Physical Layer
With physical layer we define resources to communication with mobile medium and with technically resources of boundary. Next we define physical, electronical, mechanical and functional parameters concerning physical connection single arrangement.
It is hardware layer and we can say about this too, that it have the undertaking transfer and set up the electronicle or opticle signals used to communication between PCs.
On this layer is created physical circle, on which they use to be put next machine, for expamle modem, which modulate signal on telephone link and the like.
Her purpose is activation and holding in activ level the connection term for transmision bites and marks. After transmission ending of course even deactivation.
The way of interface modems to PCs and terminals define standard RS-232-C (Recommended standard no. 232, revision C), comes from american standard organization EIA. It limits exactly boundary between 2 kinds of arrangement, which are marked in terminology of this standard like DTE (Data Terminal Equiplment) and DCE (Data Circuit-Terminating Equipment). Arrangement DTE is for example PC or terminal, and arrangement DCE is modem. Standard RS_232-C define not just count and meaning of single boundary signals between arrangement DTE-DCE, plug connection, but also electric parametrs transmission signals (voltage level, which says single status of this signals).
Today are this standards used to another function and it perhaps to connecting the printer or another periphery and also to relative PC dotting.
Disadvantages of standards are maximal reach just about 15 meters, and maximal transmission speed about 20 kbit/second. That´s why was created next, which are alternativ for this. They suppose the use of more powerful excitors and acceptors and the allow speed to 10 Mbit/second.
Abreast physical layer define references X.21 CCITT count and meaning of transmissioned signals even connector connection. It doesn´t define already electric properties of transmissioned signals- to this function come to use references V.10/X.26 and V.11/X.27.
Self specific resolution on physical layer level demand also sone new transmission technologies and resources. Physical laywer cover even standards, resp. references, which define useing this resources resp. technologies, for example for optical transmission references ISO 9314, which define standard FDDI (Fiber Distributed Data Interface), fot net ISDN references CCITT 1.430, which is for physical layer.
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