Broadcast channels allow one-to-many communication, so LANs using broadcast channels are called ****-enabled LANs. Nowadays switched LANs with higher performance using point-to-point links and link switches have completely replaced ****-enjoyment LANs in the wired domain. However, wireless LANs still use ****-enabled media technologies.

Connecting multiple sites using a broadcast channel must address the problem of conflicting signals on the ****hedge channel if more than two sites are sending data at the same time. Therefore *** enjoyment channel to focus on the consideration of a problem is how to coordinate multiple sending and receiving sites on a *** enjoyment of the transmission media occupation, that is, media access/access control (MAC) Medium Access Control or multiple access, multiple access Multiple Access, media access control technology is divided into the following two categories:

The most important feature of LANs is that they can be used to connect multiple sites at the same time, but the most important feature of LANs is that they can be used to connect multiple sites at the same time. p> The most important characteristic of a LAN is that the network is owned by a single entity and is limited in geographic scope and number of sites.

LANs have some of the following major advantages:

1. With broadcast capabilities, the entire network can be easily accessed from a single site. Hosts on a LAN can **** enjoy a variety of hardware and software resources connected to the LAN.

2, easy to expand the system and gradually evolve, the location of each device can be flexibly adjusted and changed.

3. Improved system reliability, availability, and survivability.

Now that Ethernet has taken over the LAN market, twisted pair is the dominant transmission medium in LANs, and fiber optics is used when the data rate is very high.

In order to make the data link layer more adaptable to a variety of LAN standards, the 802 Committee split the data link layer of the LAN into two sublayers:

1. Logical Link Control (LLC) sublayer

2. Media Access Control (MAC) sublayer. Control) sublayer.

Everything related to access to the transport media is placed in the MAC sublayer, while the LLC sublayer has nothing to do with the transport media, and LANs are transparent to the LLC sublayer no matter what protocol is used

Because the TCP/IP system often uses DIX Ethernet V2 instead of several LANs in the 802.3 standard, the 802 Committee has developed a Logical Link Control sublayer, which is now called the MAC sublayer. The logical link control sublayer LLC (i.e., the 802.2 standard) developed by the 802 Committee is no longer very useful.

Many vendors produce adapters with only the MAC protocol and no LLC protocol.

A network interface board is also known as an adapter or network interface card (NIC), or "network card".

Important functions of an adapter:

1. Serial/parallel conversion.

2. Cache data.

3. Install device drivers in the computer's operating system.

4. Implement the Ethernet protocol.

Ethernet uses a coordinated approach that uses a special protocol, CSMA/CD, known as Carrier Sense Multiple Access with Collision Detection, which is called Carrier Sense Multiple Access with Collision Detection.

Important features: Ethernet using the CSMA/CD protocol does not allow full-duplex communication but only alternating bidirectional communication (half-duplex communication).

For a short period of time after each station sends data, there is a possibility of a collision.

This sending uncertainty makes the average communication across Ethernet much smaller than the maximum data rate of Ethernet.

The first station to send a data frame will know if it has suffered a collision at most 2τ (twice the end-to-end round-trip delay) after sending the data frame.

The end-to-end round-trip delay of 2τ for Ethernet is called the contention period, or collision window.

After the contention period has elapsed and no collision has been detected, it is certain that no collision will occur on this transmission.

Minimum valid frame length: If a collision occurs, it must be within the first 64 bytes of the transmission

Since the transmission is aborted as soon as a collision is detected, the data that has already been sent must be less than 64 bytes.

The Ethernet specifies a minimum valid frame length of 64 bytes, and any frame that is less than 64 bytes long is an invalid frame that was abnormally aborted due to a conflict.

Therefore, if a frame is sent that is too short, it is possible that a collision will not be detected

Enhanced collision: when a station sending data stops sending data as soon as it realizes that a collision has occurred;

it then continues to send a jamming signal of a number of bits to let all users know that a collision has now occurred. all users know that a collision has now occurred.

Traditional Ethernet uses a star topology, and in the center of the star is added a very reliable device called a hub, which requires two pairs of unshielded twisted-pair cables per station, for sending and receiving.

In 1990, the IEEE developed the standard 802.3i for Star Ethernet 10BASE-T. 10BASE-T has a slightly shorter range, with the distance from each station to the hub being no more than 100 m.

The emergence of 10BASE-T twisted-pair Ethernet was a very important milestone in the history of LANs, and it established the dominance of Ethernet in LANs. dominance of Ethernet in LANs has laid a strong foundation. Some of its features are as follows:

Addresses are not required in data links using point-to-point channels, whereas when multiple sites are connected on the same broadcast channel and want to communicate with two sites, each site must be uniquely identified with a data link layer address, and each frame sent must carry an address that identifies the receiving site and the sending site. In each frame sent, it must carry an address that identifies the receiving site and the sending site, which is called a MAC address because it is used for media access control, or, in the case of a LAN, a hardware or physical address.

The RA, the registry management organization of the IEEE, is responsible for assigning the first three bytes (i.e., the high 24 bits) of the address field to the manufacturer.

The last three bytes (the lower 24 bits) of the address field are assigned by the manufacturer and are called extended identifiers, which must ensure that no duplicate addresses are produced for the adapter.

An address block can generate 2^24 different addresses. This 48-bit address is called a MAC-48, and its generic name is EUI-48. The MAC address is actually the adapter address or adapter identifier EUI-48.

Every time an adapter receives a MAC frame from the network, it first hardware-checks the MAC address in the MAC frame. If it is a frame destined for this site it accepts it and proceeds with the rest of the process. Otherwise, the frame is discarded without further processing.

The term "frames destined for this site" includes the following three types of frames:

1) unicast frames (one-to-one)

2) broadcast frames (one-to-all)

3) multicast frames (one-to-one)

4) multicast frames (one-to-one)

5) multicast frames (one-to-one)

6) multicast frames (one-to-one)

7 (multicast) frames (one-to-many)

There are two commonly used Ethernet MAC frame formats:

1, DIX Ethernet V2 standard (the most commonly used, the following is the description of this kind of frames)

2, the IEEE 802.3 standard

Invalid MAC frame:

Extending the LAN at the data link layer is done using bridges.

Bridges work at the data link layer and forward received frames based on the destination address of the MAC frame.

The bridge has the ability to filter frames. Instead of forwarding a frame to all interfaces, when a bridge receives a frame, it checks the destination MAC address of the frame and then determines which interface to forward the frame to

The most commonly used bridge today is the transparent bridge.

"Transparent" means that the stations on the LAN do not know which bridges the frames being sent will pass through, because the bridges are invisible to the stations. Transparent bridges are plug-and-play devices that are standardized to IEEE 802.1D

Transparent bridges use the Spanning Tree Algorithm: this is designed to avoid generating forwarded frames that are constantly circling around in the network

1. source route A bridge sends frames with the detailed routing information placed in the beginning of the frame. in the first part of the frame.

2. The source sends a discovery frame in a broadcast to the destination that it wants to communicate with, and each discovery frame records the routes it has traveled.

3. When the discovery frames reach the destination, they are returned to the source along their respective routes. The source station learns these routes and selects the best route from all possible routes. Any frame sent from this source to this destination must carry this routing information determined by the source in the first part of the frame.

The introduction of the switching hub in 1990 significantly improved the performance of LANs. A switching hub is often referred to as an Ethernet switch or Layer 2 switch (indicating that the switch operates at the data link layer). Ethernet switches usually have more than a dozen interfaces. Therefore, an Ethernet switch is essentially a bridge with multiple interfaces, which shows that the switch works at the data link layer.

A virtual LAN VLAN is a logical group of LAN segments that are independent of their physical location. These network segments have certain ****same requirements.

Each VLAN frame has a clear identifier that specifies which VLAN the workstation sending the frame belongs to. virtual LANs are really just a service that LANs provide to their users, not a new type of LAN.

1. Workstations B2 and B3 receive a broadcast message when B1 sends data to a member of the VLAN2 workgroup.

2. When B1 sends data, workstations A1, A2, and C1 do not receive broadcast messages from B1.

3. The VLAN limits the number of workstations that can receive broadcast messages so that the network does not suffer from performance degradation due to excessive broadcast messages (i.e., "broadcast storms").

The VLAN protocol allows a 4-byte identifier, called a VLAN tag, to be inserted into the Ethernet frame format to indicate which VLAN the workstation sending the frame belongs to.

Ethernet with a rate of 100 Mb/s or more is called High Speed Ethernet.

Star topology Ethernet, which carries 100 Mb/s baseband signals over twisted pair, still uses the CSMA/CD protocol of IEEE 802.3. 100BASE-T Ethernet is also known as Fast Ethernet.

1. can work in full duplex without conflicts. Therefore, the CSMA/CD protocol is not used.

2. The MAC frame format is still specified in the 802.3 standard.

3. The minimum frame length remains unchanged, but the maximum cable length of a segment is reduced to 100 m.

4. The inter-frame time interval has been changed from 9.6 μs to 0.96 μs.

Both full-duplex and half-duplex operation at 1 Gb/s are allowed. Uses the frame format specified by the 802.3 protocol.

Uses the CSMA/CD protocol in half-duplex mode (full-duplex mode does not require the CSMA/CD protocol).

Backward compatible with 10BASE-T and 100BASE-T technologies.

Full-duplex mode: When Gigabit Ethernet operates in full-duplex mode (i.e., both communicating parties can send and receive data at the same time), carrier extension and packet bursting are not used.

10 Gigabit Ethernet has the same frame format as 10 Mb/s, 100 Mb/s, and 1 Gb/s Ethernet.

10 Gigabit Ethernet also retains the Ethernet minimum and maximum frame lengths specified in the 802.3 standard, allowing for easy upgrades.

10 Gigabit Ethernet no longer uses copper wires, but only fiber optics as the transmission medium.

10 Gigabit Ethernet operates only in full duplex, so there are no contention issues and the CSMA/CD protocol is not used

LAN PHY. The data rate for the LAN physical layer is 10.000 Gb/s.

Optional WAN PHY. The WAN physical layer has an alternative data rate, which is the same as the so-called "Gigabit" rate. In order to connect to the so-called "Gb/s" SONET/SDH (i.e. OC-192/STM-64).

(In order for a 10 Gigabit Ethernet frame to be inserted into the payload of an OC-192/STM-64 frame, an optional WAN physical layer is used, with a data rate of 9.95328 Gb/s.)

Ethernet has been successful in increasing the rate to between 1 and 10 Gb/s, and has extended its geographic reach to both metropolitan and wide area networks. As a result, people are now experimenting with Ethernet for broadband access.

An important feature of Ethernet access is that it provides bi-directional broadband communications and the flexibility to upgrade bandwidth as users demand it.

Using Ethernet access enables end-to-end Ethernet transmission without the need for frame format conversion. This improves data transmission efficiency and reduces transmission costs.