The Internet Encyclopedia (Volume 3)

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REFERENCES 659

for law enforcement agencies where they have a warrant

to wiretap a telephone conversation.

Toll fraud is the problem of unauthorized persons mak-

ing telephone calls over the network. This can be caused

by unauthorized users originating calls on the IP network

either on legitimate phones or over illegitimate phones. As

with conventional telephony, this can be combated with

security codes and other authentication methods.

Identity spoofing refers to a hacker tricking a remote

user into believing he or she are talking to the person he

or she dialed on the IP network, when, in fact, he or she

is talking to the hacker. Again, security codes and other

authorization methods are helpful here.

IP spoofing refers to theft of IP identity, where one end

device is able to convince the IP network that its IP ad-

dress is the same as a legitimate device’s IP address. This

allows the device with the fraudulent IP address to inter-

cept calls and/or perform toll fraud using that IP address.

Spoofing the IP address of a gateway allows eavesdrop-

ping on telephone calls.

Some generally accepted recommendations for min-

imizing many of these security problems are to disable

self-registration of VOIP end devices after initial network

installation, to segregate voice from data traffic at level 2

or 3, and to use a stateful firewall at the PSTN gateway.

As noted above, segregating data from voice services also

provides the added benefit of maintaining different qual-

ity of service for data and voice.

CONCLUSION

VOIP holds great promise where the convergence of data

and voice can occur. Internetworking and overcoming

QOS issues remain some of the biggest challenges.

GLOSSARY

Analog signal A continuous signal that, at any point in

time, can have an infinite number of possible values

and that is typically analogous in some characteristic

to another signal or physical phenomenon.

Asynchronous transfer mode (ATM) A network trans-

fer method, employed at the data link layer (Level 2),

for high-speed switched packet routing, which can es-

tablish virtual channels with a specified QoS.

Channel capacity The theoretical upper rate limit, in

bits per second, at which information can be transmit-

ted over a transmission channel.

Circuit switch A switch that makes a temporary or

permanent dedicated transmission path between two

transmission links that are attached to that switch,

based on signaling information received prior to es-

tablishing the dedicated path.

Digital encoding Encoding a signal in the form of a

string of 1s and 0s.

Digital transmission Transmission of information en-

coded as 1s and 0s.

Internet A global public network based on the “Inter-

net protocol,” connecting millions of hosts worldwide,

and for which users often pay a flat fee to access, with

little or no charge for transmitting each packet of in-

formation. (Outside the U.S., Internet access is often

measured and charged on a usage basis, e.g., minutes

or units of data.)

Internet protocol (small “i”) or IP A packet switch-

ing protocol used for routing packets over and between

and private networks that is “connectionless” (i.e., each

packet making up the same message may take a differ-

ent route to reach the ultimate destination).

Node A point of connection between transmission links,

which may contain switches, and/or may contain con-

verters to interconnect transmission links with differ-

ing modalities (e.g., for connecting wire links to wire-

less links, non-digitally encoded links to digitally en-

coded links, or fiber links to copper wire links).

Packet router A type of packet switching device that

typically routes based on Level 3 network address in-

formation (such as an IP address) and typically has the

ability to choose optimal routing based on dynamically

changing criteria and routing tables.

Packet switch A type of packet switching device that

routes packets of data between links based on address

information associated with each packet. A Level 3

switch uses network addresses, such as IP addresses,

to route packets of data. Level 2 switches uses data link

layer addresses (which are typically local and/or hard-

coded) for routing.

Public switched telephone network (PSTN) A circuit-

switched network that is provided by regulated com-

mon carriers who offer their voice telephone services

to the general public.

Quality of Service (QoS) A set of performance param-

eters or criteria, such a bandwidth, jitter, packet loss,

and delay, prespecified for a service.

Transmission The movement of information (whether

or not digitally encoded) from one point to another via

a signal carried over a physical medium, such as wires,

fiber, radio, or light.

Transducer A device actuated by signal power from one

system and supplying signal power in another form

to a second system (e.g., a telephone receiver earpiece

actuated by electric power of a received transmission

signal and supplying acoustic signal power to the sur-

rounding air, which the telephone user can hear, or a

telephone microphone that has a quartz crystal that

produces electrical signal power for transmission over

wires from the mechanical acoustic power originating

from the telephone user’s voice).

Transmission link A transmission path connecting two

nodes.

Voice communication The transmission of informa-

tion contained in a voice signal.

CROSS REFERENCES

SeeCircuit, Message, and Packet Switching; Digital Com-

munication; Internet Literacy; Public Networks; TCP/IP

Suite; Web Quality of Service; Wide Area and Metropolitan

Area Networks.

REFERENCES

ASCII Table. Retrieved March 23, 2003, from http://web.

cs.mun.ca/∼michael/c/ascii-table.html