The term narrow band, also narrow band communication, English narrow bandis used with different meanings depending on the context in the field of communications technology and in the field of Internet access.
In the field of communications engineering, the term narrowband describes a transmission channel whose bandwidth is so small that the frequency response can be assumed to be almost constant. This is equivalent to a group delay that is constant throughout the band. In narrowband transmission, it is in principle possible to do without channel equalization because of the frequency-independent group delay. If a transmission channel does not have a constant frequency response, as is the case in wideband communication, the transmitted useful signals are distorted differently depending on the level of their frequencies. Equalizers that reverse these distortions at the receiver of the wanted signals can be implemented using adaptive filters, for example.
In the case of digital data transmissions, a narrow-band transmission is defined equivalently via the symbol rate. Each transmitted symbol requires a certain symbol duration for transmission
. If the symbol duration is longer than the transmission time
at the transmission channel, a narrowband communication is present:
It is essential that the definition of a narrowband transmission is not fixed to concrete numerical values of a certain bandwidth, but is oriented to the respective marginal circumstances, such as the physical parameters of a radio link.
From the field of telephone networks and with the advent of Internet access starting in the 1990s, the term narrowband is used to refer to narrowband networks or narrowband access characterized by a bandwidth of less than or equal to 3.1 kHz or a data transmission rate of 64 kbit/s or less. This rigid definition is based, on the one hand, on the bandwidth of analogue fixed network telephony. For the transmission of analogue voice signals via copper wires in good quality, a frequency band of 300 Hz to 3.4 kHz was already defined in the early days of telephony for transmission-related reasons. A similar twin wire is also used for theUK0 interface in the digital transmission of voice signals in ISDN; the required bandwidth here is 40 kHz (for the 2B1Q code with 80 kBaud) or more, depending on the coding. In addition to the signalling channel (D-channel), two so-called basic channels can be transmitted in it, each with a data transmission rate of 64 kbit/s. ITU-T Recommendation I.113 defines the term narrowband service as having a data transmission rate of a basic channel of less than or equal to 64 kbit/s. In mobile telephony, also a narrowband service, digital voice is transmitted at a much lower data rate – in the case of the Full Rate Codec in GSM, the rate is 13 kbit/s. (The bandwidth required here is 2 kbit/s.) The data rate of a base channel is defined by the ITU-T Recommendation I.113. (The bandwidth required here is 2 – 200 kHz, and it can be used by eight connections simultaneously)
Narrowband services for Internet access include analog telephone systems and telephone modems as well as ISDN and mobile networks such as GSM with GPRS. Internet access systems such as DSL belong to broadband Internet access. They require wider frequency bands with more than 40 kHz – with corresponding restrictions on the part of the network operator with regard to line length.
- Karl-Dirk Kammeyer, Martin Bossert: Message transmission. 5. Auflage. Vieweg + Teubner, Wiesbaden 2011, ISBN 978-3-8348-0896-7.
- Jochen Seitz, Maik Debes, Michael Heubach, Ralf Tosse: Digitale Sprach- und Datenkommunikation: Netze, Protokolle, Vermittlung. Carl Hanser Verlag, 2006, ISBN 978-3-446-22979-2, p. 183.