Telephone system rules

When you enter the telephone room , You will see well thought out safety measures 、 Rows of battery switches 、 Neat wires and diesel generators on the roof . All this is to ensure the high reliability of the telephone system . Because telecommunication plays a decisive role in today's society .
There are many written and unwritten regulations on how to provide telephone services , The first guiding principle about the telephone network written in the law and accepted by all is ： Provide uninterrupted service . Telecom service providers try their best to ensure the security of their physical lines , And provide redundant spare lines and procedures , For dynamic routing when a cable is interrupted . however , The caller usually doesn't know all this .

The second self-evident rule in the telephone system is ： You cannot interrupt a call that has occurred . It means , If the telephone access is occupied by others , No matter how anxious you are, you can't grab it . Another specific example ： Mother's Day , You may not be able to dial when you call home , Display on the phone “ Busy ”（ Tell the caller that the line is busy ）, In fact, no matter how anxious you are, you can't drive away those callers who are occupying the line .
Last , The third rule is ： The telephone service should be open to everyone . At least let those who are willing and able to afford a reasonable monthly service fee use it . This last rule reflects “ universal service ” The requirements of . Although telephone service is not universally available （ Because there is no unified standard for the concept of universal service ）, However, this requirement still has an important impact on the U.S. telecommunications industry . Universal service requires simple telephone operation , Should let 3.4 All children can use .
In addition to providing for telephone services , There are also corresponding regulations on the working mode of telephone network equipment . for instance ： How to confirm a call 、 How can a call coexist with other calls on the same network 、 How do switches and other devices inform each other of the arrival of a call . From these operations and functions of the telephone system, we can see , Americans are very reliable , At least , They require a high degree of reliability of the public telephone network . The telephone was invented soon , People realize that they cannot live without it . therefore , Scientific and technological personnel continue to develop new telecommunications equipment , To meet people's needs .

The above questions about telephone communication and Internet Protocol （IP） What does it matter ？ Even though Internet Only 30 Years of history , But people have long been in love with it . However , Even in those daily use Internet Of the people , Few people will think that in daily life Internet With the public switched telephone network （PSTN） As important as . because PSTN Because the network is busy, the communication speed of users will be reduced , So that “ Operation timeout ”“ The connection fails ” And so on . Although these phenomena will not invalidate the whole network , But often let Internet Users feel unbearable . The reason for such a different result is simple ： Flowers in PSTN A lot of money , And invest in Internet The funds on the are relatively small . In the past few years ,Internet The number of users has increased dramatically , Although network service providers （ISP） Expanding the capacity and efficiency of the network at an incredible speed , But this expansion speed is far behind the needs of users , They expect to get something like PSTN The same network service , In fact, it's impossible . because Internet The development speed of began to slow down , And people are increasingly dependent on it in daily life and business affairs ,Internet The quality of the infrastructure will improve , Make it as reliable as the telephone network . However , Does this mean , In a reliable , Omnipresent Internet When it appears , People can chat in any two places simply by plugging in the phone ？ Is it simply because of reliability and capacity problems that Internet Can't it become the backbone telephone network in the future ？
actually , Give Way Internet Provide PSTN What kind of function is not so simple . High reliability and large capacity are two essential features of any large telephone system , But this is only a necessary feature . stay PSTN in , Due to personal messages （ Or telephone information ） Very important , So it is necessary to spend a lot of money and resources to ensure its safety , but Internet Never to adapt PSTN Standard and designed .Internet Users do not need a single message transmission with high reliability , And the cost of obtaining high reliability in the telephone network is too human , therefore Internet Engineers are designing Internet in , Don't pay too much attention to a single message . If there is any fault in the network , This message can be routed to bypass the fault point , You can also resend . Although the network designed according to this design idea is very suitable for e-mail and file transmission , However, it is not suitable for transmitting information with strict delay requirements , Especially the real-time conversation communication such as telephone communication . Thus, there are two different ways to deal with the information on the network , Based on them, two kinds of communication protocols have been produced .
These two data communication methods are usually called connection oriented and connectionless oriented . The concept of connection has been introduced , Now let's see Internet How to handle connection oriented phone calls （ Although there is no connection between routers ）. First , Let's briefly review the operation of the next connected physical network .