Remote Access Connection Methods

Because a computer using remote access is not connected to your network, it will not use LAN technologies to connect to the network. The remote computer will instead use connection methods to connect to the LAN, including:

• Public Switched Telephone Network (PSTN, also called plain old telephone services, or POTS)

• Integrated Services Digital Network (ISDN)

• Other digital connection methods (including one of the digital subscriber lines, or DSLs, and T-series connections)

The Public Switched Telephone Network (PSTN)

Almost everyone outside the phone companies refers to PSTN as POTS. This is the wiring system that runs from your house to the rest of the world. It is the most popular method for connecting a remote user to a local network because of its low cost, ease of installation, and simplicity.

Note 

Even the employees of most phone companies refer to PSTN as POTS when discussing work inside the phone company. The only time the acronym PSTN is used is when making a technical presentation or when a phone company’s marketing department is making a public statement.

Attributes of PSTN

Two key concepts when discussing PSTN are public and switched . Public, of course, is the opposite of private and means that, for a fee, anyone can lease the use of the network, without the need to run cabling. The term switched explains how the phone system works. Although one or more wires are connected to your home and/or office, they are not always in use. In effect, your wiring and equipment is offline , or not part of the network. Yet, in this offline state, you have a standing reservation so that you can join at almost any time. Your identification for this reservation is your phone number, which is what makes the phone companies a viable communications network. You initiate a connection by dialing a phone number. Can you see how it would be technically impractical if every phone number were connected all the time? The cabling issues would be almost impossible.

Let’s take an example from the U.S. telephone system. The actual numbering sequence varies in other countries, though the concept is identical. The phone company runs a UTP (unshielded twisted-pair) cable (called the local loop ) from your location (called the demarcation point , or demarc for short) to a phone company building called the Central Office . All the pairs from all the local loop cables that are distributed throughout a small regional area come together at a central point, similar to a patch panel in a UTP-based LAN.

This centralized point has a piece of equipment attached, which is called a switch. This switch functions almost exactly like the switches mentioned in Chapter 2, “The OSI Model,” in that a communications session, once initiated by dialing the phone number of the receiver, exists until the “conversation” is closed. The switch can then close the connection. On one side of the switch is the neighborhood wiring. On the other side are lines that may connect to another switch or to a local set of wiring. The number of lines on the other side of the switch depends on the usage of that particular exchange. Figure 7.1 shows a PSTN system that utilizes these components.

Figure 7.1: A local PSTN (or POTS) network

When you want to make a call, you pick up the phone. This completes a circuit, which in most cases gives you a dial tone. The tone is the switch’s way of saying, “I’m ready to accept your commands.” Failure to get a dial tone indicates either a break in the equipment chain or that the switch is too busy at the moment processing other commands. In many areas of the world, you may hear a fast on-and-off tone after giving a command string (phone number) to the local switch. This means that other switches with which the local switch is attempting to communicate are too busy right now. Recently, this has been replaced with a localized voice, which typically says, “We’re sorry. All circuits are busy. Hang up and try your call later.” This happens frequently on holidays or during natural disasters. The phone company in a local area uses only a few wires (called trunk lines ) for normal capacity and some auxiliary lines for unexpected usage. This is because wiring and switches are very expensive. It is a trade-off between 100-percent uptime and keeping the costs of leasing the connection from the phone company affordable.

Warning 

Use caution when working with bare phone wires, as they may carry a current. In POTS, the phone company uses a battery to supply power to the line, which is sometimes referred to as self-powered . It isn’t truly self-powered, however, as the power comes from the phone system.

As a remote access connection method, POTS has many advantages, including:

• It is inexpensive to set up. Almost every home in the U.S. has or can have a telephone connection.

• There are no LAN cabling costs.

• Connections are available in many countries throughout the world.

POTS is the most popular remote access connection method because only one primary disadvantage is associated with it: limited bandwidth and thus a limited maximum data transfer rate. At most, 64Kbps data transmissions are possible, though rarely achieved by the traveling user connecting remotely to the corporate network.

Integrated Services Digital Network (ISDN)

ISDN is a digital, point-to-point network capable of maximum transmission speeds of about 2Mbps, although speeds of 128Kbps are more common. Because it is capable of much higher data rates at a fairly low cost, ISDN is becoming a viable remote user connection method, especially for those who work out of their homes. ISDN uses the same UTP wiring as POTS, but can transmit data at much higher speeds. But that’s where the similarity ends. What makes ISDN different from a regular POTS line is how it uses the copper wiring. Instead of carrying an analog (voice) signal, it carries digital signals. This is the source of several differences.

A computer connects to an ISDN line via an ISDN Terminal Adapter (often incorrectly referred to as an ISDN modem). An ISDN Terminal Adapter is not a modem because it does not convert a digital signal to an analog signal; ISDN signals are digital.

An ISDN line has two types of channels. The data is carried on special Bearer channels , or B channels , each of which can carry 64Kbps of data. A typical ISDN line has two B channels. One channel can be used for a voice call while the other is being used for data transmissions, and this occurs on one pair of copper wires. The second type of channel is used for call setup and link management and is known as the signal , or D channel (also referred to as the Delta channel ). This channel has only 16Kbps of bandwidth.

To maximize throughput, the two Bearer channels are often combined into one data connection for a total bandwidth of 128Kbps. This is known as bonding or inverse multiplexing. This still leaves the Delta channel free for signaling purposes. In rare cases, you may see user data, such as e-mail, on the D line. This was introduced as an additional feature of ISDN, but it hasn’t caught on.

The main advantages of ISDN are:

• It has a fast connection.

• It offers higher bandwidth than POTS. Bonding yields 128Kb bandwidth.

• There is no conversion from digital to analog.

However, ISDN does have a few disadvantages:

• It’s more expensive than POTS.

• Specialized equipment is required at the phone company and at the remote computer.

• Not all ISDN equipment can connect to every other type of equipment.

• ISDN is a type of dial-up connection and, therefore, the connection must be initiated.

Other Digital Options

Digital connections provide one main benefit to remote access users: increased bandwidth over older technologies. The digital nature of ISDN and other digital connection types makes them excellent choices for remote access connections. Some of the more important types are:

• xDSL

• Frame relay

• T-series

• Asynchronous Transfer Mode (ATM)

• FDDI

xDSL Technology

xDSL is a general category of copper access technologies that is becoming popular because it uses regular POTS phone wires to transmit digital signals, and is extremely inexpensive compared with the other digital communications methods. xDSL implementations cost hundreds of dollars instead of the thousands that you would pay for a dedicated, digital point-to-point link (such as a T1). They include digital subscriber line (DSL), high data-rate digital subscriber line (HDSL), single-line digital subscriber line (SDSL), very high data-rate digital subscriber line (VDSL), and asymmetric digital subscriber line (ADSL), which is currently the most popular. It is beyond the scope of this book, however, to cover all of the DSL types.

ADSL is winning the race because it focuses on providing reasonably fast upstream transmission speeds (up to 640Kbps) and very fast downstream transmission speeds (up to 9Mbps). This makes downloading graphics, audio, video, or data files from any remote computer very fast. The majority of web traffic, for example, is downstream. The best part is that ADSL works on a single phone line without losing voice call capability. This is accomplished with what is called a splitter, which enables the use of multiple frequencies on the POTS line.

As with ISDN, communicating via xDSL requires an interface to the PC. All xDSL configurations require a modem, called an endpoint, and a NIC. Often the modem and NIC are on a single expansion card.

Frame Relay Technology

Frame relay is a WAN technology in which variable-length packets are transmitted by switching. Packet switching involves breaking messages into chunks at the sending router. Each packet can be sent over any number of routes on its way to its destination. The packets are then reassembled in the correct order at the receiver. Because the exact path is unknown, a cloud is used when creating a diagram to illustrate how data travels throughout the service. Figure 7.2 shows a frame relay WAN connecting smaller LANs.

Figure 7.2: A typical frame relay configuration

Frame relay uses permanent virtual circuits (PVCs). PVCs allow virtual data communications circuits between sender and receiver over a packetswitched network. This ensures that all data that enters a frame relay cloud at one side comes out at the other over a similar connection.

The beauty of using a shared network is that sometimes you can get much better throughput than you are paying for. When signing up for one of these connections, you specify and pay for a Committed Information Rate (CIR) or, in other words, a minimum bandwidth. If the total traffic on the shared network is light, you may get much faster throughput without paying for it. Frame relay begins at this CIR speed and can reach as much as 1.544Mbps, the equivalent of a T1 line, which we’ll discuss next.

Note 

Contrast this technology with circuit switching, in which a dedicated path from sender to receiver is established and maintained throughout the conversation.

T-Series Connections

The T-series connections are digital connections that you can lease from the telephone company. They can use regular copper pairs like regular phone lines, or they can be brought in as part of a backbone (also called a trunk line). At this point, T-series connections use time division multiplexing (TDM) to divide the bandwidth into 24 channels plus a control line.

The T-series connection types are rated by the letter T plus a number. Each connection type differs primarily in its speed. Table 7.2 lists some of the T-series connections and their maximum data rates. The most commonly used T-series lines are T1 and T3.

Table 7.2: T-Series Connections

Connection	Maximum Speed
T1	1.544Mbps
T1C	3.152Mbps
T2	6.312Mbps
T3	44.736Mbps
T4	274.176Mbps

The T1 Connection

A T1 is a 1.544Mbps digital connection that is typically carried over two pairs of UTP wires. This 1.544Mbps connection is divided into 24 discrete, 64Kbps channels (called DS0 channels). Each channel can carry either voice or data. In the POTS world, T1 lines are used to bundle analog phone conversations over great distances, using much less wiring than would be needed if each pair carried only one call. This splitting into channels allows a company to combine voice and data over one T1 connection. You can also order a fractional T1 channel that uses fewer than the 24 channels of a full T1.

Note 

The European version of the T1 is the E1, which operates at 2.048 Mbps.

Real World Scenario: What’s a Good Speed for a Business?

Many of you who are in charge of setting up your company’s Internet connection may think that a T1 is the best speed for your business. Unfortunately, T1 connections to the Internet are very expensive. If your business is selling Internet connections (such as an ISP), you could justify spending the money on it. Or, if you have many users (more than 50), you could also make a case for buying one. Otherwise, you may want to check out alternatives for your business that have a similar speed but a lower cost, such as DSL, a cable modem, or ISDN.

The T3 Connection

A T3 line works similarly to a T1 connection, but carries a whopping 44.736Mbps. This is equivalent to 28 T1 channels (or a total of 672 DS0 channels). Currently this service requires fiber-optic cable or microwave technology. Many local ISPs have T3 connections to the major ISPs, including SprintNet, AT&T, and MCI. Also, very large, multinational companies use T3 connections to send voice and data between their major regional offices.

Note 

As with the T1, the T3 has a European counterpart, the E3, which operates at 34.368Mbps.

Asynchronous Transfer Mode (ATM)

ATM ( asynchronous transfer mode, not to be confused with automated teller machines) first emerged in the early 1990s. If networking has an equivalent to rocket science, then ATM is it. ATM was designed to be a high-speed communications protocol that does not depend on any specific LAN topology. It uses a high-speed cell-switching technology that can handle data as well as real-time voice and video. The ATM protocol breaks up transmitted data into 53-byte cells. A cell is analogous to a packet or frame, except that an ATM cell does not always contain source or destination addressing information; also, the ATM cell contains neither higher-level addressing nor packet control information.

ATM is designed to switch these small cells through an ATM network very quickly. It does this by setting up a virtual connection between the source and destination nodes; the cells may go through multiple switching points before ultimately arriving at their final destination. The cells may also arrive out of order, so the receiving system may have to reassemble and correctly order the arriving cells. ATM is a connection-oriented service in contrast to most network architectures, which are broadcast-based.

Data rates are scalable and start as low as 1.5Mbps, with speeds of 25Mbps, 51Mbps, 100Mbps, 155Mbps, and higher. The common speeds of ATM networks today are 51.84Mbps and 155.52Mbps. Both of these speeds can be used over either copper or fiber-optic cabling. An ATM with a speed of 622.08Mbps is also becoming common but is currently used exclusively over fiber-optic cable. ATM supports very high speeds because it is designed to be implemented by hardware rather than software; faster processing speeds are therefore possible. Soon, fiber-based ATM networks will be operating at data rates of 10Gbps.

In the U.S., the standard for synchronous data transmission on optical media is SONET (Synchronous Optical Network); the international equivalent of SONET is SDH (Synchronous Digital Hierarchy). SONET defines a base data rate of 51.84Mbps; multiples of this rate are known as optical carrier (OC) levels, such as OC-3, OC-12, etc. Table 7.3 gives common OC levels and their associated data rates.

Table 7.3: Common Optical Carrier levels (OC-x)

Level	Data Rate
OC-1	51.84Mbps
OC-3	155.52Mbps
OC-12	622.08Mbps
OC-48	2.488Gbps

FDDI

The Fiber Distributed Data Interface (FDDI) is a network technology that uses fiber-optic cable as a transmission medium and dual counter-rotating rings to provide data delivery and fault tolerance. FDDI was developed as a way to combine the high-speed capabilities of fiber-optic cable and the fault tolerance of IBM’s Token Ring technologies. An FDDI network is based on a standard introduced by the ANSI X3T9.5 committee in 1986. It defines a high speed (at 100Mbps), token-passing network using fiber-optic cable. In 1994, the standard was updated to include copper cable (called CDDI, or Copper Distributed Data Interface). FDDI was slow to be adopted, but has found its niche as a reliable, high-speed technology for backbones and high-bandwidth applications that demand reliability.

FDDI is similar to Token Ring in that it uses token passing for permission to transmit. Instead of a single ring, however, FDDI uses two rings that counterrotate. That is, the token is passed clockwise in one ring and counterclockwise in the other. If a failure occurs, the counter-rotating rings can join together forming a ring around the fault, thus isolating the fault and allowing communications to continue.

Additionally, stations on an FDDI network can be categorized as either dual-attached stations (DAS) or single-attached stations (SAS). DASes are attached to both rings, whereas SASes are attached to only one of the rings. DASes are much more fault tolerant than SASes.