Compiled some scattered knowledge about the computer network, first to share your own learning content, the second is a retrospective review of their notes. Please forgive any errors, contact the bloggers to modify, thank you for your browser.
1. Bandwidth:
Analog signals: bandwidth of the communication line minus the minimum unit of frequency is the highest frequency is HZ
Computer networks: the bandwidth of a communication line network indicates capacity per unit of data is transmittable bit / s
2, Nyquist theorem:
Title no noise is given, then the sampling Cmax = F * log2 N = 2 * f * log2 N (bit / s)
f represents an ideal low-pass signal, N represents the number of discrete levels per symbol (e.g. 4 gives the title phases, each phase amplitude 4, indicates information 16bit with one symbol, i.e. N in this case 16), N is large enough i.e. the coding good enough, Cmax may be infinite.
3, Shannon's theorem (used more):
First understand SNR = 10 * log10 (S / N) unit is db
Shannon formula: Cmax = W * log2 (1 + S / N) is the unit bit / s
S / N: average power in the average power of the signal to noise ratio
W: channel bandwidth
4, the relationship between bit rate and baud
Data transmission rate and bit rate = baud * log2 N topics given xbit / s, is generally said that the bit rate.
The bit rate in units of bit / s, the unit is the baud rate B. 1bps = 1bit / s
Example: If the baud rate of a network is 10KB, and the bit rate is 50KB, then the effective number of levels of each wave is ___ 32_ _
Solution: 50 = 10 * log2 N ---> N = 32
5, the baseband signal corresponding to a digital signal, the wideband signal is represented by an analog signal.
Refers to a 10Base-T: 10Mbit / s, using a baseband signal (digital signal), twisted-pair, T is the English word Twist
6, data transmission: circuit switching, packet switching, packet switching.
(1), circuit switched: establishing a bidirectional communication prior to the communication to be exclusive both physical channels.
Advantages: small delay, direct access to the data, the communication at any time, and real time, transmitted data sequence, absence of disorder to avoid conflicts.
Is generally used for telephony, the communication must be both connection-oriented.
(2), message exchange: the thermal exchange unit data, store and forward
Advantages: no need to establish a connection, you can always send a message, the dynamic allocation line
Cons: There are delays, larger cache storage space
(3), packet switching: store and forward for Internet technology, may appear out of order, lost or duplicate packets phenomenon, there is a delay.
Packet-switched virtual circuit can be divided into connection-oriented and form connectionless datagrams
Datagram: without establishing a connection, do our best to be delivered, does not guarantee the reliability, not necessarily sequential delivery, when a switch failure, can be updated accordingly forwarding, find another one alternative path.
VC: divided into three stages: virtual circuit establishment, data transfer, a virtual circuit is released. The user must establish a connection, transmission path without looking, arrive out of order, malfunction somewhere, all finished, packet overhead is relatively small.
Summary: large (1) the amount of data transmitted, transmission time is greater than the call time, circuit switched
(2) end of the link segment composed of many, uses packet switching
(3) is smaller than the packet-switched packet-switched, delay, burst mode is more suitable for communication between computers.
7, the data transfer: (1) parallel transfer: from the short, fast
(2) serial transmission: long distance, slow
Communication: (1) Synchronous transmission: the data transmission unit frames, adding header, frame end.
(2) asynchronous transmission: a transmission unit in bytes, each byte to add a start bit, a stop bit.
8, the transmission medium: coaxial cable: 50 ohm coaxial cable is mainly used for transmitting baseband signals, mainly 75 ohm coaxial cable for transmitting a wideband signal
Coaxial faster and farther than twisted pair, shielded as higher, anti-dry properties.
Fiber (fiber): error rate, single-mode fiber for long-distance, reflection occurs. The actual rate is mainly limited by the optical fiber to a photoelectric conversion rate
9, capable of carrying Manchester encoded clock signal is an Ethernet encoding Manchester encoding.
10, the channel is reliable, and the communication of high real-time requirements of the network using unacknowledged connectionless service is appropriate, because the channel is reliable, difficult to loss of data, while real-time requirements better, connectionless unacknowledged speed can a little faster.
11, the retraction stop and wait protocol protocol acceptance window frame N is 1,
Stop and wait protocol: transmission window = 1, 1 = acceptance window
Back N frames: The cumulative acknowledgment, the sending window: 1 <Wt <= 2 ^ n - 1, 1 = acceptance window;
ARQ: not supported cumulative acknowledgment, the sending window: Wt <= 2 ^ (n - 1), acceptance window> 1, and the disorder is acceptable.
12, MAC frame: Destination address (6 bytes) + source address (6 bytes) + type (2 bytes) + FCS (4 bytes) of data +> = 64B wherein the data part of the MAC frame is IP data , an IP packet that is greater than the minimum frame length 46B can satisfy a physical frame.
Ethernet frame length is the shortest to detect conflicts, the maximum distance of the same collision domain, the transmission rate is increased, the shorter the transmission time of one frame, the minimum frame length should be increased.
13, the same two devices in a local area network have the same MAC address is static, the two devices can not communicate,
However, if the IP address has the same two devices in the LAN, that the station can be switched to communicate, not the boot.
14, the card: the function is implemented in the physical layer and the data link layer
Switch: each port is a collision domain, so the switch never lost frames due to collisions. You can isolate collision domains, but can not isolate broadcast domains
Router: You can isolate broadcast domains, you can isolate forward broadcast packets. The main features include: routing and packet forwarding.
Is given only to the router to the destination network IP address of the next hop router, the IP address can be forwarded. Realizes the functions of the physical layer, data link layer, the network layer.
15, ARQ protocol including N frames back (transmit window: 2 ^ n - 1) and selection of a retransmission protocol (transmission window: 2 ^ (n -1)) , so when the sliding window number of digits n, the transmission window maximum is ^ 2 (n--1) .
16, MAC:
Medium access control channel is divided into: FDM, TDM, CDM, STDM, WDM, CDMA (physical layer)
Statistical time division multiplexing is a dynamic time allocation.
Random access MAC:
(1) ALOHA protocol: transmitting data to any node in the network without any detection test on the transmission data. If no confirmation is considered
For the conflict, a conflict that requires random node to wait some time before sending data until sent successfully.
(2) slot ALOHA: the difference is that when the conflict ALOHA be a fixed period of time.
(3) CSMA: On the basis of ALOHA improved on the following three:
1>, 1 insist CSMA: sending node listens to the channel is idle, sent immediately, or continue to monitor
2>, p- adhere CSMA: sending node listens to the channel is idle, the number of transmission probability p, or to almost (1 - p) and a time delay again listens
3>, non-stick CSMA: sending node listens to the channel is idle, data is sent immediately or delayed for a random amount of time re-listening.
(4) CSMA / CD: each station before sending the data to detect whether there are other computers to send data, if any, for the time being no hair on the bus, if not, then send the data. CSMA / CD is suitable for a wired network , and is applied to half-duplex mode, because the acceptance and use the same transmission channel. Full-duplex there is no conflict, because sending and receiving are two channels, so full duplex without restriction of CSMA / CD.
(5) CSMA / CA: a wireless local area network , if the channel is empty, the transmission data, must wait a period of time after transmitting node, checking whether the receiver sends back an acknowledgment frame.
17, the transmission medium, topology, media access control method is determined three main technical features of a LAN.
.....to be continued!
