OSI Seven-layer reference Model-physical layer

Physical layer

Establish, maintain and disconnect the physical connection, define the interface and media, and realize the transmission of the bit stream.

1. Signal

1) Information

2) data:

3) signal: the medium of information transmission

4) signal classification:

Analog signal: a continuously changing physical quantity.

Digital signal: discontinuous physical quantity, signal parameters are not continuous change, high and low fixed.

5) noise and attenuation

6) advantages of digital signals:

Strong anti-interference ability

Suitable for long-distance transmission and guarantee quality

2. Interface

Ethernet interface: RJ-45 crystal head

Optical fiber interface: FC, ST, SC, LC narrow square optical fiber connector (mainstream at present), MT-RJ

3. Twisted pair

1) twisted pair classification:

Shielded twisted pair (STP)

The wire is coated with a metal omentum, which is used in an industrial environment with a very complex electromagnetic environment.

Unshielded twisted pair (UTP)

2) Standard and classification of twisted pair:

Cat5 category 5 twisted pair, transmission frequency is 100MHz, suitable for 100Mbps network

Cat 5e has less attenuation, less crosstalk and better performance than Category 5 lines (over Category 5)

Cat 6 transmission frequency is 200MHz

Cat 7 transmission frequency is 600MHz

4. Connection specification of twisted pair

1) Line order

T568A:

White green, white orange, blue, white blue, orange, white brown, brown

T568B:

White orange, orange, white green, blue, white blue, green, white brown, brown

1, 2 send, 3, 6 receive

2) connection of cables:

Standard network cable (straight-through or straight-through): used to connect different devices (A-mai A-mai B)

Crossover cable: for connecting the same device (Amurb)

Reverse line: not used for Ethernet connection, but mainly used for connecting the serial port of a computer to the console (control port) of a router or switch. its line sequence is 1x8 at one end and 8x1 at the other.

Special case: the computer is directly connected to the router, the crossover switch is connected to the switch, and the crossover or direct connection is usually used.

5. Optical fiber

Single-mode fiber: for high speed, long distance, high cost, difficult termination, narrow core wire, need laser source, low dissipation, high efficiency

Multimode fiber: for low speed, short distance; low cost; easy termination; wide core wire, light source can be laser or LED; large dissipation, low efficiency

6. Physical layer equipment: network card, repeater hub