使用Windows Sockets进行网络编程

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P2/Vol.6/s&n6 Programming WinSock #30594-1 rob 11.14.94 CH01 LP #4

Chapter 1 ■ Networking and Network Programming

Note

The first design of a network passing a token ring is attributed to E. E. Newhall

in 1969. IBM first publicly supported a token-ring topology in March 1982,

and announced its first token-ring network product in 1984.

Data on the IBM token-ring network is transmitted at either 4 or 16 Mbps, depending

on the actual implementation. For computers to communicate with each other, all net-

work cards must be configured similarly to communicate at either 4 or 16 Mbps on the

network. Networked computers are connected by shielded and/or unshielded twisted-

pair cable to a wiring concentrator called a Media Access Unit or MAU (rhymes with

cow). Each MAU can support as many as 72 computers that use unshielded wire or up

to 260 computers using shielded wire. Each ring can have as many as 33 MAUs allow-

ing for a theoretical maximum of 8,580 computers on the network.

FIGURE 1.7.

Ring network.

Star Network

To transmit data between any two computers in a star network, shown in Figure 1.8,

requires that data be sent via the centrally located computer, called a hub. The hub

provides a common connection so that all the computers can communicate with one

another. To extend the star network, hubs can be connected to one another. The major

problem with star networks is that if the centrally located hub isn’t operating, the entire

network becomes unusable. A benefit of a star network is that no computer, other than

the centrally located hub, can interrupt network traffic.

Part I ■ Introduction to Networking

P2/Vol.6/s&n6 Programming WinSock #30594-1 rob 11.14.94 CH01 LP #4

The application programmer should make the server aware of the disparity in perfor-

mance when it communicates with the many other computers it serves. Those other

computers must likewise be knowledgeable about their connectivity to the server. The

client software running on the laptop computer may have a five-second time-out for

database access while the bus network’s Macintosh may need a 10-second time-out to

make up for the delays introduced by the satellite link. Attention to details such as this

early in a network application’s development cycle may save a lot of aggravation later.

Network Programming Models

The previous section discussed ways that computers and other resources can be attached

to a network. But what do we do now that we have networked computers that can com-

municate with one another and share common resources? We need software that can

take advantage of the network. This section begins a discussion of network program-

ming. Network programming can be thought of in two primary contexts: client/server

and distributed.

Client/Server Computing

In the client/server computing model, an application is split into two parts: a front-end

client that presents information to the user and collects information from the user, and

a back-end server that stores, retrieves, and manipulates data and generally handles the

bulk of the computing tasks for the client. In this model the server is usually a more

powerful computer than the client, oftentimes a minicomputer or mainframe, and serves

as a central data store for many client computers, making the system easy to administer.

Client/server architecture increases workgroup productivity by combining the best fea-

tures of stand-alone PCs with the best features of minicomputers and mainframes.

Client/server architecture makes the best use of high-end server hardware and reduces

the load on client PCs. Load reduction, in turn, provides superior performance and mini-

mizes network traffic. Figure 1.4A shows one example of a client/server interaction with

a client accessing a database server.

A server is any program that runs on a networked computer and can provide a service.

A server receives a request over the network, performs the necessary processing to ser-

vice that request, and returns the result to the requester. The client is the program that

sends a request to a server and waits for a response.

For a client and server to communicate and coordinate their work, an interprocess com-

munication (IPC) facility is needed. The subject of this book, WinSock, can be used to

satisfy this requirement. Chapter 15, “Practical Client/Server Database Application,”

introduces an example that will demonstrate client and server database implementations.

P2/Vol.6/s&n6 Programming WinSock #30594-1 rob 11.14.94 CH01 LP #4

Chapter 1 ■ Networking and Network Programming

One server program can service several client requests at the same time. For this reason,

implementing servers tends to be more difficult than implementing clients. To provide

the capability of supporting several client requests simultaneously, servers are usually

built in two parts: a single master that accepts requests and one or more slaves that ac-

tually process and respond to the individual requests.

Client/server architecture contrasts with the classical centralized architecture popular-

ized by typical mainframe installations. In a centralized environment, the “clients” are

little more than dumb terminals that act as simple data entry/display devices. There’s a

minimum of work done at the terminal. The user typically fills in the fields of a form

before sending the field data to the central computer. All processing and screen format-

ting is done on the central computer, and the dumb terminal simply displays the

preformatted data. In a client/server environment, the client has much greater intelli-

gence and more freedom with the final visual presentation of the data to the user. In-

stead of the data being preformatted to match the way it will be viewed, it’s sent back in

its “raw” format, and the application running on the client computer “decides” how to

display that data. Thus the “front end” that the user sees can be customized while the

“back end” remains unchanged.

Distributed Computing

The distributed architecture can be thought of in two different ways: precollection and

parallel processing.

Precollection is the act by which background processes on networked computers con-

currently collect and propagate information before that information is requested. An

example would be a program that requests the status of every other computer on the

local network. In the client/server environment, the client program would have to send

a request to each computer on the network and wait for a response. This procedure is

potentially very time-consuming. In a distributed implementation, each computer on

the local network would have a process that runs continually in the background and

that reports status information to every other computer on a regular interval. When the

program is run to request the status information of every other computer on the local

network, the response comes back immediately because the information was precollected

in each local computer. Of course this solution wouldn’t work well if the information

being requested was time sensitive, because the delay in the updates would make the

response outdated. Shortening the time between the updates sent by the networked

computers wouldn’t work well either because of the possibility of saturating the network’s

data-handling capabilities.

When most people think of parallel processing, they think of a computer that has more

than one processing unit. Parallel computing in a distributed system environment means

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