The data link layer above the physical layer in the layer, which is divided into two types of channels and point channels broadcast channels. They distinguish to see is one-way communication point to point, or one to many broadcast mode.
The main point of the agreement for the PPP, before there is a reliable transport protocol HDLC, now use has been relatively less. The main role of the PPP protocol to connect to a PC user's ISP, and then by connecting to the Internet through your ISP. When the user dials the access ISP, a physical connection is established from the user's PC to the ISP. In this case, the PC sends a series of LCP (Link Layer Control Protocol) packet (PPP encapsulated into a plurality of frames) to the ISP, to establish LCP connector. These selected packets and their responses to a number of parameters to be used in the PPP. Then also the network layer is disposed, NCP new access to the user's PC to assign a temporary IP address. In this way, the user's PC becomes a host on the Internet, I have the address. When the user of the communication is completed, the NCP (Network Control Protocol) network layer connection release, recovered out assigned IP address. Next, LCP release the data link layer connection. Finally, the release of the connection to the physical layer. When the network layer configuration is completed, the link will enter the "link open" state for data communication. PPP links two endpoints may send packets to each other. Two endpoints may also send PPP LCP echo request packet and answer LCP packets, to detect the state of the link. After completion of the data transfer may be issued by the end of the link LCP termination request packet, the link request to terminate the connection, upon receipt of the acknowledgment sent by the LCP terminate packets, to link termination status. If a link fails, the link will open the state to terminate the state from the link. Day after stopping the carrier demodulator, a link back to the rest state. No link between the devices from the start, to establish a physical link, then LCP link establishment. After identification after the establishment of NCP link before it can exchange data. Thus, PPP protocol is not purely a data link layer protocol, it also contains the contents of the physical layer and network layer.
There are about P2P, the channel will introduce so many, currently focuses on broadcast channels. About topological classification of things well understood. Here there is not much to do introduced. Direct introduction of two standard Ethernet
conventional Ethernet MAC frame has two standards, one is DIX ethernet V2 Another standard is the IEEE 802.3 standard
V2 Ethernet MAC frame is relatively simple, consists of five fields. The first two fields are 6 bytes long destination address and source address fields. The third field is a 2-byte type field, is used to mark the protocol layer, the MAC frame data so received to the upper layer protocol. The fourth field is the data field. A length between 46 to 1500 bytes. The last field is a 4-byte frame check sequence FCS.
V2 Ethernet MAC frame format, which does not have a frame length header field. Then the mac sub-layer How do you know how many bytes of data are removed from the received Ethernet frame to deliver it on the floor? The size of between 46-1500 in the end how much is it? Manchester encoding this point there is to know, when there will be electricity transmission symbol hopping levels. After sending a frame, we not transmitted, so that we can determine the number 4 from the end of the field end position forward.
When the field is less than 46 bytes of data, will be added MAC sublayer a positive number of bytes of the data field after the field is filled in, in order to ensure the Ethernet MAC frame is not less than 64 bytes, the upper course of the recording IP datagrams byte packet length, so when removing the frame header, the IP data segment can be removed to obtain the correct end of the frame.
In fact there are eight bytes in front of the MAC frame transmission media, the role of these eight bytes the adapter is to achieve clock synchronous with the bit stream, the first seven bytes of code are alternately 1 and 0, the first preganglionic six eight words are alternately 0,1, 1 and finally tell the two network adapters, to the information of the MAC frame. It should be emphasized that within the 8-byte FCS no longer test range. And transmitting data in units of frames on the Ethernet is transmitted, during transmission of Ethernet frames, must have a little gap between the frames, so long as the receiving end of a frame start delimiter found, which reaches behind the continuous bitstream belong to the same MAC frame. Visible without the use of an Ethernet frame end delimiter is not required to guarantee the transparent transmission of the byte inserted.