1-Computer Networks and the Internet

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1.1 What Is the Internet?

1.1.1 A Nuts-and-Bolts Description

• When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packages of information, known as packets, are then sent through the network to the destination end system, where they are reassembled into the original data.
• A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Packet switches have two main types: link-layer switches and routers. Both types of switches forward packets toward their ultimate destinations.
  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 a route or path through the network.
• End systems access the Internet through Internet Service Providers (ISPs). Each ISP is in itself a network of packet switches and communication links. ISPs provide a variety of types of network access to the end systems and provide Internet access to content providers, connecting Web sites directly to the Internet.
• Lower-tier ISPs are interconnected through national and international upper-tier ISPs such as AT&T. Each ISP network is managed independently, runs the IP protocol, and conforms to certain naming and address conventions.
• End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as TCP/IP.
• Internet standards are developed by the Internet Engineering Task Force (IETF). The IETF standards documents are called requests for comments (RFCs). RFCs started out as general requests for comments to resolve network and protocol design problems that faced the precursor to the Internet. They define protocols such as TCP, IP, HTTP (for the Web), and SMTP (for e-mail).

1.1.2 A Services Description

1.1.3 What Is a Protocol?

• All activity in the Internet that involves two or more communicating remote entities is governed by a protocol.
• A protocol defines the format and the order of messages exchanged between two or more communicating entities, and the actions taken on the transmission and/or receipt of a message or other event.

1.2 The Network Edge

• End systems are also referred to as hosts because they host (run) application programs such as a Web browser program. that is, host = end system. Hosts are further divided into two categories: clients and servers. Clients tend to be desktop and so on, whereas servers tend to be more powerful machines that store and distribute Web pages and so on. Most of the servers reside in large data centers.

1.2.1 Access Networks

• Access network: the network that physically connects an end system to the first router (= “edge router”) on a path from the end system to any other distant end system.
  
  Home Access: DSL, Cable, FTTH, Dial-Up, and Satellite
• Two main types of broadband(宽带) residential access are digital subscriber line (DSL数字用户线路) and cable(电缆).
• A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. When DSL is used, a customer’s telco is also its ISP.

• Each customer’s DSL modem uses the existing telephone line (twisted-pair copper wire) to exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The home’s DSL modem takes digital data and translates it to high-frequency tones for transmission over telephone wires to the CO; the analog signals from many such houses are translated back into digital format at the DSLAM.
• The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies:
  • A high-speed downstream channel, in the 50 kHz to 1 MHz band
  • A medium-speed upstream channel, in the 4 kHz to 50 kHz band
  • An ordinary two-way telephone channel, in the 0 to 4 kHz band
• On the customer side, a splitter separates the data and telephone signals arriving to the home and forwards the data signal to the DSL modem. On the telco side, in the CO, the DSLAM separates the data and phone signals and sends the data into the Internet.
  
• A residence obtains cable Internet access from the same company that provides its cable television.
• Because both fiber(纤维) and coaxial cable(同轴电缆) are employed in this system, it is often referred to as hybrid fiber coax (HFC).
• Cable internet access requires cable modem which is typically an external device and connects to the home PC through an Ethernet port. At the cable head end, the cable modem termination system (CMTS) turning the analog signal sent from the cable modems in many downstream homes back into digital format.
• Cable Internet access is a shared broadcast medium(媒介). Every packet sent by the head end travels downstream on every link to every home and every packet sent by a home trave