There are several different technologies that you can use to connect a stand-alone computer to a network at a remote location. From the network layer up, a remote connection is no different than a direct LAN connection, but the data-link and physical layers can take several different forms. This lesson examines some of the connection types most commonly used for remote networking and discusses the issues involved in installing and configuring them.
The following sections examine the physical layer options that you can use for remote network connections. The interface to the computer can vary from a serial port to a bus slot to a standard network interface adapter, but the actual network medium is the service that carries the signals for most of their journey. These technologies are considered in this lesson as a means to connect a single computer to a remote network, but it is also possible (in most cases) to use them to connect two LANs at different locations.
The Public Switched Telephone Network (PSTN) is just a technical name for Plain Old Telephone Service (POTS). This is the standard voice telephone system, found all over the world, which you can use with asynchronous modems to transmit data between computers at virtually any location. The PSTN service in your home or office probably uses copper-based twisted pair cable, as do most LANs, and RJ-11 jacks, which are the same as the RJ-45 jacks used on twisted pair LANs, except that RJ-11 jacks have four (or sometimes six) electrical contacts instead of eight. The PSTN connection leads to a central office belonging to the telephone company, which can route calls from there to any other telephone in the world. Unlike a LAN, which is digital and uses packet switching, the PSTN is an analog, circuit-switched network.
For more information about packet switching and circuit switching, see Lesson 1: Network Communications, in Chapter 1, "Networking Basics."
To transmit computer data over the PSTN, the digital signals generated by your computer must be converted to analog signals that the telephone network can carry. A device called a modulator/demodulator, more commonly known as a modem, handles this conversion. A modem takes the digital signals fed to it through a serial port or the system bus, converts them to analog signals, and transmits them over the PSTN (see Figure 12.1). At the other end of the PSTN connection, another modem performs the same process in reverse, converting the analog data back into its digital form and sending it to another computer. The combination of the interfaces to the two computers, the two modems, and the PSTN connection form the physical layer of the networking stack.
Figure 12.1 Modems convert digital signals to analog signals that the PSTN can carry, and then convert the analog signals back to digital signals
At the data-link layer, remote network connections that use modems and the PSTN typically use the Point-to-Point Protocol (PPP) to communicate. In a few cases, computers still use the Serial Line Internet Protocol (SLIP) at the data-link layer. For more information about these protocols, see Lesson 2 later in this chapter.
The first modems used proprietary protocols for the digital/analog conversions, but this meant that users had to use the same manufacturer's modems at each end of the PSTN connection. To standardize modem communications, organizations like the Comité Consultatif International Télégraphique et Téléphonique (CCITT), now known as the International Telecommunication Union (ITU), began developing specifications for the communication, compression, and error-detection protocols that modems use when generating and interpreting their analog signals. Today, virtually all available modems support a long list of protocols that can serve as a history of modem communications. The current industry standard modem communication protocol is V.90, which defines the 56 kilobytes per second (Kbps) data transfer mode that most modem connections use today.
The PSTN was designed for voice transmissions, not data transmissions. As a result, connections are relatively slow, with a maximum speed of only 33.6 Kbps when both communicating devices use analog PSTN connections. A 56-Kbps connection requires that one of the connected devices have a digital connection to the PSTN. The quality of PSTN connections can also vary widely, depending on the location of the modems and the state of the cables connecting the modems to their respective central offices. In some areas, the PSTN cabling can be many decades old, and connections suffer as a result. When modems detect errors while transmitting data, they revert to a slower transmission speed. This is one reason that the quality of modem connections can vary from minute to minute. Dedicated, permanent PSTN connections between two locations, called leased lines, are also available (in both analog and digital forms) and provide a more consistent quality of service, but they lack the flexibility of dial-up connections and they are quite expensive. For more information on leased lines, see Lesson 3, later in this chapter.
As with most computer peripherals these days, the majority of available modems support the Plug and Play standard, which enables operating systems to detect the modem's presence, identify its manufacturer and model, and install and configure the appropriate driver for it. As with most hardware peripherals, modems use an interrupt request (IRQ) line and an input/output (I/O) port address to send signals to the computer. With external modems, the IRQ and I/O address are assigned to the serial port that you use to connect the modem to the computer. Most computers are equipped with two serial ports, which are assigned to two of the computer's four default communications (COM) ports, COM1 and COM2. Each COM port has its own I/O port address, but COM1 and COM3 share IRQ4, and COM2 and COM4 share IRQ3.
Internal modems plug into a bus slot instead of a serial port, so you must configure the modem itself to use a particular COM port, which specifies the IRQ and I/O address assignments. If you have other devices plugged into any of the computer's serial ports, you must be sure that the modem is not configured to use the same IRQ as the ports in use.
The other configuration parameter you should be familiar with is the maximum port speed. Serial ports use a chip called a universal asynchronous receiver-transmitter (UART) to manage the communications of the device connected to the port. Most computers today have 16550 UART chips for both of their serial ports, which can run as fast as 256 Kbps. Older computers might have slower UART chips, such as the 16450, which runs at a maximum of 115.2 Kbps. Some computers even have a 16550 UART on one port and a slower chip on the other. For today's high-speed modems, you should always use a 16550 UART. Internal modems have their own UART chips built onto the card, which are nearly always 16550 UART chips.
One of the advantages of using the PSTN to connect a computer to a distant network is that no special service installation is required and the only hardware you need is a modem and a telephone jack. This means that users with portable computers can dial into their office networks wherever they happen to be. However, dialing into a distant network using the PSTN can be an expensive proposition, especially when a company has a large number of network users traveling to distant places. One way to minimize these long-distance telephone charges is to use what is known as a virtual private network (VPN) connection.
A VPN is a connection between a remote computer and a server on a private network that uses the Internet as its network medium. The network is permanently connected to the Internet and has a server that is configured to receive incoming VPN connections through the Internet. The remote user connects to the Internet by using a modem to dial in to a nearby ISP. There are many ISPs that offer national and even international service, so the user can connect to the Internet with a local telephone call. The remote computer and the network server then establish a secured connection that protects the data exchanged between them, using the Internet as the network medium. This technique is called tunneling, because the connection runs across the Internet inside a secure conduit, protecting the data in the way that a tunnel under a river protects cars from the water around it.
The primary protocol that makes this tunneling possible is the Point-to-Point Tunneling Protocol (PPTP). PPTP works with PPP to establish a connection between the client computer and a server on the target network, both of which are connected to the Internet. The connection process begins with the client computer dialing up and connecting to a local ISP using the standard PPP connection establishment process. When the computer is connected to the Internet, it establishes a control connection to the server using the Transmission Control Protocol (TCP). This control connection is the PPTP tunnel through which the computers transmit and receive all subsequent data.
When the tunnel is in place, the computers send their data through it by encapsulating the PPP data that they would normally transmit over a dial-up connection within Internet Protocol (IP) datagrams. The computer then sends the datagrams through the tunnel to the other computer. Although it violates the rules of the Open Systems Interconnection (OSI) model, you actually have a data-link layer frame being carried within a network layer datagram. The PPP frames are encapsulated by IP, but at the same time, they can also contain other IP datagrams that contain the actual user data that one computer is sending to the other. Thus, the messages transmitted through the TCP connection that forms the tunnel are IP datagrams that contain PPP frames, with the PPP frames containing messages generated by IP or any network layer protocol. In other words, because the PPP user data is secured within the IP datagrams, that data can be another IP data-gram or an Internetwork Packet Exchange (IPX) or NetBIOS Enhanced User Interface (NetBEUI) message, as shown in Figure 12.2. Because the tunnel is encrypted and secured using an authentication protocol, the data is protected from interception. After the IP datagrams pass through the tunnel to the other computer, the PPP frames are extracted and processed by the receiver in the normal manner.
Figure 12.2 The PPTP violates data encapsulation rules by carrying PPP frames within IP datagrams
Although it has only recently achieved modest popularity in the United States, the Integrated Services Digital Network (ISDN) has been around for several decades, and is especially popular in Europe, where leased telephone lines are prohibitively expensive. ISDN is a digital communications service that uses the same network infrastructure as the PSTN. It was designed as a complete digital replacement for the analog telephone system, but it had few supporters in the United States until relatively recently, when the need for faster Internet connections led people to explore its capabilities. However, other high-speed Internet access solutions, such as Digital Subscriber Line (DSL) and cable television (CATV) networks, have also become available in recent years. These other solutions are generally faster and cheaper than ISDN and have largely eclipsed it in popularity.
ISDN is a dial-up service, like the PSTN, but its connections are digital, so no modems are required. Although ISDN can support specially made telephones, fax machines, and other devices, most ISDN installations in the United States are used only for computer data transmissions. Because it's a dial-up service, you can use ISDN to connect to different networks. For example, if you have an ISDN connection to the Internet, you can change ISPs simply by dialing a different number. No intervention from the telephone company is required. However, because ISDN needs special equipment, it cannot be used in mobile devices, such as laptop computers.
ISDN also delivers greater transmission speeds than PSTN connections. The ISDN Basic Rate Interface (BRI) service consists of two 64-Kbps channels (called B channels) that carry the actual user data, plus one 16-Kbps channel (called a D channel) that carries only control traffic. Because of these channel names, the BRI service is sometimes called 2B+D. The B channels can function separately or be combined into a single 128-Kbps connection. A higher grade of service, called Primary Rate Interface (PRI), consists of 23 B channels and one 64-Kbps D channel. The total bandwidth is the same as that of a T1 leased line. PRI is not often used in the United States.
ISDN uses the same wiring as the PSTN, but additional equipment is required at the terminal locations. The telephone company provides what is called a U interface, which connects to a device called a Network Terminator 1 (NT-1). The NT-1 can provide a four-wire connection, called an S/T interface, for up to seven devices, called terminal equipment (TE). Digital devices designed for use with ISDN, such as ISDN telephones and fax machines, connect directly to the S/T interface and are called TE1 devices. A device that can't connect directly to the S/T interface is called a TE2 device, and requires a terminal adapter, which connects to the S/T interface and provides a jack for the TE2 device (see Figure 12.3).
Figure 12.3 The NT-1 provides connectors for the terminal equipment that will use the ISDN service
Because of the increased speed at which ISDN operates, the length of the connection is limited. Your home or office must be within 18,000 feet of the telephone company's nearest central office. For longer distances, an expensive repeater is required, which makes the service impractical for most users.
When you plan to connect multiple devices to the ISDN service, you purchase an NT-1 as a separate unit. However, most U.S. ISDN installations use the service solely for Internet access, so there are many products on the market that combine an NT-1 and a terminal adapter into a single unit. These combined ISDN solutions can take the form of expansion cards that plug into a bus slot or separate units that connect to the computer's serial port.
ISDN has never become hugely popular in the United States, partly because of its reputation for being expensive and for installation and reliability problems. Most telephone companies that provide ISDN service charge both a monthly subscription fee and a per-minute rate (usually about 1 cent per minute). If you will be connecting to the Internet using ISDN, you must also pay a monthly fee to an ISP for high-speed Internet access. All together, this can be quite expensive when compared to services like DSL and CATV.
Many ISDN users can tell stories of difficult ISDN installations, service outages, and repeated technical support calls. To some extent, ISDN's reputation for technical difficulties is justified, but the whole installation process has become more user-friendly in recent years. Some ISPs now offer a complete turnkey ISDN service in which they arrange for the service installation by the telephone company and provide Internet access using that service, all for one fee.
Digital Subscriber Line (DSL) is a blanket term for a variety of digital communication services that use standard telephone lines and provide data transfer speeds much greater than the PSTN or even ISDN. The various DSL service types each have a different descriptive word added to the name, which is why some sources use the generic abbreviation xDSL. Some of the many DSL services are shown in Table 12.1.
Table 12.1 DSL Services and Their Properties
|Service||Transmission Rate||Link Length||Applications|
High-bit-rate Digital Subscriber Line (HDSL)
1.544 Mbps full- duplex (using two wire pairs) or 2.048 Mbps full- duplex (using three wire pairs)
12,000 to 15,000 feet
Used by large networks as a substitute for T1 leased line line connections, LAN and Private Branch Exchange (PBX) interconnections, or frame relay traffic aggregation
Symmetrical Digital Subscriber Line (SDSL)
1.544 Mbps full- duplex or 2.048 Mbps full-duplex (one wire pair)
Same as HDSL
Asymmetrical Digital Subscriber Line (ADSL)
1.544 to 8.448 Mbps downstream; 16 Kbps to 640 Kbps upstream
10,000 to 18,000 feet
Internet/intranet access, remote LAN access, virtual private networking, video on demand, Voice over IP
Rate-Adaptive Digital Subscriber Line (RADSL)
640 Kbps to 2.2 Mbpsdownstream; 272 Kbps to 1.088 Mbps upstream
10,000 to 18,000 feet
Same as ADSL, except that the transmission speed is dynamically adjusted to accommodate the link length and signal quality
Up to 1 Mbps downstream; up to 512 Kbps upstream
Internet/intranet access, remote LAN access, IP telephony,videoconfer encing
Very high-bit-rate Digital Subscriber Line (VDSL)
12.96 to 51.84 Mbps downstream; 1.6 to 2.3 Mbps upstream
1000 to 4500 feet
Multimedia Internet access, high- definition television delivery
ISDN Digital Subscriber Line (IDSL)
Up to 144 Kbps full-duplex
Internet/intranet access, remote LAN access, IP telephony, videoconferencing
As seen by the transmission rates listed in Table 12.1, many DSL services run at different upstream and downstream speeds. These are called asymmetrical services. This happens because the nature of some DSL signals causes greater levels of crosstalk in the data traveling from the customer site to the central office than in the other direction. For end-user Internet access, this is usually not a problem, because Web surfing and other common activities generate far more downstream than upstream traffic. However, if you plan to use DSL to connect your own servers to the Internet, make sure that you obtain a service that is symmetrical or that offers sufficient upstream bandwidth for your needs. DSL services are also subject to distance restrictions, just like ISDN.
DSL provides higher transmission rates by utilizing high frequencies that standard telephone services don't use and by employing special signaling schemes. For this reason, in many cases, you can use your existing telephone lines for a DSL connection and for voice traffic at the same time. The most common DSL services are HDSL, used by phone companies and large corporations for wide area network (WAN) links, and ADSL, which is the service that ISPs use to provide Internet access to end users. DSL is an excellent Internet access solution, and it can be suitable for connecting a home user to an office LAN, as long as the upstream bandwidth is suitable for your needs.
The additional hardware needed for an ADSL connection is an ADSL Termination Unit-Remote (ATU-R), sometimes called a DSL transceiver or a DSL modem, plus a line splitter if you will also be using the line for voice traffic. A DSL modem is not really a modem, as it does not convert signals between digital and analog formats (all DSL communications are digital). The ATU-R connects to your computer using either a standard Ethernet network interface adapter or a universal serial bus (USB) port. At the other end of the link at the ISP's site is a more complicated device called a Digital Subscriber Line Access Multiplexer (DSLAM), shown in Figure 12.4. Unlike ISDN connections, DSL connections are direct, permanent links between two sites that remain connected at all times. This means that if you use DSL to connect to the Internet, the telephone company installs the DSL connection between your home or office and the ISP's site. If you want to change your ISP, the phone company must install a new link. In many cases, however, telephone companies are themselves offering DSL Internet access, which eliminates one party from the chain.
Figure 12.4 An ADSL connection is a direct link between your home or office and an ISP or other network site
All of the remote connection technologies described up to this point rely on cables installed and maintained by telephone companies. However, the CATV industry has also been installing a vast network infrastructure throughout most of the United States over the past few decades. In recent years, many CATV systems have started taking advantage of their networks to provide Internet access to their customers through the same cable used for the TV service. CATV Internet access is very fast—sometimes as fast as 512 Kbps or more—and usually quite inexpensive. CATV networks use broadband transmissions, meaning that the one network medium carries many discrete signals at the same time.
Each of the TV channels you receive over cable is a separate signal, and all of the signals arrive over the cable simultaneously. (If you have two or more TVs in your home, you prove this every day by watching two different programs at the same time using the same CATV connection.) By devoting some of this bandwidth to data transmissions, CATV providers can deliver Internet data at the same time as the television signals. If you already have CATV, installing the Internet service is simply a matter of connecting a splitter to the cable and running it to a device called (again, erroneously) a cable modem, which is connected to an Ethernet card in your computer, as shown in Figure 12.5.
Figure 12.5 CATV data connections use the same cable that delivers television signals to carry Internet data
CATV data connections are different from both ISDN and DSL connections because they are not dedicated links. In effect, you are connecting to a metropolitan area network (MAN) run by your cable company. If you run Microsoft Windows on your computer and attempt to browse the network, you will see your neighbors' computers on the same network as yours. This arrangement has the potential to cause two major problems. First, you are sharing your Internet bandwidth with all of the other users in your area. During peak usage periods, you might notice a significant slowdown in your Internet downloads. ISDN and DSL, by contrast, are not shared connections, so you have the full bandwidth you're paying for available at all times. The second potential problem is one of security. If you share a drive on your computer without protecting it with passwords, anyone else on the network can access your files, modify them, or even delete them. Computers connected to the Internet with cable modems are also prone to attack from outside. Many users are duped into downloading programs that enable malicious outside users to take over their computers and use them for nefarious purposes. The installers from the cable company are usually careful to disable file sharing on your computer, however, and there are personal firewall products that you can use to provide yourself with additional protection.
Like most DSL services, CATV data connections are asymmetrical. CATV networks are designed to carry signals primarily in one direction, from the provider to the customer. There is a small amount of upstream bandwidth, which some systems use for purposes such as ordering pay-per-view movies from your remote control, and part of this upstream bandwidth is allocated for Internet traffic. In most cases, the upstream speed of a CATV connection is far less than the downstream speed, making the service unsuitable for hosting your own Internet servers, but still faster than a PSTN connection.
CATV connections are an inexpensive and fast Internet access solution, but you can't use them to connect your home computer to your office LAN, unless you use a VPN connection through the Internet, as described earlier in this chapter. If you plan to implement VPNs, be sure that the cable modem you are using supports them.
Geosynchronous communications satellites are another means for connecting stand-alone computers to the Internet. With a satellite dish like those used for TV reception, a computer can receive downstream traffic from an ISP's network at speeds comparable to those of DSL and CATV networks. However, satellite connections are one-way only; there is no upstream traffic from the subscriber's computer to the satellite. Therefore, you must maintain a standard dial-up connection to the ISP's network to transmit signals to the Internet. As with CATV network connections, a satellite link is not suitable for remote connections to a private network, and the use of a PSTN line for upstream traffic makes even VPN connections unlikely to be practical.
There is another type of remote connection that some networks use within a single site, instead of between sites. Thin client computing involves the use of a terminal client program running on a low-end computer or a dedicated network client device that communicates with a terminal server elsewhere on the network. The role of the client is to provide the interface to the operating system and nothing more; the actual operating system and all applications run on the terminal server. The client and the server communicate using a specialized protocol, such as Independent Computing Architecture (ICA), developed by Cyrix Systems, Inc. This protocol carries keystrokes, mouse actions, and screen updates between the client and the server, enabling a user at the client side to function as though the applications are running locally, when they are actually running at the server. Thin client computing enables a network to use inexpensive machines for its clients, leaving most of the computing environment on the server, where administrators can easily monitor and maintain it.
In addition to a physical layer connection, there are other elements you need to establish a remote network connection, including the following:
Specify which of the remote connection technologies (PSTN, ISDN, DSL, CATV, and/or satellite) discussed in this lesson are associated with each of the following concepts.