computer network
Chapter 2 Physical layer
The basic concept of the physical layer 2.1
2.2 Basics of data communication
2.2.1 Data Communication System Model
A data communication system can be divided into three parts, i.e., the source system (or sending end, the transmission side), a transmission system (or transport network) and a destination system (or reception side, receiving side)
The system generally comprises a source of two parts
- Source
- Transmitter
- receiver
- end
Signals can be divided into two categories:
- Simulation Model (continuous signal)
- Digital signal (discrete signal)
2.2.2 few basic concepts related to channel
From both sides communicate information exchange point of view, there are three basic ways you can
- One-way communication, also known as simplex communication
- Way alternate communication is also known as half-duplex communication
- At the same time two-way communication, also known as full-duplex communication
This source signal to a baseband signal is referred. Often is modulated baseband signal.
Modulation can be divided into two categories:
- Converting the digital baseband modulation signal to another form of digital signals, to be encoded,
- Carrier
1. The common encoding
- Non Return to Zero
- Return to Zero
- Manchester encoding
- Differential Manchester encoding
2. Basic bandpass modulation method
- AM AM
- FM FM
- PM PM
2.2.3 channel capacity limit
1. The frequency range of the channels through
2. Signal to Noise Ratio
2.3 The following physical layer transmission medium
2.3.1-guided transmission medium
1. Twisted Pair
In order to improve resistance to electromagnetic interference twisted pair, shielded twisted pair (STP).
| Strand type | bandwidth | Cable Features | typical application |
|---|---|---|---|
| 3 | 16MHz | Analog telephone, conventional Ethernet (10Mbit / s) | |
| 4 | 20MHz | LAN has been used for tokens | |
| 5 | 100MHz | Application of speeds up to 100Mbit / s in | |
| 5E | 125MHz | Application of speeds up to 1Gbit / s of | |
| 6 | 250MHz | Application rate is higher than 1Gbit / s of | |
| 7 | 600MHz | Application rate is higher than 10Gbit / s of |
2. coaxial cable
Coaxial cable having good interference characteristics, are widely used for data transmission of higher rate.
The bandwidth depends on the quality of coaxial cable. The current high-quality coaxial cable bandwidth is close to 1GHz
3. The fiber optic cable
Commonly used optical fiber communication bands are located in three 850nm, 1300nm and 1550nm. In both cases after attenuation are small. Three bands have a bandwidth of 25000-30000GHz.
2.3.2 Non-guided transmission medium
The free space is called "non-guided transport media"
Band Category:
- Very low frequency VLF
- Very low frequency ULF
- Ultra-low frequency SLF
- ELF Extremely Low Frequency
- LF low frequency
- IF MF
- High Frequency HF
- (VHF)
- UHF UHF
- UHF SHF
- EHF EHF
Fading phenomenon multipath effects and ionospheric reflection wave communication (communication frequency) mainly by ionospheric reflection, but unstable ionosphere generated generated. The communication quality is poor HF channel. Therefore, data transfer must be used shortwave radio, they are generally low speed transmission.
Microwave radio communication plays an important role in the communication data. Microblogging in the space mainly straight lines. Since microwaves penetrate the ionosphere into the space, so that he can not short to ground propagate long distances by ionospheric reflection.
Traditional microwave communication mainly in two ways, i.e., terrestrial microwave relay communication and satellite communication.
2.4 channel multiplexing
2.4.1 frequency division multiplexing, time division multiplexing and time division multiplexing statistics
The most basic multiplexing is frequency division multiplexing and time division multiplexing. Frequency division multiplexing the simplest, the user is assigned to a certain frequency band during communication occupies throughout the frequency band. All visible frequency division multiplexing users occupy different bandwidths at the same time resource (bandwidth here refers to the rate of transmission of data rather than a frequency bandwidth)
All users are time-division multiplexing is occupied by the same frequency bandwidth at different times.
2.4.2 Wavelength Division Multiplexing
Is a wavelength division multiplexed WDM optical frequency division multiplexing.
2.4.3 Code Division Multiplexing
Code Division Multiplexing (CDM) is another method for a shared channel.
Digital transmission system 2.5
2.6 Broadband Access Technology
2.6.1 ADSL technology
ADSL Asymmetric Digital Subscriber Line technology to transform the existing analog telephone digital subscriber line technology, it is possible to carry broadband digital service.
2.6.2 Hybrid Fiber Coax (HFC network)
To improve the quality and reliability of the transmitted television signal, HFC network the existing coaxial trunk portion to the optical fiber cable television network. Light fiber node connected to the head end. In an optical fiber node are converted to electrical signals, and then transmitted to each user's home through a coaxial cable.
2.6.3 FTTx technology
Fiber to the home FTTH, fiber is to lay up to the user's home, only after the fiber into the user's home, and only then optical signals into electrical signals, in order to obtain the highest speed Internet access.