Network(1)

What is the Internet

The Internet is a computer network that interconnects billions of computing devices throughout the world.

All of these devices are called hosts or end systems. End Systems are connected together by a network of communication links and packet switches.

Communication links includes coaxial cable(同轴线), copper wire(铜线), optical fiber(光纤), and radio spectrum(射频频谱).

Different links have different transmission rate, measured in bits/second.

The sending end system segments the data and adds header bytes to each segment. The resulting packages of information, packets(数据包), are sent to the destination and are reassembled into the original data.

Packet switch: takes a packet arrving on one of its incoming communication links and forwards that packet on one of its outgoing communication links.

Two types of packet switches: routers(路由器) and link-layer switches(链路层交换机).

Link-layer switches are typically used in access networks, while routers are typically used in the network core.

The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network.

Cargo -> segmented -> transport -> unloaded and grouped

End systems access the internet through Internet Service Providers(ISPs).

• residential ISPs: local cable or telephone companies
• corporate ISPs
• university ISPs
• ISPs that provide WiFi access in airports, hotels, coffee shops and other public places
• cellular data ISPs

Each ISP is in itself a network of packet switches and communication links.

Network access:

• residential broadband access(民用宽带) such as cable modem(有线调制解调器) or DSL. DSL(Digital subscriber line): transmit digital data over telephone lines. ADSL(asymmetric digital subscriber line).
• high-speed local area network access
• mobile wireless access
• ISPs also provide Internet access to content providers, connecting Web sites and video servers directly to the Internet.

The ISPs must also be interconnected.

Protocols control the sending and receiving of information within the Internet. TCP/IP: Transmission Control Protocol(TCP) and Internet Protocol(IP). The IP protocol specifies the format of the packets that are sent and received among routers and systems.

Internet standards are developed by the Internet Engineering Task Force(IETF). The IETF standards documents are called requests for comments(RFCs). They define protocols such as TCP, IP, HTTP, and SMTP.

A Services Description

Internet is also an infrastructure that provides services to applications.

The applications are said to be distributed applications, since they involve multiple end systems that exchange data with each other.

End systems attached to the Internet provide a socket interface that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.

What is a Protocol?

Transmitted and received messages, and actions taken when these messages are sent or received or other events occur, play a central role in a human protocol. The same is true in networking–it takes two (or more) communicating entities running the same protocol in order to accomplish a taks.

• hardware-inplemented protocols in two physically connected computers control the flow of bits on the “wire” between the two network interface cards;
• congestion-control protocols control the rate at which packets are transmitted between sender and receiver;
• protocols in routers determine a packet’s path from source to destination.

A protocol defines the format and the order of messages exchaged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.