2 MAC, LAN, Layer and other strange words...

• Bandwidth : commonly it is the size of the channel used by the radio (the amount of frequency available to the system). By extension, it can also sometimes refer to the speed of the system (the bit rate).
• Bit-rate : speed at which bits are transmitted over the physical layer, also called signalling rate. Quite different from throughput (see chapter 5.4.1).
• Carrier : the base frequency used by the system. The modulation process will generate a signal centered on the carrier, of width equal to the bandwidth.
• Carrier Sense : checking the transmission medium to assess if it is free or if there is a transmission going on. Usually measure of the received power. See CSMA.
• CDMA (Code Division Multiple Access) : technique used to share the same bandwidth between different channels using codes. The code is a signature multiplexed with the signal and used to recover it. See chapter 4.3.1.
• CSMA (Carrier Sense Multiple Access) : using carrier sense to access the medium. One of the main MAC methods, see a verbose explanation in chapter 5.1.2.
• Cell : radio neighbourhood, area where all nodes can communicate with each other. As the range over radio is limited, the network is split into independent cells and a cell to cell communication is provided (via access point or internal routing).
• Channel : On the radio, this is usually synonym of a specific frequency, and by extension the communication medium. It can also mean a stream of data between two nodes (a point to point link in connection oriented systems).
• dB (decibel) : logarithmic way to express a value. Usually the signal strength (transmitted and received power) is expressed in dBm (the reference is 1 mW - 0 dBm). A difference between two values in dBm is without unit, in dB (in fact, this is a factor between the two values). See chapter 4.6.
• Ethernet : standard wired LAN protocol. Includes physical and link layers.
• Fading : variation in channel performance due to the dynamicity of the environment, make the receive signal strength change. See chapter 4.8.1.
• FEC (Forward Error Correction) : technique used to overcome some type of errors created by transmission on noisy channels, by adding redundancy bits to the main data transmission. See chapter 4.8.3.
• Frequency band : portion of the radio spectrum delimited for a particular use. For example, most wireless LANs use the 2.4 to 2.48 GHz band. A frequency band is usually divided in channels.
• Header : informations added by the protocol in front of the payload in the packet for its own use (addresses, packet type, sequence number, CRC...). Each protocol adds a different header, so in a typical TCP/IP packet as transmitted, we have a MAC header, an IP header and a TCP header, followed by the payload.
• IP : see TCP/IP.
• IPX : network protocol used in Netware, usually with SPX.
• LAN (Local Area Network) : network on a short distance, as opposed to WAN (typically inside a building).
• Latency : measure of the performance of a network for short requests and multimedia traffic. There is no real standard measurement, it might be the time to send and transmit a packet, or the time spent in the transmit queue, or the time for an answer to come back, or a number of requests per second...
• Layer : this terminology comes from the OSI specification. It divides any communicating system into 7 layers, each having a different functionality. Layer 1 is the physical layer, and layer 2 is the link layer. IP could be assimilated as layer 3 (network layer), and TCP as layer 4 (transport layer).
• Link layer : This is the part of the protocol managing the direct delivery between two devices on a specific physical layer (coaxial bus, point to point link, radio...). This includes packetisation and addressing. Most of this is implemented in the MAC.
• MAC (Medium Access Control) : this is the part of the radio device managing the protocol and the usage of the link. The MAC decides when to transmit and when to receive, creates the packets headers and filters the received packets. See chapter 5.1 for the main examples of MAC protocols.
• Medium : name to describe the mean used to transfer information. This could be a wire (twisted pairs, coax...), an optic fibber, the radio waves (the air), infrared light...
• Modem (modulator/demodulator) : in a radio device, this is the part converting the bits to transmit into a modulation of the radio waves and the reverse at the reception. It does the analog to digital conversion, the generation of the frequency, the modulation and the amplification.
• Modulation : specific way of coding information on a radio frequency. Basically, there is amplitude modulation (AM - change waveform strength) and frequency/phase modulation (FM - change waveform timing), but there exist many variations and combinations each designated by a specific acronym. See chapter 4.7.
• NetBeui : network protocol used in Lan Manager.
• Node : a device part of the network, source or destination of the data. For us, a computer with a radio card in it.
• Noise : any unwanted signal. Background noise, interferences, transmissions from nodes not belonging to the network... See chapter 4.8.
• Packet : Unit of transmission over the network. The data to be transmitted is split into packets, which are sent individually over the network.
• Protocol : specification of the interactions between systems and the data manipulated. This describes what to do and when (the rules), and the format of the data exchanged on the lower communication layer.
• Physical layer : this is the part of the device interacting with the medium. For a radio LAN, the physical layer is also called modem.
• Radio : electromagnetic waves. By extension, a device transmitting or receiving radio waves.
• Roaming : ability to move between cells of the same network. See chapter 5.3.2.
• SNR (Signal to Noise Ratio) : difference in strength between the signal we want to receive and the background noise (or any unwanted signal). See chapter 4.6.4.
• TCP/IP : network protocol used by Unix and Internet. Better in some respects than NetBeui and IPX (allows routing, for example).
• TDMA (Time Division Multiple Access) : technique used to share the same bandwidth between different channels using periodic time slots. See chapter 5.1.1.
• Throughput : measure of the performance of a network for large data transfer (such as FTP, NFS, HTTP 1.1). This speed is expressed in bits per seconds or a multiple.
• WAN (Wide Area Network) : network on a large scale : a town, a country or the world. Definitely not a LAN.
• Wired : using a wire.
• Wireless : not using a wire. For networks, it might be radio or infrared.

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Linux Wireless LAN Howto - jt@hpl.hp.com Converted to html from Frame Maker - 25 August 98 Updated 3 August 00 Copyright © 1996-2004 Jean Tourrilhes

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