[笔记][Computer Networks] Chapter 1

Computer Networks (Fifth Edition)
Chapter 1

Difference between distributed system and computer network:
In a distributed system, a collection of independent computers appears to its users as a single coherent system. Usually, it has a single model of paradigm that it presents to the users. Of a layer of software on top of the operating system, called middleware, is responsible for implementing this model. A well-known example of a distributed system is the World Wide Web. It runs on top of the Internet and presents a model in which everything looks like a document.
In a computer network, this coherence, model and software are absent. Users are exposed to actual machines, without any attempt by the system to make the machine look and act in a coherent way. If the machines have different hardware or different operating systems, this is fully visible to the users. If a user wants to run a program on a remote machine, he has to log onto that machine and run it here.

1.1 Uses of Computer Networks

Client-Server Model:
In this model, the data are stored on powerful computers called servers. Often there are centrally housed and maintained by a system administrator.
In contrast, the employees have simpler machines, called client, on their desk, with which they access remote data, for example, to include in spreadsheet they are constructing.
Peer-to-peer communication:
In this form, individuals who form a loose group can communicate with others in the group. Every person can, in principle, communicate with one or more other people; there is no fixed division into client and servers.

1.2 Network Hardware

Broadly speaking, there are two types of transmission technology that are in widespread use: broadcast links and point-to-point link.
Point-to-pint links: exactly one sender and exactly one receiver, packets may have to first visit one or more intermediate machines, multiple routes (finding a good one is important)
Broadcast links: packets sent by any machine are received by all the others
Classification of Network by Scale:
(1) PAN: Bluetooth to connect mouse to computer, RFID on smart card
(2) LAN: IEEE 802.11(WiFi), switched Ethernet
Static and dynamic allocation of channel
(3) MAN: cable television networks
(4) WAN: span a country or continent

1.3 Network Software

Fig 1-13
The entities comprising the corresponding layers on different machines are called peers. It is peers that communicate by using the protocol to talk to each other.
Each layer passes data and control information to the layer immediately below it. Between each adjacent layer is an interface.

Fig 1-15
Service: Connection-oriented vs. connection-less
The relationship of Services to Protocols:
A service is a set of primitives (operations) that a layer provides to the layer above it. A service relates to an interface between two layers, with the lower layer being the service provider and the upper layer being the service user.
A protocol, in the contrast, is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer.

1.4 Reference Models

1.4.1 The OSI Reference Model (seven layers)

(1) The Physical Layer: transmitting raw bits over a communication channel
(2) The Data Link Layer: transform a raw transmission facility into a line that appears free of undetected transmission errors.
(3) The Network Layer: controls the operation of the subnet. (determine how packets are routed from source to destination)
(4) The Transport Layer: is to accept data from above it, split it up into units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.
(5) The Session Layer: allows users on different machines to establish sessions between them
(6) The Presentation Layer: concerned with the syntax and semantics of the information transmitted
(7) The Application Layer: contains a variety of protocols that are commonly used by users, e.g. HTTP

1.4.2 The TCP/IP Reference Model
Fig 1-21