Remote Access Connection Methods Networking

Because a computer using remote access is not a part of your network, it will not use local area network (LAN) technologies to connect to the network. The remote computer will instead use other kinds of connection methods to connect to the LAN, including the following:

  • Public Switched Telephone Network (PSTN, also called plain old telephone service, or POTS)
  • Integrated Services Digital Network (ISDN)
  • Other digital connection methods (including one of the digital subscriber lines, or DSLs, broadband cable, and T-series connections)

The Public Switched Telephone Network (PSTN)

The portion of the PSTN that runs from your house to the rest of the world is known as plainold telephone service (POTS). 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. However, your connection to the PSTN may be ISDN, DSL, cellular, or some other method.

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 backbone 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 cable consisting of a pair of copper wires (called the local loop ) from your location (called the demarcation point, or demark for short) to a phone company building called the Central Office (CO) . 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. The 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.

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.

A local Connection to the PSTN

A local Connection to the PSTN

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

  • It is inexpensive to set up. Almost every home in the United States has or can have a telephone connection.
  • There are no LAN cabling costs.
  • Connections are available in many countries throughout the world.

POTS is a popular remote access connection method because few minor disadvantages are associated with it. The disadvantages are limited bandwidth, and thus a limited maximum data transfer rate, and the inferior analog signal when compared to digital methods, such as ISDN and DSL. At most, 53Kbps (limited by the FCC) data transmissions are possible though rarely achieved by the traveling user connecting remotely to the corporate network. These days, to gain access to corporate remote access services, many travelers use VPN connections over hotel or other public wired or wireless networks, which most often connect to service providers using DSL or T-carrier digital circuits.

Integrated Services Digital Network (ISDN)

ISDN is a digital, point-to-point network capable of maximum transmission speeds of about 2Mbps (Primary Rate Interface [PRI]), although speeds of 128Kbps (Basic Rate Interface [BRI]) are more common in a small-office, home-office (SOHO) environment.

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 it 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 the 128Kbps ISDN line via an ISDN Terminal Adapter, or TA (often incorrectly referred to as an ISDN modem). An ISDN TA is not a modem because it does not convert a digital signal from the computer to an analog signal on the subscriber line; ISDN signals are digital on the subscriber line. A TA is technically an ISDN-compatible device that has one or more non-ISDN ports for devices like computer serial interfaces and RJ-11 analog phones, allowing these non-ISDN devices access to the ISDN network.

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 BRI 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 through time division multiplexing on one pair of copper wires. The second type of channel is also multiplexed onto the one copper pair, is used for call setup and link management, and is known as the signaling channel, or D channel (also referred to as the Delta channel). This channel has only 16Kbps of bandwidth.

To maximize throughput, the two B channels are often combined into one data connection for a total bandwidth of 128Kbps. This is known as BONDING (which stands for Bandwidth on Demand Interoperability Group) or inverse multiplexing. This still leaves the D channel free for signaling purposes. In rare cases, you may see user data, such as credit card verification, on the D channel. This was introduced as an additional feature of ISDN, but it hasn’t caught on.

These are the main advantages of ISDN:

  • It has a fast connection.
  • It offers higher bandwidth than POTS. BONDING yields 128Kbps 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. The following are some of the more important types:

  • xDSL
  • Cable modem (broadband cable)
  • Frame Relay
  • T-series
  • Asynchronous Transfer Mode (ATM)

1. xDSL Technology

xDSL is a general category of copper access technologies that has become popular because it uses regular PSTN 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.

ADSL has become the most popular xDSL 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, and 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 DSL modem, called an endpoint, and a network interface card (NIC) in the computer. The NIC is able to be connected directly to the DSL modem using a straight-through Ethernet UTP patch cord with standard RJ-45 connectors on each end, but if other connecting devices are between the computer and the cable modem, either a special switchable port or an Ethernet crossover cable will be required for proper functionality.

2. Cable Modem

Another digital remote access connection device is the cable modem. A cable modem is a device that is used to connect computers and other devices to a cable television data network. Cable modem technology is one example of high-speed Internet access, also known as broadband Internet access (a.k.a. “broadband”). Because the cable company already has a cabled infrastructure in most cities, it was a natural choice to bring high-speed Internet connections to homes. The cable television companies got together and developed the Data over Cable Service Interface Specification (DOCSIS) that, among other things, specifies how to allow data services over a cable system. Most cable modems adhere to this standard.

A DSL modem's connection's

A DSL modem's connection's

A cable modem's conections

A cable modem's conections

Cable modems are simply small boxes that have a cable connection and a connection to the computer . There are two ways to connect a cable modem to a computer: USB and Ethernet (usually 10Base-T). Most cable modems can use either one of these methods and some modems have both types.

If you are going to connect the cable modem via USB, simply install the software driver for the cable modem on the computer, and then plug in the cable modem. The computer will detect the modem and configure it automatically.

If you are using Ethernet to connect, you must have an Ethernet NIC in your computer that is properly installed and configured. Then, once the NIC is installed, all you need to do is connect the cable modem to the NIC with an appropriate 10Base-T patch cable (RJ-45 connector on both ends, usually supplied with the cable modem). The thing to keep in mind is that the cable modem’s Ethernet connection is physically and electronically the same as a medium dependent interface-crossover(MDI-X) port on a hub or switch, meaning that you can connect your computer, which has a medium dependent interface (MDI) to the cable modem with a straight-though cable.

If you are using a hub or switch on your network and you have the cable modem plugged into the hub or switch, you may experience a connectivity issue because you are connecting an MDIX port to an MDI-X port, facing the transmit wire pair n one device to the transmit wire pair on the other device. The receive wire pairs are facing each other as well. If either your cable modem or your hub or switch has a switchable port, toggle the associated button from the MDIX setting to the MDI setting so that the transmit and receive pairs are swapped in the electronics of the one device, making a proper connection between the devices. You’ll notice your Link LEDs on the switch port and on the cable modem illuminate in approval. If you cannot find such a button on either device, it will be necessary to use an Ethernet crossover cable to connect the switch or hub to the cable modem. This is no different from connecting two hubs or switches back to back.

3. Frame Relay Technology

Frame Relay is a wide area network (WAN) technology in which variable-length packets are transmitted by switching. Packet switching involves breaking messages into chunks at the sending device. 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.

A typical Frame Relay configuration

A typical Frame Relay configuration

Frame Relay uses permanent virtual circuits (PVCs). PVCs allow virtual data communications circuits between sender and receiver over a packet-switched 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.

4. 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 channels of equal bit rate.

The T-series connection types are denoted by the letter T plus a number. Each connection type differs in its speed and in the signal used to multiplex the channels. The most commonly used T-series lines are T1 and T3.

T-Series connections

The T1 Connection

A T1 is a 1.544Mbps digital connection that is typically carried over two pairs of copper wires. This 1.544Mbps connection uses a signal known as a digital signal level 1 (DS1) and aggregates 24 discrete, 64Kbps channels that use a signal known as a digital signal level 0 (DS0). Each channel can carry either voice or data. In the POTS world, T1 lines are used to convert and bundle analog phone conversations over great distances due to the better quality of a digital signal and using much less wiring than would be needed if each pair carried only one call. This splitting into independent channels also allows a company to combine voice and data over one T1 connection or to use the T1 as if it were an unchannelized 1.544Mbps pipe. You can also order a fractional T1 (FT1) circuit, which is delivered on a T1 but does not allow the use of all 24 channels.

The T3 Connection

A T3 line works similarly to a T1 connection but carries a whopping 44.736Mbps. This is equivalent to 28 T1 circuits (or a total of 672 DS0 channels). This service uses a signal known as the digital signal level 3 (DS3), which is not the same as the DS1 signal and is generally delivered on fiber-optic cable. Many local ISPs have T3 connections to their next-tier ISPs. Also, very large multinational companies use T3 connections to send voice and data between their major regional offices.

Asynchronous Transfer Mode (ATM)

Asynchronous transfer mode (ATM), not to be confused with automated teller machines, first emerged in the early 1990s. 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 is always fixed in length, whereas a frame’s length can vary.

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, like Frame Relay, is a connection-oriented service in contrast to most Data Link protocols, which are best-effort delivery services and do not require virtual circuits to be established before transmitting user data.

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. Fiber-based service-provider ATM networks are running today at data rates of 10Gbps are becoming more and more common.

In the U.S., the standard for synchronous data transmission on optical media is Synchronous Optical Network (SONET); the international equivalent of SONET is Synchronous Digital Hierarchy (SDH). 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, and so on.

common optical carrier levels

Fiber Distributed Data Interface (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 counter rotate. 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 and thus isolating the fault and allowing communications to continue.

Additionally, stations on an FDDI network can be categorized as either dual-attached station (DAS) or single-attached station (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.


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