2016.6.20 Review Essentials of Computer network chapter III of the CSMA/CD agreement

1. The earliest Ethernet is to connect many computers to a bus:

(1) The characteristic of the bus is: When a computer sends data, all the computers on the bus detect this data, this is the mode of broadcasting communication;

(2) In order to achieve one-to-two communication on the bus, each computer's adapter can have a different address than the other adapter, when the destination address in the data frame and the hardware address stored in the adapter ROM is consistent, the adapter can receive the data frame;

(3) People usually refer to the computer on the LAN as "host", "Workstation", "site" or "station"

2. Ethernet for communication convenience to take measures:

(1) Adopt a more flexible and non-connected work mode, that is, do not have to establish a connection to send data directly, the adapter to send the data frame is not numbered, and does not require the other party to send back confirmation.

* * The services provided by Ethernet are delivered with the utmost effort, i.e. unreliable delivery;

* * The need to retransmit the error frame is determined by the high-level, but the Ethernet does not know the retransmission frame, but as a new data frame to send;

* * As long as there is a computer on the bus to send data, the bus transmission resources will be occupied, at the same time can only allow a computer to send data;

* * The conflict resolution method used by Ethernet is: CSMA/CD, Carrier Monitor multi-point access/collision detection

(2) The data sent by Ethernet is a signal that is encoded using Manchester.

* * Binary baseband digital signal is usually a high, low voltage alternating signal;

* * Manchester code: 1 is "front high after Low", while 0 is "before the first high";

* * The disadvantage of Manchester code is that it occupies a bandwidth that is one-fold higher than the original baseband signal (since the number of yards transmitted per second doubles);

Key points of the 3.CSMA/CD Agreement:

(1) "Multipoint access": That is, this is the total Linetype network, many computers are connected to a bus in a multi-point way. The essence of the agreement is "carrier monitoring" and "Collision detection";

(2) "Carrier monitoring": detection channel, either before sending, or in the transmission, each station must constantly detect the channel.

* * In the detection of the channel before sending, is to obtain the right to send, if detected already have other stations in the send, then they are temporarily not allowed to send data, must wait until the channel becomes idle to send;

* * Detect the channel in the transmission, in order to find out in time there are no other stations and the transmission of the collision;

(3) "Collision detection": Side of the transmission edge monitoring;

* * That is, the adapter side sends the data edge detection channel signal voltage changes, in order to determine whether the other stations in the sending data is also sending data;

* * When several stations at the same time on the bus to send data, the bus signal voltage change amplitude will increase (overlapping), when the adapter detects a signal voltage change amplitude exceeds a certain threshold, it is believed that the bus at least two stations at the same time transmitting data, indicating that a collision occurred;

(4) The propagation of electromagnetic wave in 1Km cable is Siyanyo 5us;

(5) In the LAN analysis, often the bus on the one-way, end-to-end transmission Shiyank for the r,a to send the data at the latest after the time to know that the data they sent and other stations sent data collision, this time up to 2r (twice times the bus end-to-end propagation delay)

(6) Obviously, when using the CSMA/CD protocol, it is impossible for a station to send and receive simultaneously (but must listen on the side of the sending side). Therefore, Ethernet using the CSMA/CD protocol is not possible for full-duplex communication and only half-duplex communication (bidirectional alternating communication);

(7) Each station after sending the data for a short period of time, there is the possibility of encountering collisions, this small period of time is uncertain, it depends on the other station to send data distance to the station;

(8) The first station to send the data frame, after sending the data frame up to the time of 2r to know whether the data frame sent to the collision; therefore, the Ethernet end-to-end round trip time 2r is called the Contention period (collision window);

* * After the contention period has not detected a collision, to be sure this send will not send a collision;

(9) Ethernet uses truncated binary exponential backoff algorithm to determine the time of retransmission after collision;

* * Truncated binary exponential backoff algorithm: Let the collision station stop sending the data, not wait for the channel to become idle, immediately, then send the data, but postpone (Backoff) a random time

* * The basic Backoff time is contention period 2r, the specific contention time is 51.2us;

* * A random number is taken from the discrete set of integers "0,1 ..., (2k-1)", recorded as R. The time that retransmission should be postponed is the contention period of R times;

**k is the number of collisions. K=min[retransmission times, 10], when the number of retransmissions more than 10 o'clock, K is not increasing but has been equal to 10;

* * When the transmission is not successful 16 times, discard the frame and report to the top

(10) Each time the adapter sends a new frame, it is necessary to execute a CSMA/CD algorithm, the adapter has no memory function for the collision sent by the past;

(11) In order to avoid collision detection before the transmission is complete:

* * Ethernet Specifies a minimum frame length of 64 bytes, or 512bit, by filling the frame length of not less than 64 bytes;

* * for 10MB/S ethernet, sending 512bit time requires 51.2us, which is the contention period mentioned above;

* * Any frame with a length of less than 64 bytes is an invalid frame that is aborted due to a conflict, and should be discarded as soon as the invalid frame is received;

(12) The signal on the Ethernet transmission 1Km about 5US, the largest Ethernet end-to-end delay must be less than half of the contention period (that is, 25.6us), which is equivalent to the maximum Ethernet end-to-end length of about 5Km, the actual Ethernet coverage is not so large;

(13) Strengthen the collision: is when the station to send the data when the collision, in addition to immediately stop sending data, but also continue to send 32-bit or 48-bit artificial interference signal, so that all users know that now has sent a collision;

The main points of the CSMA/CD agreement are summarized as follows:

① ready to send: The adapter obtains a packet from the network layer, plus the Ethernet header and tail, which make up the Ethernet frame, into the adapter's cache, but before sending it, it must first detect the channel;

② Detection channel: If the channel is detected busy, it should be constantly detected, waiting for the channel to become idle. If the channel is detected to be idle and the channel remains idle within 96-bit time, the frame is sent;

③ in the transmission process still constantly detect the channel, that is, the network adapter to the side of the transmission side monitoring, there are only two possibilities:

④ one is sent successfully: During the contention period has not detected a collision, this frame will certainly be able to send a successful;

⑤ Two is the transmission failure: During the contention period detects a collision, which is immediately stop sending data, and in accordance with the provisions of the transmission of human interference signal; The adapter then executes the exponential backoff algorithm, waits for R times 512 bit time, returns to step 2, continues to detect the channel, but if the retransmission 16 times can not be successful, then stop retransmission and to

⑥ Ethernet each time a frame is sent, be sure to keep the sent frame for a moment.

2016.6.20 Review Essentials of Computer network chapter III of the CSMA/CD agreement