Network Plus 2005 In Depth

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Voice signals can be carried over TCP/IP networks in a variety of configurations. To converse, VoIP callers can use either a traditional telephone, which uses analog signals, a telephone specially designed for TCP/IP transmission, or a computer equipped with a microphone, speaker, and VoIP client software. And on any VoIP network, a mix of these three types of clients is possible.

If a VoIP caller uses a traditional telephone, signals issued by the telephone must be converted to digital form before being transmitted on a TCP/IP-based network. This conversion can be accomplished in several ways. One way is by using an adapter card within a computer workstation. The traditional telephone line connects to an RJ-11 port on the adapter card. The adapter card, along with its device drivers and software on the computer, converts the voice signals to IP packets, and then issues the packets to the data network.

A second way to achieve this conversion is by connecting the traditional telephone to a switch or router capable of accepting traditional voice signals, converting them into packets, then issuing the packets to a data network. One example of such a switch is a digital PBX or, more commonly, an IP-PBX. (PBX stands for private branch exchange, which is the term used to describe a telephone switch used to connect calls within a private organization.) In general, an IP-PBX is a private switch that accepts and interprets both analog and digital voice signals. Thus, it can connect with both traditional PSTN lines and data networks. An IP-PBX transmits and receives IP-based voice signals to and from other network connectivity devices, such as routers or gateways.

In a third scenario, the traditional telephone connects to an analog PBX, which then connects to a voice-data gateway. In this case, the gateway connects the traditional telephone circuits with a TCP/IP network (such as the Internet or a private WAN). The gateway digitizes incoming analog voice signals, compresses the data, assembles the data into packets, and then issues the packets to the packet-switched network. This process relies on special VoIP compression and digitizing protocols. In addition, to translate between the PSTN and VoIP networks, gateways follow special VoIP signaling protocols. A discussion of these protocols is beyond the scope of this book. However, if you choose to specialize in VoIP networking, you need to understand such protocols thoroughly. When transferring calls from a packet-switched network to a circuit-switched network (for example, if you call your home telephone number from your office’s IP telephone), a gateway performs the same functions in the reverse order.

Figure 11-16 depicts the different ways traditional telephones can be used to access a VoIP network.

Rather than traditional telephones, most new VoIP installations use IP telephones (or IP phones), which transmit and receive only digital signals. When a caller uses an IP telephone, his voice is immediately digitized and issued from the telephone to the network in packet form. To communicate on the network, each IP telephone must have a unique IP address, just as any client connected to the network has a unique IP address. The IP telephone looks like a traditional touch-tone phone, but connects to an RJ-45 wall jack, like a computer workstation. Then, its connection may pass through a connectivity device, such as a hub or switch, before reaching the IP-PBX. An IP-PBX may contain its own voice-data gateway, or it may connect to a

FIGURE 11-16 Accessing a VoIP network from traditional telephones

separate voice-data gateway, which is then connected to the network backbone. Figure 11-17 illustrates different ways IP telephones can connect with a data network.

IP telephones act much like traditional telephones. For example, they feature speed-dialing, call hold, transfer, and forwarding buttons, conference calling, voice mail access, speakers and microphones, and an LCD screen that displays caller ID and call hold information. They come in both mobile and wire-bound styles. More sophisticated IP telephones offer features not available with traditional telephones. Because IP telephones are essentially network clients, like workstations, the number and types of customized features that can be programmed for use with these phones is limitless. Makers of IP telephones include Alcatel, Avaya, Cisco, Mitel, NEC, Nortel, and Siemens. In the United States, an IP telephone can cost between $150 and $750.

Rather than using traditional telephones or IP telephones, a third option is to use a computer programmed to act like an IP telephone, otherwise known as a softphone. Softphones and IP telephones provide the same calling functions; they simply connect to the network and deliver services in different manners. Before it can be used as a softphone, a computer must meet minimum hardware requirements (which any new workstation purchased at an electronics store would likely meet), be installed with an IP telephony client, and communicate with a digital telephone switch. In addition, softphone computers must have a sound card capable of full-duplex transmission, so that both

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FIGURE 11-17 Accessing a VoIP network from IP phones

the caller and the called party can speak at the same time. Finally, a softphone also requires a microphone and speakers or a headset.

Despite all the advantages to using VoIP, it is more difficult to transmit voice signals over a packet-switched network than data signals, which are designed for packet-switched transmission. First, more so than data transmissions, voice conversations can easily be distorted by a connection’s quality of service. When you talk with your friend, you need to hear his syllables in the order in which he mouthed them, and preferably, without delay. Therefore, packets carrying voice signals must be received in the same order in which they were issued and reassem-

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bled quickly. (In contrast, data packets do not necessarily have to be received in the same order in which they were transmitted, because the destination node will sort the information when it arrives.) Also, voice transmissions are subject to distortion if the connection becomes too noisy. In general, to prevent delays, disorder, and distortion, a voice connection requires more dedicated bandwidth than a data connection.

When VoIP is carried via the Internet, it is often called Internet telephony. But not all VoIP calls are carried over the Internet. In fact, VoIP over private lines is an effective and economical method of completing calls between two locations within an organization. And because the line is private, its network congestion can be easily controlled, thus resulting in better sound quality than an Internet telephone call can provide. But given the Internet’s breadth and low cost, it is appealing to consider the Internet for carrying conversations that we currently transmit over the PSTN.

Chapter Summary

Subnetting separates one network or segment into multiple logically defined segments, or subnets. A network administrator might subnet a network to achieve simpler troubleshooting, enhanced security, improved performance, and easier network management.

A subnet mask provides clues about the location of network information in an IP address. Bits in a subnet mask that equal 1 indicate that corresponding bits in an IP address contain network information. Bits in a subnet mask that equal 0 indicate that corresponding bits in an IP address contain host information.

To create subnets, some of an IP address’s bits that would, by default, represent host information are changed to represent network information instead. The change is indicated by a change in the subnet mask’s bits.

If you use subnetting on your LAN, only your LAN’s devices need to interpret your devices’ subnetting information. External routers, such as those on the Internet, pay attention to only the network portion of your devices’ IP addresses—not their subnet masks—when transmitting data to them.

A newer variation on traditional subnetting is provided by CIDR (Classless InterDomain Routing). CIDR offers additional ways of arranging network and host information in an IP address. In CIDR, conventional network class distinctions do not exist.

CIDR allows the creation of supernets, or subnets established by using bits that normally would be reserved for network class information. By moving the subnet boundary to the left, more bits are made available for host information, thus increasing the number of usable host addresses on a subnetted network.

Gateways facilitate communication between different subnets. Because one device on the network cannot send data directly to a device on another subnet, a gateway (usually in the form of a router interface) must intercede and hand off the information.

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Every device on a TCP/IP-based network has a default gateway, the gateway that first interprets its outbound requests to other subnets, and then interprets its inbound requests from other subnets.

Internet gateways maintain default routes to known addresses to expedite data transfer. The gateways that make up the Internet backbone are called core gateways.

NAT (Network Address Translation) allows a network administrator to “hide”

IP addresses assigned to nodes on a private network. In NAT, gateways assign transmissions valid Internet IP addresses when the transmission is sent to the Internet.

ICS (Internet Connection Sharing) is a service, included with Windows 98, Me, 2000, and 32-bit versions of XP operating systems, that allows a network of computers to share a single Internet connection through an ICS host computer.

Many private organizations use browser-based services for communication among authorized employees of the organization over an intranet. For communication with authorized personnel both from the organization and external to the organization, they may use an extranet.

All Internet mail services rely on the same principles of mail delivery, storage, and pickup, though they may use different types of software to accomplish these functions.

Mail client software can communicate with various types of mail server software, because the TCP/IP Application layer protocols used for this communication are standard.

SMTP (Simple Mail Transfer Protocol) is responsible for moving messages from one e-mail server to another over TCP/IP-based networks. SMTP operates through port 25, with requests to receive mail and send mail going through that port on the SMTP server. SMTP is used in conjunction with either POP or IMAP. MIME operates over SMTP to enable mail messages to contain non-ASCII content, such as graphics, audio, video, and binary files. Most modern e-mail clients support MIME encoding.

POP (Post Office Protocol) is a mail retrieval protocol. The most current and commonly used version of POP is called POP3. Using POP3, messages are downloaded from the mail server to a client workstation each time the user retrieves messages.

IMAP (Internet Message Access Protocol) is another mail retrieval protocol. Its most current version is IMAP4. IMAP4 differs from POP3 in that it allows users to store messages on the mail server, rather than always having to download them to the local machine. This is an advantage for users who do not always check mail from the same computer.

The netstat utility displays TCP/IP statistics and the state of current TCP/IP components and connections. It also displays ports, which can signal whether services are using the correct ports.

The nbtstat utility provides information about NetBIOS names and their addresses. If you know the NetBIOS name of a workstation, you can use nbtstat to determine the workstation’s IP address.

The nslookup utility allows you to look up the DNS host name of a network node by specifying the node’s IP address, or vice versa. Nslookup is useful for troubleshooting host configuration and DNS resolution problems.

The dig utility, like nslookup, queries the network’s DNS database to return information about a host given its IP address, or vice versa. In its simplest form, or when used with one of its many switches, dig provides more information than nslookup.

The whois utility allows you to obtain DNS registration information for a secondlevel domain.

The traceroute utility, known as tracert on Windows-based systems, uses ICMP to trace the path from one networked node to another, identifying all intermediate hops between the two nodes. This utility is useful for determining router or subnet connectivity problems.

Typing ipconfig at the command prompt of a system running Windows NT, 2000, XP, or Server 2003 reveals the TCP/IP settings for that computer.

You can view TCP/IP settings on a system that uses the Windows 9x or Me operating system by typing winipcfg at the command prompt.

Ifconfig is the utility that establishes and allows management of TCP/IP settings on a UNIX-type of system.

VoIP (voice over IP) is the use of packet-switched TCP/IP-based networks to carry voice signals. An organization may use VoIP to save money on telephone calls, centralize management of voice and data services, or take advantage of customizable call features.

Many types of clients and network designs are available with VoIP networks. Clients can be traditional telephones, IP telephones, or softphones (a computer running telephony software and connected to a microphone and headphones).

Analog VoIP clients may connect to traditional PBXs (private telephone switches), which then connect to a voice-data gateway that digitizes call information. Digital VoIP clients typically connect to a digital PBX or a router with VoIP capabilities.

Key Terms

ANDing—A logical process of combining bits. In ANDing, a bit with a value of 1 plus another bit with a value of 1 results in a 1. A bit with a value of 0 plus any other bit results in a 0.

CIDR (Classless Inter-domain Routing)—An IP addressing and subnetting method in which network and host information is manipulated without adhering to the limitations imposed by traditional network class distinctions. CIDR is also known as classless routing or supernetting. Older routing protocols, such as RIP, are not capable of interpreting CIDR addressing schemes.

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CIDR block—In CIDR notation, the number of bits used for an extended network prefix. For example, the CIDR block for 199.34.89.0/22 is /22.

CIDR notation—In CIDR, a method of denoting network IDs and their subnet boundaries. Slash notation takes the form of the network ID followed by a /, followed by the number of bits that are used for the extended network prefix.

classful addressing—An IP addressing convention that adheres to network class distinctions, in which the first 8 bits of a Class A address, the first 16 bits of a Class B address, and the first 24 bits of a Class C address are used for network information.

Classless Inter-domain Routing—See CIDR.

classless routing—See CIDR.

convergence—The use of packet-switched networks to carry data, plus video and voice signals.

core gateway—A gateway that operates on the Internet backbone.

default gateway—The gateway that first interprets a device’s outbound requests, and then interprets its inbound requests to and from other subnets. In a Postal Service analogy, the default gateway is similar to a local post office.

default router—See default gateway.

dig (domain information groper)—A TCP/IP utility that queries the DNS database and provides information about a host given its IP address or vice versa. Dig is similar to the nslookup utility, but provides more information, even in its simplest form, than nslookup can.

digital PBX—See IP-PBX.

domain information groper—See dig.

extended network prefix—The combination of an IP address’s network ID and subnet information. By interpreting the address’s extended network prefix, a device can determine the subnet to which an address belongs.

extranet—A network that uses browser-based services to exchange information within an organization and with certain, authorized users outside of that organization.

HTML (Hypertext Markup Language)—The language that defines formatting standards for Web documents.

Hypertext Markup Language—See HTML.

ICS (Internet Connection Sharing)—A service provided with Windows 98, Me, 2000 and 32-bit versions of XP operating systems that allows one computer, the ICS host, to share its Internet connection with other computers on the same network.

ICS host—On a network using the Microsoft Internet Connection Sharing service, the computer whose Internet connection other computers share. The ICS host must contain two network interfaces: one that connects to the Internet and one that connects to the LAN.

ifconfig—A utility that establishes and allows management of TCP/IP settings on UNIXtype of systems.

IMAP (Internet Message Access Protocol)—A mail retrieval protocol that improves on the shortcomings of POP. The single biggest advantage IMAP4 has relative to POP is that it allows users to store messages on the mail server, rather than always having to download them to the local machine. The most current version of IMAP is version 4 (IMAP4).

IMAP4 (Internet Message Protocol, version 4)—The most commonly used form of the Internet Message Access Protocol (IMAP).

Internet Connection Sharing—See ICS.

Internet Message Access Protocol—See IMAP.

Internet Message Access Protocol, version 4—See IMAP4.

Internet telephony—The provision of telephone service over the Internet.

intranet—A network or part of a network that uses browser-based services to exchange information within an enterprise. Intranets may be contained within a LAN or may be accessible via a WAN or the Internet.

IP-PBX—A private switch that accepts and interprets both analog and digital voice signals (although some IP-PBXs do not accept analog lines). It can connect with both traditional PSTN lines and data networks. An IP-PBX transmits and receives IP-based voice signals to and from other network connectivity devices, such as a router or gateway.

IP phone—See IP telephone.

IP telephone—A telephone used for VoIP on a TCP/IP-based network. IP telephones are designed to transmit and receive only digital signals.

IP telephony—See Voice over IP.

MIME (Multipurpose Internet Mail Extensions)—A standard for encoding and interpreting binary files, images, video, and non-ASCII character sets within an e-mail message.

Multipurpose Internet Mail Extensions—See MIME.

NAT (Network Address Translation)—A technique in which IP addresses used on a private network are assigned a public IP address by a gateway when accessing a public network.

nbtstat—A TCP/IP troubleshooting utility that provides information about NetBIOS names and their addresses. If you know the NetBIOS name of a workstation, you can use nbtstat to determine its IP address.

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netstat—A TCP/IP troubleshooting utility that displays statistics and the state of current TCP/IP connections. It also displays ports, which can signal whether services are using the correct ports.

Network Address Translation—See NAT.

network number—See network ID.

network prefix—See network ID.

nslookup—A TCP/IP utility that allows you to look up the DNS host name of a network node by specifying its IP address, or vice versa. This ability is useful for verifying that a host is configured correctly and for troubleshooting DNS resolution problems.

PBX (private branch exchange)—A telephone switch used to connect calls within a private organization.

POP (Post Office Protocol)—An Application layer protocol used to retrieve messages from a mail server. When a client retrieves mail via POP, messages previously stored on the mail server are downloaded to the client’s workstation, and then deleted from the mail server.

POP3 (Post Office Protocol, version 3)—The most commonly used form of the Post Office Protocol.

Post Office Protocol—See POP.

Post Office Protocol, version 3—See POP3. private branch exchange – See PBX.

Simple Mail Transfer Protocol—See SMTP. slash notation—See CIDR notation.

SMTP (Simple Mail Transfer Protocol)—The Application layer TCP/IP subprotocol responsible for moving messages from one e-mail server to another.

softphone—A computer programmed to act like an IP telephone. Softphones present the caller with a graphical representation of a telephone dial pad and can connect to a network via a LAN, WAN, PPP dial-up connection, or leased line.

supernet—A type of subnet that is created using bits that normally would be reserved for network class information—by moving the subnet boundary to the left.

supernet mask—A 32-bit number that, when combined with a device’s IP address, indicates the kind of supernet to which the device belongs.

supernetting—See CIDR.

toll bypass—A cost-savings benefit that results from organizations completing long-distance telephone calls over their packet-switched networks, thus bypassing tolls charged by common carriers on comparable PSTN calls.

traceroute (tracert)—A TCP/IP troubleshooting utility that uses ICMP to trace the path from one networked node to another, identifying all intermediate hops between the two nodes. Traceroute is useful for determining router or subnet connectivity problems. On Windowsbased systems, the utility is known as tracert.

Voice over IP (VoIP)—The provision of telephone service over a packet-switched network running the TCP/IP protocol suite. One form of VoIP (pronounced “voyp”) is Internet telephony, though VoIP is frequently used over private networks to circumvent long-distance toll charges.

VoIP – See voice over IP.

winipcfg—The TCP/IP configuration and management utility for use with Windows 9x and Me systems. Winipcfg differs from ipconfig in that it supplies a graphical user interface.

whois—The utility that allows you to query ICANN’s DNS registration database and find the information as a domain.

Review Questions

1._________________________ separates a network into multiple logically defined segments.

a.Classless routing

b.Subnetting

c.ANDing

d.Classful addressing

2.A(n) _________________________ facilitates communication between different networks or subnets.

a.gateway

b.switch

c.IP telephone

d.CIDR block

3._________________________ is a simple subprotocol, incapable of doing anything more than transporting mail or holding it in a queue.

a.NAT

b.VoIP

c.TCP/IP

d.SMTP