A communications channel has an important dependence for the channel capacity (C, in bps) to channel bandwidth (W, in Hz) ratio; this is capacity per unit bandwidth on signal to noise ratio (S/N, power of the signal over power of noise). Shannon’s formula gives an upper limit for this dependence (Shannon & Weaver, 1949), C/W=log2 (1+S/N), which represents channel efficiency. Phase lock loops for waves and data symbols in the presence of noise have been given (Reis, Rocha, Gameira, & Carvalho, 2005).
Key Terms in this Chapter
Band Pass Filter: A filter whose response is non-null for a specified frequency band and practically null for all others. It only allows for that frequency band to pass from input to output.
RADIUS: Remote Authentication Dial-In-User Service. A protocol for communications to a server that has a user directory to permit user identity verification.
dB: Decibel. Power ratio dB = 10 log10(P2/P1). Voltage ratio dB = 10 log10(V2/V1). Current ratio dB = 10 log10(I2/I1).
ISM band: Industrial, Scientific and Medical band. A license free frequency band for operation of industrial, scientific, and medical equipment.
Wired Communications: Communications where information is transported along a guided medium, through an electromagnetic wave or an electrical signal.
Wireless Communications: Communications where an electromagnetic wave transports information through an unguided medium, such as atmosphere.
FDDI: Fiber-Distributed Data Interface. Optical fibre network operating at 100 Mbps, with a maximum of 500 stations spaced up to 2 km and a maximum fiber length of 100 km. It has a dual ring topology composed of the primary ring and the secondary (or backup) ring.
bps: Bits per second. Unit of data transfer speed in a communications channel.
MAN: Metropolitan Area Network. A data communications network that covers a geographic area ranging between 10 km and 100 km.
FCC: Federal Communications Commission. The regulatory agency for United States.
IDS: Intrusion Detection System. A system that analyzes network activity and compares it to databases of attack signatures to identify a network or system attack. The IDS can be passive or reactive.
Antenna: Telecommunication device that may be able to transmit and receive electromagnetic waves. As a transmitter, an electric signal is transformed to a wave. As a receiver, the inverse process occurs. An antenna has a characteristic frequency related to its dimensions. It is designed to operate in a relatively low frequency band. Antennas can be directional (they radiate in a preferential direction) or omnidirectional (they radiate uniformly in a plane perpendicular to the axis).
ATM: Asynchronous Transfer Mode. A high speed packet switching technology where fixed size cells of 53 bytes are used to transmit information.
Autonegotiation: It detects the various communication modes that exist for two network devices, automatically configuring the highest mode of interconnection.
PEAP: Protected Extensible Authentication Protocol (Protected EAP). A protocol developed jointly by Microsoft, RSA Security, and Cisco for transmission of authentication data in 802.11 wireless networks. Clients are authenticated using server based digital certificates. An encrypted SSL/TLS tunnel is used.
Optical Fiber: A guided medium for information transmission through electromagnetic waves. It consists of a core made of glass, where light travels, surrounded by a cladding layer to reflect light. The cladding is covered by a protective and light absorbing jacket.
WEP: Wireless Equivalent Privacy. A security protocol for WLANs existing in 802.11b.
dBi: Represents the gain of an antenna compared to the gain of an isotropic antenna. It is decibels with reference to an isotropic antenna. An isotropic antenna has a gain of 1 (0 dB). It radiates uniformly in all directions of space.
Hertz (Hz): A unit to measure frequency; cycles per second of a periodic channel; and inverse of the period.
ADSL: Asymmetric Digital Subscriber Line (Asymmetric DSL). By using a technology that adds high-speed data communications to a standard telephone line, asymmetric downstream (e.g., from the Internet to a computer) and upstream (e.g., from a computer to the Internet) speeds are possible.
Planar Antenna: A directional antenna in which all the elements are in one plane.
Parabolic Grid Antenna: A directional antenna with a parabolic reflector, but instead of a dish, it forms a grid. It has the radiating or receiving element at or near the focus.
Full-duplex: Operation mode of a communications channel where information is simultaneously transmitted in both directions.
TLS: Transport Layer Security. A cryptographic protocol developed as a successor to SSL. It is defined in RFC 2246, RFC 3546.
Microwaves: Electromagnetic waves in the 2-40 GHz frequency band.
FCCN: Portuguese Foundation for National Scientific Computing.
Router: Active network device that interconnects distinct networks. It performs forwarding and packet switching for those networks. It works at OSI model level 3. There are multitechnology and multiprotocol routers.
IEEE: Institute of Electrical and Electronic Engineers, Inc. Technical, professional, nonprofit organization.
pps: Packets per second. A packet is a block of data transmitted over a network, consisting of a header, a data area, and a trailer area. Depending on protocol, packets have specific formats.
EAP: Extensible Authentication Protocol. A general authentication protocol defined in RFC 2284, RFC 3748 that supports multiple authentication methods. It is an extension to PPP protocol.
Half-duplex: Operation mode of a communications channel where information is transmitted in each direction at a time.
Token Ring: A LAN having star wired ring topology, with speeds of 4, 16, and 100 Mbps. It was developed by IBM and follows IEEE 802.5.
SSL: Secure Sockets Layer. It is used, for example, to create an encrypted connection for an Internet transaction.
Point-to-Point Wireless Link: Wireless communications where a transmitter/receiver communicates with a receiver/transmitter only, through a fixed path called a line of sight path. Two directional antennas are used.
Switch: Active network device that operates at the OSI model level 2. Having affinities with repeaters and bridges, traffic is isolated for network segments. Several simultaneous communications are possible for different network nodes. Collisions can be reduced and eliminated for full-duplex operation. If a switch has also routing capabilities, it is named switch/router.
Electromagnetic Interference: Unwanted electromagnetic radiation that superimposes to the electromagnetic waves of interest in a certain frequency band.
WPAN: Wireless Personal Area Network. Short range wireless network for data exchange between devices in a personal space, within a range of 10-20 m.
Authentication: A process of identifying secure users in a network.
dBm: Represents decibels (power level) compared to 1 mW. Power ratio dBm = 10 log10(P(mW)/1 mW).
CSMA: Carrier Sense Multiple Access. A medium access control method for multiple access transmission media. When a network node is about to transmit, it first senses the medium and only transmits if the medium is idle.
CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance. When compared to CSMA/CD, now the strategy is collision avoidance rather than collision detection.
Encryption: A process of converting the information in a not readable format, by using a reversible algorithm. By using decryption, the information becomes readable again.
Wi-Fi: Wireless Fidelity. A logo from Wi-Fi Alliance that certifies network device compliance with IEEE 802.11.
EAP-TTLS: EAP-Tunnelled TLS. An authentication method similar to EAP-TLS. Only the server uses a certificate to authenticate to the client. The client uses EAP or a legacy method.
Bandwidth: The difference between the upper and lower frequencies of a continuous frequency band.
OSI model: Open Systems Interconnection reference model. It describes seven layers: (1) physical layer, (2) data link layer, (3) network layer, (4) transport layer, (5) session layer, (6) presentation layer, and (7) application layer.
Yagi Antenna: A directional variation of the dipole antenna, consisting of three or more half-wave elements: one driven, one reflector, and one or more directors. They are connected together by a long element called the boom.
LASER: Light Amplification by Stimulated Emission of Radiation. A laser generates light, either visible or infrared, through stimulated emission.
WLAN: Wireless Local Area Network. Wireless data communication system implemented as an extension or as an alternative to a wired LAN. User mobility is guaranteed in accessing the network resources. There are infrastructure mode and ad-hoc mode WLANs.
WiMAX: Worldwide Interoperability for Microwave Access. IEEE 802.16 standard based WAN wireless broadband technology.
RCCN-RCTS: Portuguese Academic National Network. National Scientific Community Network. Science, technology, and society network.
Access Point: A device that communicates with wireless stations and connects them to the LAN.
Omnidirectional Antenna: It radiates uniformly in a plane perpendicular to the antenna axis.
dBW: Represents decibels (power level) compared to 1 W. Power ratio dBW = 10 log10(P(W)/1 W).
CSMA/CD: Carrier Sense Multiple Access with Collision Detection. A modification to CSMA. If the network node which is about to transmit detects a busy medium, it waits a random amount of time before retrying. If a collision is detected in the medium, the node stops transmission, emits a jam signal to notify other active nodes to stop transmission, and defers retransmission by a random amount of time. If successive retries result in collisions, the random retry interval is doubled after each collision. This is known as the exponential back off procedure.
LDAP: Lightweight Directory Access Protocol. A protocol, compatible with X.500, for information retrieval from a common database. Especially useful for user authentication.
ETSI: European Telecommunications Standards Institute created in 1988 by CEPT-European Conference of Postal and Telecommunications Administrations.
VLAN: Virtual Local Area Network (Virtual LAN). Applies to a set of network nodes that behave as if they are connected to the same wire even though they may actually be physically located on different segments of a LAN. VLANs are based on IEEE 802.1Q. They are configured through software. Each VLAN has its own ID. Active equipment supporting VLANs is required.
Mbps: Megabits per second. Unit of data transfer speed in a communications channel.
Pan: Personal Area Network. A data communications network that covers a personal area, within a range of 10-20 m.
Signal to Noise Ratio: Expresses in dB the amount by which a signal level exceeds the respective noise. Otherwise, it represents power of signal over power of noise.
TCP/IP: Transmission or Transfer Control Protocol/Internet Protocol. TCP works at OSI layer 4, dealing with data delivery. IP works at OSI layer 3, dealing with addressing and routing.
Port: The input and output point for information to enter or leave a communication device.
Internet: A worldwide network, based on TCP/IP that interconnects thousand of heterogeneous networks and millions of users. Routers are used for network interconnection.
Point-to-Multipoint Wireless Link: Wireless communications where a transmitter/receiver communicates with several receivers/transmitters through unguided media paths. An omnidirectional antenna and several directional antennas can be used.
LAN: Local Area Network. A data communications network that covers a limited geographic area of up to 10 km.
WAN: Wide Area Network. A data communications network that covers a wide geographic area, larger than 100 km.
EAP-TLS: An authentication method where client and server use digital certificates. It is defined in RFC 2716.