Data link layer (5 messages) Computer network data link layer_Put on straw sandals to travel blog-CSDN blog
Table of contents
1. Extend Ethernet at the physical layer
2. Extend Ethernet at the data link layer
2.1 Characteristics of Ethernet switches:
2.2 The biggest advantages of Ethernet switches
2.3 Self-learning function of Ethernet switch
3.10 Gigabit Ethernet and Faster Ethernet
4. Use Ethernet for broadband access
1. Extended Ethernet
Sometimes the range of Ethernet is extended. This extended Ethernet is still a network to the network layer.
1. Extend Ethernet at the physical layer
Due to the limitations of the CSMA/CD protocol, the distance between Ethernet hosts cannot be too far. Optical fiber can be used to extend the distance between the host and the hub . Because the delay of optical fiber is small and the bandwidth is wide, the distance between the host and the hub can be easily extended to several kilometers. Multiple Ethernet networks can be connected through a backbone hub to form a larger Ethernet network . The advantage of extending multiple Ethernets through connections is that communication between different Ethernets can be achieved and the geographical range can be expanded. The disadvantage is that the collision domain will increase and the probability of collision when sending data increases.
2. Extend Ethernet at the data link layer
Extending Ethernet is more about extending at the data link layer. Ethernet is extended at the data link layer through Ethernet switches (also called Layer 2 switches).
2.1 Characteristics of Ethernet switches:
1. An Ethernet switch is actually a multi-interface bridge. Each interface is directly connected to a host or another switch, and generally works in full-duplex mode.
2. Ethernet switches are parallel and can connect multiple pairs of interfaces at the same time, allowing multiple pairs of hosts to communicate at the same time. Hosts communicating with each other all occupy the transmission medium exclusively and transmit data without collision.
3. The Ethernet switch is a plug-and-play device, and its internal frame switching table (also called the address table) is automatically and gradually established through a self-learning algorithm.
2.2 The biggest advantages of Ethernet switches
The biggest advantage of a switch: its parallelism. Simultaneous communication between multiple pairs of hosts will not divide the total bandwidth equally, because each pair of hosts exclusively occupies the bandwidth of its transmission medium, so the bandwidth of each pair of hosts is still the original bandwidth. For a traditional 10Mbit/s shared Ethernet, if there are 10 users, the average bandwidth of each user is 1Mbit/s. If an Ethernet switch is used to connect these hosts, the bandwidth of 10 users is 10Mbit/s. Equivalent to a total bandwidth of 100Mbit/s.
2.3 Self-learning function of Ethernet switch
Two key points to achieve self-learning:
1. If a host sends data, the host's MAC address and corresponding interface are stored in the exchange table.
2. If the corresponding interface of the data receiver cannot be found in the exchange table, broadcast will be performed on all interfaces.
Because the host connected to the interface may change, and the network adapter of the host may also change, each item in the exchange table has a valid time and will be deleted after the time has passed. However, as the number of stations on the Ethernet increases, the reliability of the bus-structured Ethernet decreases. At the same time, the development of large-scale integrated circuits and special-purpose chips has made star-structured Ethernet switches cheap and reliable . Bus Ethernet uses the CSMA/CD protocol and works in half-duplex mode . The Ethernet switch does not use a shared bus and has no collision problem, so it does not use the CSMA/CD protocol, but works in full-duplex mode, but the frame format remains unchanged.
3. Virtual LAN
Virtual LAN VLAN can be easily implemented using an Ethernet switch. Virtual LAN VLAN: It is a logical group composed of some LAN segments that have nothing to do with physical location. These network segments have certain common requirements. Each VLAN frame has a clear identifier that indicates which VLAN the computer sending the frame belongs to. Virtual LAN is a service provided by LAN to users, not a new type of LAN. The computers in each VLAN in the figure below can be in different LANs.
2. High-speed Ethernet
The current Ethernet rate has developed from the traditional 10Mbits/s to 1Gbit/s Gigabit Ethernet.
1.100BASE-T Ethernet
100BASE-T is a star topology Ethernet that transmits 100Mbit/s baseband signals on twisted pairs. It still uses the CSMA/CD protocol and is also called Fast Ethernet. 100BASE-T can use an Ethernet switch. When using an Ethernet switch, it works in full-duplex state and does not use the CSMA/CD protocol. The MAC frame format used by Fast Ethernet is still the frame format specified by the IEEE802.3 standard.
2. Gigabit Ethernet
Gigabit Ethernet has the following characteristics:
- Allows to work in both full-duplex and half-duplex modes at 1Gbit/s.
- Use the frame format specified by the IEEE 802.3 protocol.
- The CSMA/CD protocol is used in half-duplex mode but not in full-duplex mode.
- Backwards compatible with 10BASE-T and 100BASE-T.
In half-duplex mode, Gigabit Ethernet adopts the " carrier extension " method to extend the contention period and the minimum length of the MAC frame sent to 512 bytes. This creates additional overhead when sending short frames requiring a lot of padding. A "packet burst" function has also been added. When many short frames are to be sent, the first short frame is filled using the carrier extension method, and the subsequent ones are sent one after another without filling.
3.10 Gigabit Ethernet and Faster Ethernet
The frame format of 10GE is exactly the same as that of 10Mbit/s, 100Mbit/s and 1Gbit/s, and the minimum and maximum frame lengths are also the same. 10GE only works in full-duplex mode and does not use the CSMA/CD protocol, which greatly improves its transmission distance. Ethernet technology develops rapidly, and 40GE and 100GE standards were developed after 10GE, both of which only work in full-duplex mode. The transmission distance can reach tens of kilometers. Now the working scope of Ethernet has been expanded to metropolitan area networks and wide area networks. Its advantages are:
- The technology is mature.
- Good interoperability.
- cheap price. When Ethernet is used in a wide area network, the price is much cheaper than SONET, a synchronous optical fiber network.
- The end-to-end Ethernet connection ensures that all frames are in Ethernet format and does not require frame format conversion.
The development of Ethernet has proved the advantages of Ethernet:
- Scalable (from 10Mbit/s to 10 Gbit/s).
- Flexible (multiple media, full/half duplex, sharing/switching).
- Easy to install.
- Good robustness.
4. Use Ethernet for broadband access
Ethernet is also now used for broadband access to the Internet. Ethernet access can provide two-way broadband communication and can be flexibly upgraded as needed (such as from 10M to 10G). However, there is no user name field in the Ethernet frame format and the process of allowing the user to enter a password to authenticate the user's identity . So PPPoE (PPP over Ethernet) was born, which encapsulates PPP frames in the PPP protocol into Ethernet for transmission. Current fiber optic broadband access FTTx uses PPPoE.