Network Plus 2005 In Depth

2.15subscribe to a service. An average subscription costs $20 to $30 per month.

Organizations that require a service subscription often require users to log on via a Web page to gain access to the service. Alternatively, they might provide users with client software that manages the client’s connection to the provider’s wireless service. This software allows the user to log on to the network and secures data exchanged between the client computer and the access point, where transmissions are most vulnerable to eavesdropping. As an added security measure, a wireless access provider might configure its access point to accept a user’s connection based on his computer’s MAC address, in addition to the user’s logon id and password. Wireless security measures are discussed in detail in Chapter 14.

At each hot spot, the access point available for public use is connected to the Internet using technology other than 802.11. For example, a local coffee shop might lease a DSL line that terminates at a combined access point and router behind the counter. That device can connect the coffee shop with its ISP while allowing patrons within the access point’s range to log on to the Internet, as shown in Figure 7-20. At T-Mobile hot spots, access points are connected (via routers) to T1 links.

FIGURE 7-20 A hot spot providing wireless Internet access

In general, to access the Internet from an 802.11 hot spot, you must:

Configure your wireless connection’s TCP/IP properties to use DHCP. (In Windows XP, for example, check the “Obtain an IP address automatically” option in the Internet Protocol TCP/IP Properties dialog box.)

Make sure your computer is not configured to automatically use a dial-up connection.

324 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY

NET+ Satellite Internet Access

2.15In 1945, Arthur C. Clarke (the author of 2001: A Space Odyssey) wrote an article in which he described the possibility of communication between manned space stations that continually orbited the earth. Other scientists recognized the worth of using satellites to convey signals from one location on earth to another. By the 1960s, the United States was using satellites to transmit telephone and television signals across the Atlantic Ocean. Since then, the proliferation of this technology and reductions in its cost have made satellite transmission appropriate and available for more regional (or even local) consumer voice and data services.

You are probably familiar with satellites used to present live broadcasts of events happening around the world. Satellites are also used to deliver digital television and radio signals, voice and video signals, and cellular and paging signals. And they provide homes and businesses— most notably in rural or hard-to-reach locations—with Internet access. This following sections describe how satellite technology works.

Satellite Orbits

Most satellites circle the earth 22,300 miles above the equator in a geosynchronous orbit. Geosynchronous orbit GEO) means that satellites orbit the earth at the same rate as the earth turns. Consequently, at every point in their orbit, the satellites maintain a constant distance from a specific point on the earth’s equator. Because satellites are generally used to relay information from one point on earth to another, information sent to earth from a satellite first has to be transmitted to the satellite from earth in an uplink. An uplink is the creation of a communications channel for a transmission from an earth-based transmitter to an orbiting satellite. Often, the uplink signal information is scrambled (in other words, its signal is encoded) before transmission to prevent unauthorized interception. At the satellite, a transponder receives the uplink signal, then transmits it to an earth-based receiver in a downlink. A typical satellite contains 24 to 32 transponders. Each satellite uses unique frequencies for its downlink. These frequencies, as well as the satellite’s orbit location, are assigned and regulated by the FCC (Federal Communications Commission). Back on earth, the downlink is picked up by a dish-shaped antenna. The dish shape concentrates the signal so that it can be interpreted by a receiver. Figure 7-21 provides a simplified view of satellite communication.

An alternative to geosynchronous satellites are LEO (low earth orbiting) satellites. LEO satellites orbit the earth with an altitude roughly between 700 and 1400 kilometers, not above the equator but closer to the earth’s poles. Because their altitude is lower, LEO satellites cover a smaller geographical range than GEO satellites. However, less power is required to issue signals between earth and an LEO satellite versus a GEO satellite.

In between the altitudes of LEO and GEO satellites lie MEO (medium earth orbiting) satellites. MEO satellites orbit the earth between 10,350 and 10,390 kilometers above its surface. As with LEO satellites, MEO satellites are not positioned over the equator, but over a latitude between the equator and the poles. MEOs have the advantage of covering a larger area of the earth’s surface than LEO satellites while at the same time using less power and causing less signal delay than GEO satellites.

NET+

2.15

FIGURE 7-21 Satellite communication

Geosynchronous orbiting satellites are the type used by the most popular satellite Internet access service providers. This technology is well established, and is the least expensive of all satellite technology. Also, because they remain in a fixed position relative to the earth’s surface, stationary receiving dishes on earth can be counted on to receive satellite signals reliably.

Satellite Frequencies

Satellites transmit and receive signals in any of following five frequency bands:

L-band—1.5–2.7 GHz

S-band—2.7–3.5 GHz

C-band—3.4–6.7 GHz

Ku-band—12–18 GHz

Ka-band—18–40 GHz

Within each band, frequencies used for uplink and downlink transmissions differ. This variation helps ensure that signals traveling in one direction (for example from a satellite to the earth) do not interfere with signals traveling in the other direction (for example, signals from the earth to a satellite). Satellite Internet access providers typically use frequencies in the C- or Ku-bands. Newer satellite Internet access technologies are currently being developed for the Ka-band.

Satellite Internet Services

A handful of companies offer high-bandwidth Internet access via GEO satellite links. Each subscriber uses a small satellite dish antenna and receiver to exchange signals with the service provider’s satellite network. Subscribers can choose one of two types of satellite Internet access service: dial return or satellite return. In a dial return arrangement, a subscriber receives data from the Internet via a satellite downlink transmission, but sends data to the satellite via an analog modem (dial-up) connection. With dial return, service providers advertise downstream (or downlink) throughputs of 400–500 Kbps, though in practice, they may be as high as 1 Mbps.

2.15lower. Therefore, dial return satellite Internet access is an asymmetrical technology. In a satellite return arrangement, a subscriber sends and receives data to and from the Internet using a satellite uplink and downlink. This is a symmetrical technology, in which both upstream and downstream throughputs are advertised to reach 400–500 Kbps. In reality, throughputs are often higher.

To establish a satellite Internet connection, each subscriber must have a dish antenna, which is approximately two feet high by three feet wide, installed in a fixed position. In North America, these dish antennas are pointed toward the southern hemisphere (because the geosynchronous satellites travel over the equator). The dish antenna’s receiver is connected, via cable, to a modem. This modem uses either a PCI or USB interface to connect with the subscriber’s computer. In a dial return system, an analog modem is also connected to the subscriber’s computer to handle upstream communications.

Figure 7-22 illustrates how a home user with dial return satellite Internet access service connects with a satellite Internet service provider.

Costs for popular Internet access services in the United States are approximately $200 for installation (which must be performed by a professional) plus a monthly service fee of $20 to $30.

FIGURE 7-22 Dial return satellite Internet service

WAN Technologies Compared

2.14dial-up connection to potentially 39.8 Gbps for a full-speed SONET connection. Table 7-4

2.15summarizes the media and throughputs offered by each technology discussed in this chapter. Bear in mind that each technology’s transmission techniques (for example, switching for Frame Relay versus point-to-point for T1) will affect real throughput, so the maximum transmission speed is a theoretical limit. Actual transmission speeds will vary.

Table 7-4 A comparison of WAN technology throughputs

Remote Connectivity

2.16continuous, dedicated access to the WAN. For example, when a user in Phoenix wants to open a document on a server in Dallas, she needs only to find the Dallas server on her network, open a directory on the Dallas server, and then open the file. The server is available to her at any time, because the Phoenix and Dallas offices are always connected and sharing resources over the WAN. However, this is not the only way to share resources over a WAN. For remote users (such as employees on the road, off-campus students, telecommuters, or staff in small, branch offices), intermittent access with a choice of connectivity methods is often more appropriate.

As a remote user, you must connect to a LAN via remote access, a service that allows a client to connect with and log on to a LAN or WAN in a different geographical location. After connecting, a remote client can access files, applications, and other shared resources, such as printers, like any other client on the LAN or WAN. To communicate via remote access the client and host need a transmission path plus the appropriate software to complete the connection and exchange data.

Many remote access methods exist, and they vary according to the type of transmission technology, clients, hosts, and software they can or must use. Popular remote access techniques, including dial-up networking, Microsoft’s RAS (Remote Access Service) or RRAS (Routing and Remote Access Service), remote control, terminal services, Web portals, and VPNs (virtual private networks), are described in the following sections. You will also learn about common remote access protocols PPP and SLIP.

Dial-up Networking

Dial-up networking refers to dialing directly into a private network’s or ISP’s remote access server to log on to a network. Dial-up clients can use PSTN, X.25, or ISDN transmission

2.16computers using the PSTN—that is, regular telephone lines. To accept client connections, the remote access server is attached to a group of modems, all of which are associated with one phone number. The client must run dial-up software (normally available with the operating system) to initiate the connection. At the same time, the remote access server runs specialized software to accept and interpret the incoming signals. When it receives a request for connection, the remote access server software presents the remote user with a prompt for his credentials—typically, his user name and password. The server compares his credentials with those in its database, in a process known as authentication. If the credentials match, the user will be allowed to log on to the network. Thereafter, the remote user can perform the same functions she could perform while working at a client computer in the office. With the proper server hardware and software, a remote access server can offer multiple users simultaneous remote access to the LAN. Many Internet subscribers use dial-up networking to connect to their ISP.

Advantages to using dial-up networking are that the technology is well understood and its software comes with virtually every operating system. (On the other hand, this option is more expensive than other options when a client travels far from the network and must dial into the network using a long-distance or 1-800 number supplied by the organization’s headquarters.) Connecting to a remote access server can be slow, however, when it relies on the PSTN. Also, it requires a significant amount of maintenance to make sure clients can always connect to a pool of modems. One way to limit the maintenance burden is for an organization to contract with an ISP to supply remote access services. In this arrangement, clients dial into the ISP’s remote access server, and then the ISP connects the incoming clients with the organization’s network.

The dial-up networking software that Microsoft provided with its Windows 95, 98, NT, and 2000 client operating systems and with its Windows NT and 2000 network operating systems is called RAS (Remote Access Service). For the Network+ exam, you will need to be familiar with the term “RAS” and be aware that, as with other dial-up networking services, RAS requires software installed on both the client and server, a server configured to accept incoming clients, and a client with sufficient privileges (including user name and password) on the server to access its resources. In the Windows XP and Server 2003 operating systems, RAS has been incorporated into a more comprehensive remote access package called the RRAS (Routing and Remote Access Service). RRAS is described in the following section.

Remote Access Servers

The previous section described dial-up networking, a type of remote access method defined by its direct, PSTN-based connection method. However, users who previously depended on dialup connections are increasingly adopting faster broadband connections, such as DSL and broadband cable technology. This section and following sections describe services that can accept remote access connections from a client, no matter what type of connection it uses.

2.16WAN to connect and log on to that network. A remote client attempting to connect to a LAN or WAN requires a server to accept its connection and grant it privileges to the network’s resources. Many types of remote access servers exist. Some are devices dedicated to this task, such as the Cisco 2500 series routers or the Cisco AS5800 access servers. These devices run software that, in conjunction with their operating system, performs authentication for clients and communicates via dial-up networking protocols. Other types of remote access servers are computers installed with special software that enables them to accept incoming client connections and grant them access to resources.

RRAS (Routing and Remote Access Service) is Microsoft’s remote access software available with the Windows Server 2003 network operating system and the Windows XP client operating systems. RRAS enables a Windows Server 2003 computer to accept multiple remote client connections over any type of transmission path. It also enables the server to act as a router, determining where to direct incoming packets across the network. Further, RRAS incorporates multiple security provisions to ensure that data cannot be intercepted and interpreted by anyone other than the intended recipient and to ensure that only authorized clients can connect to the remote access server.

Figure 7-23 illustrates how clients connect with a remote access server to log on to a LAN.

Remote access servers depend on several types of protocols to communicate with clients, as described in the following section.

FIGURE 7-23 Clients connecting with a remote access server

NET+ Remote Access Protocols

2.16To exchange data, remote access servers and clients require special protocols. The SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol) are two protocols that enable a workstation to connect to another computer using a serial connection (in the case of dial-up networking, “serial connection” refers to a modem). Such protocols are necessary to transport Network layer traffic over serial interfaces, which belong to the Data Link layer of the OSI Model. Both SLIP and PPP encapsulate higher-layer networking protocols, such as TCP and IP, in their lower-layer data frames.

SLIP is an earlier and much simpler version of the protocol than PPP. For example, SLIP can carry only IP packets, whereas PPP can carry many different types of Network layer packets, such as IPX or AppleTalk. Because of its primitive nature, SLIP requires significantly more setup than PPP. When using SLIP, you typically must specify the IP addresses for both your client and for your server in your dial-up networking profile. PPP, on the other hand, can automatically obtain this information as it connects to the server. PPP also performs error correction and data compression, but SLIP does not. In addition, SLIP does not support data encryption, which makes it less secure than PPP. For all these reasons, PPP is the more popular communications protocol for remote access communications.

Another difference between SLIP and PPP is that SLIP supports only asynchronous data transmission, and PPP supports both asynchronous and synchronous transmission. As you learned earlier, in synchronous transmission, data must conform to a timing scheme. Asynchronous refers to a communications method in which nodes do not have to conform to any predetermined schemes that specify the timing of data transmissions. In asynchronous communications, a node can transmit at any instant, and the destination node must accept the transmission as it comes. To ensure that the receiving node knows when it has received a complete frame, asynchronous communications provide start and stop bits for each character transmitted. When the receiving node recognizes a start bit, it begins to accept a new character. When it receives the stop bit for that character, it ceases to look for the end of that character’s transmission. Asynchronous data transmission therefore occurs in random stops and starts. In fact, asynchronous transmission was designed for communication that happens at random intervals, such as sending the keystrokes of a person typing on a remote keyboard. Thus, it is well suited to use on modem connections.

When PPP is used over an Ethernet network (no matter what the connection type), it is known as PPPoE (PPP over Ethernet). PPPoE is the standard for connecting home computers to an ISP (Internet Service Provider) via DSL or broadband cable. When you sign up for broadband cable or DSL service, the ISP supplies you with connection software that is configured to use PPPoE. Figure 7-24 illustrates the how the protocols discussed in this section and commonly used to establish a broadband Internet connection fit in the OSI Model. (The Application layer protocol RDP is discussed in the following section.)