2016.6.19 Review Essentials of Computer network The 3rd chapter of Data link layer using broadcast channel

1. The data link layer of the LAN

* * The main features of LAN: The network is a unit, and the geographical scope and number of sites is limited;

* * The main advantages of LAN:

(1) With broadcast function, from a site can easily access the whole network, the host on the local area network can share various hardware and software resources connected on the LAN;

(2) To facilitate the expansion of the system and gradually evolve, the location of the equipment can be flexibly adjusted and changed;

(3) Improve the reliability, usability and survivability of the system;

2. LAN classification (by network topology):

(1) Star nets: hubs in the middle;

(2) Ring network

(3) bus network: Each station directly connected to the bus, the bus at the end of the matching resistor to absorb the transmission of electromagnetic waves on the bus signal energy, to avoid harmful electromagnetic wave reflection on the bus. The bus network is most famous for its traditional Ethernet .

3. LAN transmission media: Twisted pair has become the mainstream transmission media in the LAN, when the data rate is very high, often need to use optical fiber as the transmission medium;

4. Technical method of LAN shared channel:

(1) static partition channel: Frequency Division Multiplexing, time division multiplexing, Wavelength Division multiplexing, Code Division multiplexing, high cost, not suitable for local area network;

(2) Dynamic media access control: Multi-point access, which is characterized by the channel is not in the user communication fixed allocation to the user, divided into two categories:

① Random access: All users can send information randomly. However, if two or more users happen to send information at the same time, then there will be collisions on the shared media (that is, conflicts), so that these users fail to send, so there must be a collision resolution network protocol; (belonging to the random access Ethernet)

② controlled access: The user cannot randomly send information and must obey certain control; Typical representatives of this class have decentralized control of the token ring LAN and centralized control of the multipoint line inquiry or called polling.

5. Two standards for Ethernet:

(1) DIX Ethernet V2

(2) 802.3 lan abbreviation "Ethernet"

6. In order to make the data link layer better adapt to a variety of LAN standards, the data link layer of the LAN is divided into two sub-layers:

(1) Logical Link Control LLC sublayer: The LAN for the LLC Sublayer is transparent regardless of the transmission media and the Mac Sublayer;

(2) Media access control Mac sub-layer: With the access to media-related content is placed on the Mac sub-layer;

(3) Due to the rapid development of LAN and TCP/IP system is often used in LAN only dix Ethernet V2 instead of 802.3 standard LAN, so the role of the LLC sub-layer almost disappeared, many manufacturers of adapters on the only installed MAC protocol and no LLC agreement;

7. The role of the adapter:

* * The connection between the computer and the external LAN is through the communication adapter, the adapter is a network interface board inserted in the host box, this interface board is also known as the Network interface card NIC (i.e. NIC)

* * This adapter is already embedded on the computer's motherboard and no longer uses a separate NIC! Therefore, the use of the adapter name, instead, "Network card";

(1) The composition of the adapter: processor and memory (including RAM and ROM) are installed on the adapter

(2) Important functions of the adapter:

* * Communication between the adapter and the local area network is carried out in serial mode via a cable or twisted pair;

* * The communication between the adapter and the computer is carried out in parallel by the I/O bus on the motherboard of the computer;

* * Important function: The conversion of data serial transmission and parallel transmission;

* * Because the data rate on the network and the data rate on the computer bus is not the same, so the adapter must be equipped with a memory chip to cache the data;

* * If the adapter is plugged into the motherboard, the device driver that manages the adapter must also be installed in the computer's operating system;

* * Adapters are also capable of implementing Ethernet protocols

(3) How the adapter works:

* * The adapter does not use the computer's CPU when receiving and sending various frames

* * When the adapter receives a wrong frame, the frame is discarded without notifying the computer;

* * When the adapter receives the correct frame, it uses interrupts to notify the computer and deliver the network layer in the protocol stack;

2016.6.19 Review Essentials of Computer network The 3rd chapter of Data link layer using broadcast channel