Server

This section tells you how to set up the server side of things.  I figured that this should go first since without a server, your client is kind of useless.

Security − keeping people out
Security is very important for a VPN.  That's why you're building one in the first place, isn't it?  You need to keep a few things in mind while setting up your server.

What is a VPN?

What is a VPN?
VPN stands for Virtual Private Network.  A VPN uses the Internet as it's transport mechanism, while maintaining the security of the data on the VPN.

But really, what IS a VPN?
There are several answers to that question.  It really depends on your network layout.  The most common configuration is to have a single main internal network with remote nodes using VPN to gain full access to the central net.  The remote nodes are commonly remote offices or employees working from home.  You can also link two small (or  large) networks to form an even larger single network.

the ISDN transport

Work on developing ISDN began in the  1970s but  would  not  be sold  to the  bandwidth-hungry customer until the  early  to mid 1990s. Many said it was too  little too  late, and  the  consumer market for ISDN never took  off. After the  news  of the  first VoIP telephony call over  the Internet spread in 1995, a renewed interest in ISDN emerged for a short while. But by this  time our attention was turned to the  emerging DSL technology first deployed in 1998.

The eventual ISDN standard provided for two flavors of ISDN: Basic Rate Interface (BRI) and  Primary Rate Interface (PRI). The ISDN standard defines the  basic unit of bandwidth as a B channel, which provides 64 Kbps of band-  width. B stands for bearer channel, which is another name for the  channel that carries POTS calls over  the  PSTN.

The POTS transport

As you already know, POTS is a transport that runs through the  circuit- switched PSTN. All transport lines  in the  PSTN have  a circuit-identification number, which is either all numeric or alphanumeric. For example, a POTS telephone number has  an area  code, a prefix, and  a suffix that correspond to the  physical circuit and  the  lines that make up that circuit.

Understanding How the PSTN Supports VoIP

No other network in the  world can compare to the  reliability of the  U.S. switched network. (Granted, a handful of disasters have  disrupted PSTN services in specific regions, but  these are the  exceptions, not  the  rule.) Such reliability, however, comes at a high price: The cost of the  switched network, particularly recurring (per-minute) charges, is the  highest in the world. Regional  toll and  international calling  using  the  PSTN are the  most highly  reg- ulated switched network services. This means high regulatory fees in addi- tion to recurring usage charges. But VoIP greatly reduces and may eliminate these types of charges.

How VoIP and the Internet Fit the CSI Picture

To understand how the  Internet relates to the  five CSIs, it helps to first recog- nize what  the  Internet is. No doubt Bell would  be captivated by the enormity of what  we call the  Internet. By definition, it is a network of networks. But just  like VoIP can run on any of the  five CSIs in varying  degrees of quality, the  Internet can also  be accessed from any of the  five CSIs in varying degrees of quality and  security.

VoIP over Internet
Yes, the  Internet is a network of networks, and  the  Web is one of its largest applications. But the  Internet is also  a network that is accessible through all five CSIs.

Summing up the CSIs

A CSI is like a highway system that lays out  all the  many  roads that enable people to drive  to their destinations. Within our highway system, we could characterize some roads as being large or wide, some roads as small or narrow, and  some roads as being  between these two extremes. Roads may be further broken  down  by type  of surface, that is, asphalt, concrete, gravel, or dirt.

The wireless CSI

Wireless telecommunications have  been around for more  than eight decades. First we had  the  radio in the  1920s. During World War II, we had the  inception of walkie-talkies. These  led to the development of cell division multiple access (CDMA), one of the  most popular carrier services supporting cell phone net- working  today. In the  1960s, the  first wireless transports connected mobile telephones using  radio telemetry, which connected the  caller (using radio fre- quency channels) to the circuit-switched PSTN.
 

The hybrid fiber-coaxial CSI

When fiber-optic cable began to be deployed widely, the  cable companies started using  fiber to build  out  their infrastructure. But by that time,  much of the  coaxial cable infrastructure supporting localized connections had already been established. This is why a large share of today’s cable customers have  coaxial cable coming into their premises from the  nearby telephone pole.