Inside Look at the Internet

1.3.1 Circuit Switching and Packet Switching
Chapter 1
1.1 What Is the Internet?
1.1.1 Nuts-and-Bolts Description
1.1.2 A Service Description
1.1.3 What Is a Protocol?
1.2 The Network Edge
1.2.1 End Systems, Clients, and Servers
1.2.2 Connectionless and Connection-Oriented Service
1.3 Network Core
1.3.1 Circuit Switching and Packet Switching
1.3.2 Packed-Switched Networks: Datagram Networks and Virtual-Circuit Networks
1.4 Access Networks and Physical Media
1.4.1 Access Networks
1.4.2 Physical Media
1.5 ISPs and Internet Backbones
1.6 Delay and Loss in Packets-Switched Networks
1.6.1 Types of Delay
1.6.2 Queuing Delay and Packet Loss
1.6.3 Delay and Routes in the Internet
1.7 Protocol Layers and Their Service Models
1.7.1 Layered Architecture
1.7.2 Layers, Messages, Segments, Datagrams, and Frames

Two fundamental approaches to building a network core: circuit switching and packet switching. In circuit-switched networks, the resources needed along a path to provide for communication between the end systems are reserved for the duration of the communication session. In packet-switched networks, these resources are not reserved; a session's messages use the resources on demand, and as a consequence, may have to wait for access to a communication link.

When two hosts want to communicate, the network establishes a dedicated end-to-end connection between two hosts.

A circuit in a link is implemented with eitherfrequency-division multiplexing (FDM) or time-division multiplexing (TDM). With FDM, the frequency spectrum of a link is shared among the connections established across the link. For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time solts.

Proponents of packet switching have always argued that circuit switching is wasteful because the dedicated circuits are idle during silent periods.

In mordern computer networks, the source breaks long messages into smaller chuncks of data knkown as packets. Between sources and destination, each of these packets travels through communication links and packet switches. Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link. Most packet switches use store-and-forward transmission at the inputs to the links. Store-and-forward transmission means that the switch must receive the entire packet before it can begin to ransmit the first bit of the packet onto the outbound link.

Each packet switch has multiplw links attached to it. For each attached link, the jpacket switch has an output buffer (also called an output queue), which stores packets that the router is about to send into that link. The output buffers play a key role in packet switching. In addition to the store-and-forward delays, packets suffer output buffer queuingf delays. These delays are variable and depend on the level of congestion in the network. Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely filled with other packets waiting for transmission. In this case, packet loss will occur--either the arriving packet or one of the already-queued packets will be dropped.