Devices——hosts or end systems

End systems are connected together by a network of communication links and packet switches.

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.

Two most prominent types of packet switches——routers and link-layer switches. 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 known as a route or path through the network. The

Packet-switched networks (which transport packets) are similar to transportation networks. Packet takes a path through a computer network.

End systems access the Internet through Internet Service Providers(ISP).

Each ISP is in itself a network of packet switches and communication links. ISPs also provide Internet access to content providers, connecting Web sites directly to the Internet.

ISPs that provide access to end systems must also be interconnected. These lower-tier ISPs are interconnected through national and international upper-tier ISPs. An upper-tier ISP consists of high-speed routers interconnected with high-speed fiber-optic links.

Protocols control the sending and receiving of information within the Internet.

The IP protocol specifies the format of the packets that are sent and received among routers and end systems.

peer-to-peer (P2P)

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

The networking takes two (or more) communicating entities running the same protocol in order to accomplish a task.

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, as well as the actions taken on the transmission and/or receipt of a message or other event.

End systems are also referred to as hosts. Hosts are sometimes further divided into two categories: clients and servers.

The two most prevalent types of broadband residential access are digital subscriber line (DSL) and cable.

A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. Thus, when DSL is used, a customer's telco is also its ISP. As 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,数字用户线接入复用器) 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.

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

Fiber optics connect the cable head end to neighborhood-level junctions, from which traditional coaxial cable is then used to reach individual houses and apartments. Each neighborhood junction typically supports 500 to 5,000 homes. Because both fiber and coaxial cable are employed in this system, it is often referred to as hybrid fiber coax (HFC).

Cable Internet access requires special modems, called cable modems. As with a DSL modem, the cable modem 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) turns the analog signal sent from the cable modems in many downstream homes back into digital format.

Cable modems divide the HFC network into two channels, a downstream and an upstream channel.

As with DSL, access is typically asymmetric, with the downstream channel typically allocated a higher transmission rate than the upstream channel.

Cable Internet access is that it is a shared broadcast medium（介质）. In particular, every packet sent by the head end travels downstream on every link to every home and every packet sent by a home travels on the upstream channel to the head end.

FTTH concept— provide an optical fiber path from the CO directly to the home.

It is not until the fiber gets relatively close to the homes that it is split into individual customer-specific fibers. There are two competing optical-distribution network architectures that perform this splitting: active optical networks (AONs) and passive optical networks (PONs). AON is essentially switched Ethernet.

Each home has an optical network terminator (ONT), which is connected by dedicated optical fiber to a neighborhood splitter. The splitter combines a number of homes (typically less than 100) onto a single, shared optical fiber, which connects to an optical line terminator (OLT) in the telco's CO. The OLT, providing conversion between optical and electrical signals, connects to the Internet via a telco router. In the home, users connect a home router to the ONT and access the Internet via this home router. In the PON architecture, all packets sent from OLT to the splitter are replicated at the splitter.