2Lesson 2: Wide Area Networks (WANs)
Objectives
By the end of this lesson, you will be able to:   Describe the characteristics and functions of wide area networks (WANs). Define basic signaling and transmission terminology, including analog and digital signals, Digital Signal Level Zero (DS0), broadband, baseband, multiplexing, and Digital Signal Hierarchy (DSH). Distinguish between circuit switching and packet switching, and identify various circuit-switching and packet-switching technologies. Describe the characteristics and performance of dial-up connections, including POTS and Integrated Services Digital Network (ISDN). Describe the characteristics and performance of direct connections, including leased T-carrier and E-carrier lines, cable Internet, Digital Subscriber Line (DSL), LAN connections, and fiber to the x (FTTx). Describe the characteristics and functions of Synchronous Optical Network (SONET). Describe the characteristics and functions of Asynchronous Transfer Mode (ATM).

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Pre-Assessment Questions
1. Which of the following is faster than T1? a. b. c. d. 2. E1 ISDN BRI DS0 DS1

Which type of transmission uses a single channel? a. b. c. d. Broadband Digital Subscriber Line (DSL) Baseband Cable modem

3.

How are digital signals measured?

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Introduction to Wide Area Networks (WANs) wide area network (WAN) A group of computers connected over an expansive geographic area so their users can share files and services.

A wide area network (WAN) consists of two or more LANs that cover a wide geographic area (e.g., a city, state or country). Consider a large business with offices in several locations worldwide. Each office has its own LAN, which it can use to share resources and data locally. If the LANs can be connected, each of the company offices can share its resources with the other offices. Figure 2-1 illustrates a WAN.

Figure 2-1: WAN example

LANs can be connected using communication lines provided by a public carrier (such as the phone company or an Internet service provider). When two or more LANs are connected using a public network, a WAN is created. The largest WAN on the planet is the Internet. The primary difference between a LAN and a WAN is the fact that a WAN involves two separate networks. Either a router or a switch is required to join these two networks. (You will learn about routers and switches later in the course.) Other features that distinguish LANs from WANs are:  A LAN is confined to local cabling that a consumer has installed within a home or that an IT department has routed through an office. In a LAN, the organization owns all the components. In a WAN, an organization usually leases some of the necessary components that are required to transmit data (such as a cable modem and cable Internet service, or high-speed telecommunications lines). LANs are also usually much faster than WANs. Most Ethernet NICs transfer data at 10 or 100 Mbps, or even at 1 Gbps or 10 Gbps in some newer installations. WAN speeds vary greatly depending on the technology used, but a typical WAN connection might be in the range of 1.5 Mbps to 6 Mbps.

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Many technologies are available to connect wide area networks. We will explore several of these technologies in this lesson.

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Networking and the public switched telephone network (PSTN) infrastructure The underlying physical structure or framework required for the operation of a service or enterprise.

The public switched telephone network (PSTN) — one of the oldest existing networks – provides telephone service around the world and is integral to wide area networking because of its infrastructure. For nearly a hundred years, telecommunications companies worldwide have been laying countless thousands of miles of telephone cable, trunk lines, switches, multiplexers and fiber optics. Public carriers lease their lines for private use to companies or individuals. These leased lines offer high-speed data transfer and guaranteed bandwidth. Although Internet service providers (ISPs) provide Internet access to their customers, this access is often made available through high-speed lines that are leased from public carriers. Transparent to the individual user, communications lines from public carriers furnish most of the long-distance connections that power the Internet.

Signaling Terminology
Before exploring the specifics of various WAN technologies, it is important to understand basic signaling and transmission terminology.

Analog and digital signals
Two types of signals are used to transmit information electronically: analog and digital. analog signal An electrical signal that varies continuously in amplitude and frequency.

Analog signals are patterns that vary continuously and form smooth waves. They are measured in cycles per second, or Hertz. Broadcast radio and television and CATV (community antenna television, or cable television) traditionally use analog signals. Figure 2-2 illustrates an analog signal.

Figure 2-2: Analog signal

Originally, all telephone service was analog. digital signal An electrical signal that varies in discrete steps.

In contrast, digital signals vary sharply between the predefined binary values 1 and 0. They are measured in bits per second (bps). Figure 2-3 illustrates a digital signal.

Figure 2-3: Digital signal

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Digitizing
Once mutually exclusive, analog and digital transmission systems are now compatible because of the ability to digitize analog signals. Digitizing can be accomplished through hardware such as modems or through signal processing software, as illustrated in Figure 2-4. LANs use both techniques.

Figure 2-4: Analog signals are easily digitized

The modern PSTN central office (CO) Telephone company building where subscriber lines (local loop) are connected to switching equipment for local and long-distance telephone transmissions.

Today, the public switched telephone network is almost entirely digital, except for the portion that extends from the central office (CO) of the local telephone company to the user (the local loop). The local loop is the analog portion of a voice circuit between the CO and the subscriber. When a signal passes from a customer's local loop, it is in analog format. Once the voice signal reaches the central office, it passes through a switch and is converted from analog to digital format. The digital data is a 64-Kbps bit stream known as Digital Signal Level Zero (DS0). The digital signal is sent across the major portion of the telephone network, and then, as necessary, digital signals are converted back into analog at the central office to which the destination telephone is linked.

Digital Signal Level Zero (DS0)
The links used on wide area networks are digital in nature (except for the analog local loop phone lines that connect users' homes and offices to the digital portion of a telecommunications network). multiplexing The process of combining several sets of signals onto a high-bandwidth transmission.

Regardless of the transmission standard in use, all levels of digital signaling are based on a 64-Kbps (64 thousand bits per second) channel known as Digital Signal Level Zero (DS0). DS0 is the basic level of digital communication upon which all other digital signaling levels are built. Transmissions at speeds faster than 64 Kbps require multiplexing, which is the process of combining several DS0 channels onto a highbandwidth line.

Broadband and baseband technologies channel A path for communication between two devices. The path may provide communication in both directions or in a single direction only.

In networking, bandwidth is the amount of information (or traffic) that can be carried on a given network connection at one time. A transmission medium's bandwidth can be divided into channels, and each channel is a portion of the total capacity available to transmit data. The terms baseband and broadband refer to signaling methods that use single and multiple channels, respectively.

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broadband A transmission method in which the media bandwidth is allocated to multiple channels and each channel carries a separate signal.

Broadband is a technology in which the media bandwidth is divided into multiple channels. Each channel carries a separate signal and operates in a frequency range distinct from the other channels on the same wire. This method enables a single transmission medium to carry several communications (e.g., voice, video, data) simultaneously over long distances without interference. At one time, broadband systems carried only analog signals. Today, broadband signals may be analog or digital. Familiar broadband technologies include cable Internet and DSL. Loosely, the term broadband is used to describe any high-speed data transmission that provides speeds of 1.544 Mbps and higher. The term is also loosely applied to any connection that is always "on."
Originally, broadband referred to a network architecture that uses analog signals for transmitting a variety of voice, data, video, etc., information in a local area network. It is based on the same technology used by cable television.

baseband LAN transmission methods using digital signals at speeds of 2 to 100 Mbps.

Baseband transmissions carry a single data stream. That is, the entire media bandwidth is used for a single channel. As with broadband, the signal can be either analog or digital. Baseband signaling is used for relatively short distances at average speeds of 10 Mbps to 100 Mbps. Most LANs, such as Ethernet networks, use digital baseband signaling.
The terms baseband and broadband are not applied consistently. A system that is described as baseband by one manufacturer could be described as broadband by another. There is a wide, but not universal, consensus that baseband should be used when there is a single information channel and broadband should be used when multiple information channels are available,

Multiplexing
As you have learned, multiplexing is the process of combining several sets of signals onto a high-bandwidth line. There are several multiplexing technologies in use today. These include:  Time division multiplexing (TDM) — used in baseband signaling. TDM interweaves multiple signals over a single transmission path. For example, three signals (X, Y and Z) can be sent as XXYYZZXXYYZZ. The receiving device separates this single stream into its original three signals. Frequency division multiplexing (FDM) — used in broadband signaling. FDM transmits multiple signals over a single transmission path by placing each signal within a unique frequency range, or carrier, as illustrated in Figure 2-5. FDM is used in cable modems and digital subscriber lines (DSL).

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Figure 2-5: Signals in different frequency bands

Digital Signal Hierarchy (DSH)
In North America, the Digital Signal Hierarchy (DSH) is used to classify the speed capacities of multiplexed lines. Each level in the hierarchy is formed by multiplexing lower-level lines to create higher-level ones. For example, a DS1 contains 24 (64-Kbps) DS0 channels. A DS1 transfers data at 1.544 Mbps. Table 2-1 describes the various DS levels.

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Table 2-1: Digital Signal Hierarchy (DSH)

Digital Signal Level DS0 DS1 DS2 DS3 DS4

Data Rate
64 Kbps 1.544 Mbps 6.312 Mbps 44.736 Mbps 274.176 Mbps

Equivalent to

24 DS0 channels 96 DS0 channels 672 DS0 channels 4,032 DS0 channels

Circuit Switching and Packet Switching
Early WAN standards relied on circuit-switching technologies. Today, however, most WAN protocols rely on packet-switching technologies, as opposed to circuit-switching technology. circuit-switching A technology that uses a dedicated physical path to send and receive information.

With circuit switching, a circuit is established and all information travels on the same path. In a circuit-switched network, a connection must be established before data can be transferred. A telephone call is a readily understood example of circuit switching. When you make a telephone call, you dial the number of the intended call recipient. When the call recipient answers the phone by picking up the receiver, a connection is established and a circuit is opened. The circuit remains open for the duration of the call, and as long as the circuit remains open, no one else can use the telephone line. When the call is ended (i.e., one or both parties hang up), the circuit is disconnected. In a circuit-switched WAN connection, after a circuit has been established, data traffic is transmitted and received between source and destination systems. For as long as the circuit is open, no one else can use the line. When the transfer of data is complete, just as when a phone call is finished, the circuit is disconnected and the network resources servicing the connection are released. Circuit-switched WAN technologies include:     Plain Old Telephone Service (POTS). T1/E1. T3/E3. ISDN.

packet-switching A technology that places addressing information into data packets, instead of relying upon a dedicated physical path.

With packet switching, data is divided into units called packets, which include addressing information. These packets are routed through the network based on their address information. Each packet being sent from Point A to Point B may travel a different path, depending on the traffic and conditions on the network. Packet assemblers/disassemblers (PADs) are needed at both the sending end (Point A) and the receiving end (Point B). At Point A, a PAD divides data into standard packets for transmission, and at Point B, a PAD reassembles the data packets. There is no guarantee in a packet-switched network that packets will arrive at the receiving end in the original sequence they were sent or that they will arrive at all. The PAD must put them back in order and request a retransmission of any missing packets.

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virtual circuit A path between hosts that appears to be a discrete dedicated connection when in fact it is not. Describes connections between hosts in a packet-switching network.

Packet-switching technologies rely on virtual circuits. A virtual circuit is a path between points in a network that appears to be a discrete, dedicated connection but is actually a managed pool of circuit resources from which specific circuits are allocated as needed to transfer traffic. In other words, although the packets may travel several different routes to get from source to destination, the source and destination hosts can communicate as if they had a dedicated connection in place. Virtual circuits consume bandwidth only when they transport data, and several virtual circuits can exist simultaneously over a given transmission line. Packet-switched WAN technologies include:     Digital Subscriber Line (DSL). Cable modem. SONET. ATM.

You will examine both circuit-switched and packet-switched WAN technologies in the remainder of this lesson.

WAN Connections
Consumers and enterprises have several choices for connecting to wide area network. WANs have commonly been connected using dedicated hard-wired or switched data lines. WANs can also be connected using an analog option over the public telephone network. It is becoming more and more common, however, to use high-speed data lines to connect to the Internet and let the Internet act as the carrier backbone for a WAN. Figure 2-6 illustrates WAN connections.

Figure 2-6: WANs use many different connection methods
Internet Service Provider (ISP) An organization that provides access to the Internet and usually e-mail as well.

Connectivity is generally purchased through a telecommunications company or an Internet Service Provider (ISP). Some providers offer dial-up connection, and most offer direct connection through DSL, cable or wireless connection such as satellite. (You will learn about these connections shortly.) ISPs provide an entry point, or portal, onto the Internet. An individual user or an enterprise connects to its ISP. The ISP in turn is either connected directly to the Internet backbone or is connected to a public carrier, which in turn is connected to the Internet backbone. Providers offer two principal options for connecting to the Internet or WAN:   Dial-up connection Direct connection

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Dial-Up Connections
The first type of Internet connection available to consumers was the dial-up connection. Such connections are very slow, outdated and rarely used anymore. However, some users still use dial-up because it is the least expensive method of obtaining Internet access. In a dial-up connection, you establish a connection over the phone line, conduct the data transfer, and then disconnect.

POTS
Although the PSTN is almost entirely digital in nature, the connection to users’ homes and offices is usually an analog line providing what is known as plain old telephone service (POTS). A POTS line operates at 64 Kbps. modem An analog device that enables computers to communicate over telephone lines by translating digital data into audio/ analog signals (on the sending computer) and then back into digital form (on the receiving computer).

A dial-up connection on a POTS line uses a modem (modulator/demodulator), which is a device that enables computers to transmit data over standard analog telephone lines, as illustrated in Figure 2-7. A modem converts (modulates) digital data from a computer into an analog signal to be transmitted through the telephone line to another modem. The receiving modem converts (demodulates) the analog signal back into digital code and transmits it to the receiving computer.

Figure 2-7: WAN connection over PSTN using modems

You gain access to the Internet by dialing a phone number that connects your computer with an ISP, which in turn connects your computer to the Internet. When finished, you disconnect from the ISP. The speed with which you can access the Internet is determined primarily by the speed capability of the modem. However, the maximum possible speed for data transfer over a standard analog telephone line (allowing time for modulation and demodulation) is 56 Kbps.
The term modem is widely used, but does not always assume an analog-todigital translation. For instance, DSL and ISDN modems (presented later in this lesson) are used on all-digital networks — no translation to analog is required. In this sense, DSL and ISDN modems are not actually modems. In many cases, the term modem has been used to describe any device that adapts a computer to a phone line or cable TV network, whether it is digital or analog. The term analog modem is often used to indicate a traditional modem.

Integrated Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) An international standard that defines the transmission of data, voice and video over digital telephone lines at 64 Kbps. An ISDN line consists of several 64-Kbps channels, which can be combined for faster speeds.

To gain faster dial-up connections, you can go through a local phone company to install an Integrated Services Digital Network (ISDN) line, which is a digital telephone line. ISDN is a completely digital service capable of carrying voice, fax, imaging or data communications (hence the term "integrated services"). ISDN (with Basic Rate Interface or BRI) turns a single twisted-pair (copper wire) phone line into multiple lines that can be used for voice, data, fax or video transmission at much higher data rates. ISDN uses 64-Kbps channels for voice and data, and a 16-Kbps channel for signaling and call control. The 64-Kbps channels are called bearer channels or "B" channels. The 16Kbps channel is called the delta channel or "D" channel.

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Although an ISDN modem is required to use ISDN services, the “modem” is technically a digital terminal adapter that attaches to the computer and prepares the computer data for transmission over the ISDN line. ISDN in various configurations has been available throughout most of the world. Available configurations include:  2B+D — also known as Basic Rate Interface or BRI. Intended for home use and can use standard telephone line. Uses two 64-Kbps B channels and one 16-Kbps D channel. BRI service provides data transfer at about 128 Kbps. 23B+1D — known as Primary Rate Interface or PRI in the United States and Japan. Intended for business use. Designed for 23 B channels plus one D channel. The data transfer rate is 1.544 Mbps. 30B+2D —known as Primary Rate Interface or PRI in Europe. Intended for business use. Designed for 30 B channels and two D channels. Channel 16 is used for signaling and Channel 0 is used for synchronization. The data transfer rate is 2.048 Mbps.

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Today, ISDN has been largely superseded by cable and DSL services.

Direct Connections
In contrast to dial-up connections, which require that a connection be established for each usage, direct connections provide continuous access to the Internet or wide area network through permanent network connections. That is, direct connections are always active. Direct connections are more desirable than dial-up connections because they are generally capable of handling high bandwidth. Direct connections can be obtained in various ways, including leased T-carrier and E-carrier lines, cable Internet service, digital subscriber lines and LAN connections. If your company's network is connected to the Internet, or you have a cable or DSL modem installed, then your computer has a direct connection to the Internet.

Leased lines
A leased line is a permanent connection between two or more locations that an individual or enterprise can lease from a telecommunications company or service provider. When you lease a line, you do not share it with other consumers; it is private and available exclusively to you. Therefore, leased lines offer both guaranteed bandwidth and a way to create a secure wide area network. Typically, leased lines are used by businesses to connect offices that are geographically far apart. Leased lines are also commonly used by companies for Internet access because they offer high bandwidth and can be cost-effective for heavy Internet traffic, depending on the number of users and the bandwidth requirements. Leased lines are reliable and secure, but they are expensive. Competing technologies such as DSL and cable are more cost-effective for small businesses. Leased lines include T1 and T3 in North America and Japan, and E1 and E3 in Europe and almost all countries outside the United States.

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T-carrier
T-carrier system A North American high-speed digital carrier system used to transmit data.

The T-carrier system describes leased lines available in North America and Japan. The T-carriers available to the public are:   T1 — consists of 24 channels for voice or data, and can carry digital signals over twisted-pair cable at 1.544 Mbps. T3 — consists of 672 channels for voice or data, and can transmit data at 44.736 Mbps over optical fiber or coaxial cable.

The T-carrier system is based on multiples of DS0 (64 Kbps). The basic unit is T1, which is equivalent to 24 DS0 lines. The DSH and T-carrier designations are equivalent, as shown in Table 2-2.
Table 2-2: North American Digital Signal Hierarchy (DSH)/T-carrier

DSH DS0 DS1 DS2 DS3 DS4

T-Carrier
N/A T1 T2 T3 T4

Data Rate
64 Kbps 1.544 Mbps 6.312 Mbps 44.736 Mbps 274.176 Mbps

Comments

Equivalent to four T1 channels; not available to the public Equivalent to 28 T1 channels Equivalent to 168 T1 channels

Many WAN connections use a T1 or T3 circuit. Figure 2-8 illustrates T1 links used across expansive geographic areas.

Figure 2-8: International WAN using T1 links

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T1 technology was first developed by the telephone companies to carry multiple digitally encoded voice signals throughout the network. T1 circuits were divided into 24 channels of 64 kbps each (leaving 8 kbps for signaling and network management). These channels are equivalent to DS0 channels.

T1 technology is used for many different kinds of information transport, including WANs. The T1 (or T3) circuit can be divided in several ways. For example, a T1 connecting two networks may be a single 1.536-Mbps channel (1.544 Mbps less the 8 kbps dedicated to signaling and network management), as shown in Figure 2-9.

Figure 2-9: T1 connection using single channel

Other WANs may divide the circuit depending on the kind of signal they carry (e.g., voice, data, video or other digital information), as shown in Figure 2-10.

Figure 2-10: T1 using separate channels for voice, video and data

Many businesses lease T1 lines from phone companies to connect to ISPs. The ISPs use T3 lines to connect to the Internet backbone. A T1 line can cost several hundred dollars per month. A T3 line can cost several thousand dollars per month. To connect a T1 line to a corporate LAN, the following equipment is required:  Channel Service Unit (CSU) — a device used between a public carrier's WAN and the customer's equipment. The CSU is the first point of contact for the T1 wires; it prepares the signals on the line for use on the LAN. Data Service Unit (DSU) — a device used between a customer's equipment and a public carrier's WAN. The DSU connects to the CSU and converts LAN signals to T1 signaling formats. Multiplexer (mux) — provides a mechanism to load multiple voice and data channels onto the digital line. Router — provides the interface between the LAN and the T1 line.

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Fractional T1 / T3
The ability to divide a T1 line into channels has important benefits for users. Instead of having to lease an entire T1, users can lease part of a T1 line, in multiples of 64-Kbps DS0 channels. This service is called fractional T1. Figure 2-11 illustrates the difference in bandwidth for fractional T1, T1 and T3.

Figure 2-11: Comparison of bandwidth for FT1, T1 and T3

Statistical multiplexing statistical multiplexing (stat mux) A method of sharing a transmission channel by using statistical techniques to allocate bandwidth.

As new uses for T1 technology evolved, statistical multiplexing (stat mux) was developed. Instead of giving equal bandwidth to each channel, stat mux is demanddriven. For example, if a company’s voice channel is busier than its data channel at a particular time, the statistical multiplexer would devote more bandwidth to carrying voice. A statistical multiplexer can analyze traffic density and automatically switch to a different channel pattern to speed up transmission. Data flow is prioritized based on demand rather than giving all users equal time (as with standard TDM). See Figure 2-12.

Figure 2-12: Statistical multiplexing conserves bandwidth

Statistical multiplexing is more cost-effective than standard TDM, and can respond to the changing needs of the network. Statistical multiplexing has numerous applications for users of voice/data transmission services:  Businesses that transmit mostly during certain times of the day would get priority at those peak times.

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Businesses that have alternate high and low voice and data transmission periods could use a single T1 line. Small businesses can save money and maximize their available bandwidth by using a cost-effective service that responds to their changing voice and data transmission needs.

We will look at a simple example. Telemarketing, Inc., transmits most of its voice traffic during the day, performing phone sales and taking orders. After business hours, in the evening, it uses data transmission to process the day’s merchandise orders. With fractional T1 service, the company would need to pay for two T1 DS0 channels: one for voice traffic and one for data, as illustrated in Figures 2-13 and 2-14.

Figure 2-13: FT1 requires two T1 DS0 channels, one for voice and one for data

Figure 2-14: Voice line used during day, data line used at night

With a statistical multiplexer, one line can be switched to handle voice traffic during the day and data traffic in the evening, as illustrated in Figure 2-15. The company benefits by leasing only one fractional T1 DS0 channel instead of two.

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Figure 2-15: Voice and data use same channel, based on demand

E-carrier
E-carrier system A European highspeed digital carrier system used to transmit data in almost all countries outside the United States, Canada and Japan.

The E-carrier system describes leased lines available in Europe (and almost all countries outside the United States, Canada and Japan). The E-carriers available to the public are:   E1 — consists of 30 channels for voice or data, and can transmit data at 2.048 Mbps E3 — consists of 480 channels for voice or data, and can transmit data at 34.368 Mbps.

An E1 circuit transmits data at a rate of 2.048 Mbps. It can carry 32 channels of 64 Kbps each over two pairs of wires, usually coaxial cable. Of the 32 channels, 30 are available for voice (Channel 0 is reserved for synchronization and Channel 16 is reserved for signaling). Each transmission speed is a multiple of the E1 format. E2 is a line that carries four multiplexed E1 signals, and E3 carries 16 E1 signals. An E4 line carries four E3 channels, and an E5 carries four E4 channels. Table 2-3 describes the five E-carriers.
Table 2-3: E-carrier speeds

Level E1 E2 E3 E4 E5

Data Rate (Mbps)
2.048 8.448 34.368 139.264 565.148

Voice Channels
30 120 480 1,920 7,680

Comments

Equivalent to four E1 circuits Equivalent to 16 E1 circuits Equivalent to 64 E1 circuits Equivalent to 256 E1 circuits

As is the case with T lines, fractional E lines provide less bandwidth than their undivided counterparts and are much less expensive to lease.

Cable Internet
Cable Internet service is a broadband technology. Instead of copper phone lines, a cable TV (CATV) system uses coaxial (“co-ax”) cables to transmit signals. Coax cables can transmit data and streaming audio and video images at much faster speeds than are possible using standard telephone lines. You can connect to the Internet through your cable TV system by attaching a cable modem to the computer's NIC and to a cable television outlet. By so doing, the computer becomes connected to the cable TV system's Internet server, which is in turn connected to the Internet backbone. Cable Internet essentially uses your television as the monitor by connecting your TV to the Internet with an enhanced set-top cable box.

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Figure 2-16 shows an example of a cable modem. Although you cannot see the ports, the modem is connected to the cable service through a standard cable TV coaxial cable.

Figure 2-16: Cable modem

The cable modem is an adapter that provides a connection point between the computer and the cable service. When used in a home setting, it is often referred to as a residential gateway. Outside the customer’s premise, the cable modem connects to the provider’s cable modem termination system (CMTS). The CMTS routes the traffic coming from a group of cable subscribers to an ISP for connection to the Internet.

Cable modem limitations
Cable providers generally route a coax loop through a neighborhood to support the subscribers in the general vicinity. Because cable modem users share this network loop, they share the available bandwidth, and performance slows down as new users come online. For example, some cable customers perceive a significant slowdown in access during peak hours (for example, between 5 and 8 p.m.). There are two speed measurements for broadband Internet service:   Downstream (download) — Data moves downstream as it reaches you from another location (for example, when a Web server sends a Web page that you have requested). Upstream (upload) — Data moves upstream when you send or upload information (for example, when you enter a URL in the browser’s address bar or submit an online form).

Providers of high-speed Internet service assume that most home and small-business users download far more than they upload. Usually, uploading is used only to establish and maintain connections. Larger businesses may need to upload files more often, which should be a consideration when considering cable Internet service for an enterprise. Cable modem technology uses a standard called Data Over Cable Service Interface Specification (DOCSIS). Depending on the provider and how much a customer is willing to pay, cable can theoretically provide speeds up to 100 Mbps. However, it much more common for providers to offer services between 1 to 6 Mbps downstream, and 128 to 768 Kbps upstream. Actual speeds vary widely.

Advantages over leased lines
Average cable Internet speed surpasses T1, and for home-based or small businesses that do not require exclusive guaranteed bandwidth, cable is a connection alternative worth considering. Cable Internet is less expensive that leasing a T1 and it requires less equipment: simply a cable modem and a router, or a modem/router combination.

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Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) A high-speed direct Internet connection that uses all-digital networks. xDSL Collectively, the variations of Digital Subscriber Line (DSL), which include ADSL, RADSL and HDSL.

Another direct connection method is Digital Subscriber Line (DSL), a high-speed connection that uses digital phone lines and an xDSL modem. Several variations of the DSL modem exist, hence the term xDSL. Some of the more popular versions include the following:     Asymmetric Digital Subscriber Line (ADSL) — used primarily in homes and small businesses. Rate-Adaptive DSL (RADSL) — a version of ADSL that allows the transmission rate to vary depending on your phone line capabilities. Very-High-Bit-Rate DSL (VDSL) — can support high-definition television, digital telephone and high-speed Internet access. High-bit-rate Digital Subscriber Line (HDSL) — the original version of DSL. It was used for wideband digital transmissions between the phone company and customers, as well as within large corporations. It provided 1.544 Mbps both upstream and downstream in North American, and 2.048 Mbps upstream and downstream in Europe.

You must check whether your local phone company supports xDSL. If so, its technicians may need to visit your location to install xDSL, depending on the type it offers. You will also need an xDSL modem and possibly a NIC to attach your computer to the xDSL modem. xDSL competes directly with cable modems. The xDSL modem connects to the digital telephone circuit using a standard telephone cable. The modem also includes an Ethernet port for connecting to your computer. Some models also include a USB port as an alternative method of connecting to a computer. Figure 2-17 shows a typical ADSL modem.

Figure 2-17: ADSL modem

DSL service uses a dedicated connection to the provider’s DSL Access Multiplexer (DSLAM), which combines the signals from multiple subscribers onto a single highcapacity high-speed connection to the Internet. Because of this dedicated connection, DSL subscribers do not experience a slowdown in network response time when more users connect to the network (as is the case with cable Internet subscribers).

Limited availability
Your distance from the provider’s central office determines whether you can receive DSL service. If your home or office is too far away from the central office, DSL service will not be available to you. The limit distance for ADSL service is 18,000 feet (5,460 meters). You must check with a service provider to find out if DSL is available in your area.

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DSL speeds
Different speeds are available with DSL service, depending upon the type of service you use and how far you are from the provider’s central office. As distance increases, signal quality decreases and connection speed decreases as well. The quality of the phone lines can also affect signal quality and speed. Table 2-4 summarizes xDSL speeds.
Table 2-4: xDSL speeds

Type ADSL ADSL2 ADSL2+ VDSL VDSL2

Max Downstream
8 Mbps 12 Mbps 24 Mbps 55 Mbps 100 Mbps

Max Upstream
640 Kbps 1 Mbps 3 Mbps 4 Mbps 100 Mbps

Symmetric DSL (SDSL) service is also available. With SDSL, the upstream and downstream rates are the same, and this version is used mainly by small businesses. SDSL service does not allow you to use the phone at the same time, however.

Advantages
Where it is available, DSL is a less expensive connection method than a leased line, and may be suitable for home-based or small businesses. It also requires less equipment than a leased line: simply a DSL modem and a router, or a modem/router combination

Connecting cable and DSL modems
When connecting a home office or small business to the Internet using a cable or DSL modem, you will need the following equipment:  A modem — This modem is actually a router that connects your home computer or network to the Internet. The ISP offering digital access to the Internet often supplies this modem. You can purchase or rent the modem from the ISP. You can also purchase your own router at a store. However, you should make sure the router is compatible with your ISP's network. Network cables — If you are using a DSL modem, you will use two standard Ethernet cables (for example, Category 5, 5e or 6 cables) with RJ-45 connectors. An RJ-45 connector is similar to a standard North American telephone cable connector (an RJ-11), but the RJ-45 is larger. You will attach one cable from the wall of your home to the modem, and attach another cable from the modem to your computer. If you are using a cable modem, you will attach a coaxial cable from the wall to the modem, then connect a computer to the modem using a standard networking cable (e.g., a Category 5 cable).

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It is also possible to obtain equipment that combines wireless connections with modems that are compatible with your ISP. In these cases, you must also equip each of your computers with a wireless NIC and connect your wireless access point to the modem. Figure 2-18 illustrates a broadband connection used in a home.

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Figure 2-18: Residential broadband connection

Fiber to the x (FTTx) fiber to the x (FTTx) Generic term for a family of broadband network configurations that use fiber-optic cable for last-mile telecommunication. Includes FTTN (fiber to the node), FTTC (curb), FTTB (building), FTTH (home), FTTP (premises) and FTTD (desk).

Fiber to the x (FTTx) is a generic term for a broadband network that uses optical fiber to replace all or part of the usual metal local loop used for last-mile telecommunications (the connection from the telecommunications switch to the customer premises). There are several configurations of fiber-optic deployment:  FTTN (fiber to the node/fiber to the neighborhood) — Fiber is terminated in a street cabinet up to several kilometers away from the customer premises. Customers connect to the cabinet using traditional coaxial cable or twisted-pair wiring. The area served by the cabinet is usually less than 1,500 meters in radius and can contain several hundred customers. (If the cabinet serves an area of less than 300 meters in radius then the architecture is typically called fiber to the curb.) DSL or cable access services are used between the cabinet and the customers. FTTN is used to deliver triple-play (Internet, television, telephone) services. FTTC (fiber to the curb/fiber to the cabinet) — Similar to FTTN, but the street cabinet is closer to the customer premises (usually within 300m), within range for both wired Ethernet and WiFi. Usually existing wire in the local loop is used, but where it is feasible to run new cable, both fiber-optic and twisted-pair can be used to provide a 100-Mbps or 1-Gbps connection. FTTC also supports Powerline (HomePlug) networking, so existing AC power lines can also be used to provide the last mile of service. FTTB (fiber to the building/ fiber to the basement) — Fiber is terminated at the boundary of the building, such as at the basement in a multiple-dwelling unit or the electrical room in a building. The final connection to the individual living or office spaces can be made with twisted-pair or coaxial cable, wireless, or power line communication. FTTH (fiber to the home) — Fiber reaches the boundary of the living or office space, such as a box on the outside wall of a home or a panel at each subscriber's apartment or office suite. Once at the subscriber's living or working space, the signal may be conveyed throughout the space using any means, including twisted-pair or coaxial cable, wireless, power line communication, or optical fiber. FTTP (fiber to the premises) — A blanket term for both FTTH and FTTB, or where the fiber network includes both homes and small businesses.

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FTTD (fiber to the desk) — Fiber connection is installed from the main computer room to a terminal or fiber media converter near the user's desk.

Benefits of FTTx
The speed of copper cable is limited by length. For example, Gigabit Ethernet runs over category 5e, category 6 or augmented category 6 unshielded twisted-pair copper cabling but only for a distance of 100 meters. If fiber is used instead, Gigabit Ethernet can easily reach tens of kilometers. Many WAN links based on telephone or cable television services are several kilometers long. Replacing part of these links with fiber shortens the remaining copper segments and allows them to run much faster. Configurations that bring fiber right into the building offer the highest speeds.

LAN connections
A LAN can be connected to the Internet by a router, provided that the LAN uses TCP/IP. The LAN router is connected to another router at the ISP by a high-speed data link. A high-speed data link can be fiber-optic cable or wireless media (such as radio or microwave signals) that are provided by telephone companies, cable TV services and other suppliers. The line between the LAN and the ISP is leased monthly, and can cost from U.S.$250 or less per month for a small-business connection, to $40,000 or more per month for a large corporation, depending on the speed and the amount of traffic. If LANs do not use TCP/IP, they can be connected to the Internet by gateways, which convert the protocol used by a LAN to and from TCP/IP.

Synchronous Optical Network (SONET)
Synchronous Optical Network (SONET) North America highspeed fiber-optic system used as a network and Internet backbone. Similar to European Synchronous Digital Hierarchy (SDH).

Synchronous Optical Network (SONET) is a North American standard for optical transmission. SONET lines are high-speed, fiber-optic networks usually organized in rings (although SONET can be deployed in single strand linear architectures as well). The similar technology used in Europe is Synchronous Digital Hierarchy (SDH), which will be discussed shortly. The typical SONET ring is often described as a self-healing network technology. Normally, one strand of fiber (the working ring) handles all data traffic, while a second strand of fiber (the protection ring) remains on standby. If the working ring fails, SONET includes the capability to automatically detect the failure and transfer control to the protection ring. SONET commonly transmits data at speeds between 155 Mbps and 2.5 Gbps. To create these high-bandwidth data streams, SONET multiplexes together channels having bandwidth as low as 64 Kbps into data frames sent at fixed intervals. SONET defines how data is transported in a framed structure over optical fiber at or above DS3 speeds (45 Mbps). The information field of a SONET frame is called the synchronous payload envelope (SPE). The SPE is surrounded by SONET frame overhead to create a complete SONET frame, called a Synchronous Transport Signal (STS) frame, as illustrated in Figure 2-19.

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Figure 2-19: Synchronous Payload Envelope (SPE) surrounded by STS frame

SONET speeds are defined by the American National Standards Institute (ANSI). The basic unit of transmission for a SONET signal is the Synchronous Transport Signal (STS)-1 frame, which travels at 51.84 Mbps. SONET speed can be measured by STS designation (which refers to the frame format) or by the corresponding Optical Carrier (OC) levels, as shown in Table 2-5.
Table 2-5: SONET speeds

Synchronous Transport Signal (STS) Frame Format
STS-1 STS-2 STS-3 STS-9 STS-12 STS-18 STS-24 STS-36 STS-48 STS-96 STS-192

Optical Carrier (OC) Level
OC-1 OC-2 OC-3 OC-9 OC-12 OC-18 OC-24 OC-36 OC-48 OC-96 OC-192

Speed (in Mbps)

51.84 103.68 155.52 466.56 622.08 933.12 1244.16 1866.24 2488.32 4976.64 9953.28

Benefits of using SONET/SDH
The SONET standard allows many different types of signals to be mapped into a SONET frame. It is possible to use multiplexers and routers to combine different data lines and streams onto one line, as illustrated in Figure 2-20.
T1 Line E3 Line DS4 Line Multiplexer

SDH Line

Figure 2-20: SONET/SDH and multiplexing different data streams

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add/drop multiplexer (ADM) A multiplexer capable of extracting or inserting lower-rate signals from a higher-rate multiplexed signal without completely demultiplexing the signal.

SONET add/drop multiplexers (ADMs) provide the capability to insert or extract part of the transmission payload without the overhead of completely demultiplexing and remultiplexing the entire signal. Other advantages of using SONET include the following:    It allows multiple technologies and vendor products to interoperate by defining interoperability standards. It can transport other protocols, including Asynchronous Transfer Mode (ATM) and Internet Protocol (IP). It includes built-in support for ease of management and maintenance.

SONET and Synchronous Digital Hierarchy (SDH)
Synchronous Digital Hierarchy (SDH) International highspeed fiber-optic system used as a network and Internet backbone. Similar to North American SONET.

Although SONET is a North American standard, it is closely compatible with the equivalent international standard, Synchronous Digital Hierarchy (SDH). The Synchronous Digital Hierarchy (SDH) standard is the international standard for fiber-ring networks. The basic unit for SDH is the Synchronous Transport Module (STM)-1 frame, which travels at 155.52 Mbps (three times the speed of an STS-1 frame). Table 2-6 compares SDH levels with SONET speeds.
Table 2-6: Comparison of SDH and SONET

Synchronous Digital Hierarchy (SDH) Level STM-0 STM-1 STM-2 STM-3 STM-4 STM-8 STM-16 STM-32 STM-64 STM-256
Asynchronous Transfer Mode (ATM) A scalable, connectionoriented network technology used for LANs and WANs that supports real-time video and voice as well as data. ATM organizes data into 53-byte fixed-length cells.

SONET

Speed (Mbps)

STS-1 (OC-1) STS-3 (OC-3) STS-6 (OC-6) STS-9 (OC-9) STS-12 (OC-12) STS-24 (OC-24) STS-48 (OC-48) STS-96 (OC-96) STS-192 (OC-192) STS-768 (OC-768)

51.84 155.52 311.04 466.56 622.08 1244.16 2488.32 4976.64 9953.28 39,813.12

Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) is a packet-switching technology that transmits data in uniformly sized frames called cells. Other packet-switching technologies, such as Ethernet, use variable-sized packets. Each and every ATM cell is a fixed 53 bytes long. As illustrated in Figure 2-21, 48 bytes are used for data, and five bytes are used for the header, which supports control functions.

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Figure 2-21: 53-byte ATM cell

The “one size fits all” approach, coupled with the small cell size, ensures that delays will be uniform and small. The use of a fixed frame size helps to control delay because it is much faster to process a known packet size than it is to figure out the start and end of variable packet lengths. connectionoriented A network service in which there is an end-to-end communication path between the sending and receiving devices.

ATM is primarily a connection-oriented technique that can transport both connectionoriented services (such as real-time voice and video) and connectionless services (such as IP). A connection-oriented service is one in which there is an end-to-end communication path between two communicating devices. This path is established in the setup phase of the communication and released in the teardown phase of the communication. In contrast, a connectionless service is one in which each packet entering the network is treated independently, even if multiple packets are part of the same communication. Because there is no setup or teardown to manage an end-to-end path, each packet must contain all the addressing and control information required to route it to its destination. ATM also provides a good compromise between the requirements of constant bit rate (CBR) and variable bit rate (VBR) applications. Voice and video communications are examples of constant bit rate applications. They require a continual stream of data and are highly intolerant of delay because delay negatively impacts the quality of a voice or video stream. Transferring a file across a LAN or WAN is an example of a variable bit rate application. The rate at which the packets are sent and received does not impact the quality of the file. Figure 2-22 illustrates how ATM can accommodate both types of applications.

constant bit rate (CBR) Communications that require a continual stream of information and are intolerant of delay. variable bit rate (VBR) Communications that can tolerate delays in the delivery of information.

Figure 2-22: Video and voice require constant data rate

ATM is highly scalable and supports transmission speeds ranging from 1.5 Mbps to 9.93 Gbps. It is widely used as a backbone technology in carrier networks and ISPs because of its quality of service. ATM is also used on sections of the Internet backbone where ISPs interconnect with one another.

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ATM converts its cells into SDH/SONET frames or T-carrier frames to be transported over those carriers. ATM offers several advantages:       ATM’s packet-switching ties up resources only as needed. ATM provides bandwidth-on-demand (bandwidth can be changed within given communications). ATM can redirect extra bandwidth to users who need it. ATM can handle any mix of voice, data, and video or image traffic. ATM scales from Mbps transmission for desktop users to Gbps for public network switches. ATM guarantees Quality of Service (QoS), allowing voice and video to be transmitted smoothly.

Lesson Summary
Application project
Suppose that you are about to set up a home office for yourself and another user. At the very least, you will each require Internet access and e-mail. You will likely be transferring files to and from various customers as part of your business tasks, and will probably participate in online training from time to time to keep current on your job skills. What factors should you consider when determining the type of Internet access to purchase?

Skills review
In order for LANs to communicate with other LANs across long distances, WAN links are required. These links are provided by public carriers using various WAN technologies. Following are the key points presented in this lesson:  The primary difference between a LAN and a WAN is the transport distance and ownership of the networking equipment. The technology used for transport within LANs differs from that used in WANs, which results in a difference in data transfer speeds. The infrastructure of the public switched telephone network (PSTN) is used for WAN links and for most of the long-distance connections that power the Internet. Analog and digital signals have very different waveforms. Today, analog signals are easily digitized for transport across digital networks. All levels of digital signaling are based on a 64-Kbps channel known as Digital Signal Level Zero (DS0). Although most of the PSTN is digital, the local loop (the portion that extends from the CO to the customer premises) is still analog in many regions. Broadband is a technology that divides the available media bandwidth into channels of different frequencies, enabling a single wire to carry multiple communications

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simultaneously. Frequency division multiplexing (FDM) is used to place multiple communications on a broadband wire.  Baseband is a technology that allocates the entire media bandwidth to a single communication channel. Time division multiplexing (TDM) is used to place multiple communications on a baseband wire. The Digital Signal Hierarchy (DSH) is a North American hierarchy used to classify the speed capacity of multiplexed lines. The DSH is based on multiples of DS0. Circuit switching is a technology that uses a dedicated physical path or connection to send and receive information. Plain old telephone service (POTS), T1, E1, T3, E3 and Integrated Services Digital Network (ISDN) use circuit switching. Packet switching is a technology that divides data into packets and places addressing information into each packet, thus enabling it to be routed across a network. No dedicated physical path is involved in packet switching. Digital Subscriber Line (DSL), cable modem, Synchronous Optical Network (SONET) and Asynchronous Transfer Mode (ATM) use packet switching. Although WANs have commonly been connected using hard-wired or switched data lines, it is becoming more common to the use the Internet as a carrier backbone for a WAN. Dial-up connections on a POTS line require the use of an analog modem. Dial-up connections on an ISDN line require an ISDN modem, which is a terminal adapter that attaches to the computer and prepares the computer data for transmission over the ISDN line. ISDN is available as Basic Rate Interface (BRI) at 128 Kbps for home use over standard telephone wire, or as Primary Rate Interface (PRI) at 1.544 Mbps in North America over a T1 line, and at 2.048 Mbps in Europe over an E1 line. Leased lines guarantee bandwidth, availability and secure connections. In North America and Japan, the T-carrier system is used. In Europe and other parts of the world, the E-carrier system is used. A T1 offers speeds of 1.544 Mbps over twistedpair wire, and an E1 offers speeds of 2.048 Mbps over coaxial cable. A T3 offers speeds of 44.736 Mbps, and an E3 offers speeds of 34.368 Mbps, both over either optical fiber or coaxial cable. Special equipment (CSU/DSU) is required to connect a LAN to a T1/E1 or T3/E3 circuit, and multiplexers are required to load multiple channels onto the T-carrier or E-carrier line. Separate channels with a T1/E1 can be leased as opposed to leasing the entire circuit. This is called fractional T1 or E1. Statistical multiplexing is demand-driven rather than giving equal bandwidth to each channel. With a statistical multiplexer, one line can be switched to handle voice traffic during the day and data traffic during the evening. Statistical multiplexers allow businesses to save money and maximize their available bandwidth. Cable Internet allows users to connect to the Internet through a cable TV system. Its speed surpasses that of T1/E1, and it requires only simple equipment. Digital Subscriber Line (DSL) allows users to connect to the Internet using a highspeed digital phone line and a DSL modem. Its speed surpasses that of T1/E1 and it does not require specialized equipment other than the modem. A building's distance from the CO determines whether DSL service can be provided.

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High-speed fiber-optic or wireless data links can be leased from a service provider to provide guaranteed high-speed access. High-speed data links are very expensive. Broadband fiber-optic lines are being brought closer and closer to consumer dwellings and office spaces in various configurations, including fiber to the neighborhood (FTTN), fiber to the building (FTTB) and fiber to the desk (FTTD). The closer the fiber comes to the actual LAN, the faster the connection because more of the link is carried over fiber than over copper wire. SONET defines how data is transported in a framed structure over optical fiber at or above DS3 speeds (45 Mbps). The SONET standard allows many different types of signals to be mapped into a SONET frame. ATM is a packet-switching technology that replaces variable length packets with fixed length 53-byte cells. It is primarily a connection-oriented technique that can transport both connection-oriented and connectionless services. ATM is also a good compromise between the requirements of constant bit rate and variable bit rate applications.

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Now that you have completed this lesson, you should be able to:   Describe the characteristics and functions of wide area networks (WANs). Define basic signaling and transmission terminology, including analog and digital signals, Digital Signal Level Zero (DS0), broadband, baseband, multiplexing, and Digital Signal Hierarchy (DSH). Distinguish between circuit switching and packet switching, and identify various circuit-switching and packet-switching technologies. Describe the characteristics and performance of dial-up connections, including POTS and Integrated Services Digital Network (ISDN). Describe the characteristics and performance of direct connections, including leased T-carrier and E-carrier lines, cable Internet, Digital Subscriber Line (DSL), LAN connections, and fiber to the x (FTTx). Describe the characteristics and functions of Synchronous Optical Network (SONET). Describe the characteristics and functions of Asynchronous Transfer Mode (ATM).

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Lesson 2 Review
1. What is a T1 circuit?

2. A statistical multiplexer provides each user with __________.

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What does a SONET add/drop multiplexer do?

4.

Describe an ATM cell.

5.

Which technologies provide affordable high-speed connections for home-based networks?

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