大学物理

骣 the average signal power ÷ ç SNR = 10lg ç ÷ ÷ ç the average noise power 桫
Sources of Information
The telecommunications environment is dominated
by four important sources of information:
Wireless broadcast 377 ohm channels Mobile radio channels Satellite channels 377 ohm 377 ohm
•Classifying communication channels:1.Linear and nonlinear;2.Time invariant and time varying;3.Bandwidth limited and power limited
RS232
JPEG image coding standard(JPG)
Computer data multimedia

Stands for Joint Photographic Experts Group; 8*8blocks; DCT(discrete cosine transform); Data compression: DCT coefficients should be uncorrelated; DCT coefficients should be packed into the smallest number of coefficients possible; DCT coefficients is represented by an 8-bit code word.
Primary Communication Resources
Channel bandwidth Transmitted power Power limited example: a space communication link Band limited example: the telephone circuit SNR:signal-to-noise ratio (unit:dB)
QoS (Quality of Service): Cell loss ratio Cell delay Cell delay variation Hierarchy of SONET data rates
Level OC-1 OC-3 OC-9 OC-12 OC-18 OC-24 Data Rate(Mb/s) 51.84 155.52 466.56 622.08 933.12 1,244.16



Speech Music Pictures Computer data
Sources of Information
Speech:
telephone;
rຫໍສະໝຸດ Baidudio;
Music:
radio;
music on internet;
Sources of Information
Pictures:
Movies TV
Encoding
transmitter channel receiver
Transmission
Decoding Re-creation
The Communication Process
Two basic modes of communication:

broadcasting; point –to –point communication
TV
interlaced fields; Frame; PAL:
312.5 lines/field; 625lines/frame; 25frames/second;
NTSC:
262.5 lines/field; 525lines/frame; 30frames/second;
Sources of Information Computer data:
Background and Preview
COMMUNICATION SYSTEMS
•The communication process •Primary communication resources •Source of information •Circuit switching and packet switching •Communication channels •Modulation process •Analog and digital types of communication systems •Shannon’s information capacity theorem •A digital communication problems
Communication networks Broadband networks
BISDN(broadban d integrated services digital network) ATM(asynchro nous transfer mode) SONET(synchr onous optical network )
OC-36
OC-48
1,866.24
2,488.32
Communication Channels
•Difference of logical channels and physical channels •Popular physical channels: Physical channels Twisted pair Coaxial cable Optical fiber Characteristic impedance 120 ohm 50 or 75 ohm / Range of frequency Ultimate feild
Communication networks
Routers Boundary of subnet
hosts
Communication Networks Circuit switching Telephone PSTN(Public Switched Telephone Network )
PSTN
MPEG video coding standard(MPG)
Stands for Motion Photographic Experts
Group; Exploiting four basic forms of redundancy: Interframe redundancy; Interpixel redundancy within a frame; Phychovisual redundancy; Entropic coding redundancy.
Circuit switching
Process of Circuit switching
Communication Networks Packet switching(分组交 换）
Packet A
Packet B
Packet switching(分组 交换） Data network
a layered architecture
The Communication Process
Transmission of
information

a message signal a set of symbols. Communication system



Generation of a message signal. Description of the message with a set of symbols. Encoding of the symbols. Transmission of the encoded symbols through a physical medium. Decoding and reproduction of the original symbols. Re-creation of the original message .
OSI （open Systems interconnect ion） reference model
Data network Internet
IP(internet protocol) TCP(Transmissi on control protocol) UDP(User datagram protocol)