WAN Technologies and Internet Access(ADSL,SDSL,VDSL,Troubleshooting )

WAN Technologies

Many of today’s network environments are not restricted to a single location or LAN. Instead, many networks span great distances, becoming wide area net- works (WANs). When they do, hardware and software are needed to connect these networks. This section reviews the characteristics of various WAN tech- nologies. Before we go on to discuss the specific WAN technologies, we must look at an important element of WAN technologies—switching methods.

Switching Methods

For systems to communicate on a network, the data needs a communication path or multiple paths on which to travel. To allow entities to communicate, these paths move the information from one location to another and back. This is the function of switcfting. Switching provides communication pathways between two endpoints and manages how data is to flow between them. Two of the more common switching methods used today are

.  Packet switching

. Circuit switching

Packet Switching

In packet switching, messages are broken into smaller pieces called packets. Each packet is assigned source, destination, and intermediate node addresses. Packets are required to have this information because they do not always use the same path or route to get to their intended destination. Referred to as independent routing, this is one of the advantages of packet switching. Independent routing allows for better use of available bandwidth by letting packets travel different routes to avoid high-traffic areas. Independent routing also allows packets to take an alternate route if a particular route is unavailable for some reason.In a packet-switching system, when packets are sent onto the network, the send- ing device is responsible for choosing the best path for the packet. This path might change in transit, and it is possible for the receiving device to receive the packets in a random or nonsequential order. When this happens, the receiving device waits until all the data packets are received, and then it reconstructs them according to their built-in sequence numbers.

Two types of packet-switching methods are used on networks:

. Virtual-circuit packet switching: When virtual-circuit switching is  used, a logical connection is established between the source and the des- tination device. This logical connection is established when the sending device initiates a conversation with the receiving device. The logical communication path between the two devices can remain active for as long as the two devices are available or can be used to send packets once. After the sending process has completed, the line can be closed.

. Datagram packet switching: Unlike virtual-circuit packet switching, datagram packet switching does not establish a logical connection between the sending and transmitting devices. The packets in datagram packet switching are independently sent, meaning that they can take dif- ferent paths through the network to reach their intended destination. To do this, each packet must be individually addressed to determine where its source and destination are. This method ensures that packets take the easiest possible routes to their destination and avoid high-traffic areas.

Datagram packet switching is mainly used on the Internet.

Circuit Switching

In contrast to the packet-switching method, circuit switcfting requires a dedicat- ed physical connection between the sending and receiving devices. The most commonly used analogy to represent circuit switching is a telephone conversa- tion in which the parties involved have a dedicated link between them for the duration of the conversation. When either party disconnects, the circuit is bro- ken, and the data path is lost. This is an accurate representation of how circuit switching works with network and data transmissions. The sending system establishes a physical connection, and the data is transmitted between the two. When the transmission is complete, the channel is closed.

Some clear advantages to the circuit-switching technology make it well suited for certain applications such as PSTN and ISDN. The primary advantage is that after a connection is established, a consistent and reliable connection exists between the sending and receiving device. This allows for transmissions at a guaranteed rate of transfer.

Like all technologies, circuit switching has its downsides. As you might imagine, a dedicated communication line can be very inefficient. After the physical con- nection is established, it is unavailable to any other sessions until the transmis- sion is complete. Again, using the phone call analogy, this would be like a caller trying to reach another caller and getting a busy signal. Circuit switching there- fore can be fraught with long connection delays.

Comparing Switching Methods

Table 6.1 is an overview of the various switching technologies.

Switching Method Pros Cons Key Features

Table 6.1 Comparison of Switching Methods

Packet	Packets can be	Packets can become	The two types of
switching	routed around	lost while taking	packet switching are
	network congestion.	alternate routes	datagram and virtual
	Packet switching	to the destination.	circuit. Virtual circuit
	makes efficient	Messages are divided	uses a logical
	use of network	into packets that	connection between
	bandwidth.	contain source and	the source and the
		destination	destination device.
		information.	With datagram circuit
			switching, packets are
			independently sent
			and can take different
			paths through the
			network to reach their
			intended destination.

Switching Method Pros Cons Key Features

Table 6.1 Comparison of Switching Methods              Continued

Circuit	Offers a dedicated	Dedicated channels	Offers the capability of
switching	transmission	can cause delays	storing messages
	channel that is	because a channel	temporarily to reduce
	reserved until	is unavailable until	network congestion.
	it is disconnected.	one side disconnects.
		Uses a dedicated
		physical link between
		the sending and
		receiving devices.

Integrated Services Digital Network  (ISDN)

ISDN has long been an alternative to the slower modem WAN connections, but at a higher cost. ISDN allows the transmission of voice and data over the same physical connection.

ISDN connections are considerably faster than regular modem connections. To access ISDN, a special phone line is required. This line usually is paid for through a monthly subscription. You can expect these monthly costs to be sig- nificantly higher than those for traditional dialup modem connections.

To establish an ISDN connection, you dial the number associated with the receiving computer, much as you do with a conventional phone call or modem dialup connection. A conversation between the sending and receiving devices is then established. The connection is dropped when one end disconnects or hangs up. The line pickup of ISDN is very fast, allowing a connection to be estab- lished, or brought up, much more quickly than a conventional phone  line.

ISDN has two defined interface standards—Basic Rate Interface (BRI) and Primary Rate Interface (PRI).

BRI

BRI ISDN uses three separate channels—two bearer (B) channels of 64Kbps each and a delta channel of 16Kbps. B channels can be divided into four D channels, which allows businesses to have eight simultaneous Internet connec- tions. The B channels carry the voice or data, and the D channels are used for signaling.

PRI

PRI is a form of ISDN that generally is carried over a T1 line and can provide transmission rates of up to 1.544Mbps. PRI is composed of 23 B channels, each providing 64Kbps for data/voice capacity, and one 64kbps D channel, which is used for signaling.

Characteristic	BRI	PRI
Speed	128 Kbps	1.544 Mbps
Channels	2B+D	23B+D
Transmission carrier	ISDN	T1

Internet Access Technologies

Internet access has become an integral part of modern business. You have sever- al ways to obtain Internet access. Which type you choose often depends on the cost, as well as what technologies are available in your area. This section explores some of the more common methods of obtaining Internet access.

xDSL Internet Access

DSL is an Internet access method that uses a standard phone line to provide high-speed Internet access. DSL is most commonly associated with high-speed Internet access; because it is a relatively inexpensive Internet access, it is often found in homes and small businesses. With DSL, a different frequency can be used for digital and analog signals, which means that you can talk on the phone while you’re uploading data.

When it comes to DSL services, there are two types of systems—Asymmetric Digital Subscriber Line (ADSL) and  High-Rate  Digital  Subscriber  Line  (HDSL). ADSL provides a high data rate in only one direction. It allows for fast download speeds but significantly slower upload speeds. ADSL is designed to work with existing analog telephone service (POTS) service. With fast down- load speeds, ADSL is well suited for home-use Internet access where uploading large amounts of data isn’t a frequent task.

In contrast to ADSL, HDSL provides a bidirectional high data rate service that can accommodate services, such as videoconferencing, that require high data rates in both directions. A variant of HDSL is VHDSL (Very High-Rate Digital Subscriber Line), which provides an HDSL service at very high data transfer rates.

DSL arrived on the scene in the late 1990s, and it brought with it a staggering number of flavors. Together, all these variations are known as xDSL:

. Asymmetric DSL (ADSL): Probably the most common of the DSL varieties is ADSL. ADSL uses different channels on the line. One chan- nel is used for plain old telephone service (POTS) and is responsible for analog traffic. The second channel provides upload access, and the third channel is used for downloads. With ADSL, downloads are faster than uploads, which is why it is called asymmetric DSL.

. Symmetric DSL (SDSL): SDSL offers the same speeds for uploads and downloads, making it most suitable for business applications such as web hosting, intranets, and e-commerce. It is not widely implemented in the home/small business environment and cannot share a phone line.

. ISDN DSL (IDSL): ISDN DSL is a symmetric type of DSL that is commonly used in environments where SDSL and ADSL are unavail- able. IDSL does not support analog phones.

. Rate-Adaptive DSL (RADSL): RADSL is a variation on ADSL that can modify its transmission speeds based on signal quality. RADSL sup- ports line sharing.

. Very High Bit Rate DSL (VHDSL or VDSL): VHDSL is an asym- metric version of DSL and, as such, can share a telephone line. VHDSL supports high-bandwidth applications such as VoIP and HDTV. VHDSL can achieve data rates up to approximately 10Mbps, making it the fastest available form of DSL. To achieve high speeds, VHDSL uses fiber-optic cabling.

. High Bit Rate DSL (HDSL): HDSL is a symmetric technology that offers identical transmission rates in both directions. HDSL does not allow line sharing with analog phones.

Why are there are so many DSL variations? The answer is quite simply that each flavor of DSL is aimed at a different user, business, or application. Businesses with high bandwidth needs are more likely to choose a symmetric form of DSL, whereas budget-conscious environments such as home offices are likely to opt for an option that allows phone line sharing at the expense of band- width. In addition, some of the DSL variants are simply older technologies. Although the name persists, they have been replaced with newer DSL imple- mentations. When you’re working in a home/small office environment, you should expect to work with an ADSL system.

ADSL                                      1Mbps                                           3Mbps

SDSL                                      1.5Mbps                                       1.5Mbps

IDSL                                       144Kbps                                       144Kbps

RADSL                                   1Mbps                                           7Mbps

VHDSL                                   1.6Mbps                                       13Mbps

HDSL                                     768Kbps                                       768Kbps

DSL Troubleshooting Procedures

Troubleshooting DSL is similar to troubleshooting any other Internet connec- tion. The following are a few things to check when users are experiencing prob- lems with a DSL  connection:

. Physical connections: The first place to look when troubleshooting a DSL problem is the network cable connections. From time to time,   these cables can come loose or inadvertently be detached, and they are often overlooked as the cause of a problem. DSL modems typically have a minimum of three connections: one for the DSL line, one for the local network, and one for the power. Make sure that they are all plugged in appropriately.

. The NIC: While you’re checking the cable at the back of the system, take a quick look to see whether the network card LED is lit. If it is not, something could be wrong with the card. It might be necessary to swap out the network card and replace it with one that is known to be working.

. Drivers: Ensure that the network card is installed and has the correct drivers. Many times, simply using the most up-to-date driver can resolve connectivity  issues.

. Protocol configuration: The device you are troubleshooting might not have a valid IP address. Confirm the IP address by using the appropriate tool for the operating system being used—for example, winipcfg, ipconfig, or ifconfig. If the system requires the automatic assignment of an IP address, confirm that the system is set to obtain an IP address automatically. It might be necessary to use the ipconfig /release and ipconfig /renew commands to get a new IP address.

. DSL LEDs: Each DSL box has an LED on it. The light sequences are often used to identify connectivity problems or problems with the box itself. Refer to the manufacturer’s website for specific information about error codes and LEDs, but remember the basics. A link light should be on to indicate that the physical connection is complete, and a flashing LED indicates that the connection is active.

Cable Internet Access

Cable Internet access is an always-on Internet access method that is available in areas that have digital cable television. Cable Internet access is attractive to many small businesses and home office users because it is both inexpensive and reliable. Most cable providers do not restrict how much use is made of the access. Connectivity is achieved by using a device called a cable modem. It has a coaxial connection for connecting to the provider’s outlet and an Unshielded Twisted Pair (UTP) connection for connecting directly to a system or to a hub or switch.

Cable providers often supply the cable modem, with a monthly rental agree- ment. Many cable providers offer free or low-cost installation of cable Internet service, which includes installing a network card in a PC. Some providers  also

do not charge for the network card. Cable Internet costs are comparable to DSL subscription.

Most cable modems supply a 10Mbps Ethernet connection for the home LAN, although you wouldn’t expect the Internet connection to reach these speeds. The actual speed of the connection can vary somewhat, depending on the uti- lization of the shared cable line in your area. In day-to-day application, data rates range from 1.5Mbps to 7Mbps.Cable Troubleshooting Procedures

In general, cable Internet access is a low-maintenance system with few prob- lems. When problems do occur, you can try various troubleshooting measures:

. Check the user’s end: Before looking at the cable modem, make sure that the system is configured correctly and that all cables are plugged in. If a hub or switch is used to share the cable Internet access among a group of computers, make sure that the hub or switch is on and func- tioning correctly.

. Check the physical connections: Like DSL modems, cable modems have three connections: one for the cable signal, one for the local net- work, and one for the power. Make sure that they are all plugged in appropriately.

. Ensure that the protocol configuration on the system is valid: If an IP address is assigned via DHCP, the absence of an address is a sure indi- cator that connectivity is at fault. Try obtaining a new IP address by using the appropriate command for the operating system platform you are using. If the IP addresses are statically configured, make sure that they are set correctly. Trying to use any address other than that specified by the ISP might prevent a user from connecting to the network.

. Check the indicator lights on the modem: Most cable modems have indicator lights that show the modem’s status. Under normal conditions, a single light labeled Ready or Online should be lit. Most cable providers give the user a modem manual that details the functions of the lights and what they indicate in certain states. Generally, any red light is bad.

Flashing LEDs normally indicate traffic on the  connection.

. Cycle the power on the modem: Cycling the power on the modem is a surefire way of resetting it.

. Call the technical support line: If you are sure that the connectors are all in place and the configuration of the system is correct, the next step is to call the technical support line of the cable provider. If the provider is experiencing problems that affect many users, you might get a message while you’re on hold, informing you of that fact. If not, you will eventu- ally get to speak to someone who can help you troubleshoot the prob- lem. One of the good things about cable access is that the cable company can remotely monitor and reset the modem. The cable company should be able to tell you whether the modem is functioning correctly.

Unless the modem is faulty, which is not that common, by this point the user should be back on the Internet, or at least you should fully understand why the user cannot connect. If the problem is with the cable provider’s networking equipment, you and the user simply have to wait for the system to come back on.

Wireless Internet Access

Not too long ago, it would have been inconceivable to walk into your local cof- fee shop with your laptop under your arm and surf the Web while drinking a latte. Putting aside the fact that beverages and laptops don’t mix, wireless Internet access is becoming much more common.

Wireless Internet access is provided by a Wireless Internet Service Provider (WISP). The WISP provides public wireless Internet access known as ftotspots. Hotspots offer Internet access for mobile network devices such as laptops, hand- held computers, and cell phones in airports, coffee shops, conference rooms, and so on. A hotspot is created using one or many wireless access points near the hotspot location.

Client systems might need to install special application software for billing and security purposes; others require no configuration other than obtaining the net- work name (Service Set Identifier [SSID]). Hotspots do not always require a fee for service, because companies use them as a marketing tool to lure Internet users to their businesses.

Hotspots are not everywhere, but finding them is not difficult. Typically, air- ports, hotels, and coffee shops advertise that they offer Internet access for cus- tomers or clients. In addition, WISPs list their hotspot sites online so that they are easily found.

Establishing a connection to a wireless hotspot is a straightforward process. If not equipped with built-in wireless capability, laptops require an external wire- less adapter card. With the physical requirements of the wireless card taken care

of, connect as follows:

1.     When you arrive at the hotspot site, power up your laptop. In some instances, you might need to reboot your system if it was on standby to clear out old configuration settings.

2.     The card might detect the network automatically. If this is the case, con- figuration settings, such as the SSID, are automatically detected, and the wireless Internet is available. If Internet access is free, there is little else to do; if it is a paid-for service, you need to enter a method of payment. One thing to remember is to verify that you are using encryption for secure data transfer.

3.     If for some reason the wireless settings are not automatically detected, you need to open your wireless NIC’s configuration utility and manually set the configurations. These settings can include setting the mode to infrastructure, inputting the correct SSID, and setting the level of encryption used.

In addition to using a WISP, some companies such as hotels and cafes provide wireless Internet access by connecting a wireless router to a DSL or cable Internet connection. The router becomes the wireless access point to which the users connect, and it allows clients to connect to the Internet through the broad- band connection. The technology is based on the 802.11 standards, typically 802.11b/g, and client systems require only an internal or external wireless adapter.