Remote Connectivity Concepts
Many technologies and functions are used for remote connectivity. One of the first networks-the telephone system-is still used today by almost everyone in the world. The telephone system concept was based on the idea of enabling two people in different physical locations to speak with each other. The same basic idea is used today for many different applications. Global networks have been created by corporations and institutions alike to enable remote communication and information sharing.
The basic functionality of remote connectivity is available in many different protocols and devices. For example, companies use network links such as Frame Relay and Asynchronous Transfer Mode (ATM), which encompass many different technologies. More common applications include Remote Access Service (RAS), which allows a remote user to use a protocol such as the Pointto- Point Protocol (PPP) to dial in to the RAS server over the public switched telephone network (PSTN).
This tutorial will first introduce you to types of connections such as PSTN and ISDN and then talk about RAS services and protocols for dial-up using those connection types. The tutorial will then focus on VPN-type connections and protocols as an alternative way to connect to a remote network.'
Public Switched Telephone Network (PSTN)
Almost everyone in the world has used a telephone at least once. Today, you can call anywhere in the world and get a direct connection almost instantly using this technology. The PSTN was originally designed as an analog switching system for routing voice calls. Because it has existed for several decades and has been used by so many, it has come to be known as plain old telephone service (POTS). Because PSTN is considered the first wide area network, it was the basis for many of the WAN technologies that exist today and has been instrumental in their evolution.
During the initial years of PSTN, digital technologies had not even been considered. The telephone network was based purely on analog signals traveling across copper wire to transport a human voice. The only repetitions of the signal that might have occurred were through one or two repeater devices. The term via net loss (VNL) was coined to calculate the signal degradation that occurred. This degradation was measured in decibels (dB). The only metering equipment needed to test connections consisted of test tones, decibel meters, and volume unit (VU) meters. VU meters were used to measure complex signals such as the human voice. These meters simply measure the loss or gain of a specific circuit.
Prior to the 1960s, PSTN lines could handle nothing more than what they were originally designed for-voice communication. Since then, many great technology leaps have helped the network progress. The beginning of this era was marked by the advent of the Bell T1 transmission system. As T1s became more frequently used in the telephone network, bandwidth and quality increased. This advent also began the true migration from using human operators to route calls to switching these functions electronically.
In the 1970s and 1980s, the phone companies began to invest more resources in improving the quality of the PSTN backbone. This backbone, also known as the digital access cross-connect system (DACCS), was a combination of all the T1 and T3 lines. Although many problems were associated with DACCS at that time, it provided a technology upgrade to help improve services all the way around. Soon companies started looking at PSTN lines as an alternative to the dedicated point-topoint links they were using.
As the industry started to move in the direction of PSTN lines, manufacturers began to market modems for this purpose. As modems became more commonplace, the manufacturers began mass-marketing them for everyday users. Today, although using a modem to dial in to a remote network is not as common as connecting through a broadband connection, it is still an important option for persons not having broadband technologies available. People in rural areas may still need to rely on PSTN and some companies may still rely on using PSTN lines to enable remote users to dial in to private networks as well as to back up data links for computer systems that require remote connectivity.
How PSTN Works
The POTS network originally began with human operators sitting at a switch, manually routing calls. The original concept of the Bell Telephone system was a series of PSTN trunks connecting the major U.S. cities. This was an analog-based system that met its requirements for human voice transmissions at the time. Since the inception of the telephone, the world has changed. PSTN systems still use analog from the end node to the first switch. Once the signal is received, the switch converts the signal to a digital format and then routes the call on. Once the call is received on the other end, the last switch in the loop converts the signal back to analog, and the call is initiated. Because the end node is still analog, modems are used in most homes to facilitate dial-up access. Faster technologies such as ISDN or T1s use a dedicated point-to-point link through a completely digital path, making higher bandwidths attainable. Currently, analog lines can reach only a maximum speed of 56 Kbps. Using digital lines, speeds in excess of 2 Gbps (gigabits per second) can be reached.
The telephone network works much like the TCP transport protocol. It is connection based, and the connection is maintained until the call is terminated. This enables you to hear the other person almost instantaneously. Telephone networks use two copper wires in most homes, but the switching medium is mainly fiber. This allows for the high-speed switching in the back end but slow response in data communications because of the modem device connected to the system.
Analog modems are used to connect to a remote network via a PSTN line. Although there are many different types and makes of modems, they can be categorized into three classes: single external, single internal, and multiline rack or shelf-mounted.
The external modem is the modem most commonly used today. Many ISPs use pools of external modems to enable dial-in access. These modems are also common in server hardware. Many IT workers include modems in production systems to allow for a backup communications link or for remote access.
The internal modem belongs to the same device type as the external modem. The only real difference is that it is located inside the computer chassis. Most companies no longer use these modems, because externals are easier to replace and troubleshoot. For example, internal modems do not have the light-emitting diodes (LEDs) that external modems have. This translates into a headache if you have to figure out why the modem won't connect to a remote host via the dialup connection. Some modem manufacturers provide software interfaces, but they are generally not as full-featured as those in external modems. A common use for internal modems is in laptop computers using PC (PCMCIA) cards. Many laptop vendors still integrate phone jacks into the chassis of their computers. In addition, PC cards can technically be classified as internal modems. These are used widely and do not include the LEDs or lamps an external modem offers. Quite a few businesses use external modems, and home users use internal modems. The cost difference is almost negligible, but home users usually opt for the cheaper of the two.
These solutions are becoming increasingly popular. Many vendors offer solutions that have a single chassis containing a certain number of modem cards that can be connected directly to the network. The modularity and size of these devices makes them much more efficient than trying to maintain a shelf with a stack of external modems sitting on it. These solutions have also been included in some new networking equipment. Manufacturers place analog modems in their equipment to facilitate redundancy features such as a backup network link.