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Novell's Networking Primer

Important LAN and WAN High-Speed Technologies

In today's business environment, among the most widely discussed networking topics are the technologies that make networks faster, such as Fast Ethernet and Gigabit Ethernet, as well as the technologies that connect geographically distant networks, such as frame relay, ATM, and SONET. As networks grow, so does the amount of information sent across these networks. For this reason, transmission speed is of utmost importance. This section will provide brief descriptions of the following technologies that are dramatically improving the transmission speed on computer networks.

The first seven items—Fast Ethernet, Gigabit Ethernet and 10 Gigabit Ethernet, Firewire and USB, Fibre Channel, Bluetooth, 802.11, and HIPERLAN/1—apply primarily to LANs. The other technologies in this list are reserved almost exclusively for WANs.

Fast Ethernet

100Base-T—also known as Fast Ethernet—is a high-speed LAN technology. It has been designated as the IEEE 802.3u standard and functions at the data-link (OSI Layer 2) layer's MAC sublayer, providing data transfer rates as high as 100 megabits per second (Mbps). Three kinds of wiring carry Fast Ethernet: 100Base-T4, which is four pairs of twisted-pair wires; 100Base-TX, which is two pairs of data-grade twisted pair wires; and 100Base-FX, which is two strands of fiber optic cable.

Distinguishing Characteristics

Like 10Base-T Ethernet, 100Base-T uses CSMA/CD as the MAC method. 100Base-T is based on the scalability of CSMA/CD. Scalability means that you can easily enlarge or downsize your network without degrading network performance, reliability, and manageability.

CSMA/CD was known to be scalable before the 100Base-T standard was created: a scaled-down version of Ethernet (1Base-5) uses CSMA/CD, provides data transfer rates of 1 Mbps, and enables longer transmission distances between repeaters. Because CSMA/CD could be scaled down, people reasoned that it could be scaled up. Specifying changes such as decreased transmission distances between repeaters produced a reliable data transfer rate of 100 Mbps, 10 times faster than traditional 10Base-T Ethernet.

Figure 37: On 100Base-T networks, the physical topology is a star and the logical topology is a bus. A broadcast signal travels to all parts of the cable

Advantages

100Base-T adapter cards and compatible cable are currently available from various vendors.

In addition, it is easy to upgrade from 10Base-T Ethernet to 100Base-T Ethernet. Both use CSMA/CD, and most network cards now support both 10 Mbps and 100 Mbps Ethernet. The adapter cards automatically sense whether it is a 10 Mbps or 100 Mbps environment and adjust their speed accordingly. Because 10Base-T and 100Base-T Ethernet can coexist, network supervisors can upgrade network stations from 10Base-T to 100Base-T one at a time, as needed. Moreover, most network supervisors are already familiar with CSMA/CD, so there is no need for expensive retraining.

100Base-T can be an inexpensive way to make your network faster. Adapter cards are not significantly more expensive than 10Base-T cards. In addition, Category 5 UTP cable (also called "CAT 5") is relatively inexpensive.

Disadvantages

100Base-T reduces the maximum network size compared to 10Base-T because the standard specifies shorter transmission distances between repeaters. Compared to 10Base-T, 100Base-T reduces the maximum network diameter from 500 to 205 meters. For existing networks that exceed 205 meters, routers must be installed between 100Base-T network segments.

Gigabit Ethernet and 10 Gigabit Ethernet

Faster standards are always being developed and established and those for Ethernet are no exception. One standard is Gigabit Ethernet, also known as 1000Base-T or 802.3z. Gigabit Ethernet increases transmission speed on a standard Ethernet network to 1000 Mbps, or ten times that of 100Base-T. It was designed to function on the same cabling as 100Base-T so that upgrades would be inexpensive and straightforward. Right now, the primary focus for Gigabit Ethernet is as a backbone service for 100Base-T networks. As the hardware becomes more prevalent, however, 1000Base-T subnetworks and workstations should become more common.

In addition to Gigabit Ethernet, there is also an emerging standard known as 10 Gigabit Ethernet or 802.3ae. 10 Gigabit Ethernet will support data transmission speeds of 10,000 Mbps. Analysts predict that in some cases it could replace high-speed WAN technologies such as ATM and SONET.

Although Gigabit Ethernet is enjoying widespread adoption, 10 Gigabit Ethernet has not yet reached wide-scale acceptance in the world of computer networking. Part of the problem is that details of the standard, such as the medium over which it will be propagated, are not yet defined. Also, because Ethernet has not been a long-range standard in the past, it does not include management-control capabilities to alert network administrators when something goes wrong. However, Gigabit and 10 Gigabit Ethernet are gaining popularity as a solution for the thin-client application-on-demand model.

IEEE 1394 (Firewire) and USB

The IEEE 1394 standard (also known as Firewire) and the Universal Serial Bus (USB) standard are two standards that apply to data transmission between computers and peripheral hardware. Although these two standards are different, their application is fundamentally related and therefore they are covered in the same section here.

Distinguishing Characteristics

The IEEE 1394 standard is a high-speed standard developed for processing-intensive peripherals such as scanners, digital cameras, and removable storage devices. As a complementary standard, USB is more suitable for peripherals that do not require as much speed, such as mice and keyboards. Both standards use a simple cable with jacks similar to telephone jacks or Ethernet RJ-45 jacks. Most newer computers include USB ports; IEEE 1394 ports are integrated into higher-end computers.

Advantages

The most obvious advantage of both IEEE 1394 and USB is the ease of use. Both standards support "hot swapping" of peripheral components. This means that one device can be unplugged from the computer and another plugged in (and recognized by the computer) without having to reboot the system. This is not possible with standard parallel port or serial port connections. In addition, both 1394 and USB standards allow you to connect peripherals according to a tree topology or as a "daisy chain" (linked in a straight line) and then attach them to a single port on the computer—each peripheral does not require its own port. The IEEE 1394 standard allows for 63 devices per port, whereas USB will support more than one hundred. In addition, both standards support data transfer speeds higher than conventional ports. USB supports 12 Mbps data transfer, USB 2.0 supports 36–48 Mbps, and IEEE 1394 supports up to 400 Mbps (with several faster versions currently under development). USB is expected to replace serial and parallel ports for most simple peripherals such as mice and keyboards, and 1394 will meet the high-throughput requirements of video and real-time equipment.

Disadvantages

Although USB has gained widespread acceptance—USB-compatible peripherals are readily available—IEEE 1394-compatible hardware is harder to find. Part of the reason is that 1394 is the intellectual property of Apple Computer, which tightly controls the dissemination of the standard. In addition, 1394-compatible peripherals tend to be on the high end of the technological spectrum and therefore are much more expensive than their USB counterparts. Firewire and USB are also not compatible, so hardware manufacturers either have to install both ports or choose between the two: given the popularity of USB, Firewire often loses.

Fiber Channel

Fiber channel refers to a relatively new application for optical fiber components. The most common usage of fiber channel is in storage area networks (SANs), where it is used to connect clustered servers to storage systems. This technology is also being considered as an internal drive interface (between the hard drive and the processor within a computer) and as a high-speed switching service to connect several server clusters and SANs into a large interconnected network.

Distinguishing Characteristics

Fiber channel technology consists of optical fiber cables, specialized hubs, and Gigabit interface converters (GBICs). The GBICs are used to convert electrical signals into optical signals and vice versa. The cabling is divided into two categories: multimode and single-mode fiber. Multimode fiber has a larger diameter core and allows multiple transmissions to travel simultaneously. Single-mode fiber allows only one transmission path.

Advantages

Fiber channel technology has several advantages over other transmission media, but the most important is the speed of data transmission. Fiber channel supports data-transmission speeds of 100 Mbps. In addition, since the data is transmitted as a pulse of light rather than an electronic signal, it can travel much greater distances (up to 10 kilometers) before suffering any signal degradation. Likewise, the data is immune to electromagnetic interference and radiates no energy (no heat-shielding required).

Disadvantages

The main disadvantage of fiber channel is the cost: optical fiber is much more costly than conventional copper cable and more expensive to install. The advantages of fiber channel, however, greatly outweigh the cost in many applications.

Bluetooth

Bluetooth, a wireless standard developed by Ericsson, IBM, Intel, Nokia and Toshiba, is named after a Danish king who united Denmark and Norway in the 10th century. It is designed for short-range radio transmissions between devices no more than 10 meters apart. Bluetooth operates at a frequency of 2.4GHz and transmits at speeds up to 1 Mbps. (A next generation of Bluetooth will transmit at 2 Mbps.) Bluetooth is primarily for use in mobile devices to provide connectivity and synchronization; for example, two Bluetooth-enabled handheld devices a few meters apart can synchronize phone lists or schedules. The devices can connect on a one-to-one or one-to-many basis so that when near each other, Bluetooth devices create a "piconet": an ad-hoc, peer-to-peer network of up to 10 nodes. For example, if all the participants at a meeting have Bluetooth-enabled laptops, they can create a piconet for sharing documents and messages. A Bluetooth-enabled printer in the room could be used by all without the need for cabling.

Advantages

Bluetooth is already a de facto standard that has garnered widespread interest and support among vendors. It supports both data and voice transmissions and does not require a line of sight.

Disadvantages

Bluetooth's limited range makes it useful only for instances when the device is near another Bluetooth transmitter. Its data transmission rates are not nearly as fast as those of 802.11. It has so far been prohibitively expensive to implement, but it is predicted that the price will go down as vendor interest increases.

802.11

IEEE 802.11 is an extension of the Ethernet standard, adapted for wireless LANs. It consists of one MAC-layer standard and three physical-layer standards: two for radio transmissions (DSSS and FHSS) and one for infrared. 802.11 operates at 2.4GHz and can transmit at speeds up to 2 Mbps at a range of 30–100 meters. IEEE 802.11b, ratified in 1999, boasts transmission rates of up to 11 Mbps, but only over the DSSS physical layer.

Distinguishing Characteristics

Similar to its wired Ethernet counterpart, the 802.11 MAC layer uses a variation of CSMA/CD called carrier sense multiple access with collision avoidance (CSMA/CA). Unlike wired devices, a wireless device is unable to "listen" and transmit at the same time because the noise from transmission drowns out incoming signals; therefore, if there is a collision during transmission the device will not detect it. To solve the problem CSMA/CA provides for explicit packet acknowledgement. After each packet is sent, the receiver sends an acknowledge (ACK) packet to confirm that the packet arrived intact. If the ACK packet does not arrive, the sender assumes the original packet did not arrive, probably because of a collision, and sends the packet again.

Wireless environments are also susceptible to the "hidden node" issue in which a wireless node can hear traffic from the AP but not from another wireless node, due to distance or an obstruction. To prevent a collision between hidden nodes, 802.11 specifies an optional Request to Send/Clear to Send (RTS/CTS) protocol, also at the MAC layer. When this protocol is engaged, a sending device sends an RTS packet to the AP and waits for the CTS packet before transmitting. Because all other devices interfacing with that AP receive the same CTS packet, they delay any intended transmissions until the medium is free.

Advantages

Because 802.11 is a true Ethernet specification, 802.11 devices can be integrated seamlessly into conventional Ethernet LANs. With a laptop and an 802.11 network adapter card, an employee can roam throughout a building, go from building to building, or even go to a remote office and always be connected to the network.

Disadvantages

Unlike Bluetooth, 802.11 does not support voice transmissions, and the additional overhead from the ACK and RTS/CTS packets means 802.11 transmissions will always be slower than wired Ethernet. It is also feared that 802.11 transmissions can inadvertently disrupt critical Bluetooth transmissions, such as wireless medical or manufacturing monitoring devices.

HIPERLAN/1

High-Performance Radio LAN, Type 1 (HIPERLAN/1) is a standard developed by the European Telecommunications Standards Institute (ETSI) to improve on the data throughput rates of 802.11. It is the first in a suite of HIPERLAN standards that operate in the 5GHz range: HIPERLAN/2 is Wireless ATM, HIPERLAN/3 (renamed HIPERAccess) is for wireless local loop (the last segment between a home and the telephone system), and HIPERLAN/4 (renamed HIPERLink) is for wireless point-to-point connections. Of the four standards, only HIPERLAN/1 has been approved; the others are still in development.

Distinguishing Characteristics

The HIPERLAN/1 transmission scheme is the same as that for GSM, which means it uses TDMA as its air interface and Gaussian Minimum Shift Keying (GMSK) as its modulation scheme. HIPERLAN/1 can achieve data transfer rates up to 23.5 Mbps.

With HIPERLAN the MAC layer is subdivided into the Channel Access Control (CAC) layer, and the MAC layer. The CAC layer defines how a given channel access attempt will be made, depending on whether the channel is busy or idle and at what priority level the attempt will be made, if contention is necessary. Packets receive higher priority as they age.

Multi-hop routing support (not included in 802.11) is part of the HIPERLAN/1 specification. HIPERLAN-enabled devices choose a nearby "controller" and forward all outgoing traffic to that controller. The controller will then route the packet toward its destination. HIPERLAN-enabled devices also employ "hello" packets to announce their presence to other devices. In this sense, HIPERLAN-enabled devices behave similarly to conventional routers and are therefore able to structure their own network.

Advantages

HIPERLAN/1 is compatible with wired and wireless Ethernet. It is very stable and flexible.

Disadvantages

HIPERLAN is an emerging technology, so only a few HIPERLAN/1-compatible devices exist.

Fiber Distributed Data Interface (FDDI)

FDDI is also a high-speed LAN technology. It is not generally used for direct connection to desktop computers, but rather as a network backbone connecting two or more LAN segments to provide a path for data transmission between them. A simple backbone might connect two servers through a high-speed link consisting of network adapter cards and cable. An example of such a backbone is illustrated in Figure 38.

FDDI has been designated ANSI X3T9.5 and operates at the physical and data-link layers (Layers 1 and 2) of the OSI model. Like 100Base-T, FDDI provides data transfer rates as high as 100 Mbps.

Figure 38: A simple server-based backbone connecting two LAN segments

Distinguishing Characteristics

FDDI networks have a dual, counter-rotating ring topology. This topology consists of two logical closed signal paths called "rings." Signals on the rings travel in opposite directions from each other. Although both rings can carry data, the primary ring usually carries the data while the secondary ring serves as a backup.

On FDDI networks every node acts as a repeater. FDDI supports four kinds of nodes: dual-attached stations (DASs), single-attached stations (SASs), single-attached concentrators (SACs), and dual-attached concentrators (DACs). DASs and DACs attach to both rings, while SASs and SACs attach only to the primary ring. Several SASs often attach to the primary ring through a concentrator, so that an SAS failure will not bring down the entire network. If the cable is cut or a link between nodes fails, DASs or DACs on either side of the failure route signals around the failed segment, using the secondary ring to keep the network functioning.

FDDI uses token passing for its MAC method and is implemented using fiber-optic cable.

Figure 39: If a cable section on an FDDI network goes down, DASs on either side of the failed section automatically reconnect the primary and secondary rings. Also note that the server has a redundant connection to improve reliability.

Advantages

FDDI is a fast, reliable standard. The dual, counter-rotating ring topology increases the network's reliability by keeping it functioning even if a cable is damaged. FDDI also offers network management support, which was designed directly into the standard. In addition, the standard includes the Copper Distributed Data Interface (CDDI) specification for building a network using UTP cable (which is less expensive than fiber-optic cable).

Disadvantages

FDDI's main disadvantages are availability and price. Because FDDI is not useful for transmitting large graphic and sound files (such as video), it is being replaced by Gigabit Ethernet. And because most fiber backbones are now running SONET, FDDI hardware is difficult to find. Furthermore, FDDI adapter cards and fiber-optic cable are both expensive compared to other technologies offering the same speed. Fiber-optic cable installation also requires technicians trained specifically for this purpose. Even CDDI adapters (for copper wire), which are less expensive than FDDI adapters, are more costly than 100Base-T adapters. It is expected that FDDI will soon be replaced by other technologies.

X.25

X.25 is an ITU standard and includes data-link and physical-layer protocols (Link Access Procedure Balanced [LAPB] and X.21). X.25 provides data transfer rates of 9.6 Kbps to 256 Kbps, depending on the connection method.

Distinguishing Characteristics

X.25 specifies the interface for connecting computers on different networks with an intermediate connection through a packet-switched network. X.25 was defined when the quality of transmission media was relatively poor. As a result, the standard specifies that each node in the packet-switched network must receive each packet completely and check it for errors before forwarding it.

Figure 40: X.25 networks are often provided by telecommunication carriers.

Advantages

X.25 is well understood and reliable. Connections to X.25 networks can be made through the existing telephone system, Integrated Services Digital Network (ISDN), and leased lines. Because access is simple, it is comparatively inexpensive. X.25 is available worldwide, although its market share in the United States is rapidly decreasing. In countries with little digital telecommunications infrastructure, X.25 is the best WAN technology available.

Disadvantages

X.25 is slow compared to newer technologies. The process of checking each packet for errors at each node limits data transfer rates. X.25 also uses variable-size packets, which can cause transmission delays at intermediate nodes. In addition, many people connect to X.25 networks through modems, which limit data transfer rates to between 9.6 Kbps and 56 Kbps. Although X.25 is still in use in some areas, newer, faster standards such as ATM and frame relay have largely replaced it.

Frame Relay

Frame relay is a WAN technology. Approved by ANSI and the ITU, frame relay works at the data-link layer (OSI Layer 2) of the OSI model and provides data transfer rates from 56 Kbps to 1.544 Mbps.

Distinguishing Characteristics

Frame relay is an interface specification for connecting LANs over public packet-switched networks. This standard can be thought of as a simplified version of X.25 designed to take advantage of digital transmission media.

Frame relay services are typically provided by telecommunications carriers. Customers install a router and lease a line (often a T1 or fractional T1 line) to provide a permanent connection from the customer's site to the telecommunications carrier's network. This connection enables frame relay to use "permanent virtual circuits" (PVCs), which are predefined network paths between two locations.

With frame relay the router encapsulates (or frames) network-layer packets, such as IP and IPX packets, directly into a data-link-level protocol and sends them on to the packet-switched network. Like X.25, frame relay uses variable-size frames but it eliminates the error checking required on X.25 networks. A frame relay switch simply reads the header and forwards the packet, sometimes without first receiving the frame completely. Intelligent end stations must identify missing or corrupted frames and request retransmission.

Figure 41: Frame relay is a WAN technology that enables companies to connect LANs through a telecommunications carrierÂ’s network.

Advantages

Frame relay uses PVCs over leased lines rather than a modem connection. PVCs transmit and receive data immediately, eliminating the call setup and handshaking that modems must perform. In addition, frame relay does not require error checking and flow control at the switches, thereby reducing overhead and leaving more bandwidth for data transmission. Frame relay is also a common standard in many countries. Finally, frame relay is less expensive than other WAN technologies because it provides bandwidth on demand rather than dedicating bandwidth regardless of whether data is being transmitted.

Although frame relay is fairly complex to implement, value-added resellers and most telephone companies will assist customers in determining their needs and will help install the technology.

Disadvantages

Frame relay's speed is limited because it uses variable-size frames, which can cause delays at switches along the frame's path. As a result, frame relay cannot support applications that require low latency (delayed response time) such as real-time video.

Asynchronous Transfer Mode (ATM)

ATM is a WAN technology that is generally implemented as a backbone technology. The exact relationship of the ATM layers to the OSI model is currently undefined, although ATM LAN emulation works at the data-link layer (OSI Layer 2).

ATM provides data transfer rates of 100 Mbps and 155 Mbps. At the high end, WAN implementations using ATM and SONET together have achieved data transfer rates of 2.4 Gbps.

Distinguishing Characteristics

ATM is a cell-relay technology, meaning that it uses standard-sized packets called "cells." The size of an ATM cell is 53 bytes.

In a LAN implementation ATM functions at the data-link layer's MAC sublayer. It further divides the MAC sublayer into three layers: LAN Emulation, ATM Adaptation Layer (AAL), and ATM. LAN Emulation enables you to integrate ATM with Ethernet and token-ring networks without modifying existing Ethernet or token-ring protocols.

On a mixed network, LAN Emulation hardware sits between the Ethernet or token-ring segment and the ATM part of the network. It uses the three layers mentioned above to convert packets moving toward the ATM segment into cells, and to assemble cells moving toward the Ethernet or token-ring segment into packets. AAL and ATM put data into standard-sized cells. In most network computing situations, an ATM adaptation layer breaks packets into 48-byte blocks that are then passed to the ATM layer, where the five-byte header is attached to form a complete 53-byte cell.

Advantages

ATM offers high data-transfer rates, which have climbed into the gigabit range and are still increasing. One reason that ATM is so fast is its use of cells: because they are a standard size, ATM networks handle data in a predictable, efficient manner at the switches. Standard-sized cells and high-bandwidth media like fiber-optic cable also enable ATM to support real-time voice, video, and data traffic.

ATM also offers flexibility in its transmission media. As many as 27 ATM specifications exist for media like UTP, shielded twisted-pair (STP), and fiber-optic cable. (ATM is generally implemented with fiber-optic cable.)

Ethernet and token-ring networks can be integrated with ATM through use of LAN Emulation. The ATM network can emulate (or imitate) enough of the MAC layer of Ethernet and token-ring technologies so that higher-layer protocols can be used without modification. This allows existing network applications and network protocols to run over ATM networks, resulting in great cost savings.

Disadvantages

ATM is more expensive than the other high-speed LAN technologies and is extremely complex to set up and maintain; the expense is preventing many companies from implementing ATM.

ISDN

ISDN is a set of protocols defined by the ITU to integrate data, voice, and video signals into digital telephone lines. It functions at the physical, data-link, network, and transport layers (Layers 1 through 4) of the OSI model. ISDN offers data transfer rates between 56 Kbps and either 1.544 Mbps or 2.048 Mbps, depending on the country where it is implemented.

Distinguishing Characteristics

ISDN makes end-to-end digital connections over telephone lines. Although many telephone networks are almost completely digital, the local loop that connects a home or office to the telephone company's network usually is not. Most local loops send analog rather than digital signals. ISDN replaces local analog signaling with digital signaling, enabling end-to-end digital communications.

ISDN offers Basic Rate Interface (BRI) for individuals or small branch offices and Primary Rate Interface (PRI) for larger companies.

BRI uses two bearer, or B, channels (providing 64 Kbps each) to transmit and receive data, and one delta, or D, channel for call setup and management.

PRI is a T1 line. A T1 line in the United States consists of 23 B channels and one D channel, providing a total data transfer rate of 1.544 Mbps. A T1 line in Europe, known as an E1 line, consists of 30 B channels and one D channel, providing a total data transfer rate of 2.048 Mbps. A fractional T1 uses only some of the B channels in a T1 line (and thus offers some fraction of the total T1 data transfer rate).

ISDN requires special equipment at the customer's site, including a digital phone line and a network termination unit (NT-1). An NT-1 converts the bandwidth coming over the line into the B and D channels, and aids the phone company in diagnostic testing. The NT-1 also provides a connection for terminal equipment, such as ISDN telephones and computers that have an ISDN interface. In addition, the NT-1 provides terminal adapter (TA) equipment to connect equipment that is not compatible with ISDN. TA equipment provides an intermediary connection point: such equipment has an ISDN interface for connection to the NT-1, and a non-ISDN interface for connection to non-ISDN equipment.

Advantages

ISDN increases speed and broadens data transmission capabilities, especially for those currently using analog modems to remotely connect to an office or to access the Internet. It offers faster call setup and data transfer rates. The transfer rates are acceptable for transmitting voice, data, limited video, fax, and images. ISDN can also be used for limited LAN-to-LAN communications.

With ISDN you can transmit voice and data traffic simultaneously: over the same telephone line you can concurrently talk on the phone and download a data file to your computer. For example, one BRI ISDN configuration enables you to use the two B channels (128 Kbps) for data and part of the D channel for a telephone conversation.

Disadvantages

To understand ISDN well enough to simply order services requires considerable effort. Furthermore, configuration can be difficult. ISDN speeds are faster than those of a conventional modem (56 Kbps), but they are not as fast as ADSL or other emerging technologies.

xDSL and Cable

xDSL refers to the various types of Digital Subscriber Line (DSL), a relatively new, high-speed Internet access technology. With the explosive growth in computer networking, business networks, and the use of the Internet, the demand for fast and cost-effective access has also been growing steadily. Although there are many technologies available to provide high-speed Internet access—such as leased lines, wireless connections, and ISDN—most are expensive to install and costly to maintain. xDSL was developed as a low-cost alternative to these technologies. xDSL is a technology that uses the existing standard telephone cable (twisted pair) to provide data transmission speeds rivaling and often exceeding those of much more expensive solutions. Cable modems, which connect to the Internet via cable TV hookups, provide speeds and service options similar to xDSL. Many modems available on the market support both xDSL and cable.

Distinguishing Characteristics

xDSL lines are always "on," which means that you do not have to establish a modem connection to use the Internet and then disconnect when you are finished—you are always connected. And unlike most dial-up Internet connections, xDSL can transmit both voice (or fax) and data signals over the same line at the same time. Conventional phone lines are capable of transmitting signals up to 1MHz, but voice transmissions only use the range between 1kHz and 4kHz. xDSL, on the other hand, transmits digital signals between 26kHz and 1.1MHz. A "splitter," installed at the user site, divides the signal into digital and analog signals for data and voice, respectively. For this reason, xDSL is especially attractive to home and small-business users who do not want to install separate lines for voice and data transmissions.

DSL comes in a variety of "flavors," the most common of which are listed below:

ADSL

The most popular kind of consumer xDSL is Asymmetric DSL (ADSL), also called G.dmt and Full-rate ADSL (ITU-T G.992.1). By asymmetrically dividing the available bandwidth, ADSL allows you to receive data much faster than you send it. With respect to an Internet connection, this is the optimal configuration: you send out much less data (about five percent of total transmission) than you download. Standard ADSL provides for downstream (data received) speeds of 8 Mbps and upstream (data sent) speeds of 1.5 Mbps.

ADSL Lite

A variant of ADSL is G.Lite, also ADSL Lite, splitterless DSL, and Universal DSL (ITU-T G.992.2). ADSL Lite provides a data transmission rate of 1.5 Mbps downstream and 512 Kbps upstream. ADSL Lite does not require a visit from the phone company because the splitting is managed at a central location, an arrangement which sacrifices some speed to save cost. It is expected to become the most widely installed form of DSL in private residences even though it does not handle voice and entertainment applications well.

RADSL

Rate Adaptive Asymmetric Digital Subscriber Line (RADSL) is similar to ADSL but it "listens" to see how much traffic is on the wire and adjusts its speed accordingly. Depending on the distance from the telephone company's central office, RADSL can transmit at speeds up to 7 Mbps downstream and 1.5 Mbps upstream (10,000 feet for the highest speed, 17,000 feet for the lowest).

HDSL

High bit-rate DSL (HDSL) is symmetric, meaning that it provides an equal amount of bandwidth upstream and downstream. Generally used for wideband connections within a corporation and between telephone companies and their customers, HDSL is often used in lieu of a T-1 connection. The data transmission rate varies depending on how many twisted-pair wires are used. Two wires provide 1.5 Mbps; three carry 2 Mbps. HDSL II, a variant of HDSL, provides the same speeds as HDSL but over a single wire.

SDSL

Symmetric DSL (SDSL) also provides the same speeds as HDSL but differs in two important ways: it is limited to 10,000 feet and it uses only one wire. It also uses the same modulation technique as ISDN. SDSL is a forerunner to HDSL II.

IDSL

ISDN DSL (IDSL) is a hybrid between ISDN and DSL. Using the same modulation technology as IDSN, IDSL bypasses voice lines and uses the less-busy data network instead. Transmission speeds are slightly higher than those of ISDN.

VDSL

Very high bit-rate DSL (VDSL) is a developing technology that will provide higher transmission rates over shorter distances. At 1,500 feet the speeds may be as high as 13 Mbps and at 1,000 feet an amazing 52 Mbps. High-definition television signals are one proposed use for VDSL.

  Type Downstream Upstream Distance Medium
ADSL Asymmetric 8.0 Mbps 1.5 Mbps 18,000 ft single twisted pair
ADSL Lite Asymmetric 1.5 Mbps 512 Kbps 18,000 ft single twisted pair
RADSL Asymmetric 7.0 Mbps 1.5 Mbps 17,000 ft single twisted pair
HDSL   Symmetric   1.5 Mbps 1.5 Mbps 15,000 ft 2 twisted-pair wires
2.0 Mbps 2.0 Mbps 15,000 ft 3 twisted-pair wires
SDSL Symmetric 1.5 Mbps 1.5 Mbps 10,000 ft single twisted pair
IDSL Symmetric 144 Kbps 144 Kbps 18,000 ft single twisted pair
VDSL Asymmetric 13–52 Mbps 1.5–3.2 Mbps 1,000 ft – 4,500 ft fiber-optic cable

Advantages

The most obvious advantage of xDSL is the low cost. Although the modems are fairly expensive, the price is still less than that of leased lines and ISDN. Also, many phone companies and ISPs are offering G.Lite service for a monthly fee with low or no installation charges. And because xDSL uses existing telephone wiring, you do not need to have new lines installed. The next most important advantage is the data transmission speed: ASDL offers 8 Mbps downstream as opposed to 128 Kbps with ISDN.

Disadvantages

The primary disadvantage of xDSL is security. Because the connection is always on, it is easier for "crackers" (unscrupulous users who break in to others' computers) to find your computer on the Internet. Those who subscribe to xDSL will need to ensure they have sufficient security in place to prevent identity theft or other mischief. Some solutions are security software, firewalls, switches, and modifying OS settings to reduce the chance of a security breach.

Another disadvantage may be availability. Because of the nature of the technology there is a limit placed on how far a subscriber can be from a major telephone line switching hub. For that reason, xDSL may not be available in your area; however, most phone companies and many ISPs are rapidly increasing the scope of xDSL availability.

Synchronous Optical Network (SONET)

SONET, also known in some countries as Synchronous Digital Hierarchy (SDH), is a WAN technology that functions at the physical layer (OSI Layer 1) of the OSI model. SONET has been accepted by ANSI and recommended by the ITU. It specifies a number of data transfer rates from 51.8 Mbps to 13.21 Gbps.

Distinguishing Characteristics

SONET defines a fiber-optic standard for high-speed digital traffic. This standard provides the flexibility to transport many digital signals with different capacities.

Data communications sometimes prove difficult because digital signaling rates can vary. For example, as stated in the above section on ISDN, in the United States a T1 line provides 1.544 Mbps, while in Europe an E1 line provides 2.048 Mbps. SONET resolves such problems by defining how switches and multiplexers coordinate communications over lines with different speeds, including defining data transfer rates and frame format.

SONET defines a number of Optical Carrier (OC) levels. Each level defines an optical signal and a corresponding electrical signal called Synchronous Transport Signal (STS). The base level is OC-1/STS-1 or 51.84 Mbps. Each level's rate is a multiple of 51.84 Mbps. The table below shows the OC levels and the corresponding data transfer rates that SONET defines.

OC Level Data Rate
OC-1 51.8 Mbps
OC-3 155.5 Mbps
OC-9 466.5 Mbps
OC-12 622.0 Mbps
OC-18 933.1 Mbps
OC-24 1.24 Gbps
OC-36 1.86 Gbps
OC-48 2.48 Gbps
OC-96 4.976 Gbps
OC-192 10 Gbps
OC-255 13.21 Gbps

SONET also provides easy access for low-speed signals such as DS-0 (64 Kbps) and DS-1 (1.544 Mbps) by assigning them to sub-STS-1 signals called "Virtual Tributaries."

Advantages

The SONET standard defines data transfer rates and a frame format that all vendors and telephone companies throughout the world can use, creating a framework for global networking. SONET also includes management capabilities for telephone company equipment. Cell relay technologies such as Switched Multimegabit Data Services (SMDS) and ATM operate above SONET, making SONET the foundation for many broadband services. And because SONET uses a ring topology, line breaks and equipment failures barely affect service.

Disadvantages

SONET is primarily a technology for voice transfer. Developments in optic fiber and data-transfer technologies such as in ultra long-haul dense wavelength division multiplexing (DWDM) may make SONET obsolete.

Wireless Packet-Switched Networks

To accommodate the demand for data transmission capabilities in mobile devices, many service providers offer access to wireless packet-switched networks. A packet-switched network does not require a continuous connection the way circuit-switched networks do (the telephone system is circuit switched). In the United States, the principal wireless packet-switched networks use one of the following technologies: DataTAC, Mobitex, Cellular Digital Packet Data (CDPD) and microcellular data network (MCDN). DataTAC was created by Motorola and IBM, Mobitex was developed in Sweden by Eritel, CDPD by the Wireless Data Forum, and MCDN by Mobitel.

Distinguishing Characteristics

DataTAC networks use one of two protocols: MDC4800 and Radio Data Link Access Procedure (RD-LAP), the former providing raw throughput rates up to 4.8 Kbps and the latter up to 19.2 Kbps (actual data rates are about half that after factoring out overhead). DataTAC networks are hierarchical networks wherein base stations connect to area communications controllers that connect to message switches. Customers' fixed-end systems connect to message switches. Depending on the area, DataTAC networks use between one and ten 25 kHz channels in the 800 MHz range. Connection to wired networks is through X.25 or TCP/IP protocols.

Mobitex, on the other hand, operates on the 400MHz or 900MHz frequency with an 8 Kbps data rate on 12.5 kHz channels. Each Mobitex packet, called an MPAK, can be no more than 512 bytes long. The Mobitex system is also hierarchical, consisting of network management centers, switches, and base stations that cover up to 30 km per cell. The standard Mobitex configuration connects via the X.25 protocol to wired networks, but IP connectivity is available as well.

CDPD differs from the other two in that it transmits over the existing analog cellular infrastructure rather than radio transmission towers. CDPD is non-proprietary, which means that you can use the same software to access any CDPD network. A faster alternative than DataTAC or Mobitex, CDPD also supports more operating systems than the other two. CDPD also uses RD-LAP and is based on TCP/IP.

MCDN, called Ricochet by Metricom, is a high-speed, "desktop quality" wireless packet-switched network designed for laptop computers. It operates at speeds of 28.8 Kbps, and a 128 Kbps implementation is now under way. MCDN uses FHSS and operates at the 900MHz, 2.3GHz, and 2.4GHz frequencies. MCDN employs microcell radio transceivers and a wired access point. The wired access point cell covers about 20 square miles, and within that cell the microcell radios are positioned in a checkerboard pattern (approximately 100 per cell), connected according to a wireless mesh topology. These radios are about as big as a shoe box and are installed on streetlight and utility poles. An MCDN user has a special modem connected to the laptop that connects with the closest microcell radio. The radio then sends the signal to the next radio and the next until it reaches the wired access point. From there, the signal is carried to the Internet or a corporate LAN via a T1 or fiber optic line. MCDN is based on IP protocols.

Advantages

Devices for all four technologies operate in an "always on" mode; that is, they do not require dedicated connections that need to be established every time they are used. Because the networks are packet switched (as opposed to circuit switched, like the conventional telephone system) the devices send data in small bursts, allowing several devices to use the same frequency at the same time. The technologies also employ store-and-forward techniques to ensure that packets are delivered even when the remote device is temporarily unavailable.

Disadvantages

Except for MCDN, the technologies are not capable of transmitting large files: video conferencing, Web browsing, file transfer, and high-speed multimedia cannot be accommodated. And although MCDN can provide these services, the network is available only in a handful of cities.

Circuit-Switched Cellular (CSC)

Circuit-Switched Cellular (CSC) technology, like CDPD, runs on the existing analog cellular infrastructure; however, it is capable of handling larger file transfers, albeit at a slower rate than CDPD.

Distinguishing Characteristics

CSC uses the established analog frequencies and infrastructure the same way a conventional modem uses land-based telephone lines: with a cellular phone and a CSC-enabled modem, you call another modem to establish a connection with it. Because many wired modems do not accept wireless protocols, many providers have set up modem pools that receive the cellular modem signals and translate them into the standard voice frequencies of the conventional modem before sending the signals to the intended modem. CSC operates at air frequencies between 824MHz and 894MHz and offers transmission rates between 14.4 Kbps and 20 Kbps.

Advantages

CSC technology has the same coverage area as standard analog cellular; in other words, it is nearly ubiquitous.

Disadvantages

Because it uses a continuous connection on the same frequencies as analog voice cellular, providers often charge the same rate for CSC as for voice. For infrequent data transfers this may not be expensive, but for lengthy Internet connections it can become so.

General Packet Radio Service (GPRS)

General Packet Radio Service (GPRS) is a connection-oriented technology that runs on top of the existing GSM and TDMA infrastructure to provide a wireless packet-switched network similar to the Internet.

Distinguishing Characteristics

GPRS has been developed as an intermediary step between second- and third-generation cellular systems. Unlike CSC, GPRS traffic does not actually go through the GSM network but only uses the GSM network to look up user profile data. It employs between one and eight channels that can be used by several devices at once and offers transmission rates between 14.4 Kbps and 115 Kbps. GPRS supports Quality of Service (QoS), and because connection setup is fast, it appears to users that their connection is always on.

Advantages

GPRS can provide most of the functionality of the current wired Internet, with services such as chat, textual and visual information, still and moving images, document sharing and collaborative work, audio, e-mail, remote LAN access, and file transfer.

Disadvantages

Voice and GPRS calls occupy the same resources, which means that if there is heavy voice traffic on the GSM or TDMA network, fewer resources will be available for GPRS links and vice versa. GPRS also does not have store-and-forward capabilities.

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