is used for collision detection, and full duplex mode is not supported.
100base-tx: Two pairs of Category 5 UTP twisted pair are used for full duplex transmission, one pair is used for sending signals to the switch, and the other pair is used for receiving signals from the switch. The maximum distance between the station and the hub/switch is 100m, and the distance between the station-hub-station shall not exceed 200m.
100base-fx, using multimode optical fiber. When the stations are directly connected, the distance between them shall not exceed 4 12m in half-duplex state and 2000m in full-duplex state. When single-mode optical fiber is used, the maximum transmission distance in full-duplex state can reach10000m.
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Before the ink of the fast Ethernet standard was dry, the 802 Committee began to develop a faster fast Ethernet, called Gigabit Ethernet. Gigabit Ethernet allows full-duplex and half-duplex operation at the rate of 1Gb/s: CSMA/CD protocol is used in half-duplex mode, but it is not needed in full-duplex mode. It can be backward compatible with 10BASE-T and 100BASE-T technologies, with good network expansibility and easy to upgrade and manage. At present, it has become a mature campus LAN backbone network technology.
Similarly, according to the different cables used, Gigabit Ethernet can be divided into two types, one is 1000base-x based on Fibre Channel and the other is 1000base-t based on twisted pair.
1000base-sx-sx stands for short wavelength (using 850nm laser). When using multimode fibers with core diameters of 62.5μm and 50μm, the transmission distances are 275 meters and 550 meters respectively.
1000base-LX-LX stands for long wavelength (using 1300nm laser). When using multimode fibers with core diameters of 62.5μm and 50μm, the transmission distance is 550m m. When using a single-mode fiber with a core diameter of 10μm, the transmission distance is 5km.
1000 base——CX——CX stands for copper wire, using two pairs of shielded twisted pair, and the transmission distance is 25m.
1000BASE-T—— Use 4 pairs of UTP twisted pair 5 for full duplex operation, and the transmission distance is1000 mm. ..
At the end of 1999, the 802 Committee studied the 10 Gigabit Ethernet technology (10Gbps), and in 2002, the optical fiber standard, the shielded copper cable standard and the copper twisted pair standard were officially released.
10 Gigabit Ethernet uses optical fiber as the transmission medium. The transmission distance of single-mode optical fiber is over 40km, and that of multimode optical fiber is 65 ~ 300 m. ..
10 Gigabit Ethernet adopts full-duplex mode instead of CSMA/CD mechanism, thus getting rid of the distance limitation of CSMA/CD. Now we are working hard to study "carrier Ethernet" and standardize 40 Gbps and 100 Gbps.
Ethernet has developed for more than 30 years, and there is no real competitor in the development process. The reason lies in its simplicity, reliability and easy maintenance, which is also a classic saying in the online world-"Keep it simple, or you will become stupid!"
Actually, token is a special frame, which is used to control the sending right of network nodes. Only nodes with tokens can send data. That is, if a station has a frame queue waiting for transmission, it can send a frame when it receives the token, and then pass the token to the next station; If it has no queued frames to transmit, it simply passes the token. Students should note that the sending node will delete the token after obtaining the right to send, and there will be no more tokens on the loop, and it is impossible for other nodes to obtain tokens, which ensures that only one node on the loop sends data at a certain moment, so there is no contention in token ring technology, which is a typical contention-free media access control method.
Fiber Distributed Data Interface (FDDI) appeared in 1990s, which is higher than Ethernet (10Mbps) and token network (4 or 16Mbps). The access mode of FDDI is similar to the access mode of token ring network, and the network communication adopts "token" transmission mode, but it is different from the standard token ring.
As can be seen from the composition structure diagram of FDDI, it adopts double-ring structure, and its network information flow consists of two similar flows, which flow around two reciprocal rings in opposite directions. One ring is called "main ring", which is sent counterclockwise, and the other ring is called "secondary ring", which is sent clockwise. Generally speaking, network data signals usually only flow on the main ring. If the ring fails, the token network will automatically reconfigure the network and the data can flow to the auxiliary ring in the opposite direction. One advantage of this double-ring structure is redundancy. One ring is used for transmission and the other ring is used for backup. That is to say, if there is a problem, the primary ring is disconnected and the secondary ring takes its place.
Wireless LAN can be divided into two categories: the first category has fixed infrastructure, and the second category has no fixed infrastructure.
The so-called "fixed infrastructure" refers to a group of fixed base stations that are established in advance and can cover a certain geographical range. The cellular phone that people often use is to connect the calls made by users' mobile phones by using a large number of fixed base stations established in advance by telecom companies and covering the whole country.
For the first kind of WLAN with fixed infrastructure, 1997 IEEE has formulated the protocol standard 802.1/series standard for WLAN, and there are two kinds of network usage-self-organization and architecture mode.
( 1) ? Self-organizing mode
The network in this mode consists of a group of interrelated computers, which can directly send data to each other for point-to-point or point-to-multipoint communication.
WPAN wireless personal area network
WPAN is a self-organizing network connecting personal electronic devices, such as our portable computers, tablet computers, portable printers and mobile phones. , using wireless technology, without using access point AP, the whole network range is about10mm ... WPAN can be used by one person or several people. These electronic devices can communicate easily, just like connecting with ordinary cables. Now the most common example of wireless personal area network is the Bluetooth system we usually use.
(1) Bluetooth system
The earliest use of WPAN is the Bluetooth system introduced by Ericsson 1994. Its standard is IEEE802. 15. 1, the transmission rate is 720kb/s, the distance range is 10 m, and it runs in 2.4GHz band.
Bluetooth technology is a low-cost, near field communication connection technology used between various fixed and mobile digital hardware devices. This connection is stable and seamless, and its program is written on a 9×9 mm microchip, which can be easily embedded in the device. At the same time, it can easily penetrate obstacles and realize all-round data transmission. If the equipment has a wide range of activities and needs to be quickly interconnected with various devices, such as notebook computers, digital cordless phones, personal digital assistants, mobile phones and so on. It is ideal to use Bluetooth or wireless personal area network.
(2) ZigBee？
Similar to Bluetooth, ZigBee is a new short-range wireless communication technology for sensor control applications. It is a wireless data terminal of the Internet of Things, and provides users with wireless data transmission function by using ZigBee network. ZigBee, the name comes from the communication mode used by bees for survival and development. Bees tell their partners the location, distance and direction of new food sources by dancing in a z-dance. ZigBee is a low-power LAN protocol based on IEEE802. 15.4 standard. Its main characteristics are short communication distance, usually 10 to 80 meters, low data transmission rate, 2~250kb/s, and low cost.
(3) high-speed WPAN
The standard of high-speed WPAN is IEEE802. 15.3, which is specially designed for transmitting data between portable multimedia devices, and supports the data rate of 1 1 to 55 MB/s ... This is especially convenient when more and more digital devices are used by individuals. For example, using high-speed WPAN can connect a PC with printers, scanners, external hard disks and other electronic devices in the same room without cables. Videos taken by others with digital cameras can be copied to the memory card of your digital camera without connecting cables; The portable computer in the conference hall can project the prepared slides onto the big screen through the projector, which is very convenient and fast without connecting wires.
(2) There are architectural models.
The network in this mode shown in the figure adopts a star topology, and each client is connected to a central access point AP, which can be connected to other networks such as wired networks through the central access point shown in the figure, and then connected to the Internet through a router.
The LAN formed by the central AP is what we usually call wifi, and the MAC layer uses CSMA/CA multi-point access carrier monitoring/collision avoidance protocol (different from CSMA/CD used by classical Ethernet). Now, WIFI is almost synonymous with WLAN.
Now, many places, such as offices, airports, fast food restaurants, hotels, shopping centers and so on. , you can provide paid or free WIFI access to the public. Such places are called hotspots and are public wireless access points. An area connected by many hotspots and access points AP is called a hotspot.