Data link layer using broadcast channel

Data link layer using broadcast channel 1. The data link layer of the LAN 1.1 The main features of LAN The main feature of LAN is that the network is owned by a unit, and the geographical range and the number of sites are limited. Note: When the local area network has just appeared, the local area network has higher data rate, lower delay and less ber than the WAN. But with the widespread use of optical fiber technology in wide area networks, the current wide area network also has high data rates and very low ber. 1.2 main advantages of LAN

(1) has broadcast function, from a site can easily access the entire network. Hosts on the LAN can share various hardware and software resources attached to the LAN.
(2) To facilitate the expansion and gradual evolution of the system, the location of the equipment can be flexibly adjusted and changed.
(3) Improve system reliability (reliability), availability (availibility), survivability (survivability).1.3 lan can be categorized by network topology star-shaped mesh:Because of the presence of hubs and the large number of twisted-pair wires used in local area networks, the Ethernet of star-shaped Ethernet and multi-stage star-shaped structure has been widely applied.Ring Net:The most typical is the token ring, which is referred to as the Token ring.Bus network:The stations are connected directly to the bus. The matching resistors at both ends of the bus absorb the energy of the electromagnetic signal transmitted on the bus, and avoid harmful electromagnetic wave reflection on the bus. The bus network can use two kinds of protocols: (1) CSMA/CD used by traditional Ethernet. (2) Token delivery bus network, that is, the bus network is physically, and logically is the token ring network. The former bus network has now evolved into a star-shaped network, the latter a token-passing bus network has long since exited the market.tree-shaped mesh:Tree-shaped network is the deformation of the bus network, all belong to the use of Broadcast channel network, but this is mainly used for frequency division multiplexing broadband LAN.
1.4 ways to share a channel static partitioning of channels
(1) such as the use of frequency division multiplexing, time division multiplexing, Wavelength division multiplexing and Code Division multiplexing. Users will not be sent conflicts with other users as long as they are assigned to the channel.
(2) This method of dividing the channel cost is relatively high, not suitable for local area network use.Dynamic media access control
(1) random access:Random access is characterized by the random sending of information to all users. But if two or more users happen to send messages at the same time, a collision (i.e. a conflict) will occur on the shared media, causing these users to fail to send. Therefore, there must be a network protocol to resolve collisions.
(2) controlled access:The characteristic of controlled access is that users cannot send information randomly and must obey certain control. Typical representatives of this class have decentralized control of token ring LANs and centralized control of multipoint line probing (polling) or called polling.

NOTE: A broadcast channel can have one-to-many communication. The LAN uses the broadcast channel. The level of LAN work has spanned the data link layer and the physical layer. Local area networks are not just about data link layers. 2. Ethernet 2.1 Two standard of Ethernet 2.1.1 DIX Ethernet V2DIX Ethernet V2, is the world's first LAN product specification.2.1.2 IEEE 802.3[w-ieee802.3]The first IEEE Ethernet standard IEEE 802.3 was developed in 1983 with a data rate of 10mb/s. The 802.3 LAN makes a minor change to the frame format in the Ethernet standard, but allows hardware implementations based on these two standards to interoperate on the same LAN. Note: Due to the fierce commercial competition, the IEEE 802 committee failed to form a unified, best LAN standard, but was forced to develop several different LAN standards, such as 802.4 token bus network, 802.5 token ring network, etc.
(1) In order to better adapt the data link layer to different local area network standards, the IEEE802 board has split the data link layer of the LAN into two sub-layers, that is, the logical link Control Sub-Layer LLC (Logical link controls) and the media access control sub-layer Mac (Medium access Control).
(2) content connected to the transmission media is placed on the Mac Sublayer, while the LLC Sublayer is independent of the transport media, regardless of the transport media and the Mac sub-layer of the LAN to the LLC sublayer is transparent.
2.2 Function of the adapterThe connection of the computer to the external LAN is through the communication adapter (adapter). An adapter is a network interface board (or a PCMCIA card inserted in a laptop) that is plugged into a computer's mainframe box. This interface board is also referred to as the network interface card NIC (NET Interface card) or simply the NIC.The communication between the adapter and the LAN is carried out serially through a cable or twisted pair, while the communication between the adapter and the computer is carried out in parallel via the I/O bus on the motherboard of the computer. an important function of the adapter is the conversion of data serial and parallel transmissions. Because the data rate on the network is not the same as the data rate on the computer bus, the memory chip that caches the data must be installed in the adapter. If you insert the adapter on the motherboard, you must also install the device driver that manages the adapter in the computer's operating system. The driver will then tell the adapter where to send a chunk of data from the memory to the local area network, or where it should store the data blocks it transmits over the LAN.The adapter also needs to be able to implement the Ethernet protocol. The adapter does not use the computer's CPU when it receives and sends various frames. The CPU can then handle other tasks. When the adapter receives a wrong frame, it discards the frame without notifying the computer. When the adapter receives the correct frame, it uses an interrupt to notify the computer and deliver it to the network layer in the protocol stack. When the computer sends the IP datagram, the IP datagram is handed down to the adapter by the protocol stack, which is assembled into a frame and sent to the local area network.The computer's hardware address--mac address, which is in the Rom of the adapter. The computer's software address,--IP address, is in the computer's memory.

Reference Documents:
[1] Computer Network (5th edition) Shehiren--Chapter 3.3